Understanding Blue Green Algae
Blooms in Myall Lakes NSW.
NSW Department of Infrastructure, Planning and Natural Resources
As of the 2nd April 2003 the Department of Land and Water Conservation (DLWC) was
abolished by the NSW State Parliament and the Department of Sustainable Natural Resources
(DSNR) established. As of the 29 th May 2003 the Department of Sustainable Natural Resources
was abolished and replaced by the Department of Infrastructure, Planning and Natural
Resource (DIPNR). Hence most references to DLWC have been changed to DIPNR. However as
many aspects of this project, including contracts between DLWC and Environment Australia,
were undertaken prior to the 2nd April, reference is still made to DLWC in this document where
appropriate.
NSW Department of Infrastructure, Planning and Natural Resources
Understanding Blue Green Algae
Blooms in Myall Lakes NSW.
Prepared by:
Matthew Dasey, Joanne Wilson, Natasha
Ryan, Graham Carter, Nick Cook, Susana
Realica-Turner and
Edited By:
Matthew Dasey, Allan Raine,
Natasha Ryan, Joanne Wilson and Nick
Cook
Supported by:
NSW Department of Infrastructure, Planning and Natural Resources
Acknowledgments
This report was prepared with technical contributions from Lee
Bowling (CNR), Peter Scanes (EPA), Anna Redden (University of
Newcastle), Graham Harris (CSIRO) and Monika Muschal
(CNR). Also DIPNR would like to thank the residents of the
Myall Lakes area who greatly assisted in providing information on
the history of the lakes.
Published by:
New South Wales Department of Infrastructure,
Planning and Natural Resources
Hunter Region
July 2004
NSW Government
ISBN 0 7347 5498 1
Cover photographs – Myall Lakes. Taken by Joanne Wilson and Graham Carter
NSW Department of Infrastructure, Planning and Natural Resources
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Contents
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1. INTRODUCTION
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1.1 Background
1.2 Scope and Objectives of the Coast and Clean Seas Project
1.3 Myall Lakes in Comparison to other Australian coastal lakes and estuaries
1.3.1 Classification of Coastal Waterways
1.3.2 Classification of the health and status of coastal waterways in Australia
1.4 Nutrient dynamics in estuaries and coastal lakes
1.4.1 The sources of nutrients
1.4.2 Nutrient limitations, cycling and algal production
1.4.3 Response of lakes and estuaries to nutrient loads
2. CHARACTERISTICS OF MYALL LAKES AND ITS CACTCHMENT
2.1 Location of Study Area
2.2 Characteristics of the Myall Lakes region
2.2.1 Climate
2.2.2 Geology and soils
2.2.3 Preliminary geomorphic status of the Myall River catchment
2.2.4 Settlement History and Landuse
2.2.5 Human Settlement
2.3 Characteristics of the Myall Lakes System
2.3.1 Geography
2.3.2 Lake Flushing
2.3.3 Lake Structure and Bathometry
2.3.4 Sedimentology of the Broadwater, Myall Lakes NSW
2.3.5 Benthic Nutrient Fluxes in Bombah Broadwater, Myall Lakes
2.3.6 Groundwater quality
2.3.6.1 Groundwater Electrical Conductivity
2.3.6.2 Groundwater pH
2.3.6.3 Groundwater Chemical composition
2.3.6.4 Indications of Possible sewage contamination
2.3.6.4.1 Faecal Bacteria
2.3.6.4.2 Nutrients
2.3.6.4.3 Nutrient Trends versus Water Level Trends
2.3.6.4.4 Nutrients and Park Occupancy
2.3.3.4.5 Synopsis
3. NUIRIENT SOURCES TO MYALL LAKES
3.1 Catchment nutrient inputs to Myall Lakes
3.1.1 Catchment Management Support System Modelling
3.1.1.1 Comparison of loads arising in Myall Lake sub-catchments
3.1.1.2 CMSS Estimates of historical and contemporary annual nutrient loads
3.1.1.3 CMSS Estimates of nutrient source arising from different land uses
3.1.2 Estimates if nutrient load from River volume
3.1.3 Measured Nutrient Exports from the Catchment - autosampler results
3.1.4 Results from Water Quality Monitoring in the Myall River
3.1.5 Atmospheric Nutrient Contributions
3.1.6 Potential nutrient contributions from National Park visitor sewage waste
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3.1.7 Estimated Phosphorus load from Groundwater inflows
3.1.8 Estimate of Myall Lakes System nutrient loading per unit lake surface area
4. PHYTOPLANKTON ABUNDANCE AND DISTRIBUTION
4.1 Background
4.1.1 Phytoplankton community as an indicator of estuarine health
4.1.1.1 Changes in phytoplankton community and ecological succession
4.1.2 Some Characteristics of Blue-Green Algae
4.1.2.1 Adaptations of Blue-Green Algae
4.1.2.2 Ambient conditions that enhance Blue-Green algal growth
4.1.2.3 Human health implications of Blue-Green algae
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4.2.1 Aims of the monitoring study
4.2.2 Algal sampling and analysis
4.2.3 Data Analyses and presentations
4.3 Results
4.3.1 Spatial Distributions of Blue-Green Algae
4.3.2 Blue-Green Algal community change and succession
4.3.2.1 Multivariate analysis of temporal and spatial trends in the Blue-Green Algae
Community
4.3.2.2 Phase 1 - Blue-Green Algal Bloom
4.3.2.3 Phase 2 - Blue-Green Algal Bloom
4.3.2.4 Phase 3 - Blue-Green Algal Bloom
4.3.3 The response of algal community composition to changing conductivity
4.3.4 Descriptions of phytoplanktonic community succession in Myall Lakes
4.3.4.1 Upper Myall Rivermouth
4.3.4.2 Bombah Braodwater
4.3.4.3Two-Mile and Boolambayte Lakes
4.3.4.4 Myall Lake
4.3.4.5 Some observed patterns of phytoplankton diversity and abundance
4.4 Discussion
4.4.1 The role of conductivity in algal community composition change
4.4.2 Patterns of nutrient concentration and the algal community in Myall Lakes
4.4.3 Other factors influencing the composition of Blue-Green Algae communities
5. WATER QUALITY
5.1 Water Nutrients
5.1.1 Introduction
5.1.2 Methods
5.1.2.1 Rainfall
5.1.2.2 Water nutrient analyses
5.1.2.3 Near-Benthic Water collection
5.1.2.4 Interpreting nutrient data against ANZECC and ARMCANZ guidelines
5.1.2.5 Data consolidation and presentation
5.1.3 Results
5.1.3.1 Rainfall
5.1.3.2 Total Phosphorus
5.1.3.3 Soluble Reactive Phosphorus
5.1.3.4 Total Nitrogen
5.1.3.5 Ammonium
5.1.3.6 Nitrate/Nitrite
5.1.3.7 Chlorophyll a
5.1.3.8 Near-Benthic Water Collection
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5.1.4 Discussion
5.1.4.1 Temporal and spatial patterns of nutrient distribution
5.1.4.2 N Near-Benthic Water Collection
5.2 Nitrogen Stable Isotope signatures of Aquatic Plants
5.2.1 Introduction
5.2.2 Methods
5.2.3 Results
5.2.4 Discussion
5.3 Physico-chemical parameters
5.3.1 Introduction
5.3.2 Methods
5.3.3 Results
5.3.3.1 Stratification
5.3.3.2 Light availability
5.3.4 Discussion Physico-chemical water quality
6. AQUATIC VEGETATION AND LAKE HABITAT
6.1 Background
6.1.1 Vegetation
6.1.2 Lakebed sediments
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6.2 Methods
6.2.1 Vegetation assessment method
6.2.2 Lakebed assessment method
6.3 Results
6.3.1 Vegetation
6.3.2 Types of aquatic plants found in Myall Lakes
6.3.3 Spatial Distribution of plants in Myall Lakes
6.3.4 Temporal Change in the Distribution of plants in Myall Lakes
6.3.4.1 Najas marina and Charophytes - Annual growth cycles
6.3.4.2 Vallisneria gigantea - Broadwater die-off
6.3.4.3 Potamogeton perfoliatus - Widespread colonisation
6.3.5 Benthic Survey - Results and Discussion
6.3.5.1 Percent Total Organic Carbon (TOC)
6.3.5.2 Total Nitrogen (TN)
6.3.5.3 Total Phosphorus (TP)
6.3.5.4 Total Sulphur (TS)
6.3.5.5 Grainsize Analysis
6.3.5.6 Benthic Microcystis sp
6.4 Discussion
6.4.1 Vegetation
6.4.2 Benthic Nutrient Content
6.4.3 General Discussion
7. SYNTHESIS OF THE DATA
7.1 What are the physical and biological conditions/processes, which contribute to
Blue-Green Algal blooms in Myall Lakes
7.2 What are the relative contributions of nutrient sources to algal growth in the lakes?
7.2.1 The role of nutrients in algal growth
7.2.2 Sources of nutrients to Myall Lakes
7.2.2.1 Rainfall
7.2.2.2 Groundwater
7.2.2.3 Catchment Inflows
7.2.2.4 Nutrient recycling within the lakes
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7.2.3 Summary of nutrient source data
7.3 What is the nutrient status of Myall Lakes
7.3.1 Techniques to assess nutrient status
7.3.2 Assessing the nutrient status of Myall Lakes
7.3.3 Risks due to increasing nutrients
7.4 Supporting the development of effective management strategies to reduce
catchment loads
7.4.1 Regional Strategic Focus: Lower North Coast Catchment Blueprint
7.4.2 Local Catchment Level: Myall Community Catchment Plan
7.4.3 Specific Initiatives: Dairy Industry trials to investigate effectiveness of buffer strips
7.5 Developing pro-active assessment protocols to assist in the management of future
algal blooms
7.6 Myall Lakes - Where to from here?
7.6.1 Continued Monitoring in Myall Lakes and its Catchment
7.6.2 Additional ongoing investigations
7.6.3 Future Studies
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8. REFERENCES
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9. APPENDICES
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Executive summary
Myall Lakes is one of the largest coastal lake systems in New South Wales boasting over ten
thousand hectares of waterways set wholly within the Myall Lakes National Park. The Myall
Lakes system comprises a series of lakes including the Bombah Broadwater (lower lake),
Two Mile and Boolambayte Lakes (mid-lakes) and Myall Lake (upper lake). Feeding this lake
system is a catchment area of 78,000 hectares. The Myall and Crawford rivers are the main
tributaries to the lake system, feeding into Bombah Broadwater, while Boolambayte Creek
also supplies fresh water. The Lower Myall River connects this unique waterbody to the
ocean, allowing saltwater exchange from Port Stephens. Myall Lakes has significant
environmental and cultural value to the local, national and international community. The lake
system is recognised internationally under the Ramsar Convention as an important wetland,
and Myall Lakes National Park is a popular tourist destination for camping, bushwalking,
fishing, boating and water sports. A healthy lake system is integral to the culture and
economy of the local area.
In early 1999 Myall Lakes began to exhibit major signs of a natural system in trouble when a
large, toxic blue-green algae bloom formed in the lower section of the lakes. Blue-green algae
are a type of bacteria that act like plants by using light for photosynthesis. When conditions
are ideal they can multiply at a prolific rate resulting in a bloom. Potentially harmful algal
scums accumulated on the shores of the lake including at many popular camping areas. The
bloom persisted on-and-off until April 2001 having a major impact on the local community –
tourist numbers dropped and the lakes were intermittently closed to commercial and
recreational fishing. Blue-green algal blooms have continued to occur in the lakes since mid
2001, although not as severely as those experienced in 1999.
The initial algal bloom in 1999 left the Myall Lakes community extremely concerned about
the future of their unique natural asset. The State Government responded to these concerns by
initiating the ‘Monitoring Blue-Green Algae in Myall Lakes’ project - a partnership between
the then Department of Land and Water Conservation (DLWC; now Department of
Infrastructure, Planning, and Natural Resources [DIPNR]) and the NSW National Parks and
Wildlife Service (NPWS: now part of the Department of Environment and Conservation
[DEC]) with funding from the Federal Government’s Coasts and Clean Seas program. The
project was designed to:
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understand the combination of factors that cause algal blooms in the lakes;
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determine the overall health of the Myall Lakes system; and
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predict the likelihood that they would occur again.
This information was considered vital to ensure future resources could be effectively targeted
to manage the lakes system, and the impact of possible future algal blooms.
Information collected during the study (2001–2002) was combined with information collected
prior to 2001, and from recent catchment studies. Interpretation of the many sources of
information was complex, and in many cases, characteristics of both eutrophic and
mesotrophic environments occurred simultaneously or sequentially in the same area.
However, a picture has emerged of the major nutrient sources, temporal and spatial patterns in
physical and ecological characteristics of Myall Lakes, and the likelihood of toxigenic algal
blooms occurring in the future.
The period over which information was available (1999-2002) encompassed both above and
below average rainfall conditions, and extended throughout the entire Myall Lakes region.
During the period of the project (2001-2002), below average rainfall was experienced, and no
toxigenic algal blooms occurred. This study highlighted the naturally high variability in
physical and ecological characteristics among the interconnected lakes and between ‘wet’ and
‘dry’ periods. During the study a benthic organic microalgal layer was discovered over much
of the bed of Myall Lake and some of the mid-lakes region. The role this layer plays in the
ecology of the lake is yet to be determined, though some speculation is made in this report.
The study showed that catchment runoff from the Upper Myall River is currently the most
important source of ‘new’ nutrients contributing to the formation of toxigenic algal blooms in
Myall Lakes (noting that a significant amount of nutrient is already ‘stored’ within the lake
sediments). Due to long flushing times, the lake acts as an effective trap for nutrients, organic
matter and sediments and is therefore highly vulnerable to increased inputs from the
catchment. The naturally low flushing also results in increased importance of the processing
of organic matter and nutrients by processes occurring in the lake sediments.
The status of the three main regions of the lake system is discussed separately below.
Broadwater
The occurrence of toxigenic algal blooms in the Broadwater in 1999 alerted the community
and natural resource managers to the potentially eutrophic status of this waterway.
Investigations during the project showed that while some characteristics of the Broadwater are
generally indicative of eutrophication, others are typical of mesotrophic waterways. Most
signs of eutrophication in the Broadwater are associated with periods of high inflow when a
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large proportion of the total nutrient load is delivered to the system. At times of low inflow, it
appears the area is able to ‘recover’ to a mesotrophic state.
Evidence for eutrophication of the Broadwater includes high loads of nutrients from the
catchment, long flushing time, the occurrence of toxigenic blue-green algal blooms, low water
clarity, sometimes high concentrations of phosphorus and nitrogen in the water column, the
presence of filamentous algal species in macrophyte beds, restriction of aquatic plants to
shallow water, and low denitrification rates in muddy sediments. However, the presence of
extensive beds of perennial macrophytes, good water quality during dry periods and high
denitrification in shallow sandy sediments indicate a mesotrophic status.
It appears then that at most times, the lake is ‘healthy’ but switches to a eutrophic state in
response to large inflows from the catchment. The extended period of above average rainfall
that occurred prior to April 1999 created favourable conditions in the Broadwater for
toxigenic blue-green algal blooms. Large inflows of freshwater from the catchment reduced
the salinity of the Upper Myall Lake and Broadwater to that of a freshwater system, carried
high loads of nutrients and sediment, and caused increased turbidity. It appears this
combination of conditions, particularly the extremely low salinity, occur infrequently in the
Broadwater and are dependant on the duration and intensity of rainfall in the catchment of the
Upper Myall River.
The flushing time of the Broadwater is long relative to other coastal lakes in NSW, and
therefore it will act as an effective sink for nutrients, organic matter and suspended sediments.
However, compared to other regions within Myall Lakes, it is most influenced by both
catchment inflows and tidal exchange. Therefore, while it receives the highest nutrient and
organic load from the catchment, a small amount will be lost through flushing. Also, the
relative influence of saline intrusion and freshwater inflows result in significant, long term
changes in salinity related to wet and dry rather than seasonal cycles. For example, over the
period of this study, salinity ranged between 1 and 18 mS/cm.
The Upper Myall River is a significant source of nutrients to the Broadwater. Levels of
phosphorus in the water column at the mouth of the Upper Myall River are typically high in
both wet and dry periods. This indicates that the Upper Myall River is an important source of
Phosphorous to the Broadwater, even in dry conditions when flows are low.
The Broadwater is naturally turbid due to the deposition of fine particles carried in the Upper
Myall River. These settle rapidly once riverine waters meet the slower moving lake waters.
Strong winds and the shallow nature of the Broadwater results in frequent resuspension of this
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fine material. Naturally turbid conditions restrict the growth of macrophytes to the shallower
areas of the lake around 1.5m depth or less. Therefore, increases in turbidity would further
restrict the distribution of macrophytes in the Broadwater to the shallow margins of the lake.
The macrophyte assemblage in the Broadwater is composed of species typical of estuarine to
brackish environments. It includes perennial macrophytes (Vallisineria, and Ruppia); and
loosely attached macroalgal assemblages (Gracillaria and Enteromorpha). These
macrophytes play a number of very important roles as primary producers, physical habitat for
fish and invertebrates, absorbing nutrients, stabilising sediments, and oxygenating the water
column. Many macrophyte species are sensitive to changes in nutrient loads or ratios, and
reduction in area or changes in species composition can be an indicator of decline. The
presence of filamentous algae amongst some areas of macrophyte beds in the Broadwater is
likely to be a sign of increasing eutrophication.
The sediment distribution and bathometry of the Broadwater is typical of other coastal lakes.
Shallow banks are composed of sandy sediments, which slope steeply to deeper areas
characterised by fine muddy sediments. Preliminary studies showed that carbon, nitrogen and
phosphorus content in the sediments were similar to other coastal lakes in NSW. While these
nutrients are generally bound tightly to sediments, studies in the Broadwater showed that
nutrients stored in muddy sediments may be released into the water column as ammonium or
nitrate, fuelling further algal growth. This release of inorganic nutrients to the water column
is enhanced under low oxygen conditions. Low oxygen near the sediments are likely to occur
in the Broadwater after inflows deliver organic matter and freshwater. This increases the
oxygen demand of the sediments due to decomposition of organic matter, and stratification
due to salinity differences. This often results in the bottom layer becoming anoxic as the
oxygen is not replaced by photosynthetic activity or wind mixing.
Myall Lake
Of all three lakes, Myall Lake appears to be the most ‘healthy’ but also the most sensitive to
any increases in nutrient loads. Myall Lake has good water clarity, low nutrient load (in terms
of delivery of ‘new’ nutrients from the catchment, particularly phosphorus [P]), generally low
water column nutrient concentrations, and diverse macrophyte assemblages. These
characteristics are indicative of an oligotrophic system but the dominance of green and small
blue-green algal species in the phytoplankton, and the highly organic content of the bed of the
lake are signs of some level of nutrient enrichment. However, toxigenic blue-green algal
species such as Anabaena and Microcystis have not been recorded in the water column of
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Myall Lake during the study period. The blue-green algal alerts were triggered by high
biovolumes of other non-toxigenic species of blue-green algae.
Myall Lake has no major freshwater inflows and is furthest from the influence of tidal
flushing. Consequently it has an extremely long flushing time which makes it highly sensitive
to any increase in nutrient loads. It also results in an extremely stable water body. The salinity
of the lake is very low and movement of saline water from the Broadwater only has a very
small influence on the Lake. Salinity changes are very small and occur over the time scale of
years. The water column is well mixed and remains clear throughout the year. Stratification
was not recorded during this study due to the lack of freshwater inflows. Therefore conditions
which favoured the development of toxigenic blue-green algal blooms in the Broadwater did
not occur in Myall Lake.
Any significant increase in the nutrient load has the potential to cause symptoms of
eutrophication, including the development of toxigenic algal blooms. The current
characteristics and composition of the phytoplankton and macrophyte communities provide
some indication as to the likely consequences of increased nutrients, particularly phosphorus.
The phytoplankton community is dominated by small blue-green and green algal species,
which are good competitors in low phosphorus environments. Diatoms are unlikely to become
a major part of the community, as there are indications of silica limitation in this part of the
lake. Silica is an essential requirement for diatom growth and values are very low in Myall
Lake. In addition, while the macrophyte community is diverse, it is dominated by
macrophyte species such as Chara and Najas. These species have a highly seasonal growth
pattern and form extensive beds which cover up to 80% of Myall Lake during summer and
autumn, and then die off over winter. At present, these macrophytes grow to the bottom of the
deeper areas of the lake due to the good water clarity and high light penetration. Species such
as Chara and Najas will only grow in low phosphorus environments. Increases in P are likely
to result in a switch from a clear, macrophyte dominated community to a more turbid system
dominated by phytoplankton species.
During this study, phosphorus concentrations in the water were very low. Values for total
nitrogen were consistently high throughout the study, although it is likely that approximately
80% of the total nitrogen is in a form (dissolved organic nitrogen) not readily bioavailable to
algae. These results probably reflect the lack of riverine inflows and the recycling of organic
matter and nutrients within this essentially closed system. The seasonal growth of
macrophytes is likely to utilise a large amount of nutrients during periods favourable for algal
growth (high light and temperature). The decay of large quantities of macrophytes may have
contributed to the high nitrogen and carbon content on the bed of this lake. The nutrients in
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the sediments and organic layer are likely to play an important role in the nutrient cycling of
Myall Lake due to its reduced flushing. Further investigations are required to determine
whether nutrients are released into the water column, and the conditions that may favour this.
This organic layer also contains a benthic Microcystis species that has not been detected in
water column samples. This benthic algal layer may be playing an important role in absorbing
nutrients generated in the sediments through remineralisation of organic matter before they
reach the water column. It is also suspected that the benthic algal layer may have a major
influence on the type of aquatic plants present in Myall Lake. It is speculated that the benthic
layer does not allow the establishment of most species, due to its mucous-like character, and
that only Najas has the ability to establish and grow within it.
Mid-Lakes
The Mid-Lakes region encompasses Boolambayte and Two-Mile Lakes. This region receives
a small riverine inflow from Boolambayte Creek and also exchanges water with the
Broadwater at Bombah Pt and Myall Lake at Violet Hill. Not surprisingly, the characteristics
of this region are intermediate between the Broadwater and Myall Lake. Two Mile Lake is
more similar to the Broadwater, while Boolambayte Lake shares characteristics similar to
Myall Lake. While the nutrient loads to this region are smaller than to the Broadwater, the
flushing time is longer and therefore this area is more susceptible to nutrient inputs.
Two Mile Lake shows signs of eutrophication similar to that of the Broadwater, particularly
during wet weather. It has experienced toxigenic algal blooms, and elevated nutrient
concentrations. However, it is less turbid than the Broadwater and has healthy and diverse
macrophyte communities. Macrophytes are composed of both perennial and ephemeral
species, especially in the southern part of the lake. There, extensive sandy areas support dense
beds of perennial species to a depth of 2m. There was some growth of the ephemeral species
Najas, over summer, but growth was not as extensive as in Myall Lake. This may be due to
competition for nutrients with other macrophyte species, and reduced water clarity which
restricted the development of Najas to shallower areas. Also, higher concentrations of
phosphorus which occur in Two Mile Lake compared to Myall Lake may not be suitable for
the growth of some species such as Chara.
Water quality of Two Mile Lake reflects the exchange with and proximity to the Broadwater.
The southern part of this lake had characteristics similar to the Broadwater i.e. higher salinity
and phosphorus and lower nitrogen, while the northern part was more similar to Myall and
Boolambayte Lakes. The conditions, which favoured the development of toxigenic algal
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blooms in the Broadwater also, occurred in Two Mile Lake. As such these occurred
concurrently in the two lakes.
The substrate and bathometry of Two Mile Lake is composed of extensive shallow sandy beds
covered in macrophytes, with steep drop offs to muddy, organic areas up to 3 m deep. High
concentrations of ammonium in waters close to the bottom in deeper areas highlights the
possibility that remineralisation of organic matter may be returning ammonium to the water
column.
Boolambayte Lake receives inflows from Boolambayte Creek. Given the longer flushing time
of this Lake compared to Two Mile and Broadwater, it is likely that this area is the most
susceptible to increased nutrient loads from the catchment. Currently, Boolambayte Lake
shows characteristics similar to Myall Lake. The lake is clear, and is dominated by ephemeral
macrophyte species Najas, and numbers of toxigenic algal species remained low even during
blooms in Two Mile Lake. Salinity is generally low but shows a greater range than Myall
Lake. Nutrient concentrations are similar to Myall Lake with generally low phosphorus, and
higher nitrogen. The phytoplankton assemblage is dominated by small blue-green and green
algal species with Violet Hill Passage recording some of the highest biovolumes of nontoxigenic blue-green algae. These non-toxigenic blue-green algae have triggered alerts after
the toxigenic algal blooms disappeared from the Broadwater and Two Mile Lakes.
Boolambayte Lake is relatively shallow and extensive areas of the bed are covered in the
organic microalgal layer. The characteristics of this material are similar to that of Myall Lake
although values for organic carbon and nitrogen content were lower than in Myall, but higher
than the rest of the lake.
Synopsis
The study has highlighted that for toxic blue green algae blooms to form, a number of factors
need to occur at the same time. The main factors include:
? freshwater inflows (floods) from the upper Myall River which are large enough to
displace saline water in the Broadwater (so that salinity levels are at least lower than
2mS/cm);
? turbid water which gives toxic blue-greens a competitive advantage over other
phytoplankton species; and
? high levels of nutrients to fuel algal growth (including those delivered by flooding and
potentially those released from sediments).
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It is likely that the conditions, which were responsible for the blooms of 1999, will occur
again, and therefore it is expected that toxic blue green algal blooms will occur in future.
Because the upper Myall River directly discharges freshwater into the Broadwater it is likely
that the conditions required for bloom formation will be limited to this area, and the adjacent
Two-mile Lake, in the short to medium term. The project has shown that a good indicator for
the potential of a bloom to form in the Broadwater is when salinity levels fall below 2mS/cm
(and conditions become more or less eutrophic).
The Broadwater’s link to Port Stephens (and hence tidal waters) ultimately increases salinity
levels (following freshwater inflows) to a point where toxic blue green algae species cannot
survive (and conditions return to mesotrophic).
The study has highlighted that the relative isolation of Myall Lake from direct freshwater
inflows from the catchment and saline inflows from the estuary, has created a stable
environment where toxic algal blooms are unlikely to occur. It has also highlighted that Myall
Lake would be very sensitive to any sustained increase in nutrient levels.
The study has found that there is no ‘quick-fix’ to remove toxic algal blooms once they form.
It also recognises that the cumulative impact of increased nutrient delivery from the
catchment (mainly during flood flows) over time may have led to increased sediment /
nutrient levels. As such, the main management action that can reduce the likelihood of
blooms occurring over the long-term is to reduce the amount of nutrient delivered to the lakes
from the catchment during flood events. Catchment management should be targeted at those
landuse activities that deliver the highest nutrient levels relative to the area they occupy.
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Chapter 1, Introduction.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
1. Introduction.
1.1. BACKGROUND.
The Myall Lakes system is a series of interconnected fresh to brackish coastal lakes that are
central to the Myall Lakes National Park. The Myall Lakes represent the only remaining
example of a large coastal brackish lake on the NSW coast that has not been greatly modified
by human activities and only one of its type in the Manning shelf bioregion (NPWS 2002).
The outstanding natural and cultural values of this lake system have been recognised
internationally, with Myall Lakes National Park being listed as a Ramsar Wetland of
International Importance (NPWS 2002).
In early April 1999, a bloom of harmful blue green algae (Anabaena spp) was reported in the
Broadwater and lower section of Boolambayte Lake (anecdotal evidence suggests that a small
outbreak could have formed some time earlier in 1999). The bloom extended from Bombah
Broadwater through to Korsmans Landing, with scums forming at foreshore sites including
popular camping areas. In response, the Manning/Karuah/Great Lakes Regional Algae
Coordinating Committee (MKGLRACC) placed the lower section of Myall Lakes up to
Korsmans Landing on ‘algal high alert’ in accordance with the Manning/Karuah/Great Lakes
Regional Algal Contingency Plan. Under this protocol, alerts (warnings and/or closures) are
placed on certain locations to minimise human contact with harmful algae.
Since the initial bloom in 1999, numerous warnings have been issued and lifted in different
areas of the lakes in accordance with the Contingency Plan, with the final warning on the
Bombah Broadwater being lifted in April 2001. In June 2000, there was a sharp decline in the
toxin producing species such as Anabaena circinalis and Microcystis aeruginosa in the
Broadwater, and an increase in non-toxin producing species such as Chroococcus sp and
Merismopedia sp (details of algal dynamics are presented in Chapter 4).
However, Myall Lake and the upper reaches of Boolambayte Lake have experienced repeated
blue-green algal blooms, composed predominantly of Chroococcus, over the past 2 years, in
particular around the Neranie area. This area of Myall Lakes is the least flushed part of the
whole system (see Chapter 2) and is more freshwater in its ecology. The last ‘high alert’ for
this area was lifted in April 2002. However, a high alert was issued for Violet Hill in
November 2002 in response to potentially harmful levels of blue green algae (Chroococcus)
being detected. The high alert status was still current in early February 2003.
NSW Department of Infrastructure, Planning, and Natural Resources
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Chapter 1, Introduction.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
The occurrence of harmful blue green algal blooms has often been associated with
eutrophication (nutrient enrichment) of inland and coastal waterways in Australia and
overseas (Livingston 2001). Prior to the recent blue green algal blooms Myall Lakes was
considered a near-pristine coastal lake and there was little information on the ecology and
nutrient status of the lakes. Atkinson et al (1981) highlighted the vulnerability of the system to
eutrophication, although recent cursory assessments considered the lakes to be in good
condition (MHL 1998). Therefore the occurrence of blooms was unexpected, and it was
difficult to move beyond speculation of the cause of the blooms, or whether blue green algal
blooms had occurred in the lakes prior to 1999.
The sustained blue-green algae blooms within Myall Lakes focused attention not only on the
environmental conditions within the lake itself, but also on the surrounding catchment area
which is the primary source of most water column nutrients. As a result, the then NSW
Department of Land and Water Conservation (now Department of Infrastructure, Planning
and Natural Resources [DIPNR]1 ) and NSW National Parks and Wildlife Service (NPWS;
now part of the Department of Environment and Conservation [DEC]2 ) initiated the Myall
Lakes Catchment Investigation to provide a basic assessment of catchment water quality and
some of nutrient cycling processes within the lake system. As part of this assessment, a longterm monitoring program was also set up to monitor phytoplankton within the lakes so that
the distribution of blue-green algal blooms within the system could be determined and to
assess risk to public health as required under the RACC protocols. Funding was also made
available for additional studies. These included; measuring nutrient loads from the catchment
in rain events, using an autosampler stationed at Bulahdelah; monthly monitoring of nutrient
concentrations at a number of sites in the Upper Myall River; and measurement of the fluxes
of nutrient from the lake sediments to the water column.
In 2001, further funding was obtained through the Federal Government’s Natural Heritage
Trust Coasts and Clean Seas (CCS) program for the project ‘Monitoring Blue Green Algal
Blooms in Myall Lakes’ to investigate the ecology of the lakes and assess its nutrient status.
This information was required to identify the factors that may have led to the blooms, assess
the likelihood of future blooms, and draft informed management options to reduce the risk of
this occurring.
1
Note that the project was undertaken by the Department of Land and Water Conservation (DLWC).
DLWC is now known as the Department of Infrastructure, Planning, and Natural Resources (DIPNR)
and is hence referred to as such in the body of this report.
2
Note that subsequent to the project being finalised the Department of Environment and Conservation
was established bringing four agencies together into a single department that includes the National
Parks and Wildlife Service.
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Chapter 1, Introduction.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
1.2. SCOPE AND OBJECTIVES OF THE COASTS AND CLEANS SEAS PROJECT.
The Coasts and Clean Seas project ‘Monitoring Blue Green Algae in Myall Lakes’ was
developed as a partnership between DIPNR and NPWS. The monitoring project was initiated
in September 2001. The main objectives of the project were to:
?
Determine the physical and biological conditions and processes that contributed to the
occurrence of blue green algal blooms in Myall Lakes;
?
Assess the relative contribution of different sources of nutrients to algal growth in Myall
Lakes;
?
Assess the nutrient status of Myall Lakes by comparing the relative biomass of different
primary producers (phytoplankton, benthic microalgae, macroalgae, seagrasses /
macrophytes) in Myall Lakes;
?
Obtain information to support the development and implementation of effective
management strategies to reduce catchment nutrient loads; and
?
Develop pro-active assessment protocols to assist in the management of future algal
blooms.
The project was an ambitious one at the outset and while the objectives of the project have
been met to varying degrees. For example plant biomass investigations were replaced with a
study of plant distribution after the discovery of a layer of Gyttja diverted resources to its
investigation there is still much to learn about nutrient dynamics and lake ecology. The results
of the monitoring program, combined with additional work carried out prior to and since the
commencement of the project, provide information on fundamental ecological processes
affecting nutrient dynamics and phytoplankton growth within Myall Lakes. Findings of the
project will also be incorporated into existing management plans and processes including the:
?
Plan of Management for Myall Lakes National Park and RAMSAR Site Management,
?
Lower North Coast Catchment Management Blueprint.
?
Myall Lakes Community Catchment Plan,
This report details the results of the CCS project and provides a starting point for promoting a
better understanding of the lakes’ dynamics and ecology.
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Chapter 1, Introduction.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
1.3. MYALL LAKES IN COMPARISON TO OTHER AUSTRALIAN COASTAL
LAKES AND ESTUARIES.
1.3.1. Classification of Coastal Waterways.
Each coastal waterways is a unique ecosystem with characteristics (eg. salinity, age, size,
catchment size, flushing time) that can make it quite distinct in character even from adjacent
waterways. The biological communities in these waterways can likewise also be unique, so it
is difficult to describe and quantify the health and character of a waterway relative to others,
based on the resident ecological assemblages. There have been recent attempts to standardise
the classification and health of estuaries in Australia.
Roy et al. (2001) has published a classification for south east Australian estuaries. Three main
types of estuary are recognised in NSW. They are:
1) Tide-dominated, drowned river valleys,
2) Wave-dominated, barrier estuary and,
3) Intermittently Closed and Open Coastal Lakes and Lagoons (ICOLLs).
The latter two estuary types are the most common in NSW and often have water residence
times3 of greater than one year (Roy et al., 2001). These systems are generally shallow, with
mixing achieved predominantly through wind-driven water circulation (Roy et al., 2001).
The Myall Lakes system is described as a brackish barrier estuary (Roy et al., 2001) and has
an estimated flushing time of between 400 and 800 days (MHL 1998; Atkinson et al., 1981).
1.3.2 Classification of the health and status of coastal waterways in Australia.
Estuaries are the end point for the runoff from coastal catchments. Due to their small
entrances in proportion to their volume and diminished tidal range, coastal lakes and lagoons
often act as sinks for inputs from their catchments. The nutrient trapping ability of coastal
lakes makes these locations very productive natural systems, and crucial components of
coastal ecology. Conversely, the ability of these estuaries and coastal lakes to trap nutrients
makes them especially prone to eutrophication and sedimentation.
Rapid declines in lake health resulting in loss of habitat and biodiversity may shortly follow
excessive nutrient loadings. Therefore, because coastal lakes are especially vulnerable, much
3
Water residence time refers to the estimated time it takes for the water in a given area to be fully
replaced.
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Chapter 1, Introduction.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
of the recent effort has been applied to rapidly identify the waterways most likely to
experience environmental decline. Two recent initiatives have provided assessments of the
health or condition of estuaries Australia wide, and of coastal lakes in NSW. These are:
1. National Land and Water Resources Audit (NLWRA) -a nationwide estuarine assessment,
2. Healthy Rivers Commission (HRC) Inquiry into Coastal Lakes –focussing on the health
and management of coastal lakes in NSW.
NLWRA - Estuarine Assessment, 2000.
The NLWRA assessed the health of 970 estuaries Australia-wide and classified them into 4
categories of Near Pristine, Largely Unmodified, Modified and Severely Modified. The initial
condition assessment results found that 28% of the estuaries assessed in Australia were
thought to be in a modified or severely modified condition. However, in NSW, a much higher
proportion (49%) of estuaries was assigned this status. A summary of the results of this
survey is shown in Table 1.1 below.
Table 1.1. Summary estuarine status for Australia (NLWRA, 2001)
Status
Total Percentage
Australia-wide
NSW
Near pristine
49%
10%
Largely unmodified
23%
40%
Modified
17%
26%
Severely modified
11%
23%
The NLWRA has classified Myall Lakes as a wave-dominated estuary in a largely unmodified
condition. Estuaries in this condition are considered by the NLWRA as generally being in
good condition, but with some catchment and estuary impacts. This category comprised 23%
of the 979 estuaries assessed Australia wide and 40% of the 133 estuaries assessed in NSW
(NLWRA, 2001). Within NSW the majority of estuaries that were assessed fell within the
Largely unmodified category (see Table 1.1)
The Healthy Rivers Commission (HRC) Inquiry into Coastal Lakes.
In undertaking their comprehensive inquiry into coastal lakes the HRC assessed 91 coastal
lakes in NSW and classified them into categories depending on the amount of management
intervention required to ensure the sustainable health of each waterway. The 4 categories of
protection needed were:
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Chapter 1, Introduction.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
1. Comprehensive Protection,
2. Significant Protection,
3. Healthy Modified Conditions and,
4. Targeted Repair.
Each of these categories represents the fundamental ‘management orientation’ for coastal
lakes required under the HRC’s Coastal Lake Strategy. A summary of the results of the
inquiry is shown in Table 1.2.
In undertaking their assessment the HRC based their classification on three broad factors:
?
Inherent natural sensitivity to human activities (eg. from potential pollutant inflows,
flushing capacity, entrance behaviour),
?
Existing condition of the catchment and lake water (eg the extent of land clearing,
potential impacts of different land uses and occurrence of water quality problems and fish
kills) and,
?
Recognised natural and resource conservation values (eg presence of listed species,
ecosystem uniqueness and representativeness, commercial values, reserves).
Table 1.2. Classification of Coastal Lakes in NSW based upon the HRC findings (HRC,
2002)
Management Orientation
Comprehensive Protection
Significant Protection
Healthy Modified Conditions
Targeted Repair
Total Percentage
18%
30%
39%
13%
According to the HRC findings Myall Lakes has been grouped within the Significant
Protection management framework which includes around 30% of the coastal lakes in NSW.
The HRC report recognised Myall Lakes as having an extreme natural sensitivity rating as
well as a high conservation value (HRC, 2002), and that it requires a high level of care to
ensure its future health.
Another important feature of Myall Lakes is the fact that the lakes are one of the few brackish
coastal lakes that exist in Australia and only one of the two that occur in NSW, Lake
Ainsworth being the only other. Lake Ainsworth is a small system receiving water from a
highly modified catchment and has been rated by the NLWRA as having a Modified condition
NSW Department of Infrastructure, Planning, and Natural Resources
6
Chapter 1, Introduction.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
and classified by the HRC within the Targeted Repair grouping (NLWRA, 2001& HRC,
2002).
Despite their size differences, both of these lakes share similar ecological attributes, and each
has experienced periodic blue-green algal blooms in recent years. An indication perhaps of
the extreme vulnerability of this specific type of waterway and a warning in general of the
consequences of catchment degradation can have on coastal lakes.
1.4. NUTRIENT DYNAMICS IN ESTUARIES AND COASTAL LAKES.
1.4.1. The sources of nutrients.
Even pristine catchments export nutrients to coastal lakes in the waters of streams and rivers
arising within them. However, human disturbances such as deforestation, agricultural
practices (eg. fertiliser application and land clearing) or point source pollution such as
stormwater run-off can lead to a far greater amount of excessive nutrient loads being
delivered into these poorly flushed water bodies (Entry & Emmingham, 1996; Mander et al.,
1997; Jorgensen et al., 2000). For example, Eyre (1997) contrasted historical and
contemporary nutrient loads (nitrate and phosphate) into the Richmond River estuary and
found that the concentrations of nitrate and phosphate were up to 3 times greater in
comparison to when the catchment was relatively unmodified in the 1940’s. It is generally
thought that Australian waterways are particularly well adapted to function optimally when
nutrients are in very short supply (Harris, 2001) in comparison to other parts of the world
because of the aridity of the climate.
According to Harris (2001), pristine catchments generally export low loads of nitrogen (N)
and phosphorus (P), with the predominant form of N being dissolved organic nitrogen (DON),
which arises naturally from the decay of plant matter. When catchments are cleared for timber
production or agricultural use there is an abrupt change to dissolved inorganic nitrogen (DIN)
being the predominant form of N (Harris, 2001).
In such modified catchments, a change in the form and increase in the annual load of nutrients
entering waterways can lead to eutrophication, where natural ecological processes cannot
function properly (Young et al., 1996; Martin et al., 1999). Nutrient enrichment may result in
the development of algal blooms through the rapid growth of opportunistic autotrophs such as
cyanobacteria and other phytoplankton, and macroalgae (Codd, 2000; Menéndez & Comín,
2000). Also, high sediment loads can lead to an increase in the turbidity of a system, which
can alter plant assemblages, through decreasing attenuated light levels and restricting
macrophyte growth (Asaeda et al., 2001).
NSW Department of Infrastructure, Planning, and Natural Resources
7
Chapter 1, Introduction.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
1.4.2. Nutrient limitation, cycling and algal production
It is well documented that nitrogen (N) and phosphorus (P) are the nutrients most likely to be
limiting primary production in aquatic and marine systems (see reviews by Young et al.,
1996; Harris, 1999). Young et al. (1996) state that in marine systems, N is the primary
limiting macronutrient, while in freshwater systems P tends to be the most likely to be the
limiting nutrient.
This occurs because in freshwater systems, rates of nitrogen fixation are relatively high,
denitrification rates are relatively low, and P is effectively bound to iron and not bioavailable.
This results in a relatively higher proportion of N to P available for algal growth. In marine
systems, nitrogen fixation rates are relatively low, denitrification rates are higher, and sulphur
binds effectively to iron in sediments, liberating P. The mechanisms underlying nutrient
limitation will also be affected by a number of factors including changes in salinity, and the
availability of other elements (Harris, 1999). Estuaries and coastal lakes experience variations
in these parameters and, accordingly, either or both of these nutrients may be limiting at
different times or under different conditions (Young et al., 1996).
In the N cycle, atmospheric N (N2 ) diffuses into the water column or soils where nitrifying
bacteria convert it into inorganic N in the form of ammonium (NH4 +), which is available for
uptake by aquatic macrophytes, macroalgae and phytoplankton (Boulton & Brock, 1999).
Through the death and decay of organic material and by bacterial nitrification, ammonium
(NH4 + ) is converted to nitrite (NO2 -) and nitrate (NO3 -), which can also be assimilated for
photosynthetic growth (Boulton & Brock, 1999). The final step in the N cycle, known as
denitrification, is the metabolic process of certain bacteria that convert NO3 - back into
nitrogen gas, which is either cycled back into the system and fixed by bacteria, or is diffused
into the atmosphere (Boulton & Brock, 1999).
In impacted systems, where excessive amounts of organic carbon is present, and low oxygen
conditions occur, the process of denitrification may be interrupted. This results in the
conversion of NO3 - to NH4 + that is released into the water column for uptake by most algae
instead of being removed from the system by denitrification.
In undisturbed systems, P usually arises from the weathering of rocks and sediments, entering
the estuary as inorganic phosphate (PO 4 3-) (van der Molen et al., 1998; Boulton & Brock,
1999). Phosphorus is rarely present in estuarine systems as dissolved PO4 3- because it is
rapidly assimilated by primary producers or readily adsorbs onto colloidal particles
(Gonsiorczyk et al., 1998; Boulton & Brock, 1999) but this process is greatly affected by
salinity. Unlike N, P is not readily recycled in the water column or lost to the atmosphere by
NSW Department of Infrastructure, Planning, and Natural Resources
8
Chapter 1, Introduction.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
microbial degradation of organic matter, but tends to remain tightly bound to sediment
particles, mainly in conjunction with iron complexes. However, in estuarine systems when
anoxic / reducing conditions occur, sulphur forms complexes with iron which can release
PO4 3- into the water column. Furthermore, PO43- precipitates into the sediment where it may
be stored or assimilated by macrophytes via their roots (Gonsiorczyk et al., 1998; van der
Molen et al., 1998; Boulton & Brock, 1999). Phosphorus can be released from estuarine
sediments when anoxic conditions are apparent, reducing conditions exist, when the ionic
bond with iron is weakened (Boulton & Brock, 1999).
1.4.3. Response of lakes and estuaries to nutrient loads
There are a number of common and well documented symptoms of excess nutrient loading to
lakes and estuaries which include: blooms of phytoplankton and macroalgae; occurrence of
blue-green algal blooms; increased turbidity; declines in distribution or health of seagrass and
macrophytes; periodic anoxia; and fish kills. However, once these symptoms occur, the
ecosystem could already be overloaded with nutrients, and restoring the health of the system
is likely to be very difficult. According to Scheffer et al. (1993) and Scheffer (1998), this is
because shallow aquatic environments tend to exist in two stable states. Harris (1999b) further
states that estuarine systems and can switch between these two states relatively quickly. The
two states are:
1) Clear and dominated by macrophytes or seagrass, or
2) Turbid and dominated by phytoplankton.
The response of a coastal waterway to an increasing or decreasing nutrient load is complex
and non-linear especially in shallow systems. Shallow lakes are very adept at processing and
cleansing themselves of nutrient loads, but when this capacity is exceeded, the ability of the
system may ‘crash’ to a fraction of it’s previous capacity. This is due to the combined failure
of nutrient cycling processes that remove nitrogen from the sediments, and the collapse of
biological processes that maintain biodiversity and prevent the domination of a few groups of
plants or phytoplankters (Harris 1999b).
Although denitrification is the main process of nitrogen loss from most estuaries, the removal
of water column nutrients through flushing to the ocean can also play a role in lowering
nutrient loads. This can be particularly true for freshwater slugs following large rain events
that can pass right through well flushed estuaries directly into the ocean. Lakes that have long
residence times trap incoming nutrients and receive less assistance in removing nitrogen from
their waters, and consequently are naturally prone to higher N concentrations in the water
NSW Department of Infrastructure, Planning, and Natural Resources
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Chapter 1, Introduction.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
column, especially where incomplete denitrification is occurring. Complete denitrification is
further inhibited by excess organic carbon loading in these places and anoxic conditions
leading to the release of ammonia from the sediments, which fuels further algal blooms.
Under some conditions, internal cycling of nutrients stored in the sediments of coastal lakes,
particularly of N, may be the main source of nutrient loads to the water column in dry
conditions, and keeps the water column nutrients elevated during periods that would normally
be oligotrophic. The high nutrient concentrations in Myall Lakes at some times in dry weather
could be a consequence of this process. The reduction of loads from the catchment may have
little effect on the nutrient status of waterways that are nourished from benthic stores of
nutrients until a significant improvement in denitrification efficiency is achieved.
NSW Department of Infrastructure, Planning, and Natural Resources
10
Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
2. Characteristics of Myall Lakes and its
catchment
2.1. LOCATION OF STUDY AREA
Myall Lakes is a large coastal lake system on the lower north coast of NSW approximately 70
kilometres north of Newcastle.
Figure 2.1. Myall Lakes Catchment.
2.2. CHARACTERISTICS OF THE MYALL LAKES REGION
The total catchment area of Myall Lakes is approximately 780 km2 and the main input of
freshwater into the lake system is from the Myall and Crawford Rivers that join at Bulahdelah
and flow into the western portion of Bombah Broadwater (see Figure 2.1). The only other
major inflow of surface waters into the lakes is from Boolambayte Creek, which flows into
the northern portion of Boolambayte Lake. Other inputs of fresh water are from rainfall upon
the lakes’ surfaces and groundwater drainage from the sand mass on the eastern shore.
Elsewhere around the lakes the watershed lies very close to the shoreline and only limited
inputs of freshwater occur from these sources (Atkinson et al. 1981).
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Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
2.2.1. Climate
Climatic influences play an important role in affecting the characteristics of Myall Lakes with
the larger El Niño and La Niña, and seasonal climatic cycles changing the balance of
freshwater inflows, evaporation and tidal flushing within the lakes.
In dry periods, Myall Lakes increases in salinity due to evaporation and greater seawater
inflow from Port Stephens. In wetter periods, larger and more frequent rainfall events result in
larger inflows from the catchment changing the lake to a brackish or freshwater dominated
system. This increased input of freshwater often carries greater nutrient loads and sediment
derived from the catchment.
The mean annual rainfall for Bulahdelah is 1328 mm with the wetter months occurring in late
summer and early autumn with mean monthly rainfall exceeding 100 mm (see Figure 2.2).
The driest months are late winter and early spring with mean monthly rainfall about 60 mm
with summer rainfall generally more reliable than winter rainfall. Rainfall decreases inland
but upland areas probably receive more rainfall (Atkinson et al. 1981).
Mean monthly maximum temperatures range from 27 °C in summer to 17 °C in winter with
minimums of 15 °C and 3 °C respectively.
Daily rainfall records have been documented for the catchment at the Bulahdelah Post Office
since 1906. These records provide an excellent picture of long-term variation in rainfall over
the previous 95 years within the Myall Lakes catchment. Figure 2.3 below shows how the
rainfall has varied over this 95 year period.
The dashed line shows the mean rainfall of 1328 mm per year which is represented as 0 mm.
Years with lower than average rainfall move the graph down by the difference between the
average and that years total, while years with above average rainfall move the graph up by the
difference.
NSW Department of Infrastructure, Planning, and Natural Resources
12
Mean Total Monthly Rainfall (mm)
Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
180
160
140
120
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
11
12
Month
2001
1996
1991
1986
1981
1976
1971
1966
1961
1956
1951
1946
1941
1936
1931
1926
1921
1916
Bulahdelah Post Office
1911
3000
2500
2000
1500
1000
500
0
-500
-1000
-1500
-2000
1906
Rainfall (mm)
Figure 2.2. Mean Monthly Rainfall for Bulahdelah Post Office (1906 to 2001).
Year
Figure 2.3. Cumulative departure from the mean for rainfall recorded at the Bulahdelah
Post Office.
NSW Department of Infrastructure, Planning, and Natural Resources
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Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
1800
1600
1400
1200
1000
800
600
400
200
0
Mean Yearly
Rainfall
Actual Yearly
Rainfall
19
90
19
91
19
92
19
93
19
94
19
95
19
96
19
97
19
98
19
99
20
00
20
01
Rainfall (mm)
Chapter 2, Characteristics of Myall Lakes and its catchment
Years
Figure 2.4. Yearly rainfall recorded at Bulahdelah Post Office between 1990 and 2001.
Figure 2.3 shows that between 1906 and 1950 fairly average rainfall was experienced with
few events of greatly above average rainfall experienced. The latter part of this period
experienced a significantly dry period between 1936 and 1950. This dry period also coincided
with an increase in agricultural activities within the catchment. Between 1950 and 1980 the
Myall catchment was significantly wetter than the previous half of the twentieth century with
a number of significantly above average rainfall events occurring during this period. Between
1980 to 2001 the trend reversed and there tended to be less rainfall on average, which is
typical of an El Niño weather pattern. Figure 2.4 shows the later half of this period in more
detail. This figure illustrates the typical long term weather pattern of below average rainfall
followed by a period of sustained above average rainfall.
2.2.2. Geology and soils
The geology of the Myall Lakes catchment consists of a series of unconsolidated Quaternary
sediment deposits associated with alluvial flats and several sand dune systems with the valley
itself developing within a large syncline of varied rock types defining the catchment
boundary. The underlying bedrock of the catchment consists of sedimentary and volcanic
rocks of the Carboniferous and Permian age.
The dominant geological structure within the catchment is the Myall Syncline, which
developed after a major folding and faulting of the bedrock. The synclinal and anticlinal axes
of this system expose rock types of the Alum Mountain and Nerong Volcanics and have a
north-west and south-east orientation (Worth, 1992). Ridges flank both sides of the Myall
NSW Department of Infrastructure, Planning, and Natural Resources
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Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
syncline and consist of Carboniferous rocks. These were formed between 290 – 345 million
years ago by a combination of sedimentary deposits and volcanic activity. They consist of
sandstones, mudstones and siltstones with some igneous intrusions.
The valley floor consists of Permian sandstones and shales exposed by extensive erosion
which were formed between 240 – 280 million years ago by sedimentary processes. These
formations were deposited by the ocean and have resulted in rolling hills near Bulahdelah and
Markwell and consists of sandstone, conglomerate, shale and some coal deposits.
The sediments of the Quaternary period are the most recent formations and are only 2 million
years old. These formations were deposited by alluvial processes, including weathering and
erosion, and occur in the lower reaches of the Myall River and Boolambayte Creek. Aeolian
(wind transported) and wave processes have lead to the formation of extensive dune systems
half way between Bulahdelah and the Bombah Broadwater and on the eastern side of the
Myall Lakes. These extensive sand dune deposits developed ridges that formed a barrier
system. They completely or partly enclose lagoons or swamps that form linear depressions
parallel to the shoreline.
2.2.3. Preliminary geomorphic status of the Myall River catchment.
In October 2002 the DIPNR undertook a preliminary geomorphic assessment of the Myall
River Catchment to determine the most likely location of sediment sources, stores and sinks
since European settlement.
There is little doubt that in many NSW catchments European activities such as deforestation,
and in particular the clearance of riparian vegetation has caused channel incision and bank
erosion that has lead to the degradation of the channel itself (see Brooks 1999 for example).
The Myall River is no exception to this and it has undergone a similar process to that of other
coastal streams in NSW, losing many of its pre European settlement geomorphic features.
During the field assessment two areas of sediment accumulation and four potential sediment
sources were identified within the Myall Catchment two of which are directly related to the
degradation of the Myall River. The potential sediment sources include:
Meander cutoffs on the alluvial reaches
The clearing of vegetation from floodplains and banks usually causes the geomorphic process
of channel straightening or meander cutoffs to occur with increased frequency. This is due to
the loss of bed and bank cohesion provided by riparian vegetation which in turn allows faster
bend migration, erosion of flood runners and bed lowering via ‘headcuts’.
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Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Once bed lowering has occurred this destabilises the unvegetated banks causing them to
collapse in on themselves resulting in channel expansion. Within the area around Markwell,
bed lowering of around 1 metre is evident and is indicated by remnant bed levels of
abandoned channels and the high location of older trees that were formerly at the toe of the
bank.
Channel expansion of about 2 to 3 times is apparent, evident in the small size of recently
abandoned channels relative to the much larger size of the present channel. All of these
effects have potentially released large volumes of floodplain sand and silt into the river which
are then available to be transported downstream. These have then been either deposited on
floodplains as overbank deposition or flushed through the system and into the Lakes.
Bed incision on partly confined reaches
Upstream of Markwell the bed of the Myall River has incised about 0.5 m, which may be the
result of the removal of logs from within the stream and by increased runoff from the cleared
catchments. This process is indicated by the exposure of large cobbles in the bed. According
to Nanson and Doyle (1999), these streams would have had sandy or gravelly beds and large
loadings of wood before European settlement.
Cobble beds with very little wood are now a common feature of the Myall Catchment with
most of the sand and gravel already being washed out of the upper reaches of the catchment.
However these reaches will still be contributing small amounts of coarse sediment.
Gullies
A few eroding gullies were evident in the upper catchment and would be contributing a small
amount of coarse sediment to the lower order tributaries of the Myall River. This observation
agrees favourably with that of Atkinson et al. (1981) who noted that no appreciable
accelerated erosion was evident throughout the Myall Lakes catchment although some
localised gully erosion was evident.
Slopes direct to streams
A few eroding slopes were evident within the Boolambayte Creek catchment. Sites such as
these would be contributing a small amount of fine sediment to the lower order streams.
Sediment Stores
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Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
The floodplains adjacent to the Myall River are functioning as sediment stores (areas where
material may be accumulated temporarily). This material would be mobilised during flow
events that flow over the floodplain. Floodplain stripping or reworking is common on these
types of rivers given the nature of flows, and high flow events may be sufficient to scour
around vegetation or to strip the top layer of floodplain surface. Flood channels are also
features within the floodplain itself, which during smaller flow events may release stored
material through the loss of vegetation and subsequent bed lowering.
Sediment sinks
Sediment sinks are the places where sediment is permanently removed from the system so
that it does not move further down the river, at least in the historical time scale. The main
catchment sediment sinks observed within the Myall catchment are the terraces in the wider
parts of the alluvial valley. These are very old deposits and are not a part of the present
sediment budget and have formed via the deposition of sediment at the edges of the valley on
the old floodplain during the Holocene period upstream of Bulahdelah (Thom, 1965). During
this period south-eastern Australia was wetter and therefore more fluvially active resulting
higher stream flows and the formation of the features (Nanson and Doyle, 1999).
Therefore from preliminary investigations, the major sources of sediment in the Myall
catchment appear to be streambeds and stream banks with a small input from cleared gullies,
with slope erosion contributing a minor amount. This is consistent with other findings for east
coast catchments (Brierley and Fryirs, 1998, Caitcheon et al., 2001). The main sources of the
stream bed and bank sediment appear to be derived from channel incision and expansion on
the alluvial reaches, which has occurred around the middle of the catchment. This too is
consistent with other east coast catchments in that the middle reaches of east coast rivers
appear to be the most impacted by anthropogenic activities. According to Nanson and Doyle
(1999), the flood events that occurred in the late 1940’s and early 1950’s were widely
reported as channel modifying events within coastal NSW. This correlates well with Figure
2.3 which shows above average rainfall within the Myall catchment coinciding with the
increased landuse within this timeframe.
The preliminary results of this qualitative study indicate that the Myall River has contributed
and potentially continues to contribute a significant quantity of material downstream during
high flow events. This conclusion is contrary to the findings of Worth (1992) and further
quantitative investigations are required.
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Chapter 2, Characteristics of Myall Lakes and its catchment
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2.2.4. Settlement History and Landuse.
European settlement in the Manning and Karuah catchments, which included the Myall
Catchment, occurred between 1804-1817 (DLWC, 2000). Prior to this the Myall catchment
was inhabited by the Worimi aboriginal people whose area extended from Port Stephens in
the south to Forster/Tuncurry in the north and as far west as Gloucester. The early European
development of the region was directly linked to the timber resources of the area, with cedar
felling preceding both agricultural and urban development.
By 1817 cedar was being transported to Port Stephens and shipped to the settlements at
Newcastle and Sydney. Timber grants were given out in the early 1830s and by the late 1830s
cedar cutters had worked their way up the coast, reaching Coolongolook. Sawmills were
established on the foreshores of the Myall Lakes and Boolambayte Creek. Most of the timber
cut in the district was used for ship construction, with the timber being hauled down the Myall
River.
In about 1825 the Australian Agricultural Company was formed and given a land grant of 404
858 hectares extending from Port Stephens to the Manning River. However the company
found that the land was largely unsuitable and a land swap was made with the Crown. The
township of Bulahdelah was established in 1857 at the junction of the Myall and Crawford
Rivers.
Initial attempts at dairying failed, due to the soil limitations and restricted transport network.
Beef cattle production therefore became the predominant farm landuse. Timber production
expanded rapidly during World War II to provide essential materials for the war effort. The
area developed into a major forestry district, however its full potential was not exploited due
to transport network limitations.
Grazing in the area was initially based on native pastures however, the introduction of the
mechanised farm machinery in the 1940’s allowed improved pasture to be sown. By 1948
virtually every property along the Myall River was a dairy farm. These were mostly small
family farms, stocking an average of 30 cows each. Corn was also grown extensively along
the river flats to provide feed for cattle during the winter.
The current landuses in the catchment still include pasture production associated with the
grazing of beef and dairy cattle, although only 5 dairies are still in operation. The major
landuse within the catchment is timber production/conservation, and recreation and tourism
with a large section of the catchment to the east, west and south covered by State Forests and
National Park (see Table 2.1).
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Chapter 2, Characteristics of Myall Lakes and its catchment
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in Myall Lakes NSW.
State Forests cover an area of 26 414 hectares. These forests include Nerong State Forest (8,
200 ha) which is situated on the western side of the Pacific Highway south of Bulahdelah and
the Myall River State Forest (19, 100 ha) is a large area of forest that extends along the
western boundary of the Myall catchment and Bulahdelah State Forest, which is located in the
middle, east and north-eastern parts of the catchment.
The Myall Lakes National Park was established in 1978 covers an area of 44 612 hectares,
and is the only national park in NSW that comprises an entire waterbody and shoreline. As
well as the conservation of native plant and animals, the national park supports a range of
tourist and recreational activities including, camping, bushwalking, picnicking, swimming,
boating and fishing.
Table 2.1. Landuses of the Myall Catchment.
Landuse
State Forest
Privately held forest lands
National Park (native vegetation)
Poultry
Grazing -Native Pasture
Grazing -Improved Pasture
Percentage of catchment area
32.5
20.2
19.4
Less than 1%
13.1
1.3
Dairy
Urban
Rural Residential
Other
Roads and Easements
Water (Rivers and Lake area)
Unknown
0.2
0.3
0.3
0.2
12.5
2.2.5. Human settlement.
There are two main population centres within the catchment, these are Bulahdelah, through
which the Pacific Highway runs, and Hawks Nest / Tea Gardens. The population of these
centres is 1161 and 2545 respectively (G. Tuckerman pers. comm). Hawks Nest and Tea
Gardens are popular tourist destinations and provide access to the Myall Lakes National Park
and eastern shore of the Bombah Broadwater.
Bulahdelah is the largest centre upstream of the lakes and the commercial centre of the upper
catchment with the remainder of the catchment dominated by rural holdings. The name
Bulahdelah comes from the Worimi people’s original name for the area and means “the place
beneath a mountain where two rivers meet” these two rivers being the Myall and Crawford
Rivers.
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Chapter 2, Characteristics of Myall Lakes and its catchment
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Downstream from Bulahdelah Mid-Coast Water operates a Sewage Treatment Plant (STP) for
the township. The STP discharges treated effluent to Frys Creek, a tributary of the Myall
River and was upgraded to tertiary level treatment in 1996 to meet the NSW Environment
Protection Authority discharge standards for protected waters. Prior to this effluent was
discharged following secondary treatment only.
2.3. CHARACTERISTICS OF THE MYALL LAKES SYSTEM.
2.3.1. Geography
The Myall Lakes system lies within 2.5 kilometres of the Tasman Sea and is separated by two
distinct dune systems with a minimum topographic relief between the lakes and the ocean of
20m (Skilbeck et al. 1999). The system is made up of a series of four interconnected brackish
lakes: Myall Lake; Boolambayte Lake; Two Mile Lake; and Bombah Broadwater which give
Myall Lakes a total water surface area in excess of 100km2 . The lakes are drained to the ocean
by the lower Myall River, which stretches 26km from Bombah Broadwater to Port Stephens.
Myall Lakes overlies the New England Fold Belt, which has largely determined the
configuration of the lakes. The western shoreline and the islands within the system are
comprised of bedrock outcrops comprised of irregular Carboniferous sediments of the New
England Fold Belt. The eastern shoreline has mainly been formed by the two distinct dune
systems of the late Pleistocene and Holocene, which link headlands formed from
Carboniferous outcrops (Skilbeck et al. in press). The lakebed is formed between the wide
Pleistocene ridge and the Holocene beach systems. This represents the most recent marine
transgression and may have reached a standstill about 5000 years ago (Worth, 1992).
Lakeshore features such as the sand and pebble dominated beaches and bluffs that occur along
the shoreline of Myall Lake in the north have been described by Thom, (1965) as relic
features representative of a period of higher energy within the lake. As no marine fauna or
extensive aeolian deposits have been found along the shorelines, Thom (1965) concluded that
both the cliffs and beaches are relic lakeshore features possibly a result of a period of
increased wave energy. The relic shorelines therefore suggest a decrease in the intensity of
wave action accompanying the siltation of the lake.
2.3.2. Lake flushing.
Unlike most open lake systems in NSW that are close to the ocean much of Myall Lakes is
not subject to semi-diurnal tidal flushing, saline water only moves up the system in extended
periods of low rainfall. The lower Myall River and southern portion of Bombah Broadwater
appear to be the only parts of the lake system that are subject to significant tidal flushing
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Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
(Atkinson et al. 1981). Therefore the amount of water able to leave the lakes each day is small
compared to the volume of the lakes. This means that due to the limited exchange with the
ocean any nutrients that enter the lake system, especially during large rain events, are likely to
be retained within the lakes.
Sampling for total salts by Atkinson et al. (1981) in the lakes between 1972 and 1978 showed
inter-lake variability in salinity with the upper portions of the lakes exhibiting a low range in
salinity while Bombah Broadwater exhibited the widest range of salinity (Ryan, 2002). Manly
Hydraulics Lab (MHL, 1998) reported that salt water entering the lakes via the lower Myall
River is trapped in Bombah Broadwater and then slowly flows up the rest of the system as a
gravity current. Wind and convection currents cause vertical mixing of lake waters.
The constriction of the lakes’ entrance has resulted in long water residence times within the
lakes system. These times have been calculated by Manly Hydraulics Lab (1999) at between
400 to 800 days which is much longer than most other NSW coastal lakes. This figure was
derived from salinity data and is an estimate of the approximate time for the whole volume of
the Myall Lakes to exchange with Port Stephens. This long flushing time also gives the lakes
a large salinity range as discussed further in Chapter 5, with the southern part of Bombah
Broadwater exhibiting estuarine characteristics and Myall Lakes freshwater characteristics.
The long water residence and flushing times are also important because it means that that the
lakes are very sensitive to human activities in the catchment and essentially act as a ‘sink’ for
nutrients. Atkinson, et al. (1981) identified this and stated that the lack of flushing of the
system upstream of Bombah Broadwater makes this area particularly susceptible to pollution
and eutrophication.
2.3.3. Lake Structure and Bathometry
In 2001 DIPNR carried out a bathometric survey of Myall Lakes as part of the NSW
Government’s Estuary Management Program (Figure 2.5). The aim of the bathometric survey
was to provide not only an accurate picture of navigable waters within the lakes system but it
will also provide catchment managers with an accurate base upon which changes in aquatic
vegetation could be mapped and areas of significant sediment inputs from the catchment
identified.
From the data collected in the survey the average depth of Myall Lakes has been calculated at
2.7m, which is shallow in comparison to other coastal lakes. Nevertheless, the bathometry of
Myall Lakes is quite varied with some areas ranging in depth quite significantly. Boolambayte
Lake for example has a maximum depth of 13 metres at Violet Hill but has an average depth
of 2.3m.
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Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
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Figure 2.5 Bathometry of Myall Lakes.
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Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
The mouth of the Myall River is another area of deep water with its deepest part 6.5m deep
although the bed level quickly shallows up 1.5 upon entering Bombah Broadwater. The
Broadwater itself is also quite interesting with large areas of shallow water on the northern
and eastern shores. These areas are sandy and flat topped and range in depth from 1 to 1.5m
on average but then quickly drop off into deeper water, which occupies the remainder of the
Broadwater. These two areas are well developed and occupy quite a significant area of the
northern and eastern shorelines of the Broadwater.
Two-Mile Lake is also dominated by a large sand bar feature on its south-western shoreline.
This feature like the ones in Bombah Broadwater rises very quickly to an average depth of
between 1 to 1.5m from about 4m in the main channel.
Smaller sand bar features also occur on the eastern shore Myall Lake in Palmers Bay and
Kataway Bay but these are not as dominant as the ones in Bombah Broadwater and Two Mile
Lake. The bar features described exhibit similar depth characteristics and rise rapidly from the
lakebed to a depth of 1 to 1.5m from and average of about 2.5 to 3m. The remainder of the
lakebed, generally below 2m is comprised of muds. Thom (1965) reported that preliminary
observations reveal that the lake bottom is comprised of soft muds and fine organic debris.
Along the western shoreline of the lakes the shallow bays are typically dominated by a muddy
substrate but this is in some cases overlayed by matter high in organic content, which will be
discussed further in Chapter 6.
Work is still ongoing with the development of a hydraulic model for the Myall Lakes system.
This will enable prediction of lake circulation patterns that are present and what affect these
movements have on the mixing of waters and the movement of algae and nutrients within the
water column.
2.3.4. Sedimentology of the Broadwater, Myall Lakes, NSW.
Worth (1992) undertook a study as part of a Graduate Diploma at the University of Newcastle
to describe the sediments of the Bombah Broadwater and investigate their possible origin and
rate of accretion. A number of possible sources of lacustrine sediment were envisaged from
the geological formations and the landuse history within the catchment. These included;
suspended material transported from the Myall River, deposited organic material and beach
sands from the Holocene and Pleistocene dune systems.
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Chapter 2, Characteristics of Myall Lakes and its catchment
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An estimation of the accretion rate of sediments was obtained using two different methods
that utilise the decay or radioactive isotopes. These were radiocarbon dating (C14 ) and
radioactive caesium dating (Cs137 ). The average accretion rate of sediment in the Broadwater
was estimated to be 0.10 cm/yr over the last 1190 years (using the radiocarbon dating) and
0.184 cm/yr over the last 38 years (for the Cs137 dating) (Worth 1992). Worth (1992) states
that the agreement between these two rates many indicate a long term stability in the system
which is supported by other evidence from the upper Myall River, although this evidence is
not discussed. Also, radiocarbon dating was only carried out on one core only at the mouth of
the Myall River whereas the radio Cs137 dating carried out on 12 cores.
According to Worth (1992), the benthic sediments of the lake and associated waterways
appear to consist mainly of silt and clay precipitated under increasing salinity as the waters
move downstream. The actual site of this precipitation may vary according to the flooding by
the river (Worth 1992). Worth concluded that since European settlement, anthropogenic
disruption to the natural regimes of erosion and sedimentation in the catchment have been
minimal, which is in conflict with the findings of Section 2.2.3, which found significant
erosion in some subcatchments had occurred. Obviously, further investigation of sediment
accretion rates within the lake (including the influences of factors such as bioturbation and
resuspension by wind) is required before an accurate picture can be acquired.
2.3.5. Benthic Nutrient Fluxes in Bombah Broadwater, Myall Lakes.
In June 2000 the Australian Geological Survey Organisation (AGSO, now Geosciences
Australia) conducted an eleven-day study in Bombah Broadwater, measuring fluxes of
nutrients from sediments of different types using benthic chambers. The aim of the study was
to provide an estimate of the fluxes of different forms of nitrogen from the sediment into the
water column. Three sites were established two on mud substratum and a third on shallow
sandy material near Mungo Brush.
It is important to note that the benthic chamber study was undertaken on a single occasion
during winter months and it is possible that the nutrient fluxes might not be typical of the
rates at other times of the year. The estimates of sediment denitrification in Myall Lakes
indicates that denitrification efficiency was low (approximately 40%) during the study at the
mud sites. The sandy location in the eastern Broadwater however, had a fairly high
denitrification efficiency (approximately 80 %, see Figure 2.6). The results suggests that at
the time of the study Bombah Broadwater was in a state in which labile nitrogen in the form
of ammonia from the sediments was contributing to plant growth. It is possible that the
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Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
extended cyanobacterial bloom experienced in Myall Lakes over the summer of 1999/2000
was exacerbated by poor sediment denitrification (AGSO, 2000).
In summary, the results from the AGSO study indicates that at the time of the study:
?
within the mud substrate, Myall Lakes exhibited high organic carbon loads and low
denitrification efficiencies;
?
within the sandy substrate areas Myall Lakes exhibited low organic carbon loads and high
denitrification efficiencies;
?
the denitrification efficiencies in the mud facies are low when compared to other similar
Australian estuaries; and
?
the contrasting poor mud denitrification and good sand denitrification may, in part,
balance the overall efficiency of the Broadwater’s denitrification.
It is difficult to generalise about the system-wide denitrification rate from the limited data
available, particularly in light of the findings of the sediment survey (Chapter 6). However,
the denitrification efficiency of the mud substrate is probably more representative of the
system as a whole than the sand facies. However, the low efficiency in muddy places may be
ameliorated to some extent by the high denitrification efficiencies within the sandy substrate
areas. The low denitrification efficiency at the mud sites is verging on values that the AGSO
PPB model of estuarine eutrophication predicts as being within the range typical of a
eutrophied waterway (Figure 2.6).
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Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Figure 2.6 The modelled relationship between residence time of nitrogen in an estuary
and sediment denitrification (AGSO, 2000). Residence time is defined as the number of
times added nitrogen is recycled between sediments and the overlying water column. (NB.
Port Phillip Bay is also an area where elevated nutrients are causing detrimental changes to its
ecology).
2.3.6. Groundwater quality.
Groundwater within the Myall Lakes Catchment occurs in both fractured rock aquifers and
unconsolidated sand deposits. Deep groundwater is generally associated with the fractured
rock formations of Carboniferous and Permian origin. These groundwaters contain higher salt
concentrations due to their contact with rock formations that originated in a marine
environment. The regional groundwater flow in these fractured rock aquifers is from the
north-west to south-east.
Unconfined perched aquifers occur almost everywhere in the Quaternary sands in the Myall
Lakes National Park. These unconsolidated sand deposits generally have a high degree of
porosity and permeability. These properties provide the sands with high storage and
transmitting abilities. The local groundwater flow in the sand dunes east of Myall Lakes is
either north-west to south-east towards the lake or north-east to south-west towards the ocean
depending on the lake and sea level fluctuations (DLWC, 2000b).
Information describing groundwater around the Myall Lakes National Park (and hence land
immediately adjacent to the lake system) was mostly derived from the results of the
groundwater study carried out by DIPNR for NPWS from 1999 to 2002 (DLWC 2000b). The
objective of the study was to describe the character of the groundwater and determine the
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Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
extent of septic effluent impacts (if any) on the groundwater systems of Myall Lakes. The
groundwater study involved a field investigation and the establishment of a groundwater
monitoring network and quarterly sampling for a period of three years to benchmark the
undisturbed condition of groundwaters and monitor changes in the groundwater quality that
may result from the septic effluent dispersal into the sub-surface. Thus, a number of bores in
Control and Impacted (near known sources of effluent containment and disposal) locations
were established and monitored over the period.
The groundwater monitoring included field measurements of the groundwater level, EC, pH
and water temperature and collection of samples for laboratory analysis. For each bore, the
first sample was analysed for major ions, trace elements, alkalinity, EC, pH, total nitrogen,
ammonia, nitrate, nitrite, total phosphorus, total reactive phosphorus, faecal coliform and
faecal streptococci. Subsequent quarterly samples were analysed for EC, pH, total nitrogen,
ammonia, nitrate, nitrite, total phosphorus, total reactive phosphorus, faecal coliform and
faecal streptococci.
The various aquifers that exist in the Myall Lakes National Park based on geology from oldest
to youngest are:
?
Permian sedimentary rocks;
?
Carboniferous sedimentary and volcanic rocks;
?
Quaternary sediments consisting of,
?
Pleistocene Inner Barrier Sand Dunes to the west of the lakes; and
?
Holocene Outer Barrier Sand Dunes to the east of the lakes (ocean side).
The Permian and Carboniferous aquifers are further subdivided into various formations based
on their origin and location. However, these subdivisions were not described here in detail as
the groundwater study concentrated mainly on the Quaternary sediments where most of the
campsites and septic toilets were established. The Quaternary sediments are further
subdivided according to age and location as this has significance on the occurrence of acid
sulphate soils mainly associated with the Inner Barrier Sand in estuarine flood plains.
The type of groundwater is a reflection of the geology of the aquifer, the source or origin of
the groundwater plus the physical, chemical and biochemical processes taking place in the
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Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
sub-surface. The groundwater in Myall Lakes National Park has been characterised based on
electrical conductivity, pH and dominant major ions.
2.3.6.1. Groundwater Electrical Conductivity
Fresh, brackish and saline groundwaters are found in the Myall Lakes area. The major
component of fresh groundwater is rain that is naturally low in salinity. The salinity of this
type of groundwater in Myall Lakes National Park ranges from 30 to 300 ?S/cm with a
median of 100 to 150 ?S/cm. Fresh groundwater can be found in the Outer Barrier sand dune
systems especially at Mungo Brush, Bombah Broadwater, Bombah Point, Myall Shores, near
the wetlands south of Boolambayte Lake, along the Old Gibber Road and along the track to
the Myall River Mouth.
The second type of groundwater is brackish groundwater, which is a mixture of fresh
groundwater and some recently intruded seawater. The conductivity of this type of water
ranges from 100 to 2300 ? S/cm and it has a median conductivity of 300 to 350 ?S/cm. The
salinity of this type of water is highly variable due to the influence of rainfall, the lake, and
sea level fluctuations. More saline groundwater occurs during dry periods when there is very
limited dilution from local rain. On the eastern side of the Outer Barrier sand dunes, seawater
exists as a wedge beneath the fresh groundwater with its thickest part towards the ocean.
Brackish groundwater occurs where these two systems mix. It is drawn closer to the surface
by pumping of the spear points supplying water around the campsites at Mungo Brush and the
NPWS depot. On the western side of the Outer Barrier sand dunes, the brackish groundwater
is hydraulically connected to the lakes. Brackish groundwater is present in the sand dunes in
Mungo Brush, White Tree Bay, Mungo Point, Bombah Point and Yagon.
The third type is saline groundwater originating from the Carboniferous and Permian
sedimentary rocks located to the west of the lakes. The maximum salinity of this type of water
is generally higher than the brackish groundwater found in Quaternary sands. The salinity
levels of Carboniferous and Permian groundwaters varies less during dry periods. This could
indicate a slower response to rainfall recharge, which only occurs in regional groundwater
systems.
All three types of groundwater are hydraulically connected with the lakes. Faults and fractures
in the Permian aquifers may also result in groundwater seepages especially in the Inner
Barrier sand dunes to the west of the lake. Groundwater from the sand dunes discharges into
the lake when lake water levels are low, and recharge when lake levels are high.
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Chapter 2, Characteristics of Myall Lakes and its catchment
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2.3.6.2. Groundwater pH
The pH of the groundwater in the Myall Lakes area is a function of any, or a combination of,
the following:
?
the presence and amount of organic substances (eg Humic and tannic acids); and
?
the chemical weathering of minerals (especially pyrite and arsenopyrite in acid sulphate
soils).
Based on pH, groundwater in Myall Lakes can be grouped into two categories. The first group
is classified as slightly acid groundwater with a pH ranging from 4.5 to 7.5 and median of 6.0.
This range of pH was encountered in monitoring bores in Mungo Brush, White Tree Bay,
Myall Shores, Korsmans Landing, Yagon, Mungo Point and Bombah Point.
The second group is classified as highly acidic groundwater with pH ranging from 3.5 to 5.0
and median of 4.0 to 4.5. This was encountered in Bombah Broadwater, near the pond at
Bombah Point, near the wetlands south of Boolambayte Lake, along the Old Gibber Road and
along the track to upper Myall River Mouth.
Both types of groundwater have a distinct odour and brown colour resembling humic acid.
This indicates the high organic content of the sands that could be due to the breakdown of the
abundant plant materials in the sediments. There was no correlation found between salinity
and pH in any of the groundwater samples.
2.3.6.3. Groundwater Chemical composition.
From the results of the groundwater monitoring project, classification of the types or classes
of groundwater in Myall Lakes National Park based on the most dominant major cations and
major anions can be developed. Results from these classifications are listed in Table 2.2.
Table 2.2. Classes of groundwater in Myall Lakes National Park identified by
groundwater monitoring bore areas.
Type
1
2
3
4
5
6
Locations of similar Groundwater type.
Mungo Brush, Broadwater, Bombah Pt, Korsmans
Landing, Mungo Point, Bombah Point, The Big Gibber
Quarry, Myall Lake area
Myall Shores, Old Gibber Rd, River Mouth
White Tree Bay
Yagon
Mungo Brush
Myall Shores Resort
NSW Department of Infrastructure, Planning, and Natural Resources
Chemical element
or compound
Na - Cl
Na – Mg - Cl
Na – Mg – Cl - HCO3
Na – Cl - HCO3
Na – Mg - HCO3 - Cl
Na – Ca - Mg - HCO3 - Cl
29
Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Further classification of groundwater type was undertaken using Piper diagrams which
assisted in classifying the groundwater according to the dominant major cation and major
anion pair (consisting of Na, Mg, Ca, Cl, SO 4 and HCO3 ). By plotting the groundwater
samples results in comparison with known compositions of other water sources (eg. seawater,
rainfall, lake water and wetland water) the dominant sources of groundwater in different
locations were identified.
Based on the Piper diagram assessment it was concluded rain and seawater are the two main
components of the groundwater of Myall Lakes. The majority of the groundwater samples
analysed were similar in type to seawater (showing a dominance of the Na - Cl ion pair).
Exceptions were found however for the samples from Myall Shores which reflected ionic
composition similar to that of rainwater. Groundwater composition in this area was found to
have Na, Mg - HCO 3 ion pair as its dominant component.
2.3.6.4. Indicators of Possible sewage contamination.
The groundwater quality parameters used in the groundwater monitoring included faecal
coliform, faecal streptococci, total nitrogen, ammonia, oxidised NO2 and NO 3, total reactive
phosphorus and total phosphorus. These parameters were used as surrogate parameters to
identify sewage effluent impacts on the groundwater.
2.3.6.4.1. Faecal Bacterial.
During the first year of groundwater monitoring faecal bacteria either in the form of faecal
coliform or streptococci, were either 1 Colony Forming Unit (CFU)/100 mL or below the
detection limit. Analysis for faecal bacteria was dropped from subsequent samples and the
monitoring was focused on the nutrient concentrations. The absence of faecal bacteria in the
groundwater could be due to the very low pH and the highly variable salinities of the
groundwater resulting into a very unstable environment not suitable for bacterial survival and
growth. Reduced or oxygen depleted groundwater systems may also inhibit bacterial survival.
2.3.6.4.2 Nutrients.
In general, the groundwater in Myall Lakes National Park has naturally high nutrient
concentrations. These concentrations are similar to those found in the surface waters of the
Myall Lake system, based on the results of the surface water monitoring (Chapter 5). The
control or reference nutrient concentrations in the groundwater from unimpacted locations are
presented in Table 2.3 below:
NSW Department of Infrastructure, Planning, and Natural Resources
30
Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Table 2.3. Nutrient concentrations in control bores in Quaternary sands of Myall Lakes
National Park.
Analyte
Minimum
(mg/L)
Maximum
Average
Median
(mg/L)
(mg/L )
(mg/L)
Total P
0.013
0.192
0.061
0.052
Soluble Reactive P
0.008
0.148
0.050
0.046
Total Nitrogen
0.200
2.100
0.878
0.930
N as Ammonia
0.020
0.270
0.154
0.165
NOx
0.010
0.160
0.040
0.020
Over the monitoring period, the median control total P and soluble reactive P concentrations
were exceeded only once at impacted bores - Bombah Point in January 2001.
The median control total nitrogen concentration was exceeded several times at impacted bores
at Myall Shores near the evapotranspiration trench in November 2000, January 2001, January
and May 2002. The concentration of ammonia (as N) was consistently higher than the control
in White Tree Bay during the entire groundwater monitoring period and was exceeded several
times at Mungo Brush and Myall Shores from October 1999 to March 2000 and November
2000 up to May 2002 (Figure 2.7). Korsmans Landing showed higher concentrations of
ammonia (as N) than the control only in January 2000. While no direct observation of
contamination of groundwater by waste-water was made, there is little doubt that the elevated
nutrient concentrations near the Myall Shores ‘Ecotourism’ resort are due to contamination,
which has been contributing to nutrients in the groundwater and probably Myall Lakes. These
loads have been partially quantified in Chapter 3, to estimate the maximum possible load
arising from visitor waste activities.
The median control oxidised nitrogen (NOx) concentration was exceeded by samples from
Bombah Point in January 2001 and Myall Shores in October 2001 and May 2002. The
concentration at the remainder of groundwater monitoring sites was consistently lower than
control or below laboratory detection limits. These data are discussed in more detail in the
groundwater study report (DWLC 2000b).
NSW Department of Infrastructure, Planning, and Natural Resources
31
Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Ammonia concentration at Control (C) and Impacted (I) bores.
3
Ammonia (mg/L)
2.5
Myall Shores (I)
2
Myall Shores (I)
White Tree Bay (I)
1.5
Mungo Pt (C)
1
Myall Rivermouth (C)
0.5
Apr-01
Jan-01
Nov-00
Jul-00
May-00
Mar-00
Jan-00
Oct-99
Aug-99
0
Date
Figure 2.7. Groundwater ammonia concentration in Control and Impacted bores.
August 1999 to April 2001.
2.3.6.4.3 Nutrients Trends versus Water Level Trends
Most of the groundwater monitoring sites shows nitrogen trends similar to the watertable
trends, ie. when water levels fall, the nutrient levels also fall.
This means that the
groundwater level fluctuation has a direct effect on the nitrogen distribution. The exceptions
were Mungo Brush (GW078018), Bombah Broadwater (GW078020) and Myall Shores
(GW078022 and GW078023). In Bombah Broadwater, there was an increase in ammonia
levels in 2001 despite the groundwater level falling. In Myall Shores and Korsmans Landing,
both total N and ammonia levels increased as groundwater levels fell.
All of the monitoring sites show total phosphorus trends opposite to the watertable trends.
Reactive P trends are highly variable both in the control and other monitoring bores. Mungo
Brush (GW078018), Bombah Broadwater (GW078020) and Bombah Pt. (GW078021) show
continuously falling reactive P trend. At White Tree Bay (GW078019), reactive P trend is
almost similar with the groundwater trend.
Myall Shores (GW078022 and GW078023)
shows almost flat linear slightly increasing trend indicating stable reactive P distribution with
no effect from watertable fluctuation. At Korsmans Landing (GW078024) reactive P trend
show continuous increase while the watertable fell.
2.3.6.4.4 Nutrients and Park Occupancy
The number of people visiting and camping at Myall Lakes National Park peaks during
Christmas, Easter, and school holidays in July and the October long weekend. The number of
NSW Department of Infrastructure, Planning, and Natural Resources
32
Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
visitors to the popular campsites and Myall Shores Resort during the 1999 peak periods were
recorded (Table 2.4).
Table 2.4 Estimate of park occupancy rate (Environmental Management Pty Ltd, 2000)
Peak Season
Duration (days)
1999 Park Total Occupants
1
(Actual)
Christmas
12 - 22
490
Easter
6 - 15
614
July School Break
15
300
October Long
3 - 14
380
Weekend
1 Data from Barry McKibbin (Myall Shores Resort Manager, pers. comm. 2001)
Nutrient levels were compared to the total number of visitors for each peak period in 1999.
The same 1999 values for park visitors were assumed and used for 2000, 2001 and 2002 due
to lack of data.
In general, nutrient level fluctuations do not show a correlation to the park’s occupancy rate
except for nitrogen levels (both total N and ammonia) in Myall Shores (GW078022 and
GW078023) (see Figure 2.7).
2.3.6.4.5 Synopsis
Despite the naturally high background nutrient levels in the groundwater, there are sampling
sites that show elevated nutrient levels that appear to be linked to effluent contamination. In
most cases, the distribution of nutrients in the groundwater indicates the influence mainly of
watertable fluctuation, which is the result of climate or rainfall. Human activities including
septic sewage disposal may have some influence on the groundwater nutrient levels and
distribution at some bores (mainly those located at Myall shores).
It is considered that when the groundwater is low (deeper) and nutrient levels are high,
nutrients are not necessarily ending up, or contributing greatly, to the lake’s total nutrient
load. The similarities in the water qualities of the lake and the groundwater plus the type of
geology of the aquifers and the lake suggest the direct hydraulic connection between these
two reservoirs. With the constant exchange in water, the groundwater and the lake are also
exchanging nutrients continuously. Groundwater recharge, in the Quaternary sand after a rain
event, is believed to be instantaneous.
Therefore, significant nutrient transfer from the
groundwater can only occur when there is significant recharge resulting in higher (shallow)
groundwater levels and a hydraulic gradient towards the lake.
NSW Department of Infrastructure, Planning, and Natural Resources
33
Chapter 2, Characteristics of Myall Lakes and its catchment
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
The movement and volume of nutrients from impacted locations to other water bodies (ie.
swamps and lakes) was not studied. However, it is assumed that part of the nutrients will be
consumed by vegetation, released back into the atmosphere, changed to stable forms, and
some portion will remain for transfer to wetlands and /or the lakes. In the case of potential
direct contamination from Myall Shores, there is a shorter pathway for the groundwater
nutrients to the lake.
As demonstrated in this study, groundwater in the Myall Lakes National Park is considered
highly vulnerable. Its strong linkage with the surrounding swamps and lakes can provide a
pathway for nutrients to reach the lake, which in significant quantities, contribute to the total
nutrient load entering the lake. In Chapter 3, nutrient loads from human activities on the lake
foreshore (which usually enter the lake via groundwater) are derived and compared to loads
from other sources.
NSW Department of Infrastructure, Planning, and Natural Resources
34
Chapter 3, Nutrient sources to Myall Lakes.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
3. Nutrient sources to Myall Lakes.
3.1. CATCHMENT NUTRIENT INPUTS TO MYALL LAKES.
The eutrophication of Australian waterways through the addition of excessive nutrient loads
arising in their catchments has led to an increase in the severity and frequency of algal blooms
(Young et al. 1996).
A number of complex processes are involved in the movement of nutrients from the landscape
into waterways. These include transport and redistribution processes changes in elemental
speciation and microbial nutrient cycling. For the development of management strategies the
establishment of estimates of annual nutrient loads provides a good first measure of
catchment condition. The modelled estimation of annual nutrient loads using generation rates
for differing land, attempts to isolate some of the release processes such as erosion,
atmospheric contribution, fertiliser and decay of organic matter. Despite its limitations
nutrient load modelling provides a means of identifying principal source areas, which is
important information for planning nutrient reduction strategies (Young et al. 1996).
3.1.1. Catchment Management Support System Modelling.
The CMSS (Catchment Management Support System) model was developed by the CSIRO
Division Of Water Resources to provide a tool to assist in the understanding of nutrient
generation on a catchment scale. The CMSS model utilises data relating to mapped land
attributes; estimation of nutrient generation rates (derived from published scientific literature
and research), land management information, and rates of instream process. The CMSS model
has been developed to consider catchment-scale issues and hence provides a method that can
be used to compare the relative significance of contributions from nutrient sources in a
catchment (Farley and Davies 1993).
The CMSS modelfor the Myall Lakes catchment was developed from field-verified landuse
mapping information (1997 aerial photography). In the development of the model reference
was made to geology, land capability information, and soil landscape mapping information
held by DIPNR. Nutrient generation rates for various landuse activities were developed from
published Australian information and considered in context of the mapped landuses.
In the Myall Catchment a number of point-source nutrient loads were identified. These
included dairy shed effluent, poultry sheds, and the Bulahdelah STP. The treatment plant
removes nearly all phosphorus from effluent, with the nitrogen component remaining.
NSW Department of Infrastructure, Planning, and Natural Resources
35
Chapter 3, Nutrient sources to Myall Lakes.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Table 3.1. Land use nutrient generation rates used for Myall Catchment CMSS model.
Land Use
Phosphorus kg/ha/yr
Nitrogen kg/ha/yr
Urban
Improved (Dairy) Pasture *
1.30
1.5
9.0
6.0
Roads
1.80
6.0
Rural residential
Timbered Land (not NP, not SF)
0.60
0.20
4.0
2.0
Native Pasture *
National Park (NP)
0.15
0.08
2.0
1.5
State Forest (SF)
0.12
1.5
0.15
0.0047
1.5
0.0225
0.5 Tonnes / yr
1.75 Tonnes / yr
Easements
Dairy cow unit (500 kg)#
**
Poultry manure per 120 m X 12m
shed
(Note: These rates indicate potential contribution developed from Hunter Catchment CMSS Model Hancock 1997)
Reference * Bagsinka et al. (1998) # 10% of actual manure produced.** Poultry manure : 5 batches
year, full clean out, 105 m 3 manure produced per batch, 75 % (DM) dry matter ; Nitrogen 2.6% DM - 30
% readily available ; Phosphorus 1.8 % DM 13 % readily available. Assume available N and P
transported and lost in run-off.
3.1.1.1. Comparison of loads arising in Myall Lakes sub-catchments.
The Myall Lakes catchment can be divided into 4 sub-catchment areas. Of these the Myall
River catchment has the largest land area. The areas of each sub-catchment are shown below
(Table 3.2).
Table 3.2. Description of sub-catchments of Myall Lakes catchment.
Sub-catchment
Land Area Ha
Myall River Catchment
Nerong Area Catchment
Boolambayte Creek Catchment
Myall Lakes and Lake's edge Catchment
44 362
3 938
9484
23584
Percentage of
Catchment Area
54.5
4.8
11.7
29.0
The CMSS model can be used to develop a comparison between the sub-catchment areas to
compare estimate of nutrient delivery. The results in Table 3.3, and Table 3.4 summarise the
modelled nutrient load arising in the 4 main subcatchments of the Myall Lakes system. It is
clear from these tables that approximately 70% of the total annual load of N and 80% of the
total annual load of P entering the Myall Lakes arises in the Myall River catchment.
Table 3.3. Comparison of Sub-Catchment CMSS calculated nutrient loads - Nitrogen.
Sub -Catchment
Myall River Catchment
Nerong Area Catchment
Boolambayte Creek Catchment
% Land Area
54.5
4.8
11.7
Nitrogen Load
kg/ yr
96500
5 900
15 300
NSW Department of Infrastructure, Planning, and Natural Resources
% of Catchment
Nitrogen Load
69.6
4.3
11.1
36
Chapter 3, Nutrient sources to Myall Lakes.
Myall Lake (Lake's Edge) Catchment
Total
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
29.0
20 700
138 400
15.0
Table 3.4. Comparison of Sub-Catchment CMSS calculated nutrient loads - Phosphorus.
Sub -Catchment
% Land Area
Myall River Catchment
Nerong Area Catchment
Boolambayte Creek Catchment
Myall Lake (Lake's Edge) Catchment
Total
54.5
4.8
11.7
29.0
Phosphorus Load
kg /yr
12 800
440
1 500
1 300
16 040
% of Catchment
Phosphorus Load
79.6
2.8
9.5
8.1
3.1.1.2. CMSS estimates of historical and contemporary annual nutrient loads.
When considering the contemporary nutrient loading of the Myall Lakes system it is
important to recognise that the nutrient loading from the catchment to the lake system has
changed considerably over the past 200 years. Consequently, the nutrient regime under which
the lakes’ were subject to prior to European settlement is probably quite different to what is
currently observed. This could be of great significance in setting targets for future nutrient
loads from the catchment.
To put these changes in perspective, a simple comparison can be made between estimates of
nominal nutrient load if the whole catchment were under native undisturbed forest and with
estimates from loadings under the current land uses. From these estimates it can be seen that
nitrogen loading for the lake system has increased approximately ten fold, while phosphorus
loadings have tripled (Figure 3.1).
Myall Lakes Catchment CMSS Model
Nutrient Load Comparison.
160
Tonnes per Year
140
Pre-1788
120
100
Year 2002
80
60
40
20
0
Nitrogen
Phosphorus
Figure 3.1. Myall River Catchment Annual Nutrient Load Comparison (1788 and 2002)
NSW Department of Infrastructure, Planning, and Natural Resources
37
Chapter 3, Nutrient sources to Myall Lakes.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
3.1.1.3. CMSS estimates of nutrient source arising from different land uses.
Estimates of nutrient loads from various landuses have been developed from published
information relating to those landuses. Landuse is recognised as a good overall predictor of
nutrient load potential (Young
et al.
1995). A limitation with such methodology however, is
that it does not deal explicitly with individual site characteristics, but instead creates relative
comparisons between land use types. For this reason the estimates derived for various
landuses in the Myall catchment should not be regarded as absolute, but rather, as a guide to
which particular landuse may be contributing a relatively higher nutrient load to the lake
system.
In Table 3.5 a comparison of estimated nutrient loads from individual land uses is presented.
From a catchment management perspective, knowing the magnitude of loads arising from
different landuses provides invaluable information. This data can then assist in the targeting
of strategies to reduce loads from specific sources.
Table 3.5 .CMSS derived land use annual nutrient loads from the Myall Lakes
Catchment. (NB. Bulahdelah STP was upgraded in the 1990s – prior to this it would have released
relatively higher P and N loads).
Landuse
State Forest
Percentage of
catchment area
32.5
Percentage of
Nitrogen Load
29.0
Percentage of
Phosphorus Load
19.8
Privately held forest lands
National Park (native vegetation)
Poultry - assuming litter is spread
on-site.
Grazing -Native Pasture
Grazing -Improved Pasture
Dairy Operations
20.2
19.4
Less than 1%
18.0
17.0
13.0
10.2
7.9
31.2
13.1
1.3
Unkown - small
11.0
5.0
5.0
9.9
10.2
5.0
Urban
Rural Residential
Other
Roads and Easements
Bulahdelah STP
0.2
0.3
0.3
0.2
Less than 1%
1.0
1.0
Less than 1%
Less than 1%
Less than 1%
1.3
1.0
2.5
1.0
NPWS Tourism
Water (Rivers and Lake area)
Total Nutrient Load (kg/yr)
Unknown
12.5
Less than 1%
Unknown
138, 400
Less than 1%
Unknown
16, 040
To provide an assessment of accuracy of the data generated by the CMSS modelling fieldvalidations were undertaken which included catchment water quality sampling and eventbased load measurements.
NSW Department of Infrastructure, Planning, and Natural Resources
38
Chapter 3, Nutrient sources to Myall Lakes.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
3.1.2. Estimates of nutrient load from River volume.
An estimate of the annual nutrient delivery (TN and TP) by the Myall River system was
developed from the water nutrient concentration data collected for the Myall River at
Bulahdelah and estimates of annual flow of the Myall River. A limitation of such an estimate
is that it will underestimate the true nutrient loading because of the non-normal distribution
(skewness) of the data. That is, infrequent flood events may import disproportionately larger
loads than can be predicted by these measurements. The estimates of annual river nutrient
loads provide only rough values however they give a means to verify the estimated loads.
Results of these estimates are shown in Table 3.6 below.
Table 3.6. Empirical estimates of annual Myall River TP and TN load to Myall Lakes.
Analyte
Total Phosphorus
Estimated
Annual
Discharge
ML (106 L)
Median Annual
Concentration
(mg/l)
Estimated Annual load
of (Tonnes / yr)
0.117*
15.5 (TP)
0.7*
93.1 (TN)
133 000**
Total Nitrogen
*Unpublished DLWC data (1999 – 2002).
** From Myall Lakes and Port Stephens Estuary Process Study (MHL 1999).
When comparing this flow estimated nutrient load with data generated by CMSS it can be
seen that there is general agreement between the two methods.
3.1.3. Measured Nutrient Exports from the Catchment - autosampler results
To provide field verification of the CMSS modelling results a storm event sampling program
was undertaken to report on rain event nutrient loads. In late 2000 an auto sampler was
installed at the Bulahdelah flow-monitoring site with sampling triggered by rainfall and / or a
change in river height.
Water samples were collected and analysed for four rain events over the period January 2001
- June 2002, samples being collected every 2 hours for the duration of the event. A summary
of measured results from these events is shown in tables below.
Table 3.7. Summary results from several rain event sampling - Myall River at
Bulahdelah.
Date
Rainfall
(mm)
River Flow
(ML)
Event Phosphorus
Load (kg)
Event Nitrogen
Load (kg)
11 -12th February 2001
25
330
4
21
NSW Department of Infrastructure, Planning, and Natural Resources
39
Chapter 3, Nutrient sources to Myall Lakes.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
20-22nd February 2001
55
3400
1170
6500
6 - 11th March 2001
267
32333
9400
45100
1- 13th February 2002
266
32632
2400
21000
Results from this rain event sampling have helped to quantify the magnitude of nutrient loads
from the catchment. In large (over ~100 mm) rain events there is a disproportionate increase
in the load of nutrients transported from the Myall River catchment into the lake system. The
quantities of nutrients measured during these storm events are consistent with predicted
quantities estimated by CMSS modelling, and thus provide validation of the modelled results.
It is apparent from the results summary in Table 3.7 that there were large differences in the
quantity of nutrients delivered to the lake during two events of similar size. In the rain event
of 1- 13th February 2002 and 6 - 11th March 2001 similar quantities of rainfall fell (approx 260
mm). Nominal riverine nutrient loads however were considerably higher in the March 2001
event, most probably caused by the greater intensity of rainfall in this event. The 260 mm rain
in March 2001 being recorded over a five day period, where as the rainfall in the February
2002 event was recorded over a 10 day period. Factors such as the intensity of rainfall, the
time since last flood (eg. an intense storm following prolonged drought will deliver more
nutrients than a storm following recent flooding, because more nutrients will have available in
the landscape), and duration of rain event, will all influence the processes which mobilise
nutrients into the river system and the amount delivered.
3.1.4. Results from Water Quality Monitoring in the Myall River .
Monthly water sampling has been undertaken by DIPNR as part of a broader program to
investigate river health in the river systems of the Lower North Coast. The program reports on
a variety of parameters at end-of-system monitoring sites for major rivers in the Lower North
Coast Catchment area. The sampling program has been designed to provide general
information on river nutrient levels, catchment loads and changes in condition over time.
For the Myall River catchment sampling is undertaken on the Myall River at Bulahdelah
Bridge where continuous river height monitoring equipment is installed. A summary of the
findings from samples collected 1/1/2000 - 30/6/2002 are presented below (Table 3.8 and
Table 3.9).
NSW Department of Infrastructure, Planning, and Natural Resources
40
Chapter 3, Nutrient sources to Myall Lakes.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Table 3.8. Summary of physical water quality characteristics.
Statistic
Parameter
Number of samples
Electrical
Conductivity
(µS/cm)
30
Turbidity (NTU)
Temperature
30
30
28
Mean
825
6.9
22.1
20.3
Minimum value
79
6.2
2.9
11.4
Maximum vale
4999
7.7
107.0
28.7
S.D.
1035
0.3
22.9
4.7
Median
284
6.9
13.6
20.8
pH
The physical water quality characteristics are consistent with characteristics of streams in the
Lower North Coast area. The pH values recorded reflecting the acid to neutral status typical
of coastal stream systems. The median turbidity value was found to be 13.6 NTU with the
colouration present generally arising from humics of dissolved organic matter. The electrical
conductivity readings were generally low, however some higher values were recorded in the
drier times. In late 2002 saline water was observed intruding up the Myall River as far as
Bulahdelah Bridge which is thought to be the approximate limit for the Port Stephens salt
wedge.
Table 3.9. Summary of riverine nutrient concentrations.
Parameter
Number of samples
NOx Nitrogen
(mg/L)
Total Nitrogen
(mg/L)
30
30
FRP Soluble P
(mg/L)
29
Total
Phosphorus
(mg/L)
29
Mean
0.048
0.687
0.035
0.097
Minimum
0.000
0.420
0.002
0.024
Maximum
0.180
1.270
0.141
0.281
S.D.
0.044
0.217
0.031
0.058
Median
0.040
0.690
0.031
0.096
The median total phosphorus level was found to be 0.096 mg/L. The SRP (soluble reactive
phosphorus - the soluble phosphorus component that is most readily used by plants and algae)
was found to be 0.031 mg/L with this representing some 32% of the phosphorus present. The
median total nitrogen value was found to be 0.69 mg/L with the NOx nitrogen value of 0.04
mg/L. The soluble NOx component representing around 5% of the nitrogen present.
In Figure 3-2 a plot of the monthly results of total nitrogen and total phosphorus is presented
from samples taken from the Myall River at Bulahdelah. It can be seen that over the
monitoring period a large variation in the Total Phosphorus and Total Nitrogen concentrations
NSW Department of Infrastructure, Planning, and Natural Resources
41
Chapter 3, Nutrient sources to Myall Lakes.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
were recorded, reflecting the influence of rainfall on nutrient concentration and loads. The
graph however shows a general parity between changes in riverine total nitrogen and total
phosphorus concentrations.
Myall River at Bulahdelah
Total Phosphorus and Total Nitrogen Concentrations
Total P
Total N
0.25
0.2
0.15
0.1
0.05
May-02
Mar-02
Jan-02
Nov-01
Sep-01
Jul-01
May-01
Mar-01
Jan-01
Nov-00
Sep-00
Jul-00
May-00
Mar-00
0
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Total Nitrogen mg/L
0.3
Jan-00
Total Phosphorus mg/L
Jan 00 - June 02
Figure 3.2. Monthly TN and TP concentrations for Myall River at Bulahdelah.
There is a strong correlation between river total nitrogen and total phosphorus concentrations
r2 = 0.91 (Figure 3.3). This suggests, as might be expected, that the transport processes
relating to the movement of these nutrients are related.
Correlation Matrix for Total Nitrogen vs.Total Phosphorus
Myall River at Bulahdelah (Jan 00 -June 02)
TOTAL_P = -.0720 + .24571 * TOTAL_N
Correlation: r = .91254
0.30
Regression
95% confid.
Total Phosphorus mg/L
0.24
0.18
0.12
0.06
0.00
0.3
0.5
0.7
0.9
1.1
1.3
1.5
Total Nitrogen mg/L
Figure 3.3. Correlation of riverine TN and TP concentration at Buladelah.
NSW Department of Infrastructure, Planning, and Natural Resources
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Chapter 3, Nutrient sources to Myall Lakes.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
3.1.5. Atmospheric Nutrient Contributions
In rainfall there is a small nutrient component present, often associated with airborne dust and
soil particles being washed down in the first flush of rainfall. These small quantities when
considered over such a large surface area such as 100 km2 of the Myall Lakes system could
make a significant annual contribution to system’s nutrient load.
Analysis from coastal rainfall (Data from metrological station at Lake Ainsworth North Coast
NSW) found total phosphorus concentrations of around 0.01 mg/L Total P. Annual rainfall
for Bulahdelah is around 1328 mm year and the lake surface area approximately 100 km2 .
This equates to an atmospheric delivery to the lake of 1330 kg Phosphorus / yr.
Data for rates of atmospheric Nitrogen deposition in the southern hemisphere identify a range
of between 50 -150 kg / km2 / year (Harris, 1999). For the purposes of this assessment the mid
value of 100 kg/ km2 /year will be used. From these figures an estimate of 10 tonnes per year
was delivered based upon a surface area of 100 km2 . These levels are considered insignificant
compared to other sources.
3.1.6. Potential nutrient contribution from National Park visitor sewage waste
When the blue green algal bloom was first detected in Myall Lakes, the initial reaction of
many people familiar with the lake system was to assume that the more obvious sources of
nutrients, such as waste-water treatment facilities on the lake’s foreshore, were the main cause
of the bloom. To place in perspective the potential nutrient load arising from tourismgenerated sewage an assessment, which assumed that all toilet waste and grey water
ultimately found its way into the lake’s waters (ie. worst case scenario), was performed. The
theoretical assessment shown in Table 3.10 concluded that fairly small contributions would
arise (less than 5%), in comparison to the catchment load estimates for nitrogen and
phosphorus that are also represented in Table 3.3 and Table 3.4.
Table 3.10. Potential nutrient load from visitor sewage estimates in comparison with
catchment loads
Site
Total direct waste input
CMSS Estimate of catchment
load
Nitrogen kg / yr
2, 089
138, 400
Phosphorus kg / yr
646
16, 040
3.1.7. Estimated Phosphorus load from Groundwater inflows
No nutrient load estimates from groundwater were undertaken as part of this project. However
Manly Hydraulics Lab (MHL) estimated the rate of ground water inflow into the lake system
as part of the Myall Lakes and Port Stephens Estuary Process Study (MHL 1998). Estimates
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Chapter 3, Nutrient sources to Myall Lakes.
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
were developed based on published information describing the hydraulic conductivity of
similar aquifer types, and assume a high hydraulic gradient. Loads were calculated for
phosphorous, but not for nitrogen. Results of the MHL phosphorous load estimates are
presented in (Table 3.11), however in light of further groundwater work, it is regarded that
these are likely to be an over-estimate and further work is required to provide reasonable
certainty for load figures.
While no verification of the nutrient flux data has been performed and the figure of 4.7 t/year
is not intended to be an accurate measurement, it provides a ‘ballpark’ maximum figure of
annual groundwater phosphorus load based on some fairly coarse estimates. The wastewater
component of this figure (assuming all human waste is discharged directly into the lake) is
relatively small (see section 3.1.6).
Table 3.11. Estimate of annual groundwater phosphorus load. Based on inflow rates
developed by MHL in Port Stephens and Myall Lakes Estuary Process Study (1998).
Analyte
Maximum possible Annual Load
(Tonnes)
4.7*
Total Phosphorus
* Tidal/ seasonal influence will possibly change loadings, with the movement (loss) of phosphorous to
groundwater system possible when outflows from the lake to the aquifer system occur.
3.1.8. Estimate of Myall Lakes System nutrient loading per unit lake surface area.
The Myall Lakes system is estimated to have a water surface area of approximately 100 km2 .
When considering nutrient loading impacts it is important to consider the physical
characteristics of the estuary system. A shallow waterway with a large surface area for
example, can accommodate a greater nutrient load than a waterway of the same volume, but
lesser area. In theory, the greater area available for denitrification in the shallower waterway
will cause a lesser loading of organic carbon and greater denitrification efficiency, all else
being equal. The measure of nutrient load delivered per unit area can thus assist in describing
the nutrient status of a waterbody. Table 3.12 below presents estimates of nutrient loading per
surface area of lake system currently, and pre-European settlement.
Table 3.12. Estimated annual catchment nutrient load for Myall Lakes.
CMSS Estimate pre 1788
CMSS Estimate 2002
Nitrogen
Tonnes /km2 /year
0.1
1.38
NSW Department of Infrastructure, Planning, and Natural Resources
Phosphorus
Tonnes /km2 /year
0.05
0.16
44
Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
4. Phytoplankton Abundance and Distribution
4.1. BACKGROUND
Phytoplankton is the collective term used to describe mainly unicellular algae less than 50?m
in diameter that live within the water column, but may have a resting stage or spore cycle
within the sediments. Some of the larger colonial forms may possess individual cells of
uniform structure within filaments, chains or loosely bound cells within mucilage.
Identification of these species is based on the cell shape, cell dimensions, cell wall
composition, mucilage layers, chloroplast structure, flagella and storage mechanisms (Boney,
1988). On the basis of these physical characteristics phytoplankton are grouped into classes.
The major classes covered in this report are:
1. Blue-green algae (Cyanophyceae);
2. Diatoms (Bacillariophyceae);
3. Dinoflagellates (Dinophyceae); and
4. Green algae (Chlorophyceae).
Along with macroalgae and macrophytes, phytoplankton is a principal source of primary
productivity (organic material) in aquatic ecosystems, and forms the basis of food webs. This
primary productivity of phytoplankton is dependant on adequate light, inorganic carbon,
dissolved mineral nutrients, suitable salinity, water and suitable ambient temperature to
sustain metabolism (Boney, 1988). The particular interaction of these limnological conditions
and the morphology and ecophysiology of phytoplankton species (by virtue of their size,
shape, habitat/niche preference, nutrient requirements, movement abilities) determine the
distribution and persistence of particular classes and species of phytoplankton. Consequently,
phytoplankton community composition can reflect strongly the physical and chemical
characteristics of a water body.
4.1.1. Phytoplankton community as an indicator of estuarine health.
A change in species richness and an increase in the abundance of phytoplankton is often
associated with eutrophication. Globally, Anderson et al., (2002) reported strong correlations
between total phosphorus input and phytoplankton production in freshwaters, and total
nitrogen input and phytoplankton production in marine waters. As well, they noted that
blooms of blue-green algae in estuarine and brackish coastal waters in Australia and
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Scandinavia have also been linked to phosphorus enrichment. So, enrichment both of these
macronutrients can play a role in exacerbating estuarine algal blooms.
Commonly, green algae (Chlorophyceae) and blue-green algae dominate the phytoplankton in
degraded systems (e.g. Boney 1988). It is thus possible to use the observed occurrence,
biomass and extent of phytoplankton as good bioindicator of trophic status of water bodies.
The response of phytoplankton to changes in water quality however, is not necessarily linear
or easily interpreted.
While the general principles underlying algal blooms are reasonably well understood, the
reliable prediction of algal blooms in rivers, estuaries and coastal waters with complex
mixing, circulation and flushing remains an inexact science (Anderson et al., 2002). The
competitive interactions between algal species as well as many other biological factors (eg
presence or abundance of grazers, water clarity / nutrients, and weather conditions), which are
rapidly changing, interact to determine the composition of the dominant phytoplankton
community.
This report focuses mainly on blue-green algae due to the development of the potentially toxic
bloom of Anabaena circinalis and Microcystis aeruginosa in April 1999 in Myall Lakes.
4.1.1.1. Changes in phytoplankton community and ecological succession.
Any ecological community present somewhere at a point in time is not only a result of the
environmental conditions that currently prevail, but is also a consequence of the conditions
and organisms that preceded its occurrence (Krebs, 1985). Consequently, while the
requirements of a particular taxa might, in principle, be ideally suited to the observed
conditions another seemingly inferior taxa may actually predominate because of the sequence
of events or conditions that have occurred previously. This principle, that the sequence of
events (the succession) are as important as the current conditions is one of the tenets of
ecology and is important in describing the observed changes in the phytoplankton community
in Myall Lakes.
Phytoplankton succession is thought to be, in part, attributable to changes in ratios of
nitrogen, phosphorus and silica (Anderson et al., 2002), changes in flow conditions causing
steep environmental gradients (Fabbro & Duivenoorden, 2000), and the differential effects of
these and other physical, chemical and biological factors on individual species (Hallegraeff &
Reid, 1986). Coastal lakes are physically dynamic systems (changing due to rainfall, tidal
flushing etc.) and the phytoplankton in these places also changes rapidly in response. The
algal taxa that dominate at one point in time, perhaps in response to a large disturbance or
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
brief event, can modify the environmental conditions (e.g. nutrient regime) to change the
community composition that occurs later (Krebs, 1985). Consequently there is often a
stochastic component that plays an unknown role in algal ecology.
While there may be spatial and temporal differences imposed on phytoplankton succession in
flowing rivers, reservoirs, coastal lakes, estuaries and the sea, the patterns of development and
trigger mechanisms for algal blooms appear to be universal. For example, according to
Hallegraeff & Reid (1986) the species patterns in the sea off Sydney on the east coast of
Australia resemble spring and autumn patterns experienced in European waters.
On the east coast of Australia peaks in phytoplankton abundances in oceanic waters are often
related to nutrient enrichment through periodic upwelling of deep ocean waters which are
high in nutrients. Although upwellings occur throughout the year, marked changes in
phytoplankton assemblages can be initiated in Spring with the appearance of high numbers of
small chain-forming diatoms, followed by larger centric diatoms in November and later by
dinoflagellates. Diatoms decline in abundance towards the end of summer reappear in late
summer and early autumn again followed by an abundance of large and small dinoflagellates.
In fresh waters the succession is similar with Boney (1988) reporting spring outbursts of
diatoms (due to their capability for rapid cell division, and high photosynthetic rate) in
response to increasing photoperiod and high nutrient concentrations. These are then followed
by slower growing and larger diatoms, then dinoflagellates and other organisms with slower
growth rates such as green algae and blue-green algae.
The summer conditions of higher temperatures, lower nutrients and water stability enable
slower growing green algae and blue-green algae to compete with diatoms, that grow quicker
in cooler waters. This has been demonstrated in the Peel Harvey Estuary in Western Australia,
where blooms of diatoms were common in winter after river inflows, and blue-green algae
(Nodularia ) growth limited by temperature in winter, invaded in summer. However this only
occurred if winter riverflows had decreased salinity to levels conducive to Nodularia.
Increased salinity was also found to contribute to a decrease in Nodularia (Lukatelich &
McComb, 1986).
Climate variability was also found to have major impacts on seasonal and interannual changes
in dominant phytoplankton biomass in Lone Pine Dam in Queensland. This was due to
variations in inflows (nutrient and silica inputs) that were correlated with the Southern
Oscillation Index cycle (El Niño) (Harris & Baxter, 1996). In general it was found that there
was an inverse relationship between diatoms and blue-green algae with the two rarely
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
coexisting. Diatoms dominated under mixed conditions while blue-greens dominated during
calm periods.
4.1.2. Some Characteristics of Blue-Green Algae
Freshwater blue-green algae (Cyanophyceae) are a type of photosynthetic bacteria
differentiated from other algae by (Fogg et al., 1973):
?
their carbohydrate storage of glycogen;
?
cell wall composition of saccharides; and
?
photosynthetic pigments of chlorophyll - ? , carotenoids and phycobilin.
Within the class are orders that are defined by their shape cell dimension, structure, motility
and presence of specialised cells (Baker & Fabbro, 1999). Across these orders, species can be
characterised further according to their ecostrategies or habitat preference (eg: planktonic or
benthic [Chorus & Bartram, 1999]) and the type of toxin they produce. Blue-green algae are
rarely dominant but tend to be opportunistic species that have adaptations that allow them to
exploit a certain set of resources and conditions, and out-compete other phytoplankton under
such conditions.
4.1.2.1. Adaptations of Blue-Green algae:
Blue-green algae are primitive algal cells and are generally not able to routinely out-compete
other groups in healthy waterways. However, they posses a number of adaptations that allow
them a competitive advantage in degraded waterways:
1. The possession of low energy requirements to maintain cell functions, when compared to
other groups;
2. The presence of gas vacuoles in some taxa, and additionally the formation of large colonies
or chains that make the cells of some taxa (e.g. Anabaena and Microcystis) very buoyant.
These taxa often form scums in the calm after a windy period due to buoyancy
overcompensation. Buoyancy regulation allows the colonies to maintain their position
within the water column at optimal levels for photosynthesis (Fay, 1983). This is
particularly an advantage in turbid waters, and where vertical mixing is low.
3. Blue-green algae also possess accessory pigments such as phycocyanin and phycoerthyrine
that allow the cells to photosynthesise in lower light conditions compared to other
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
phytoplankton. This along with point 2 above favours these cells in turbid waters
(Bowling & Baker, 1996).
4. The ability to switch between autotrophy (photosynthesising) to heterotrophy (consuming
other organisms) which allows some species to overwinter on the bottom of lakes or
survive during periods of low light availability (e.g. Microcystis).
5. A substantial storage capacity for phosphorus, allowing species to load phosphorus when it
is not limited. This storage capacity enables species to perform two to four cell divisions
resulting in a thirty two fold increase in cell numbers (Chorus & Bartram, 1999).
6. The production of akinetes or spores (by some species) allows viable populations to be
maintained in sediments for many years until suitable conditions to bloom again
eventuate.
7. Some species of blue-green algae have the ability for nitrogen fixation and produce
specialised cells (heterocysts) to facilitate this (eg Anabaena). However these heterocysts
are only developed in the absence of suitable levels of combined nitrogen in the water
(NHMRC, 1984) and some other species are also thought to fix small amounts of
atmospheric nitrogen (Microcystis). This fixation and the ability to also liberate
substantial quantities of extra cellular nitrogenous compounds (Fogg et al., 1973: Rai,
1997), means that nitrogen does not limit the occurrence of some species, and the
ammonia secretion probably assists in maintaining stable populations once the bloom has
developed.
4.1.2.2. Ambient conditions that enhance blue-green algal growth:
Although the adaptations of blue-green algae mentioned above are substantial, they do not
provide a competitive advantage over other phytoplankton in pristine waterways. To enable
blue-greens to become dominant a change in ambient water conditions towards the following
is required:
1. High nutrient concentrations,
2. Low salinity,
3. Warm waters (to increase growth rates),
4. A change in turbidity from ambient levels,
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
5. Calm water conditions and stratification. Lack of vertical mixing can cause other algae to
sink out of the illuminated water column, while buoyant blue-green algae can remain near
the surface.
6. Low amounts of grazer predation. This may in part be enhanced by the toxins produced by
the cells.
7. Low numbers of phytoplankton competitors. This will allow the cells’ low growth rates to
be less disadvantageous, as nutrients are not monopolised by other algae.
It is clear that while blue-green algae are primitive organisms and have inferior competitive
abilities under most conditions, at times water conditions exist that favour these algae over
other types. Often, the changed conditions associated with the degradation of waterways are
ideal conditions for these cells, so disturbances in a waterways catchment can be reflected in
changes in the phytoplankton community to include a greater proportion of blue-green algae.
The response is not always so predictable however, the initial algal assemblage, the presence
of elevated nutrient concentrations and complex interactions of the above limnological
conditions are required to enable bloom development (Mitrovic & Bowling, 1996).
4.1.2.3. Human Health implications of Blue-Green algae.
Human health concerns over the presence of blue-green algae are due, in part, to the presence
of lippopolysaccharides in all taxa.
These are contact irritants posing health threats to
recreational water users. Although there are limited studies on the effect of these contact
irritants, epidemiological studies by Saadi et al. (1995) and Pilotto et al. (1997) identified that
dermatological and gastrointestinal symptoms were linked to increased cell numbers of bluegreen algae.
Toxins (also odour and taste compounds) are produced by some species, that upon ingestions
can cause serious human health problems (NHMRC,1994). Microcystin toxin, which is
produced by some species of Microcystis, has a hepatotoxic effect (damaging the liver).
Others have a neurotoxic effect (affecting the nervous system) such as saxitoxins or paralytic
shellfish poisons produced by some species of Anabaena. The bloom of Anabaena and
Microcystis in the Darling River in 1991 caused numerous stock deaths in animals that had
consumed contaminated water (Blue-Green Algal Task Force 1992). Other species are also
known to produce cytoxins (e.g. Cylindrospermopsin) that have an effect on a number of
internal organs (Burch & Nicholson, 2000).
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
While the presence of blue-green algae is a health concern to recreation, particularly if
directly swallowed, some toxins also have the potential to accumulate in high concentrations
within shellfish, prawns and in the muscle of fish (Falconer & Choice, 1992). So the presence
of blue-green algal toxins in waterways can affect human health through direct contact or
through the consumption of contaminated seafood, which can swim to, and be consumed from
waterways unaffected by algal blooms.
4.2. METHODS
4.2.1. Aims of the monitoring study
The aims of monitoring blue-green algae, and the analysis of the phytoplankton data were two
fold:
1. To ensure adequate warnings to the public in response to high cell counts of potentially
toxic and non toxic algal species, through implementation of the Regional Algal Contingency
Plan (MKGLRACC, 2000). This program was initiated in 1999.
2. To develop an understanding of the ecology of phytoplankton in Myall Lakes, and utilise
algal assemblages as indicators of estuarine health. Aspects covered include the spatial and
temporal distribution (and the factors influencing this), and community dynamics.
4.2.2. Algal sampling and Analysis
Sampling was carried out by NPWS and DIPNR staff in accordance with the ‘Draft National
Protocol for Monitoring of Cyanobacteria and their Toxins in Surface Waters (ARMCANZ,
1997) using a 2m integrated water column sampler. Prior to 2001, sampling frequency varied
depending on the species present and cell counts, in accordance with state guidelines for
recreational waters. This information was initially collected for the purpose of initiating and
lifting public health warnings, so sampling intensity varied depending on extent and type of
algae present creating numerous data gaps in early records. Total algal counts were performed
on the samples by NATA1 accredited laboratories for algal enumeration that guarantees
accuracy for abundant taxa of +/- 20% (this is standard across the nation).
From January 2001 total phytoplankton counts and biovolumes were measured in accordance
with revised state guidelines for recreational waters with the frequency again depending upon
requirements of the Algal Contingency Plan but occurred at least on a monthly basis at up to
ten sites. Retrospective calculations were made to determine biovolumes based on cell counts
prior to January 2001. There is a small risk that unknown temporal or spatial differences in
NSW Department of Infrastructure, Planning and Natural Resources
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
cell sizes and colonies may result in inaccurate biovolumes for the period prior to January
2001, but these differences are likely to be trivial.
After simultaneous testing by three laboratories between January and May 2000, a change of
testing laboratory was made due to its greater ability to detect blue-green algal species with
smaller sized cells and perform counts of all algal taxa (not just blue-greens).
4.2.3. Data Analyses and presentations.
Although the Coast and Clean Seas project commenced in September 2001, the algal data
collected from April 1999 are incorporated into this report to demonstrate the significant
change in the composition of the phytoplankton assemblage in Myall Lakes between 1999
and 2002.
The average biovolumes (mm3 /L) or cell counts (cells/mL) for the eight most abundant genera
of blue-green algae at sites within four regions (River Mouth, Broadwater, Two Mile Lake
and Myall Lake)) were graphed to exhibit temporal and spatial changes from April 1999 to
September 2002. The average total biovolume of toxigenic (Anabaena, Microcystis,
Aphanizonomon) and non toxigenic species were also graphed in these regions to show the
variation in potential toxicity of the algal community over time. Additionally, for the period
from March 2001 to September 2002 the average total phytoplankton biovolumes for four
main families (Bacillariophyceae, Chlorophyceae, Cyanophyceae, and Dinophyceae) were
graphed to show the temporal and spatial variation of all phytoplankton.
Multivariate analysis of the eight blue-green algae genera, and separately all phytoplankton at
the genus level, was used to elucidate patterns of similarity in assemblage composition. A
similarity matrix was constructed using cell counts from individual samples (cells/mL) as this
avoids any error introduced by incorporating the retrospectively calculated biovolume data.
Although most of the high outliers had been removed, the data included many zero values and
occasional high cell counts. The data was transformed to the fourth root and a similarity
matrix of Bray-Curtis index scores was constructed. A cluster analysis was performed using
the similarity matrix to produce dendrograms. Group-average linkage was used in the
clustering technique so that the average dissimilarity between groups was used in the
algorithm. An ordination was performed on the similarity matrix using non-metric multidimensional scale (n-MDS) techniques. The samples were plotted in three dimensions and the
1
National Association of Testing Authorities.
NSW Department of Infrastructure, Planning and Natural Resources
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
procedure repeated ten times so that the plot with the lowest distortion was used. Samples
were labelled in the year taken and season taken to identify any seasonal or temporal trends.
Spatial maps of average monthly total blue-green algal biovolumes were created using
geospatial software (GIS) to visually show the extent of the blue-green algae bloom over the
study period.
Scatterplots of Electrical Conductivity against the logarithmic total algal cell count for five
genera for sites at The Broadwater, Bombah Point, Korsemans Landing, Violet Hill and Myall
Lake were plotted. The data from April 1999 to April 2002 were plotted to show the effect of
salinity in selecting algal species and influencing cell abundance over a period of great spatial
and temporal salinity difference.
Results and analyses from Ryan (2000) were used in discussing results here as they are
particularly focussed on the ecophysiology and morphology of blue-green algae and the
factors leading to the development of the blue-green algae bloom and succession in Myall
Lakes.
4.3. RESULTS.
4.3.1. Spatial Distribution of Blue-Green Algae.
The first indications of a blue-green algal bloom in Myall Lakes occurred in March 1999
when regular lake users noted green scums. These were similar in appearance to the later
confirmed bloom in sheltered embayments around Bombah Point. In early April 1999, a
widespread green slick and pungent earthy odour was reported to the NSW Environment
Protection Authority (EPA) by National Parks and Wildlife Service (NPWS) staff. A sample
was taken and identified as containing high numbers of the freshwater blue-green alga
Anabaena circinalis, a taxon potentially able to produce saxitoxins or paralytic shellfish
poisons that are neurotoxic. This incidence was the first scientifically documented blue-green
algal bloom in the Myall Lakes system, and anecdotal reports from the long-term community
agreed that scum-forming algae was not previously know at the location.
As no regular water quality sampling program was in place prior to April 1999, the status of
water quality and phytoplankton communities within the lakes prior to the blue-green algal
bloom were unknown (Ryan, 2000). This is a common problem throughout Australia with
Bowling (1984) concurring that monitoring of limnological conditions does not occur until a
blue-green algal bloom has developed and as such the pre-bloom conditions that facilitate
such blooms is not well understood.
NSW Department of Infrastructure, Planning and Natural Resources
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
During the three and half year study period (April 1999 to September 2002) the blue-green
algal bloom was initiated, or first reported, in the Broadwater in high biovolumes (> 2 mm3 /L,
see Figure 4.1). Through time, algal blooms were subsequently reported in Two Mile Lake,
Boolambayte Lake and then Myall Lake in April 2000, although different taxa dominated in
each area. (ie. toxic species, such as Anabaena and Microcystis, were generally restricted to
the Broadwater and mid-lakes, while smaller-celled species, such as Chroococcus, dominated
in Myall Lake). High biovolumes were not observed in Myall Lake until October but
decreased thereafter dramatically from February 2001.
NSW Department of Infrastructure, Planning and Natural Resources
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Figure 4.1. Myall Lakes Average Total Monthly Biovolumes.
4.3.2. Blue-Green Algal community change and succession
During the three and half year study, the composition of the blue-green algal community
changed dramatically. The use of multivariate analysis (of blue-green algal community
composition, the occurrence and abundance in each sample taken) was employed to identify
the main taxa present in each location as the bloom progressed, and to identify seasonal
changes imposed on the community.
4.3.2.1. Multivariate analysis of temporal and spatial trends in the Blue-Green Algal
community.
Ordination plots (Figure 4.2) highlight the inter-annual variability in the blue-green algal
community as a function of distinct wet (before July 2001) and dry periods (after July 2001).
Each point represents the similarity of a single day’s algal community to other samples.
Seasonal plots were generated but were not included, as the strong inter-annual variability
was greater than any seasonal signal.
Stress: 0.1
1999
2000
2001
2002
Figure 4.2. Temporal Changes in Blue-Green Algal Community Assemblage in the
Broadwater April 1999 - October 2002.
NSW Department of Infrastructure, Planning and Natural Resources
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Stress: 0.11
1999
2000
2001
2002
Figure 4.3. Temporal Changes in Blue-Green Algal Community Assemblage in Myall
Lake April 1999 - October 2002.
The high abundance of Anabaena in 1999 and Microcystis in 2000 were major contributors to
structuring the community in the Broadwater. These species were associated with wet periods.
The occurrence of Planktothrix and Coelosphaerium in late 2000 which was then replaced by
algae such as Merismopedia and Chroococcus in mid 2001 produced a change in community
structure during the dry years (2001-02). This was well defined in Myall Lake (Figure 4.3)
with the occurrence of very high counts of Anabaena early in 1999, and then both Anabaena
and Microcystis in 2000.
The clear dichotomy in the algal community structure in 2000 corresponds with the decline of
Anabaena and Microcystis and their replacement by the algal genera, Chroococcus and
Merismopedia. This may be due to changes in the analytic technique to determine cell counts
that occurred around this time. The technique used prior to June 2000 was biased toward
detection of larger algal species. However, the ordination seems to be more affected by a lack
of Microcystis and Anabaena after June 2000, and these results would not have been affected
by changes in laboratory technique. The following sections provide a description of the
changes over time of the blue green algal communities (as illustrated in Figures 4.2, 4.3, and
4.4). Descriptions of these communities are described below and are termed “Phases”, with
Phase 1 being the first detected bloom.
4.3.2.2. Phase 1 - Blue-Green Algal Bloom
The first species detected in April 1999 in the Broadwater was Anabaena circinalis.
Although this species has the potential to produce neurotoxins, none were detected through
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
toxicity testing (for saxitoxins) towards the end of the Anabaena bloom. Anabaena was not
detected in high numbers anywhere in Myall Lakes after July 2000.
The occurrence of large celled filamentous Anabaena as the primary coloniser (Figure 4.4)
after catchment inflows was found when total phosphorus, phosphate and total nitrogen was
greatest (Ryan, 2002). It is likely during this time that turbidity in the Broadwater was also
high due to catchment inflows, as has been observed subsequently.
4.3.2.3. Phase 2 - Blue-Green Algal Bloom
In December 1999 Anabaena was again found to be very abundant in the Broadwater and in
Two-Mile Lake. This was later (~ February 2000) replaced by Microcystis aeruginosa that
was found to be producing the hepatotoxin microcystin, in varying concentrations, throughout
the bloom. Like Anabaena, Microcystis aeruginosa has gas vacuoles and forms large colonies
(that enhance their buoyancy), and may produce surface scums that are easily blown by the
wind. After early 2000 both taxa ceased to be dominant in any location in the lakes.
NSW Department of Infrastructure, Planning and Natural Resources
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Chapter 4, Phytoplankton Abundance and Distribution
Biovolume (mm3/L)
10
4
8
3
6
2
1
0
0
10
Biovolume (mm3/L)
2
4
14.7
14.2
Anabaena
Aphanizomenon
Aphanocapsa
Chroococcus
Coelosphaerium
Merismopedia
Microcystis
Planktothrix
Broadwater
4
3
6
4
2
10
2
1
0
Mid-Lakes
4
8
9.0
3
6
4
2
0
2
1
0
10
4
Myall Lake
8
3
6
4
2
1
0
0
Ap
r-0
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Ju
l-0
0
Oc
t-0
0
Ja
n-0
1
Ap
r-0
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Ju
l-0
1
Oc
t-0
1
Ja
n-0
2
Ap
r-0
2
Ju
l-0
2
Oc
t-0
2
2
Ap
r-9
9
Ju
l-9
9
Oc
t-9
9
Ja
n-0
0
3
Biovolume (mm /L)
River Mouth
8
0
Biovolume (mm3/L)
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Figure 4.4. Changes in Blue-Green Algal Community April 1999 – October 2002
4.3.2.4. Phase 3 - Blue-Green Algal Bloom
The large-celled blue green algal taxa were replaced by Coelosphaerium and species such as
Chroococcus and Merismopedia in mid 2000. The latter species persisted in the upper lakes
(Myall Lake) over the remainder of the study period . It is interesting to note that in the spring
of 2000 the blooms were wholly dominated by Merismopedia until January 2001, (Figure 4.4)
and since that date Chroococcus has been the dominant taxa in those areas sustaining smallNSW Department of Infrastructure, Planning and Natural Resources
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
celled cyanophyte blooms. The simultaneous decline of Merismopedia in conjunction with the
sharp increase in Chroococcus abundance, while it may be coincidental closely mimics algal
succession patterns (eg. Boney 1988) reported for other locations. It should be noted that
while the general trend of change from larger celled species to the smaller celled species
appeared to occur in all systems, it was most pronounced in the Broadwater and mid-lakes.
The abundance of any given taxa across the different lake systems also varied substantially. It
should also be noted that the changes from large-celled to small celled species are not
pronounced for Myall Lake and that smaller celled species may have been common in this
area over the April 1999 to April 2000 period.
This apparent succession could indicate the response of the algal community to a disturbance,
which gradually returned to a community, dominated by Chroococcus or could have been the
start of conditions that favoured cyanophytes over other phytoplankton in these locations. It is
impossible to make conclusions however, without more information.
These genera lack gas vacuoles, but because of their small cell and colony size are not easily
sedimented-out like larger colonial algae, and do not produce scums. Small-celled algae
require less energy to maintain cell function than their larger competitors. These taxa are well
adapted to remain homogenously mixed throughout the metalimnion, and are not obvious to
the naked eye even when present in high concentrations.
4.3.3. The response of algal community composition to changing conductivity.
The occurrence of the first potentially toxic bloom in April 1999 in the Broadwater coincided
with the lowest conductivity (or salinity) recorded during the study (less than 2 mS/cm). The
initial bloom followed consecutive high monthly rainfalls (>250 mm) in May 1998,
November 1998 and April 1999 with two daily events of 65 mm and 123 mm on the 1st and
6th of April 1999 (Ryan, 2002). Maps of changes in conductivity throughout the lake and
over time are presented in Chapter 5.
The correlation between observed ambient conductivity and total cell counts for four main
blue-green algal genera of Myall Lakes are presented in Figure 4.7. A clear distinction is
shown between the range of salinities in which Anabaena and Microcystis were found and
that observed for Chroococcus and Merismopedia, the latter being found in waters far more
saline than other taxa.
Previous analysis of these data by Ryan (2002) identified that the spatial distribution of algae
was correlated with decreasing conductivity, and that a trigger for the development of
potentially toxic algal blooms were values below 2 mS/cm. This can be observed in Figure
NSW Department of Infrastructure, Planning and Natural Resources
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
4.5 for the Broadwater and in Figure 4.6 for Myall Lake, and illustrates that a drop in
conductivity to 2 mS/cm did not occur in Myall Lake until June 2000, after which a rapid
increase in cell numbers was observed. The increase of potentially toxic algae into the middle
lakes and Violet Hill appears to have coincided with declining conductivity. Whether the
mechanism for the increase is due to the transport of cells from the Broadwater or is a
response of in situ algae to fresher conditions or a combination of both is not known.
In the Broadwater, increases in conductivity as a result of low rainfall after December 2000
were followed by dramatic decreases in potentially toxic algal cell counts (Figure 4.4). This
was mirrored two months later in Myall Lake for the homogenous small celled blue-green
algae. However, in Myall Lake the rate of conductivity change was slight in comparison to
the lakes below Violet Hill, which exhibited trends similar to the Broadwater (Ryan, 2002).
Thus, the decreasing photoperiod and lower water temperatures at this time of year (that
disproportionately slow blue-green algal growth rates when compared to other phytoplankton)
may have been more significant in this decline (ie. the decline in Myall Lake) than the
gradually declining conductivity.
Salinity intrusion appeared to be an important factor in decreasing the dominance of
Anabaena and Microcystis (Ryan, 2002). The increase in salinity in the Broadwater of up to
20 mS/cm in December 2000 corresponded to a rapid decline in algal cell numbers and
cessation of the bloom in this part of the lake (Figure 4.5). Salinity increases prior to this
began in August 2000 in the Broadwater and resulted in a change in blue-green algal
community composition with an increased prevalence of small-celled species.
Total BGA Cells/mL
180000
25000
22500
20000
17500
160000
140000
120000
Total BGA
100000
15000
12500
EC
80000
10000
60000
EC s/cm
30000
27500
200000
7500
5000
2500
40000
20000
7/03/01
7/02/01
7/01/01
7/12/00
7/11/00
7/10/00
7/09/00
7/08/00
7/07/00
7/06/00
7/05/00
7/04/00
7/02/00
7/03/00
7/01/00
7/12/99
7/11/99
7/10/99
7/09/99
7/08/99
7/07/99
7/06/99
7/05/99
0
7/04/99
0
Date
Site 5 Mid Broadwater
Figure 4.5. Response of Blue-Green Algae to Salinity in The BroadwaterApril 1999 March 2001, Source: Ryan, 2002.
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Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
200000
30000
27500
25000
22500
20000
17500
15000
12500
10000
7500
5000
2500
0
Total BGA Cells/mL
180000
160000
140000
120000
100000
Total BGA
80000
EC
60000
40000
20000
7/03/2001
7/02/2001
7/01/2001
7/12/2000
7/11/2000
7/10/2000
7/09/2000
7/08/2000
7/07/2000
7/06/2000
7/05/2000
7/04/2000
7/03/2000
7/02/2000
7/01/2000
7/12/1999
7/11/1999
7/10/1999
7/09/1999
7/08/1999
7/07/1999
7/06/1999
7/05/1999
7/04/1999
0
EC s/cm
Chapter 4, Phytoplankton Abundance and Distribution
Date
Site 1 Myall Lake @ Shellys
Figure 4.6. Response of Blue-Green Algae to Salinity in Myall LakeApril 1999 - March
2001, Source: Ryan, 2002.
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Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
27000
24000
21000
18000
15000
12000
9000
6000
10
1
3000
1000000
100000
10000
1000
100
0
Cells/mL
Microcystis aeruginosa
Chapter 4, Phytoplankton Abundance and Distribution
24000
27000
24000
27000
21000
18000
15000
12000
9000
6000
3000
1000000
100000
10000
1000
100
10
1
0
Cells/mL
Anabaena circinalis
Electrical Conductivity (us/cm)
Electrical Conductivity (us/cm)
Cells/mL
Chroococcus sp .
1000000
100000
10000
1000
100
10
21000
18000
15000
12000
9000
6000
3000
0
1
Electrical Conductivity (us/cm)
1000000
100000
1000
100
10
27000
24000
21000
18000
15000
12000
9000
6000
3000
1
0
Cells/mL
Merismopedia sp.
10000
Electrical Conductivity (us/cm)
Figure 4.7. Scatterplots of Total Cell Counts (Combined Myall Lakes) and Electrical
Conductivity for 4 Algal Genera April 1999 – April 2001 Indicating the Ecological
Limitation of these Genera to Salinity: Source Ryan (2002).
4.3.4. Descriptions of phytoplanktonic community succession in Myall Lakes.
The previous section discussed blue-green algae only. The next section discusses total
phytoplankton community changes over time and their spatial pattern for the period from
January 2000 to September 2002. As mentioned in Section 4.1 a well described pattern of
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
algal succession is known to occur in marine and freshwaters in spring, often responding to
increasing daylight and high nutrient concentrations. A rapid initial bloom of small-celled
diatoms occurs, which are then followed by slower growing and larger diatoms, then
dinoflagellates and other organisms with slower growth rates such as green algae and bluegreen algae. Blue-green algae are thought to also be favoured under calm ambient conditions
and those described previously. However, from early 2001 all algal taxa from the lakes began
to be identified and thus successions after that date only are possible.
4.3.4.1. Upper Myall Rivermouth.
This site experienced diatom blooms for two months in Summer 2000-2001 and Spring 2001
that corresponded with the timing of saline intrusion into the river. Sharp increases in
dinoflagellate abundances were experienced in the upper Myall Rivermouth from Autumn
2002 with a large biovolume (13.5 mm3/L, corresponding to approximately 14,000 cells/mL)
occurring in Spring 2002 and corresponding with salinity intrusion and transient stratification
(see Figure 4.8). Trend analysis normally requires many years of data, so few trends can be
detected here due to the short study period. However from the limited data it does appear that
diatom and dinoflagellete blooms in the Myall Rivermouth may be associated with saline
intrusion and stratification that may be increasing nutrient fluxes from the sediments (see
Chapters 3 and 5 for more detail).
4.3.4.2. Bombah Broadwater.
The Broadwater was initially characterised as a community dominated by toxigenic bluegreen algae in autumn and winter of 2000 that followed a large rainfall event in autumn. The
composition of the phytoplankton community prior to this and the pre 1999 bloom are
unknown.
After this time the community composition in the Broadwater changed (Figure 4.9) and was
less dominated by blue-green algae, but exhibited some features of phytoplankton succession.
Small increases in diatom abundance were observed in winter of 2001, followed by a large
green algae bloom the following spring and a smaller blue-green algae bloom. Small increases
in dinoflagellate abundance were observed in the summer of 2001-2002, after a decline in
green and blue-green algal abundance, and an increase in conductivity in the lake. The marine
genus Gymnodinium predominated, of which a number of species can produce paralytic
shellfish poisons or neurotoxic shellfish poisons, but species-level identification was not
determined on this occasion.
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
4.3.4.3. Two-Mile and Boolambayte Lakes.
The phytoplankton composition in the Mid Lakes was similar to the Broadwater in that it was
dominated in Winter 2000 by blue-green algae. In contrast to the Broadwater, a peak in
diatom numbers preceded this blue-green algae bloom in autumn following the autumn
rainfall. Small peaks in diatom abundance were also observed in spring and summer 20002001 and low biovolumes of blue-green algae followed decreases in diatoms in summer again reflecting what was occurring in the Broadwater. Peaks in diatom abundance were
observed again in autumn and winter of 2001 after which a succession of green algae and
blue-green algae followed through spring and summer (see Figure 4.8).
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Chapter 4, Phytoplankton Abundance and Distribution
Biovolume (mm3/L)
6
River Mouth
5
4
3
Biovolume (mm3/L)
Diatoms
Green Algae
Blue Green Algae
Dinoflagellates
1
Broadwater
7.8
5
4
3
2
1
0
6
Biovolume (mm3/L)
13.5
2
0
6
Mid-Lakes
5
4
3
2
1
0
6
Biovolume (mm 3/L)
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Myall Lake
10.5
5
4
3
2
1
Oc
t-0
2
Ju
l-0
2
Ap
r-0
2
Ja
n-0
2
Oc
t-0
1
Ju
l-0
1
Ap
r-0
1
Ja
n-0
1
Oc
t-0
0
Ju
l-0
0
Ap
r-0
0
0
Figure 4.8. Changes in Phytoplankton Community April 2000 - October 2002.
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Stress: 0.19
2000
2001
2002
Figure 4.9. Temporal Changes in Phytoplankton Community Assemblage in the
Broadwater 2000 – 2002 Contrasting a Wet and Dry Phase
In contrast to the Broadwater, the Mid-Lakes exhibited higher numbers of diatoms but a less
dramatic green algal bloom, which followed the autumn rainfall events and coincided with a
drop in conductivity. Similar to the Broadwater, no peaks were observed throughout the
summer of 2002 and till the end of the project period (October 2002).
4.3.4.4. Myall Lake.
Community composition in Myall Lake in autumn 2000 (prior to the drop in conductivity in
September 2000) was dominated by a high abundance of diatoms. The phytoplankton
composition in Myall Lake exhibits high inter-annual variation (Figure 4.10) that seems to be
due to the occurrence of small celled blue-green algal species in 2001 and a high species
richness of diatoms. Small celled blue-green algae were not present in many samples in 2000,
compared to 2001 and 2002, while a high richness of diatoms were present in 2000, compared
to 2001, although they were actually more abundant at the beginning of 2001.
At the beginning of summer 2000-2001 a small peak in diatom abundance occurred, after
which blue-green algae dominated the phytoplankton composition in mid to late summer. A
smaller diatom peak was observed the following winter (2001) in Myall Lake. This was
followed by an algal succession (small diatoms ? larger diatoms ? dinoflagellates), that was
also occurring in the Broadwater and Mid-Lakes. Later a green algal bloom of very high
biovolume (10.5mm3 /L) in Spring 2001 occurred, followed by a blue-green algal bloom in
summer 2001-2002. In the following autumn and winter of 2002, no increase in diatom
abundance occurred in Myall Lake or any other part of the Lakes, apart from a small peak in
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
the Rivermouth. One possible mechanism that may be keeping diatom abundance lower in dry
weather is the exhaustion of silica in the water column causing growth limitation.
Stress: 0.16
2000
2001
2002
Figure 4.10 Temporal Change in Phytoplankton Community Assemblage in Myall Lake
2000 – 2002 Contrasting a Wet and Dry Phase
4.3.4.5. Some observed patterns of phytoplankton diversity and abundance.
Diatoms: The general spatial trend throughout the monitoring period was for decreasing
richness of diatoms from the Rivermouth to Myall Lake,. Temporally, the number of diatoms
declined between 1999 and 2002, corresponding to a decrease in freshwater inflows
(particularly in the Broadwater and mid-lakes) and perhaps indicating silica limitation,
particularly in Myall Lake. The dominant diatom in the Rivermouth, Broadwater and Mid
Lakes was Thalassiosira, a marine genus. This diatom occurred in smaller numbers in Myall
Lake where the population was dominated by the freshwater diatom Synedra.
Green algae: In contrast, green algae richness was highest in Myall Lake with the freshwater
green alga, Oocystis, being the dominant genus. This lake had the most stable conductivity,
and was generally the least saline on average of the lakes. Green algae richness was similar in
the Broadwater and Mid Lakes and was dominated by the marine green alga, Carteria in the
Rivermouth and freshwater green alga, Crucigenia , in the Broadwater and Mid Lakes.
Dinoflagellates: The Broadwater experienced the highest richness of dinoflagellates with the
genera observed mainly found in marine or brackish / marine habitats (e.g. Gymnodinium),
although the highest cell counts were observed in the Rivermouth. This richness decreased
with distance from the marine inflows, and Myall Lake contained a mixture of freshwater and
marine dinoflagellates, dominated by the freshwater alga, Peridinium.
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Blue-green algae: Blue-green algal richness was also higher in the Mid-Lakes and Myall
Lake compared to the Broadwater. Over the short monitoring period, Myall Lake was
dominated by a mainly freshwater algal community, but has some representatives of marine
and brackish taxa. In contrast, the algal community in the Broadwater was composed of a
mixture of freshwater and marine species, depending on the salinity status of the time. It is
also clear that Myall Lake has high green and blue-green phytoplankton richness with a
continuing spring bloom of small celled blue-green algae and green algae.
4.4. DISCUSSION.
4.4.1. The role of conductivity in algal community composition change.
Low conductivity appears to be a major prerequisite for the occurrence of larger-celled
potentially toxic species such as Anabaena and Microcystis. Conductivity range, however, is
an important factor in the long term regulation of small celled phytoplankton communities.
In Myall Lakes, salinity or conductivity is a function of the rates of: freshwater inflows from
the catchment or rain; loss of fresh water through evaporation from the lakes; and saline
intrusion from the Port Stephens estuary via the lower Myall River. The stenohaline (small
variation in salinity) character of Myall Lake, (see Chapter 5) appears to be important in
maintaining stable small celled blue-green algal populations. The more variable (euryhaline)
character of the Broadwater, creates a less stable salinity regime which in turn influences the
long-term stability of any given phytoplankton assemblage.
According to Paerl (1988) small shifts in salinity of between 1.5 – 3.5 mS/cm can cause a
dramatic shift in phytoplankton species composition. Previous analysis of algal data with
conductivity, from throughout the lakes, by Ryan (2002) identified that Anabaena and
Microcystis were present when conductivities were less than 6 mS/cm with a preference for 14.5 mS/cm (Figure 4.7). This is supported by Winder (1994) and Winder & Cheng (1995)
who reported between 5 and 7 ms/cm as the ecological limitation for Anabaena circinalis. In
contrast Chroococcus and Merismopedia dominated at conductivities between 1.5 – 10
mS/cm but were persistent up to 27 mS/cm., although in reduced numbers (Figure 4.7).
4.4.2. Patterns of nutrient concentrations and the algal community in Myall Lakes.
Freshwater inflows not only reduce salinity to levels that favour toxigenic blue green algal
species, they also deliver loads of nutrients and sediment to Myall Lakes, and are likely to
cause stratification of the water column. These combined effects of freshwater inflow create
conditions that favour blue green algae over other phytoplankton species. Therefore, locations
within the lakes with the greatest amount of freshwater input (i.e. the Broadwater) are also the
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
most prone to toxigenic blue-green algae blooms. While it is impossible to separate the effects
of increased nutrients, stratification and reduced salinity, examples of increased abundance of
blue-green algae following rainfall for salt-tolerant taxa have been found in this study. In
March 2000 and February 2001 rainfall events were followed by increased algal cell counts of
Chroococcus in the Broadwater and Mid Lakes, apparently enhancing a bloom that was
already underway prior to the rainfall event (Figure 4.4). The influence of these freshwater
flows on the lake nutrient concentrations are reported in Chapter 5, which discusses far lower
nutrient concentrations in the water column during dry weather, than during a period of
average rainfall. The influence of different nutrient types on algal abundance is discussed
below.
Phosphorus: A clear gradient of highest water column total phosphorus concentration in the
upper Myall Rivermouth and lowest at Neranie at most sampling times, supports the model
that the Myall River is the source of most nutrients to the system. In addition, this part of the
lakes have been found to be periodically stratified (following high catchment inflows) which
could be intermittently facilitating the release of phosphorus from the sediments in cases
when bottom waters become anoxic.
Murray & Parslow (1999) reported that the spatial variation of algal blooms is strongly
influenced by the location of major nutrient inputs, with temporal variation dominated by the
occurrence of catchment runoff events delivering nutrient loads from the catchment. This is
consistent with the occurrence of algal blooms in close proximity to freshwater inflows in the
Peel Harvey estuary in Western Australia (Lukatelich & McComb, 1986), Port Phillip Bay in
Victoria (Murray & Parslow, 1999) and Cockburn Sound in Western Australia (Hillman et al.,
1990).
Various studies (Fogg et al., 1973: Hart et al., 1985) have alluded to nutrient requirements as
the determining factor in succession from Anabaena to Microcystis. Bowling (2001) reported
that Anabaena requires higher ambient phosphorus than Microcystis because it is less able to
utilise phosphorus at low concentrations, and is thus well suited to follow shortly after a
period of high phosphorus. After its addition, phosphorus may be rapidly lost via
sedimentation or consumed by flora and depleted.
Differences in sediment chemistry processes in freshwater compared to brackish and marine
systems mean that phosphorus is less likely to be a limiting nutrient in brackish systems
compared to freshwater systems. In this study low concentrations of phosphorus in benthic
material (in comparison to nitrogen) were found in the Mid Lakes and Myall Lake. This may
suggest that productivity in Myall Lake is limited by phosphorus.
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
Nitrogen: A less-clear picture of the role of nitrogen in algal ecology has emerged in the
system. The blue-green algal bloom in Myall Lake from December 2000 (of Merismopedia
and Chroococcus) over the summer not only corresponded with a decrease in conductivity to
less than 2 mS/cm but also followed high ammonia concentrations in Myall Lake that
occurred from April-June 2000 and in December 2000 (see Chapter 5). It is widely thought
that small-celled taxa such as Chroococcus grow disproportionately well in waters that are
readily supplied with ammonia.
4.4.3. Other factors influencing the composition of blue-green algal communities.
Although it is energetically expensive, Anabaena is able to utilise atmospheric nitrogen, when
there is not sufficient in the water column. Extracellular excretion of ammonia from
Anabaena or from cell lysis (after death) may increase the concentration of combined
nitrogen in the water. Bowling and Baker (1996) identified that excretion is a process that
may elevate ammonia concentrations making limnological conditions more suitable for
Microcystis, which is able to accumulate and utilise low concentrations of phosphorus.
It can be concluded that the switch in algal assemblage from larger celled to smaller celled
blue-green algae is related to the periods of high and low freshwater inflows to Myall Lakes..
However, as this analysis has been conducted over a very small time scale in comparison to
the long term processes occurring within the lakes, it is difficult to highlight any long term
trends. This is particularly pertinent given that water exchange within the lakes can be an 800
day cycle, which is longer than the duration of the study.
Although at the time of writing, potentially toxic blue-green algal taxa were no longer
dominant in the lakes, it is possible they will re-occur in the future due to their morphological
features, which gives them an advantage over other species in certain conditions. According
to Harris (1994) it is common for larger celled blue-green algae to dominate phytoplankton
communities when nutrients are in abundance. Due to its large cell size the gas vacuoled
Anabaena, requires higher nutrient concentrations than other species of blue-green algae to
maintain cell function, and cannot dominate when nutrients are scarce. In the absence of high
combined nitrogen Anabaena can grow specialist cells (heterocysts) for nitrogen fixation
(NMHMRC, 1984). It can also produce spore-like akinetes that enable it to regenerate, when
out-competed, until conditions are favourable for the establishment and maintenance of new
populations.
The gas-vacuoled blue-green algae such as Microcystis and Coelosphaerium also have the
ability to over-winter on lake benthos and reanimate as viable cells once photosynthetic rates
exceed those of respiration (Chorus & Bartram, 1999). Consequently, gas-vacuoled algae do
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Chapter 4, Phytoplankton Abundance and Distribution
Understanding Blue Green Algae Blooms
in Myall Lakes NSW.
not readily form blooms in winter, as they are less buoyant, while small-celled algae remain
suspended within the water column. Over-wintering Microcystis, if present would be
destroyed by salinity intrusion into the lakes. As discussed previously, the ongoing spring
blooms of Chroococcus and Merismopedia in Myall Lake may persist for some time due to
their winter occurrence within the water column and efficient nutrient cycling, among a range
of other potential influences. The succession of Chroococcus over scum-forming species such
as Microcystis may be related to a variety of factors such as nutrient availability, light
availability, electrical conductivity, water chemistry stability, water column stability and by
virtue of their ecophysiology. The fact that this taxon is not buoyant makes it more suited to
the clearer waters of Myall Lake.
This type of algal species may be able to sustain large populations for extended periods of
time through senescence, efficient nutrient cycling within the water column, (e.g. ammonia
excretion and absorption), and may not be as reliant on external nutrient sources as the larger
species. It is likely that the presence of Chroococcus (although in this instance it seems to
have been facilitated by taxa that need high nutrient concentrations) might not necessarily be
an indication of elevated nutrients in Myall Lake. Once established and present throughout
winter within the water column, Chroococcus would have a competitive advantage over other
algae due to efficient nutrient partitioning and will undergo an increase in abundance in
Spring when conditions become more favourable (Ryan, 2002). Chroococcus has the ability
to scavenge Nitrogen (N) compounds from the water column. Chapter 5 discusses the
potential links between the abundance of these smaller celled species of blue green algae and
available N. This helps explain the patterns of abundance observed in spring 2000, 2001 and
2002.
Also influencing the recurrence of blooms is the ability of Anabaena to produce akinetes as
resting spores. These may be tolerant to salinity and can survive for many years in sediments
(Fay, 1983). Baker (1999) identified that akinetes in the Murray River provided the inoculum
for bloom development, but required germination conditions that depended on the interaction
of a number of environment factors including increased light availability, water temperature
and elevated nutrients. This may represent a potential future problem for the Broadwater,
where the abundance of these taxa was greatest.
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5. Water Quality
5.1. WATER NUTRIENTS
5.1.1. Introduction
While all phytoplankton and algae require nutrients for their normal growth and reproduction,
increases in nutrient runoff can lead to eutrophication characterised by excessive algal and
phytoplankton growth. Blooms of blue green algae often occur in areas with excess nutrient load. In
addition, the relative abundance of different nutrients, and the chemical form in which nutrients occur
in a waterbody play an important role in determining the composition of algal and phytoplankton
assemblages (Harris 1999, Anderson et al. 2002). For example, ammonia is preferentially taken up by
small celled blue-green algae while nitrite is better utilised by green algae.
One of the main findings of the Blue-Green Algae Task Force (1992) which investigated the bluegreen algal bloom of the Darling River system in 1991, was that higher than normal concentrations of
water nutrients, particularly phosphorus, were responsible for creating conditions that favoured bluephosphorus may also be important in some cases (see review in Chapter 1).
The most important source of nutrients to coastal waters is catchment runoff, with most nutrients
delivered to waterbodies during rain events. On the east coast of Australia, rainfall is highly seasonal
and there is large interannual variability causing large variation in the timing and amount of nutrient
delivered to coastal waters. It is important to note that due to rapid uptake by algae and phytoplankton
(and efficient nutrient cycling between algae, sediments and the water column) nutrient concentrations
in the water column are often low, even when nutrient loads delivered to a waterbody may be high.
As is discussed in Chapter 2, Myall Lakes is at the end of the Myall River catchment, and has a very
low natural flushing rate. As a consequence, most of the nutrients that enter via rainfall events in the
Myall Catchment are processed in the lakes, as exports to the lower estuary are thought to be fairly
small. As with any lake, the concentration of nutrients present in the water at any point in time are a
consequence of the rate of delivery of nutrients and the rate of consumption and loss from the water
column. During sustained periods of dry weather, the concentration of nutrients in Myall Lakes is
determined by internal recycling and processing.
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5.1.2 Methods
5.1.2.1 Rainfall
Daily rainfall records for Bulahdelah were obtained from Bureau of Meterology for the study period –
January 2000 – October 2002.
5.1.2.2 Water nutrient analyses.
Between January 2000 and October 2002, water nutrient concentrations were determined on 33
separate dates, usually for between 6 and 8 locations in the lakes. Between October 2001 and October
2002, 8 locations were sampled monthly, and a 6-week intensive phase of fortnightly sampling took
place from December 2001 to January 2002. The analytes tested for in each of the samples are
presented in Table 5.1.
Table 5.1. Analytes and detection limits for water nutrient analyses.
Analyte
Method Used*
Detection Limit
Total Nitrogen
APHA 4500-NF&-PH
0.01 mg/L
Total Phosphorus
APHA 4500-NF&-PH
0.002 mg/L
Nitrogen as Ammonia #
APHA 4500-NH3 H
0.01 mg/L
Oxidised Nitrogen
APHA 4500-NO3 I
0.01 mg/L
Soluble Reactive Phosphorus
APHA 4500-PG
0.002 mg/L
Chlorophyll - ? +
APHA 10200H
0.06 mg/m3
# While the laboratory reports Nitrogen as ammonia (NH3 ) in water samples, in natural waters where the pH is
less than 9, most ammonia exists as ammonium (NH4 +). Therefore this report will report concentrations of
ammonium (NH4 +).
* - Analyses performed by Australian Water Technologies, Sydney.
+
- As a means of assessing standing stock of phytoplankton and nutrient consumption.
Water samples were collected the top 2m of the water column using an intergrated pole sampler. Three
or four replicate samples from each site were combined in a clean, rinsed pail. Samples were
immediately extracted from this composite and chilled on ice prior to delivery to the laboratory. Prior
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to filling, sample bottles were rinsed twice thoroughly with approximately 50 mL of sample to reduce
the risk of contamination.
Samples taken for Soluble Reactive Phosphorus (SRP),ammonuim (NH4 +) and nitrate/nitrite (NOx)
were filtered immediately in the field using disposable 0.45 ? m filters and rinsed syringes. To reduce
the risk of contamination clean sample bottles were rinsed twice with 5 mL of filtrate before a 50 mL
sample was retained for analysis.
5.1.2.3. Near-Benthic Water collection.
In September 2002, water samples were collected from just above the sediment and from the surface.
Bottom water samples were collected by drawing a sample into a tube that was suspended 5-10 cm
above the lakebed. Sampling was conducted at 1 control site (Broadwater), and 2 experimental sites
(Upper Myall Rivermouth and Two-Mile Lake) in the deepest parts of the lake system. This was to
ensure that samples taken from the bed of the lake were least disturbed by wind-generated turbulence,
and most thus most likely to accumulate nutrients if benthic processes were releasing them from the
benthos.
The three replicate samples at each location were drawn to the surface and into a vacuum chamber
using a hand-operated vacuum pump. The samples were immediately filtered and chilled before
transportation to the laboratory. Positioning of the sampling tube above the lakebed was achieved with
the assistance of a closed-circuit underwater video on a boat over the sampling location. With the
video it was possible to clearly see the position of the uptake tube in relation to the benthos and avoid
contact with the sediment. In this way clean samples of the near-benthic water could reliably be taken.
Using the same method. three replicate surface samples were also taken at each location and treated in
the same manner as bottom water samples. Thus, the concentration near the lakebed could be
compared to the concentration of the remainder of the water column.
5.1.2.4. Interpreting nutrient data against ANZECC and ARMCANZ guidelines
The Agriculture and Resource Management Council of Australian and New Zealand (ARMCANZ)
National Water Quality Management Strategy (2000) sets out general annual guidelines for ambient
nutrient concentrations in freshwater, estuaries and marine systems (see Table 5.2). While there are
many steps to be undertaken in determining whether observed nutrient concentrations are higher than
is desirable for a given system (features specific to an individual waterway play a very significant
role), these values at least provide a framework for interpreting observed nutrient concentrations in a
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broad sense. For this purpose alone, monthly average nutrient concentrations are compared to the
annual guideline figures in this chapter. Understanding the significance of these comparisons and
implications for lake health are discussed in Chapter 7.
TABLE 5.2 : The Agriculture and Resource Management Council of Australian and New Zealand
(ARMCANZ) National Water Quality Management Strategy (2000) guidelines for ambient
nutrient concentrations.
Analyte
Guideline Value
Fresh
Estuarine
Total Phosphorous
0.010 mg/L
0.030 mg/L
Soluble Reactive Phosphorous
0.005 mg/L
Total Nitrogen
0.350 mg/L
0.300 mg/L
Nitrogen as Ammonium (NH4 )
0.010 mg/L
0.015 mg/L
Nitrogen as Oxidised Nitrogen (NOx)
0.010 mg/L
0.015 mg/L
Chlorophyll a
5 ?g/L
4 ?g/L
As mentioned above, two distinct periods average rainfall and lower than average rainfall, occurred
during the study period. Although the actual delivery of nutrients was not quantified during this
period, it is assumed that the two periods represent about average and then below average nutrient
loads to the system. The spatial distribution is thus discussed within the perspective of these two
regimes.
5.1.2.5. Data consolidation and presentation.
Interpreting the results of nutrient analyses from a large number of locations incorporating years of the
data is a cumbersome process. Although during the Coasts and Clean Seas Program the same 8 sites
were sampled on nearly all occasions, prior to this program samples were collected from many
different locations, at varying intervals. To avoid discarding valuable data that could otherwise be
included in the analysis, and to simplify this process, the data has been consolidated to 8 regions that
represent the main regions within the lake system. Table 5.3 outlines the area that corresponds to each
region.
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Table 5.3. Location names for reporting Water Quality.
No.
Name.
Includes samples taken in these areas.
1
Rivermouth
Upper Myall Rivermouth, western Broadwater.
2
South Broadwater
Lower Myall Rivermouth, Mungo Brush, White
Tree Bay, southern Broadwater.
3
North Broadwater
Mid-Broadwater, northern Broadwater
4
Bombah Point
Bombah Point, Southern part of Two-Mile Lake.
5
Korsmans Landing
Korsmans Landing, The Narrows, northern part
of Two-Mile Lake, Boolambayte Creek.
6
Violet Hill
Boolambayte Lake, Violet Hill.
7
Myall Lake
Myall Lake locations east of Burrah Burrah Pt.
8
Neranie
Myall Lake locations west of Burrah Burrah Pt.
As well as this spatial consolidation, the results have been averaged temporally by the calender month
in which they were sampled. Much of the water quality is presented in maps which utilise this spatial
and temporal simplification to allow the presentation of a complex dataset in a visually simple manner.
It should be remembered however, that the maps are just intended as a visual aid to the interpretation
of the data and are not intended as a definitive guide to the exact concentration in locations that were
not sampled. However, in a well-mixed waterbody without many nutrient point-sources such as Myall
Lakes, it is likely the distribution of solutes within a region would be fairly uniform. The results
summarised in Figure 5.2 to Figure 5.13 show average monthly concentration of different types and
chemical forms of nutrients in Myall Lakes from April 1999 to October 2002. Colours were selected
to represent the relative concentration of each analyte throughout the lakes as well as how these relate
to the ARMCANZ guideline.
5.1.3. Results
The results of water column nutrient analysis from April 1999 to October 2002 are presented below in
graphical and map format. The role of wet and dry periods in influencing nutrient concentration is
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highlighted and the results are compared to ANZECC and ARMCANZ guideline values for average
annual nutrient concentrations in fresh and estuarine waters.
Due to the large influence of rainfall on the export of nutrients from the catchment to waterway, the
results of nutrient sampling within Myall Lakes are presented in the context of the rainfall patterns
during the study.
5.1.3.1 Rainfall
During the study period two distinct rainfall regimes occurred. From March 2000 to July 2001,
generally close to average or above average rainfall was recorded (Figure 5.1). Although there were
months of low rainfall during this time, approximately 108% of the expected rainfall were received.
Drought conditions were experienced during the subsequent period of August 2001 to October 2002,.
Although 2 months of above average rainfall occurred, 7 consecutive months of below average rainfall
were recorded in this period with approximately 72% of the expected average rainfall in this period. It
is assumed that a higher nutrient load was being delivered to the lakes during the average rainfall
period than in the subsequent dry period
Average and Recorded Monthly Rainfall - Bulahdelah
Recorded Monthly Rainfall
450
Monthly Rainfall (mm)
Average Monthly Rainfall
Average Rainfall Period
400
350
Dry Period
300
250
200
150
100
50
Sep-02
Jul-02
May-02
Mar-02
Jan-02
Nov-01
Sep-01
Jul-01
May-01
Mar-01
Jan-01
Nov-00
Sep-00
Jul-00
May-00
Mar-00
Jan-00
0
Date
Figure 5.1. Monthly Rainfall at Bulahdelah. January 2000 to October 2002.
Because of the distinct differences in rainfall regime (and corresponding differences in water column
nutrient concentration) these two periods are dealt with separately in this chapter. The assumption is
that although flow in the Myall River was not measured, during the first period (average rainfall) a
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greater nutrient load was being delivered to the lakes than in the subsequent (dry) period. This
assumption is also supported by the nutrient load investigations discussed in Chapter 3. This may
provide an opportunity to better understand the relative contribution of internal and external nutrient
loads in promoting and sustaining algal blooms.
5.1.3.2 Total Phosphorus (TP)
Monthly average concentrations of TP in the 8 regions of Myall Lakes are shown as maps in Figure
5.2 (average rainfall) and Figure 5.3 (low rainfall). There was a general pattern of higher values of TP
in the Rivermouth and Broadwater grading to lower values in Myall Lake. This pattern was apparent
in both average and low rainfall periods. During both the average and low rainfall periods, values of
TP at the Rivermouth and Broadwater sites were often above the ARMCANZ/ANZECC guideline
values of 0.01 mg/L (freshwater), and at times above 0.03 mg/L (estuarine) and reached 0.08 mg/L in
April 2000. In contrast, the Two-Mile, Boolambayte and Myall Lakes had average monthly
concentrations of less than 0.02 mg/L, for most months.
5.1.3.3 Soluble Reactive Phosphorus (SRP)
Average monthly SRP concentrations were often below detection limit (Figure 5.14) with the
exception of sites at the Rivermouth, Broadwater, and Mid-Lakes. At the Rivermouth site, SRP values
up to 0.16 mg/L were recorded on occasions throughout the study. In the Broadwater and Mid-Lakes
values were usually low, with elevated concentrations recorded in April 2000. SRP concentrations in
Myall Lake were below detection limit on all but one sampling occasion. Due to the large number of
occasions when values were below the analytical detection limit, these data have not been presented as
maps.
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Figure 5.2: Average monthly concentration of Total Phosphorus in Myall Lakes during a period
of average rainfall April 1999 – July 2001.
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Chapter 5, Water Quality
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Figure 5.3: Average monthly concentration of Total Phosphorus in Myall Lakes during a period
of low rainfall October 2001 – September 2002 .
5.1.3.4 Total Nitrogen (TN)
Average monthly TN values were higher than ANZECC/ARMCANZ guideline values throughout the
Lakes in both average and low rainfall periods. Highest values were recorded in Myall Lake at over
1.5 mg/L with average values of 0.7 mg/L during the low rainfall period. There was a general trend of
increasing TN values from the Rivermouth up to Myall Lake.
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5.1.3.5 Ammonium (NH4 +)
Average monthly NH4 + concentrations were highly variable among locations within Myall Lakes, and
over time (Figure 5.6, 5.7, 5.11 & 5.13). During the average rainfall period, values at the Rivermouth
and Broadwater, were usually low relative to Mid-Lakes and Myall Lake. However values exceeded
ANZECC/ARMCANZ guideline values on many occasions during both average and low rainfall
periods (Fig 5.16) and reached 0.24 mg/L in April 2000. During the average rainfall period, sites in the
Mid-Lakes and Myall Lake regions showed consistently elevated NH4 +concentrations reaching
extreme values up to 0.9 mg/L. However, during the low rainfall period between October 2001 and
October 2002, values were generally below 0.05 mg/L and were usually lower than values recorded at
the Rivermouth and Broadwater.
It is clear that while the increase in ammonium concentration in March – April 2000 occurred at all
sites, the concentrations in the Broadwater declined to ‘normal’ concentrations by June 2000, while
locations in the Mid-Lakes and Myall Lake remained high or increased over the subsequent few
months. Across the lake system, the site at Violet Hill reported the highest concentrations, although
elevated concentrations were also found in Myall Lake (0.2 – 0.4 mg/L) in this period.
5.1.3.6 Nitrate/Nitrite (NOx)
NOx was sampled less intensively spatially and temporally in this period than other analytes. On one
occasion, very high concentrations were reported for the Broadwater (0.101 mg/L) but high
concentrations did not occur in other parts of the system (see Figure 5.8, 5.9 & 5.13). In general Myall
Lake had very consistently low concentrations, while other parts of the system varied considerably. In
the low rainfall period, concentrations greater than the ANZECC/ARMCANZ guideline value of 0.010
mg/L were recorded only in the upper Myall Rivermouth on 4 occasions, and at sites in the
Broadwater on 3 separate sampling days out of 17 (see Figure 5.13). For the rest of the lake
system NOx remained low, or below detection limit for the entire low rainfall period.
5.1.3.7 Chlorophyll a (chl a)
Most chlorophyll a samples were collected during the low rainfall period (see Figure 5.10). Except for
the Broadwater, chl a values were generally low and were below guideline values. In contrast, chl a
values at the Rivermouth site, were often above guideline values.
Locations that were known to possess high biovolumes of algae in the dry period (See Chapter 4), did
not show a correspondingly high Chlorophyll-? concentration at the same time. For example, Myall
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Chapter 5, Water Quality
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Lake and Neranie which in December 2001 had average monthly biovolumes of 2.67 mm3 /L for main
algal groups, had a similar concentration of Chlorophyll-? (2.63 ?g/L Chl.-? ) than the Broadwater
sites which had 0.212 mm3 /L for the same groups (2.71 ?g/L Chl.-? ). This analyte is therefore not a
reliable measure of standing-stock for this system.
Figure 5.4 Average monthly concentration of Total Nitrogen in Myall Lakes during a period of
average rainfall April 1999 – July 2001.
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Chapter 5, Water Quality
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Figure 5.5 Average monthly concentration of Total Nitrogen in Myall Lakes during a period of
low rainfall October 2001 – September 2002 .
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Chapter 5, Water Quality
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Figure 5.6 Average monthly concentration of ammonium (NH4 +) in Myall Lakes during a period
of average rainfall April 1999 – July 2001.
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Chapter 5, Water Quality
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Figure 5.7 Average monthly concentration of N as NH4 + in Myall Lakes during a period of low
rainfall October 2001 – September 2002 .
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Chapter 5, Water Quality
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Figure 5.8 Average monthly concentration of nitrate/nitrite (NO x) in Myall Lakes during a
period of average rainfall April 1999 – July 2001.
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Chapter 5, Water Quality
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Figure 5.9 Average monthly concentration of nitrate/nitrite (NO x) in Myall Lakes during a
period of low rainfall October 2001 – September 2002.
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Chapter 5, Water Quality
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Figure 5.10 Average monthly concentration of chlorophyll a (chla) in Myall Lakes from
September 2000 – September 2002.
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Chapter 5, Water Quality
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ammonia concentration (mg/L)
Mean (and Standard Error) ammonia concentrations.
Drought period October 2001 - October 2002.
0.04
0.03
0.02
0.01
0
Myall River
Mouth
S. Broadwater
MidBroadwater
Bombah Point
Korsmans
Landing
Violet Hill
Mid-Myall
Neranie
Location
Figure 5.11 Spatial distribution of average (with Standard Error) concentration of N as
Ammonium in the low-rainfall period.
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Chapter 5, Water Quality
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0.20
0.020
River Mouth
River Mouth
0.015
SRP (mg/L)
TP (mg/L)
0.15
0.10
0.010
0.05
0.005
0.00
0.000
0.20
0.020
Broadwater
Broadwater
0.015
SRP (mg/L)
TP (mg/L)
0.15
0.10
0.010
0.05
0.005
0.00
0.000
0.20
0.020
Mid-Lakes
Mid-Lakes
0.015
SRP (mg/L)
TP (mg/L)
0.15
0.10
0.010
0.05
0.005
0.00
0.000
0.20
0.020
Myall Lake
Myall Lake
SRP (mg/L)
0.015
0.10
0.010
Ap
r-9
9
Ju
l-9
9
Oc
t-9
9
Ja
n-0
0
Ap
r-0
0
Ju
l-0
0
Oc
t-0
0
Ja
n-0
1
Ap
r-0
1
Ju
l-0
1
Oc
t-0
1
Ja
n-0
2
Ap
r-0
2
Ju
l-0
2
Oc
t-0
2
0.000
Ju
l-0
2
Oc
t-0
2
0.00
Ju
l-0
1
Oc
t-0
1
Ja
n-0
2
Ap
r-02
0.005
Ju
l-9
9
Oc
t-9
9
Ja
n-0
0
Ap
r-0
0
Ju
l-0
0
Oc
t-0
0
Ja
n-0
1
Ap
r-01
0.05
Ap
r-99
TP (mg/L)
0.15
Figure 5.12. Total and soluble Phosphorus concentration in the four main regions of Myall
Lakes from April 2000 to October 2002.
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Chapter 5, Water Quality
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0.6
River Mouth
+
NH4 (mg/L)
TN (mg/L)
1.5
0.5
1.0
0.12
River Mouth
0.10
NO x (mg/L)
2.0
0.4
0.3
0.2
River Mouth
0.08
0.06
0.04
0.5
0.1
0.02
0.0
0.0
0.00
2.0
0.6
+
NH4 (mg/L)
1.0
0.5
0.10
0.3
0.2
0.06
0.04
0.02
0.0
0.00
2.0
0.6
1.0
0.5
0.0
2.0
Myall Lake
1.0
0.4
0.3
0.2
0.10
0.06
0.04
0.02
0.0
0.6
0.00
0.12
Myall Lake
0.4
0.3
0.2
Mid-Lakes
0.08
0.1
0.5
NH4+ (mg/L)
1.5
0.12
Mid-Lakes
NO x (mg/L)
+
NH4 (mg/L)
0.5
Broadwater
0.08
0.0
Mid-Lakes
TN (mg/L)
0.4
0.1
1.5
TN (mg/L)
0.12
Broadwater
0.10
NO x (mg/L)
TN (mg/L)
1.5
0.5
NOx (mg/L)
Broadwater
Myall Lake
0.08
0.06
0.04
0.5
0.00
Ap
r-9
9
Ju
l-99
Oc
t-99
Ja
n-0
0
Ap
r-0
0
Ju
l-0
0
Oc
t-0
0
Ja
n-0
1
Ap
r-0
1
Ju
l-0
1
Oc
t-0
1
Ja
n-0
2
Ap
r-0
2
Ju
l-0
2
Oc
t-0
2
0.02
0.0
Ap
r-9
9
Ju
l-9
9
Oc
t-9
9
Ja
n-0
0
Ap
r-0
0
Ju
l-00
Oc
t-0
0
Ja
n-0
1
Ap
r-0
1
Ju
l-01
Oc
t-01
Ja
n-0
2
Ap
r-0
2
Ju
l-02
Oc
t-02
Ap
r-9
9
Ju
l-9
9
Oc
t-9
9
Ja
n-0
0
Ap
r-0
0
Ju
l-0
0
Oc
t-0
0
Ja
n-0
1
Ap
r-0
1
Ju
l-0
1
Oc
t-0
1
Ja
n-0
2
Ap
r-0
2
Ju
l-0
2
Oc
t-0
2
0.0
0.1
F
Figure 5.13. Concentrations of Total Nitrogen, NH4+, and NOx in the four main regions of Myall Lakes from August 1999 to October 2002.
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Chapter 5, Water Quality
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5.1.2.8 Near-Benthic Water Collection
Figure 5.14 shows the concentrations of NH4+, NOx and SRP at bottom and surface waters at the
Rivermouth (A), Western Broadwater (B), and in Two Mile Lake (C). Water quality profiles showed
that the water column in the Western Broadwater was well mixed with only 6% difference in DO
saturation between surface and bottom waters. At this site there was little difference in nutrient
concentrations between the suface and bottom waters.
In contrast, water quality profiles at the Rivermouth and Two Mile Lake showed marked differences
between surface and bottom waters in concentrations of nutrients. At the Rivermouth, DO saturation
was only 3% at the bottom and 89% at the surface. At this site, NH4 + was more than 5 times higher in
bottom waters, in comparison to surface waters. SRP was also higher on the bottom than in surface
waters, although this was due to a single replicate having a high concentration (0.009 mg/L).
Differences were also noted at Two Mile Lake, but were smaller than at the Rivermouth. DO
saturation was 71% in bottom waters compared to 89% on the surface, NH4 + concentrations 3 times
higher on the bottom, but there was little difference in SRP concentrations.
Myall River Mouth, Sept. 24th, 2002.
Mean Concentration (mg/L)
0.050
0.047
0.040
0.030
0.020
0.008
0.010
0.010
0.005
0.005
0.002
NOx
(Surface)
SRP
(Benthic)
SRP
(Surface)
0.000
NH4+
(Benthic)
NH4+
(Surface)
NOx
(Benthic)
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Western Broadwater, Sept. 24th, 2002
Mean Concentration (mg/L)
0.050
0.040
0.030
0.020
0.010
0.008
0.007
0.005
0.007
0.001
0.001
SRP (Benthic)
SRP (Surface)
0.000
NH4+ (Benthic)
NH4+ (Surface)
NOx (Benthic)
NOx (Surface)
Two-Mile Lake, Sept. 24th, 2002
Mean Concentration (mg/L)
0.050
0.040
0.033
0.030
0.020
0.010
0.010
0.010
0.005
0.001
0.001
SRP
(Benthic)
SRP
(Surface)
0.000
NH4+
(Benthic)
NH4+
(Surface)
NOx
(Benthic)
NOx
(Surface)
Figure 5.14: Concentrations of NH 4+, NOx and SRP at bottom and surface waters at the
Rivermouth (A), Western Broadwater (B), and in Two Mile Lake (C)
5.1.4. Discussion
5.1.4.1 Temporal and spatial patterns of nutrient distribution
The main source of nutrients to Myall Lakes is freshwater inflows that carry runoff from their
catchments. Inflows are highly seasonal depending on rainfall. Therefore, the type and amount of
nutrients exported from catchments, the location of riverine inflows, and the timing of rainfall events
will greatly influence the spatial and temporal distribution of nutrient loads to Myall Lakes. Nutrient
concentrations in the water column are a result of not only the nutrient load, but uptake by primary
producers, and biogeochemical processes in the water and sediment.
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The gradient of phosphorus concentrations for both TP and SRP, from highest at the Myall
Rivermouth to lowest at Myall Lake, appears to reflect the nutrient load from the catchment associated
with freshwater inflow. Much of the Total Phosphorus loads in the waters entering the Broadwater are
likely to be bound to sediments that settle quickly to the bottom before they can reach Mid-Lakes and
Myall Lake. Dissolved phosphorus (SRP) is likely to be be taken up quickly by aquatic plants and
phytoplankton within the Broadwater before it can circulate further up the system. In contrast to
phosphorus, the concentration of total nitrogen is generally lowest at the Rivermouth site, and greatest
in Myall Lake. There are a number of potential reasons for this gradient. Much of the nitrogen may be
in the form of dissolved organic nitrogen (G. Harris pers comm) as a result of organic loading from a
largely forested catchment and long residence time. High TN values reflect large quantities of nitrogen
being cycled within the system (G. Coade pers comm). This is consistent with annual cycles of
growth and decay of large amounts of aquatic vegetation in Myall Lake and Mid-Lakes (Chapter 6).
The gradient may also reflect the importance of sediment biogeochemical processes in nitrogen
cycling, particularly the removal of nitrogen through denitrification. Differences in sediment between
the Broadwater and Myall Lake may greatly influence the rate of denitrification. In the Broadwater,
denitrification was measured on one occasion as good in the shallow sandy sediments, and relatively
low in the deep mud basin. Denitrification has not been measured in the Mid-Lakes and Myall Lake.
However, the benthos of these areas are covered in gyttja that is high in organic content and low in
oxygen. These are two factors known to inhibit the process of coupled nitrification-denitrification that
is so important in the removal of nitrogen from waterways.
These results highlight the difficulty of using nutrient concentrations as an indicator of nutrient status.
While TN is highest in Myall Lake and is often over the ANZECC/ARMCANZ guideline value, it is
likely that most of the nitrogen is in an organic form that is not bioavailable to phytoplankton and
algae.
It is clear that the concentration of all nutrients was lower and less variable during the low rainfall
period than during the period of average rainfall. This is particularly true of the reactive forms of N
and P (NOx, NH4 and SRP) which were present in very low concentrations during the low rainfall
period. In particular, the concentration of NH4 + was markedly lower during periods of low rainfall.
The elevated concentrations of NH4 + recorded at most sites in April 2000 occurred just after a period
of high rainfall in March 2000. At this time, 428 mm of rain fell in the catchment, which is
approximately 3 times the monthly average (159 mm). Although this suggests that runoff is a
significant source of NH4 + to Myall Lakes, it is likely that other mechanisms are involved given that
highest concentrations were recorded at sites remote from riverine inflows and elevated values were
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not recorded from the Rivermouth site. In addition, the mechanism for the maintenance of high NH4 +
concentrations in the months after the rainfall event is not known. NH4 + is usually taken up quickly by
phytoplankton and aquatic plants and is therefore usually in low concentration in coastal waters. One
explanation for high concentrations may be internal cycling of nutrients from the sediment to the water
column. Sampaklis (2003) found extremely high concentrations of NH4 + (up to 15mg/L) in the gyttja
overlying sediments in Mid-Lakes and Myall Lake. The gyttja is easily disturbed by wind mixing and
can increase NH4 + concentrations in the water column up to 10 times background levels during calm
weather. It is possible that disturbance of the gyttja at this time contributed to elevated concentrations
in the water column.
The influence of freshwater inflows on the delivery of SRP to Myall Lakes is demonstrated by the
consistently high values of SRP at the Rivermouth throughout the study, and the high values at sites
close to freshwater inflows (Broadwater and Mid-Lakes) after the large rainfall event in March 2000.
5.1.4.2 Near benthic water collection.
Higher concentrations of NH4 + and SRP in bottom waters were associated with reduced DO saturation.
These high levels may be due either to suboxic conditions near the sediment, stimulating the release of
NH4 + and SRP from the sediments, or to the build up of nutrients released from the sediment into
bottom waters that are not mixed into the water column. In either case, these results indicate that
sediment release of nutrients is an important process at sites in Myall Lakes. The even distribution of
nutrients through the water column at the Western Broadwater reflects the well mixed conditions at
the site.
Although this study did not attempt to quantify the loads of ammonium being delivered to the water
column, it is clear that the benthic material in the lakes is involved in the cycling and contribution of
nitrogen to the water column. The biological reduction processes that promote the incomplete
denitrification of nitrogen into ammonium operate in locations with excess organic carbon which has a
high biological oxygen demand. It is not surprising then, that low dissolved oxygen and elevated
ammonium were observed at the Myall Rivermouth site. However, it is particularly interesting to find
that the site in Two-Mile Lake which did not have low dissolved oxygen at the lakebed also had
elevated ammonium concentrations. This may be due to sediment characteristics and processes at this
site. As mentioned previously, the gyttja present in Two Mile Lake has high organic content, low
oxygen, and high ammonium content in pore waters. These characteristics suggest that coupled
nitrification-denitrification is likely to be low in these sediments due to inhibition of this process by
high organic load and low oxygen. Where organic matter is remineralised and denitrification is not
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occurring, NH4 + is released from sediments. Low oxygen, either in waters near the bottom, or in the
gyttja, may also explain why NOx is not elevated at these sites where NH4+ was higher in bottom
waters. Under these conditions oxidation of NH4 + to NOx is inhibited.
5.2. NITROGEN STABLE ISOTOPE SIGNATURES OF AQUATIC PLANTS
5.2.1. Introduction
Stable isotope ratios of nitrogen have been used to quantify the degree and extent of impact from
anthropogenic nutrient inputs in rivers and coastal waters. The technique is based on the fact that
anthropogenic sources of nitrogen, have different ‘signatures’ or ratios of stable isotopes of nitrogen
compared to natural sources. Stable isotope analysis is a useful tool in identifying the location and
extent of specific impacts (Dennison and Abal, 1999), and the likely source of the nutrients. However,
it does not directly assess the impact of the nutrient load on the ecosystem, nor provide an assessment
of the environmental health of the study area.
The ratio of stable isotopes of nitrogen (N14 : N15 ) that occurs in the atmosphere is globally quite stable
(Lajtha and Michener, 1994). However, the ratio is able to be changed by any process, physical or
biochemical, that causes a change in the chemical form of the element. For example nitrification
(oxidation), denitrification (reduction), and nitrogen fixation, can lead to a change in the relative ratios
of the isotopes in the organisms that perform these functions, and ultimately in the ambient
environment.
Many of the processes associated with human activity (eg. application of nitrogenous fertilisers,
operation of sewage treatment plants, intensive animal agriculture) facilitate processes that change the
‘natural’ ratios of these isotopes, generally increasing the proportion of N15 (?N15). Increased
‘background’ ?N15 can provide a convenient method of tracing the presence of anthropogenic waste in
the environment. This method has proved to be particularly useful for tracing sewage being discharged
into an estuarine environment (eg. Dennison and Abal, 1999). Dennison and Abal (1999) found a clear
elevation of ?N15 in experimentally deployed algae in areas that receive the enriched effluent
compared to control locations, and decreasing ?N15 with increasing distance from the discharge point.
It was intended that a similar method might be used in Myall Lakes to identify whether a significant
proportion of the nitrogen fuelling plant growth in the lakes is arising from the catchment, or is being
recycled from the lakes.
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Although human activity is known to increase the ?N15 in effluent arising from the sites mentioned
above, many other natural processes are also known to alter the atmospheric ratio of N14 to N15 which
make interpretation of ?N15 differences in the ambient environment difficult. For example the natural
range of soil ?N15 can vary from very depleted in N15 to highly enriched values, in the absence of any
sewage or other fertilisation (Lajtha and Michener, 1994). As a consequence, runoff leaving pristine
forested land without the addition of sewage effluent may be strongly enriched or depleted in N15 ,
making a positive N15 signal from a sewage treatment plant, for example, difficult to interpret at some
point downstream. This would be particularly so if the ?N15 signal in runoff from pristine land
changed through time, or if non-uniform rainfall initiates runoff from a variety of land-uses.
In the Myall Lakes catchment, pristine forest and a variety of land-uses (see Chapter 2) are present as
well as a STP at Bulahdelah. It is more than likely that the water entering Myall Lakes will have a
?N15 that varies through time depending on the amount of runoff or rainfall arising from different parts
of the catchment. Consequently, although this was not assessed in this study, it would be expected that
the water entering the system would have a highly variable and unknown ?N15 . A preliminary survey
of ?N15 in vegetation in the catchment of the Upper Myall River in 2000, found that different
subcatchments did not demonstrate a clear pattern (DLWC, unpublished data).
Because of the complexity of the system and previous difficulties in interpreting Myall Catchment
?N15 data, a simple one-off study was conducted to examine patterns of spatial ?N15 distribution in a
single ubiquitous species of macrophyte in the lakes. This was intended to provide a starting point for
future studies, and allow some simple insight to nitrogen dynamics in the lakes.
The intention was to investigate whether plant growth in different parts of the lakes are utilising
different nitrogen sources, or whether the same nitrogen source is responsible across the system, at one
point in time. The Hypothesis being tested was ‘That there would be a significant difference in the
?N15 found in the tissues of the summer ephemeral Najas between any two locations’. The Null ‘That
no significant difference would be found’.
5.2.2. Methods
The macrophyte, Najas marina, was chosen as the subject for the stable isotope study. This
macrophyte grows abundantly in all the lakes of the system, although less so in the Broadwater (see
Chapter 6). It possesses roots that probably can extract nutrients from sediments, and leaves that may
absorb dissolved nutrients from the water. The plant is a summer ephemeral, and grows from seed to
mature flowering plants over each summer, and forms continuous dense beds over 1 kilometre in
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length in Myall Lake. Following the summer bloom, the mature plants flower, produce seed, and then
die off gradually over winter (see Chapter 6). Consequently, it was possible to collect plants of similar
age and maturity from all locations in the lake, minimising any potential confounding created by
temporal differences.
On March 15th 2002, samples of Najas marina were collected from 8 locations representative of all
sections of the Myall Lakes system, except the upper Myall Rivermouth (Table 5.4). Within large
mono-specific macrophyte beds at each location several handfuls of plant were haphazardly gathered
from the topmost parts of the plants near the surface, and placed into a plastic bag that was then
sealed. Water depth at each location was approximately 3m.
Table 5.4 Sampling locations for Stable isotope study
Location
Location name and description.
A
Kataway Bay – Southern end of Eastern Myall Lake.
B
Mayers Point – Northern part of Myall Lake.
C
Mid-Myall Lake – Off Shelley’s Beach.
D
Violet Hill – In eastern channel.
E
Boolambayte Lake – Near Goat Island.
F
Two – Mile Lake – Near Korsmans Landing.
G
Bombah Point – Near Bombah Point ferry.
H
Mungo Brush – Sand bar near White Tree Bay.
Samples were kept frozen prior to analysis. In the laboratory the leaves and terminal stems were
separated from the remainder of the plants (to ensure that material of approximately equal age was
tested) and a preliminary drying of material was conducted for 48 hours at 60?C. To test the precision
of the laboratory method, 8 replicate samples from each of the 8 locations were created. Dried plant
material was analysed by the School of Plant Biology at the University of Western Australia using a
standard in-house analytical method.
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Results were analysed using a one way ANOVA to determine whether significant differences occurred
among sites, and Kruskal- Wallis test to identify which sites were different from each other.
5.2.3. Results
The mean values for ?N15 from N. marina tissue ranged from strongly negative values (-9) in Myall
Lake and Violet Hill, to slightly positive values (+3) in Two-Mile Lake and the Broadwater (Figure
5.15).
A one-way ANOVA on the effect of location on the mean for the ?N15 was highly significant,
(P<<0.001), so the null hypothesis was rejected.
A Kruskal-Wallis multiple comparison Z-Value test (regular test) in summary, showed that the mean
?N15 at Mayers Point, Myall Lake, and Violet Hill did not differ significantly (Group A in Figure
5.15), and the sites at Two-Mile Lake, Bombah Point and Mungo Brush were not statistically different
(Group B). The means of Kataway Bay and Boolambayte Lake did not differ from each other and did
not differ in some comparisons with sites in groups A and B.
Myall Lakes delta N15 ‰ – March 15th 2002
+4.0
0
0
Delta
N15
( ‰)
Group B
-5
Group A
-10.0
Mungo Br.
Bombah Pt
Two-Mile
Boolambayte
Violet H.
Myall L.
Mayers Pt
Kataway
Figure 5.15. Boxplots of N. marina tissue ?N15 recorded in the study (replicates = 8).
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5.2.4. Discussion
The strongly negative ?N15 values recorded in Myall Lake and Mid-Lakes in this study were outside
the range usually reported for aquatic plants in coastal waters in the literature. Lajtha and Michener,
(1994) describe numerous mechanisms that can decrease the ?N15 values found in plant tissues and the
ambient environment. These include isotopic fractionation where ‘lighter’ forms of ammonia or nitrate
are selectively taken up by aquatic algae. This has been shown to produce strongly negative ?N15
values in the range –10‰ to - 20‰. Also the mobilisation (ie. movement through diffusion or
evaporation etc.) of ammonia can strongly favour the availability of lighter N14 isotope. This would
also contribute to negative ?N15 values in aquatic plants.
It is possible that the negative ?N15 values found in Myall Lake are due to one or both of these
mechanisms. Studies by Sampaklis (2003) have shown that the unconsolidated benthic sediments in
Myall Lake are easily disturbed which results in the release of high levels of NH4 + into the water
column. This would be readily available for uptake by macrophytes and may contribute to lower ?N15
values. In addition, strongly negative ?N15 values in dwarf mangroves in Belize was found to be due to
plant fractionation due to slow growth and low N demand associated with phosphorus limitation
(McKee et al 2002).
Variations in ?N15 must be examined in the context of natural variations due to genotype, and
environmental conditions. Although these factors can be responsible for local variability of 2-5 ‰
?N15 the observed range in Myall Lakes Najas of over 14 ‰ is unlikely to be due to within-species
variability. By sampling plants of the same species and age in similar habitats, any within-species
variability has been minimised and may reflect different nitrogen sources for plant growth in different
parts of the lakes.
5.3. PHYSICO-CHEMICAL PARAMETERS
5.3.1. Introduction.
The physico-chemical features of the water column, as well as nutrients, play a crucial role in the
development of blue-green algal blooms. In summary, the following water quality characteristics are
known to increase the likelihood of blue-green algal species becoming dominant:
1. Low salinity;
2. Warm water;
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3. A change in the ambient turbidity; and
4. Stratification and or calm water conditions.
Low salinity is important as Anabaena and Microcystis are typically freshwater species and are
intolerant to increases in salinity (Chapter 4). The occurrence of stratification is a potentially important
feature in waterbodies that are prone to algal blooms. In short, the normal vertical mixing of the
waterbody is prevented by a much greater density of deeper waters over surface water. A number of
processes can be responsible for this discontinuity, but in shallow coastal lakes a layer of dense saline
oceanic water lying beneath fresher rain or catchment runoff water is one of the main processes that
causes stratification. The ability of some blue green species to regulate their buoyancy provides them
with an advantage over other species in turbid and/or stratified conditions. They are able to move
between well lit surface waters, and bottom waters where nutrients are available.
Subtle differences in water chemistry that can influence the health and diversity of the phytoplanktonic
community and macrophytes are also influenced by physico-chemical water quality. Variations in pH
for example, can affect the toxicity and speciation of chemicals in the water column. Reductions in
dissolved oxygen concentrations in bottom waters can enhance the release of stored nutrients (N and
P) into the water column which are then available for plant growth, and in extreme cases lead to fish
kills. Increases in turbidity decreases light penetration that may limit the production and distribution of
benthic plants and algae.
This chapter describes the physico-chemical characteristics of the water column of Myall Lakes during
the period April 1999 – October 2002.
5.3.2. Methods
Surface and bottom physico-chemical water quality measurements were made throughout Myall Lakes
from the Upper Myall Rivermouth to Neranie during the period April 1999 - September 2002. Water
quality measures generally coincided with algal sampling, and were not timed to capture specific
weather events. Water quality parameters of temperature, pH, DO and conductivity were measured in
the field using a water quality meter calibrated just prior to use. Generally data from between 6 and 8
locations for each sampling day were collected.
Light penetration through the water column was measured in different parts of the lake. Water clarity
was measured with a secchi disc during water quality and algal runs from September 2001 –
September 2002. On three occasions (February, June and September 2002), the instantaneous
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measurement of Photosynthetically Active Radiation (PAR) attenuation through the water column was
measured at 0.5-1.0 m intervals with Li-Cor Underwater Quantum Sensors.
Longer-term measures of the amount of PAR underwater were collected using light loggers
(Dataflow) for the period December 2001 to February 2002 (2 sites) and again from June to August
2002 (4 sites). In each case the illumination was related as a percentage to logged ambient illumination
in air at Bombah Point. The loggers were deployed at the same depth in each location. Due to inflows,
the lakes’ depth changed during the study (thus changing the depth of the loggers) so these measures
are best considered as means of comparing the relative clarity of the water at different places in the
lake rather than the absolute intensity of PAR.
5.3.3. Results
Preliminary investigations showed that the physico-chemical character of 4 distinct regions could be
identified. As was the case previously above for the water nutrient data, samples have been averaged
where appropriate to typify the water value of each parameter. Sites in the following waterbodies have
been consolidated to create the graphs below:
1. Myall Rivermouth,
2. Bombah Broadwater,
3. Mid-Lakes (Two-Mile and Boolambayte Lakes),
4. Myall Lake (including Violet Hill and Neranie)
Temperature: The temperature of surface waters ranged between 10?C in winter and 27?C in summer
and was similar among the 4 regions. The only exception to this was the Myall Rivermouth in June
2002 which was approximately 3?C cooler than the main body of the lake (Figure 5.17).
pH: Few pH values were recorded prior to September 2001 but showed that values ranged between
6.7 – 8.2 with lower values at Myall Rivermouth and Myall Lake, and higher values at the Broadwater
and Two Mile Lake. After September 2001, values ranged between 6.2 and 9 with highest values in
Myall Lake followed by Two Mile Lake and Broadwater, and lower values in Myall Rivermouth. The
pH varied over this period with low values in November 2001 coinciding with a minor rainfall event.
There also appeared to be a seasonal trend with highest values in summer and lower values in spring
and autumn.
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Dissolved Oxygen: Daytime DO ranged between 5-11 mg/L in the main body of the lake over the
period of the study. Values were similar among the 3 regions with slightly higher values at Myall Lake
compared to Broadwater and Two Mile Lake at times in late 2001 and 2002. Dissolved oxygen in the
Myall Rivermouth was generally lower than at other sites but varied substantially between sampling
times.
Conductivity: Conductivity ranged between 1-18 mS/cm. Values recorded between April 1999 and
September 2000 and between May and October 2001 were below 5 mS/cm at all sites in Myall Lakes
(Figure 5.16). At other times, the salinity regime was discretely different among the 4 regions. The
Broadwater and Two Mile Lake recorded highest salinity during dry periods due to saline intrusion
from the lower Myall River. Salinity in Myall Lake was low and relatively constant with a maximum
of 6 mS/cm reflecting the long distance from saline influence and absence of significant river input or
surface water flows. The upper Myall Rivermouth had low conductivity but values were highly
variable indicating the influence of both the saline intrusion during dry periods and river flows after
rainfall.
The conductivity of Myall Lake gradually increased between early 2001 to October 2002. Over the
study period Myall Lake did not display conductivities less than 2 mS/cm, which were found to be
necessary for the initiation of the Anabaena bloom of the Broadwater, and did not exceed 6 mS/cm
during dry weather. The large volume of water contained in this lake and small connection to the rest
of the system via the constricted passage at Violet Hill, means that rapid change in conductivity is
unlikely in this waterbody.
In contrast, the Broadwater displayed far more rapid and marked changes in conductivity (eg.
September – November 2000). As mentioned in Chapter 2, the Broadwater receives inputs of fresh
and saline estuarine waters and thus salinity is highly variable. The relatively small size and shallow
nature of the Broadwater means that the salinity responds quickly to inflows. The Mid Lakes (TwoMile and Boolambayte) are very small in volume compared to the other lakes. They receive direct
catchment runoff via Boolambayte Creek (as well as the lakes’ shoreline), and were found to be very
fresh for short periods (eg. September 2000) and were also at times similar in salinity to the
Broadwater(e.g. November 2001 to June 2002). It might be expected that because of the small size of
the catchment and volume of these lakes, the salinity in this part of the system is a more a reflection of
water movement from the Broadwater and Myall Lake. Strong currents (pers. obs) are often
encountered in dry weather at Violet Hill draining Myall Lake, and Bombah Point draining Two-Mile
Lake into the Broadwater. The presumed mechanism for this motion is seiching caused by windgenerated water motion on Myall Lake creating localised lake-surface anomalies. In dry weather it
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was common for there to be a gradient of increasing salinity further up the lake system, perhaps
demonstrating that this section of the lakes is a place of diffusion and mixing of water from the larger
waterbodies at opposite ends of Two-Mile and Boolambayte Lakes.
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Figure 5.16 Electrical Conductivity of surface waters. April 1999 to Oct 2002.
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Figure 5.16 (continued). Electrical Conductivity of surface waters. April 1999 to Oct 2002.
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Figure 5.16 (continued) Electrical Conductivity of surface waters. April 1999 to Oct
2002.
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Figure 5.16 (continued) Electrical Conductivity of surface waters. April 1999 to Oct 2002.
NSW Department of Infrastructure, Planning, and Natural Resources
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20
18
16
14
12
10
8
6
4
2
0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
Dissolved Oxygen (mg/L)
EC (mS/cm)
30
28
26
24
22
20
18
16
14
12
10
8
pH
o
Temperature
C)(
Chapter 5, Water Quality
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Understanding Blue Green Algae Blooms
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River Mouth
Broadwater
Mid-Lakes
Myall Lake
Temperature
Salinity
pH
Dissolved Oxygen
8
6
4
2
0
300
Rainfall (mm)
250
Rainfall
200
150
100
50
Ap
r-9
9
Ju
l-9
9
Oc
t-9
9
Ja
n-0
0
Ap
r-0
0
Ju
l-0
0
Oc
t-0
0
Ja
n-0
1
Ap
r-0
1
Ju
l-01
Oc
t-0
1
Ja
n-0
2
Ap
r-0
2
Ju
l-0
2
Oc
t-0
2
0
Figure 5.17 Summary of physico-chemical water quality and rainfall for April 2000 to
September 2002.
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5.3.3.1. Stratification
Significant differences in surface and bottom physico-chemical water quality (indicating stratification)
occurred infrequently during the study period, except in the Myall Rivermouth. This may be due to
mixing caused by horizontal wind-generated currents in the exposed and shallow waters of the lake
body. Figure 5.18 shows bottom and surface water values for temperature, salinity and dissolved
oxygen at the four main sampling regions. At the Myall Rivermouth site, there were differences in
surface and bottom temperature, salinity and dissolved oxygen on a number of sampling occasions. On
occasions when differences occurred, bottom waters were generally cooler, more saline (and thus
more dense) and as a consequence of the diminished vertical mixing, had lower dissolved oxygen than
the surface waters. The exception to this was in winter 2000 and 2002 when surface waters were
cooler than bottom waters, and in April 2000 and 2002 when dissolved oxygen was higher in bottom
than surface waters.
The Broadwater showed little evidence of stratification except in winter 2001 when bottom waters
were slightly more saline than surface waters. Slightly lower dissolved oxygen at the bottom was
recorded at the Broadwater and at Two Mile Lake in April to July 2001. The Myall Lake showed very
little difference in any parameter between surface and bottom waters on any sampling occasion during
the study.
On December 10th - 11th 2002, a wet weather event (~250 mm at Bulahdelah) sampled separately to
the C&CS program, created a freshwater layer in the Broadwater that remained intact for several days,
but did not cause anoxia in benthic waters.
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22
20
22
20
18
16
14
surface
bottom
2
0
20
18
16
14
10
8
20
18
EC (mS/cm)
Two Mile
22
20
18
16
14
8
6
4
2
10
4
2
0
12
Two Mile Lake
16
14
12
10
8
6
10
6
4
0
30
28
22
12
16
14
12
10
8
6
12
10
4
2
8
0
2
10
Myall
8
6
4
2
0
Ap
r-9
Ju 9
lOc 99
t-9
Ja 9
n-0
Ap 0
r-0
Ju 0
lOc 00
t-0
Ja 0
nAp 01
r-0
Ju 1
lOc 01
t-0
Ja 1
n-0
Ap 2
r-0
Ju 2
lOc 02
t-0
2
20
18
Myall Lake
16
14
Dissolved Oxygen (mg/L)
0
20
18
Two Mile
8
8
Myall Lake
Broadwater
6
4
2
26
24
22
surface
bottom
8
12
10
EC (mS/cm)
o
Temperature ( C)
12
22
Ap
r-9
Ju 9
l-9
Oc 9
t-9
Ja 9
n-0
Ap 0
r-0
Ju 0
lOc 00
t-0
Ja 0
n-0
Ap 1
r-0
Ju 1
lOc 01
t-0
Ja 1
n-0
Ap 2
r-0
Ju 2
lOc 02
t-0
2
o
Temperature ( C)
26
24
16
14
2
0
8
River Mouth
12
Broadwater
6
4
12
10
10
0
22
Broadwater
22
20
18
30
28
10
8
Dissolved Oxygen (mg/L)
26
24
14
12
Dissolved Oxygen (mg/L)
30
28
River Mouth
6
4
EC (mS/cm)
8
18
16
12
surface
bottom
Dissolved Oxygen (mg/L)
EC (mS/cm)
26
24
12
10
o
Temperature ( C)
River Mouth
Ap
r-9
Ju 9
l-9
Oc 9
t-9
Ja 9
nAp 00
r-0
Ju 0
l-0
Oc 0
t-0
Ja 0
nAp 01
r-0
Ju 1
lOc 01
t-0
Ja 1
n-0
Ap 2
r-0
Ju 2
lOc 02
t-0
2
o
Temperature ( C)
30
28
Figure 5.18 Surface and bottom physico-chemical water quality between April 2000 and September 2002.
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5.3.3.2. Light availability
Secchi depth readings were made for the period August 2001 to July 2002 (Figure 5.19). The
Myall Rivermouth had the least light penetration indicated by low secchi depths (0.5-1.6m),
followed by the Broadwater. Two Mile Lake and Myall Lake had similar secchi depths with
greatest water clarity occurring in April and May 2002.
Instantaneous light attenuation through the water column was measured on three occasions
throughout the study and results are shown in Figure 5.19. There was a trend of decreasing
attenuation from the Rivermouth to sites in Myall Lake, as well as temporal differences. The
compensation depths (depth to which 1% of the incident light reaches) are shown in Figure
5.20. This depth is approximately the limit of photosynthetic production - where
photosynthesis of phytoplankton is equal to respiration rate and often indicates the depth limit
of seagrass and vegetation. The compensation depth ranged from around 2m at the
Rivermouth to around 8m at Myall Lakes indicating the potential for macrophyte growth on
the bed of different areas of the lake
Figure 5.21 shows the average daily records of incident Photosynthetically Active Radiation
(PAR) between 10 am and 2pm, and underwater at approximately 1m depth at a range of
sites, the percentage of incident PAR which reached approximately 1m depth, and the daily
rainfall at Bulahdelah. Because there was shading of subaquatic and ambient light loggers
when the sun was at low incident angles, only this period (when the sun was high overhead) is
used. Incident PAR varied according to cloud cover but generally reached 1600 – 1800
?mol/m2 /s during the period December 2001 and March 2002. Values recorded during the
period showed lower levels in winter (1000 ? mol/m2 /s – June 2002) increasing in Spring
(1400 ? mol/m2 /s – September 2002).
The amount of PAR reaching a depth of approximately 1m varied widely among locations
within Myall Lakes and demonstrated the effect of rain events on water clarity and therefore
benthic illumination. In summer 2001 - 2002, water clarity was greater at Violet Hill
compared to Broadwater, with 2 - 2.5 times more PAR reaching the same depth at Violet Hill
than at the Broadwater. The reduction in water clarity in late January at both sites occurred
after a rain event. Water clarity gradually recovered over 6 weeks but took longer to recover
at the Broadwater than at Violet Hill.
In winter/spring 2002, water clarity was significantly higher at Myall Lake followed by Mid
Lakes, Broadwater and the upper Myall River mouth until mid July 2002. After July 2002
water clarity was higher in the Broadwater than Mid Lakes. Although the incident PAR
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increased from July to September 2002, the amount of light at approximately 1m depth
steadily decreased at the latter 3 sites. This may have been related to small amounts of rainfall
in late July and August 2002.
Table 5.5 presents a summary of the logged data for winter 2002. Even in this period of below
average rainfall, there was a clear gradient of more light reaching 1m depth in Myall Lake
compared to the Broadwater. Within the Broadwater, at the Rivermouth site on average only
8.9% of incident light reached 1m depth compared Mid Broadwater (14.9%), indicating the
highly turbid nature of the Upper Myall River.
Table 5.5 Summary of logged PAR records.20 June 21 July 2002.
Location Name
Mean percentage of
incident PAR at ~ 1m*
Myall Rivermouth (western Broadwater)
Middle Broadwater.
Two-Mile Lake (Korsmans Landing).
Myall Lake.
8.9
14.9
21.7
46.5
* - Depth was variable due to lake height fluctuations of +40cm.
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5
depth (m)
4
secchi depth
River Mouth
Broadwater
Two Mile
Myall
3
2
1
Ju
l-0
2
Oc
t-0
2
Ju
l-0
0
Oc
t-0
0
Ja
n-0
1
Ap
r-0
1
Ju
l-0
1
Oc
t-0
1
Ja
n-0
2
Ap
r-0
2
Ju
l-9
9
Oc
t-9
9
Ja
n-0
0
Ap
r-0
0
Ap
r-9
9
0
Figure 5.19 Secchi depths for 4 regions of Myall Lakes for the period August 2001 to
September 2002.
Attenuation coefficient (k)
2.2
Light attenuation
Feb 02
Jun 02
Sept 02
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0
Compensation depth (m)
Compensation depth
2
4
6
8
RM
RM
SB
SB
NB
NB
BP
BP
KL
KL
VH
VH
ML
ML
Figure 5.20 Light attenuation and Compensation depth for sites within Myall Lakes for
Period Feb – September 2002. RM = Rivermouth, SB = Southern Broadwater, NB =
Northern Broadwater, BP = Bombah Pt, KL = Korsmans Landing, VH = Violet Hill, ML =
Myall Lake.
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2000
Air
1800
PAR (umol/m 2/s)
1600
1400
1200
1000
800
600
400
200
0
800
Rivermouth
Broadwater
Two Mile Lake
Myall Lake
600
2
PAR (umol/m /s)
Broadwater
Violet Hill
400
200
0
100
% PAR at 1m
80
60
40
20
0
Rainfall (mm)
120
100
80
60
40
20
0
Dec 01
Jan 02
Feb 02
Mar 02
Jun-02
Jul-02
Aug-02
Sep-02
Figure 5.21 Average daily PAR intensity (between 10am and 2pm) at 1m depth and
rainfall at Bulahdelah.
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5.3.4. Discussion – Physico-chemical water quality.
Surface water quality values recorded in Myall Lakes generally fell within normal ranges for
coastal and freshwaters (ANZECC and ARMCANZ 2000). Exceptions were slightly elevated
levels of pH in Myall Lake after September 2001 that may be due either to high productivity
of macrophytes or sulphur reduction in benthic sediments. Low dissolved oxygen on
occasions in the Rivermouth may be due to high organic loading.
Spatial differences were observed among the four regions in the degree of stratification, water
quality parameters, and water clarity. These differences reflect the relative influence of
rainfall and catchment inputs, and physical and ecological processes among the regions.
The Upper Myall Rivermouth was characterised by frequent differences between surface and
bottom temperature, salinity and DO. As well, this location had low and variable salinity,
acidic pH and low DO. Low water clarity was also found here and nearby in the western
Broadwater. This site is most influenced by rainfall and runoff from the catchment. The
observed acidic pH may result from tannic acids in the river which is typical of low-gradient
coastal rivers in Australia. Low DO is likely to be a result of high biological oxygen demand
in waters from the catchment combined with a small surface area over which oxygen from the
atmosphere can diffuse. The lower water temperatures at this location may play a role in
preventing blue-green algae blooming in this location (Chapter 4).
The Broadwater and Mid Lakes are similar in that they are normally brackish although the
salinity is somewhat variable and pH and DO values were within normal ranges. There was
reduced DO near the benthos in the Broadwater on some occasions which may be due to high
rates of sediment remineralisation and organic decomposition of material transported from the
catchment in the river. It is likely that fine particles and organic matter settle to the bottom
quickly once the flow is reduced as it reaches the Broadwater. The lower water clarity in the
Broadwater compared to the Mid-Lakes is also likely to result from the abundance of fine
particles at this site which are easily resuspended in an area open to the wind. The bathometry
and sediment composition of the Mid-Lakes region makes it less susceptible to resuspension.
Myall Lake is least influenced by rainfall and catchment influences due to its distance from
major inflows. Stratification was not observed during this study. This region has low and
stable salinity, sometimes elevated pH, normal levels of DO and high water clarity. Myall
Lake is a large open area with a depth of not more than 5 m that would experience substantial
wind mixing reducing the likelihood of stratification. The elevated pH may be due to either to
sulphur reduction in sediments or high production by ephemeral macrophyte species that
reach a high biomass in this region in late spring and summer each year.
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Calculation of compensation depth shows that areas closest to river inputs (upper Myall
Rivermouth and Broadwater) which are most susceptible to reduced water clarity have the
shallowest compensation depth. This means that any decrease in water clarity in these areas
due to increased turbidity in river inputs may decrease the depth to which aquatic plants are
able to grow. Depending on the bathometry of the region, this may substantially reduce the
area available for plant growth. Macrophyte beds are an important part of the ecology of
Myall Lakes as they provide food and shelter for many species of birds and fish, stabilise
sediment, and play an important role in taking up and regulating nutrients in the water
column.
Of the physico-chemical water quality data presented here the finding that low (and
potentially) rapidly-changing conductivity, and low water clarity occur in the Broadwater is
of great significance for the development of scum-forming blue-green algal blooms in the
lakes. The coincidence of these two factors (as mentioned in Chapter 4) create a regime that
favours fresh-water algal species that can regulate their buoyancy. These are characteristics of
Anabaena, which was a primary coloniser during the 1999 bloom. It is thus no coincidence
that although the toxin-producing cells were found in the mid-lakes later, they occurred in
greatest abundance on the Broadwater. The upper Myall Rivermouth, which is the source of
nutrients and turbid water, is cooler than the Broadwater and this may influence the growth of
slower-growing blue-green algal cells. Further research would be required to confirm this
however.
The salinity regime and clear waters of Myall Lake also seems to discourage the growth of
scum-forming blue-green algae. Because this lake has no large freshwater catchment inputs,
runoff entering via the Myall River and Boolambayte Creek displaces water already resident
in the Braodwater and Mid Lakes, pushing it up into Myall Lake (pers obs – December 2002).
This mechanism appears to prevent fresh and turbid water reaching the upper parts of the
system and thus may prevent Myall Lake from providing the conditions necessary for the
scum-forming blue-green algal blooms that dominated the Broadwater in 1999-2000. The
death of phytoplankton competitors (that would otherwise out complete blue-green algae) due
to rapidly declining salinity and high turbidity following a large rain event could be a means
by which blue-green algae were able to dominate in the Broadwater (Chapter 4). The high
water clarity, stable conductivity, as well as instances of high levels of Ammonium, may
explain the dominance of the smaller-celled blue green algae (eg. Chroococcus) in Myall
Lake. As discussed in Chapter 4, these species can not control their buoyancy (thus require
good clarity and well mixed water) and are Ammonia scavengers.
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So although the low-flushing of the lakes means that the system is a sink for nutrients, this
feature also means that there is a limited ingress of runoff into the upper parts of the lakes,
which seems to buffer the areas with the least water exchange ( ie. Myall Lake) from the
effects of catchment degradation (which mainly influence the Broadwater). Although it has
not been quantified it seems likely from conductivity and water nutrient results that there is a
differential impact from catchment inputs on each lake of the Myall Lakes system. The
Broadwater, which takes high nutrient runoff directly from the Myall River, undoubtedly is
receiving a far greater nutrient load than Myall Lake which appears to mainly receive water
that has been displaced from other parts of the lake system, which has had most available
nutrients removed previously. This feature has meant that very little catchment-derived silt
has accumulated on the bed of Myall Lake, and a unique benthic flora has developed instead
(see Chapter 6). Also, it has led to a situation of phosphorous limitation in Myall Lake,
making it particularly sensitive to any changes in the amount of this nutrient that it receives.
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6. Aquatic vegetation and lake habitat
6.1. BACKGROUND
6.1.1. Vegetation
Macrophytes are an important component of both estuarine (Allanson and Baird, 1999) and
freshwater (Sainty and Jacobs, 1994) ecosystems. They play a number of important roles
including primary production, providing habitat and food for fauna, promoting denitrification
by aerating sediments, and reducing the resuspension of sediments (Allanson and Baird,
1999). Macrophytes are good indicators of aquatic ecosystem health as they are especially
vulnerable to decline and changes in species composition in response to eutrophication and
sedimentation.
Pristine waterways are commonly characterised by long-lived macrophytes, predominantly
perennial angiosperms, although some are naturally dominated by short-lived species. For
example, seagrasses are considered an indicator of good health in NSW estuaries while the
growth of extensive beds of filamentous algae is considered a sign of poor health (Astill and
Lavery, 2001). Macrophytes, like terrestrial plants, are reliant on light in the wavelengths of
400-700nm, or Photosynthetically Active Radiation (PAR), to supply their energetic needs. In
water, PAR declines sharply with depth, restricting plant growth to the well-illuminated areas.
As a consequence, the total area that is suitable for the growth of macrophytes in a given
waterbody varies depending on the clarity of the water, and the area of the lakebed that falls
within the illuminated zone. This means that areas which appear to be ideal plant habitat
might not support macrophytes because water clarity is poor, or the bed is too deep.
As well as the physical characteristics of waterways influencing bed illumination, over the
course of a year, the area of lakebed that receives enough light to support perennial plants in a
particular lake will vary considerably. Incident Photosynthetically Active Radiation (PAR) at
noon in summer is nearly three times that of mid-winter (DLWC, unpublished data). If the
water clarity remains constant throughout the year, more light will reach the lake bed in
summer than winter.
One of the possible consequences of a large seasonal change in light penetration is seasonal
changes in the distribution or density of macrophytes. A similar change in macrophyte
communities can also be expected in response to increasing turbidity (Livingston, 2001). For
this reason, the decline of perennial plants in deeper areas of lakes can be an early warning of
deteriorating water clarity. Ephemeral species that are more suited to carrying out their entire
life-cycle during the summer months may then become more abundant, under some
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circumstances, leaving bare sediment in winter when light levels are too low for plant growth.
However, the establishment of ephemeral species (eg Najas), as opposed to perennial species,
may also be influenced by the substrate type.
Numerous studies in Australia have indeed found that in waterways that have increasing
nutrient loads and declining water clarity, there is a shift in macrophyte assemblages to
species with short-lived life histories (Lavery, Lukatelich and McComb, 1991) and often a
retraction of long-lived species to shallower areas due to better light availability. While the
predominance of short-lived plants can be a key to understanding the ecological processes
operating in a waterway, naturally high variability in community composition in healthy
waterways can confuse interpretation.
Changes in the abundance, distribution and diversity of macrophyte assemblages in a
waterbody are a natural component of a normal functioning ecosystem (Allanson and Baird,
1999). Within a macrophyte bed it would be expected that individual plants that are
recruiting, maturing, and plants that are dying would be present. As well, changes in the
dominant taxa may be due to other ecological processes such as grazing or competition, or in
response to long term changes in water quality. Therefore, it is essential to sample through
time to monitor the changing composition of macrophyte beds. The main objective of this
component of the study was to provide a qualitative description of the changes in the types of
plants observed in the lakes over a 12 month period, that would provide information on the
biological characteristics of Myall Lakes and provide an indication of the likely health of this
system and a guide to where further investigation should be undertaken.
6.1.2. Lakebed sediments.
The bed of lakes and estuaries are sinks for sediment, nutrients and organic matter. However,
these nutrients are not usually available for algal growth as they are tightly bound to silt
particles, or are contained in decomposing organic matter. Therefore, most of the nutrients
entering the lake from riverine inputs are retained in the sediments and removed from the
water column for an indefinite period. Phosphorus in particular can be removed from aquatic
ecosystems in this way.
As well as capturing nutrients, sediments are able to process and remove nitrogen through
bio-geochemical processes. Microbial processes convert nitrogen in organic matter to the gas
phase (N2 ) that is then diffused to the atmosphere and removed from the system. While this
process of denitrification has the potential to cleanse lakes of nitrogen, the presence of
excessive amounts of organic carbon in the sediment leads to a decline in the rate of the
preceding step of nitrification (Waite, 1984). This can result in the release of dissolved
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inorganic nitrogen (DIN) which can be readily taken up by algae. The actual rate of
denitrification or DIN release is dependant on a variety of factors including grainsize,
microbial assemblage present, dissolved oxygen levels, and organic content. All these factors
interact in complex ways. Therefore, rates of denitrification and DIN release cannot be
predicted from sediment nutrient concentration alone.
So while the presence of high amounts of nutrients and organic matter in material on the
lakebed need not necessarily be cause for concern, these patterns could provide some insight
to the interactions occurring between the water column and lakebed. To characterise the types
of material present on the bed of the lakes and provide a starting point for further
investigations into lakebed processes, a one-off lake wide survey was made in this program.
With the use of data kindly provided by Geosciences Australia, who surveyed the Broadwater
in 2000, a more complete picture of benthic character was made.
As well as this lake-wide survey in late 2001 it was noted that an unusual benthic microbial
assemblage was present in shallow areas, which seemed to dominate the benthos in TwoMile, Boolambayte, and Myall Lakes. Some preliminary investigations into the identity and
features of this microbial assemblage were made and are reported also.
6.2. METHODS
6.2.1. Vegetation assessment method.
A preliminary survey was conducted in September 2001, where the major types of
macrophyte were collected and subsequently identified by the National Herbarium. In
November 2001, February 2002, June 2002, and September 2002 qualitative assessments
were made of the distribution of the main vegetation taxa identified.
Three methods were used to identify the main types of vegetation:
1. Visual survey. In shallow water it was possible from a boat to see and rapidly identify the
macrophytes present.
2. Rake collection. Using a tethered metal rake it was possible to gather and identify loosely
rooted vegetation. The rake was thrown and dragged towards the boat several times at
each location to ensure a representative sample was collected.
3. Sled-mounted underwater video. Areas with hard-packed sand and strongly rooted
macrophytes were not well collected with the rake and were assessed using a closedcircuit underwater video. The general method involved deploying the sled from a boat in
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deep water and towing the equipment slowly towards the shore. Transects were observed
on the monitor from the boat and taped for later analysis if required.
The general method used at each location was to perform visual surveys in shallow parts of
the area, and these were combined with transects from or towards the shore, in which the
presence of each taxa found at each 0.5m depth increment was recorded (eg. At 1m, 1.5m,
2.0m et c.). The depths were noted from an on-board depth sounder and later corrected for
variation in lake height by recording the reading at a height board at Mayers Point.
In each method a Global Positioning System (GPS) was used to record points at which
samples were taken, and in the case of the video, where a change in the benthic macrophytes
was found. In this way points were directly transferable to the Geographic Information
System GIS), which were used in the creation of the maps that follow. The distribution of
each taxa at each sampling occasion was made into a GIS shapefile for use in later
applications.
6.2.2. Lakebed assessment method.
The general method of benthos sampling was as follows:
Fifty-one locations were selected to form a rough grid pattern and the positions recorded on a
GPS. To collect material, 40 cm long, 8.5 cm diameter transparent polycarbonate cylinders
were used in a hand-operated coring device. At each location 6 replicate core samples were
taken. Care was taken to ensure undisturbed cores were collected, and that the surface/water
interface was intact. The samples were immediately sub-sampled to collect the top 2 cm of
each core. The 6 replicates were combined to form a composite sample. Samples were kept
cool and transported to the laboratory. In the laboratory, the composite from each site was
passed through a 2 mm sieve which removed seeds and recently deposited vegetation. A small
sample was mounted and examined qualitatively to check for the presence of benthic
microcystis under compound microscope. The sample was thoroughly homogenised and subsamples were taken which were then dispatched for analysis.
Analysis of sediment nutrient content was performed by AGAL (a NATA accredited
laboratory) and grainsize analysis was performed without charge by Geosciences Australia. A
summary of the analyses performed is presented in Table 6.1, below. All samples were
analysed for Total Organic Carbon (TOC), but approximately half of the samples were
analysed for nutrients to reduce the total cost of the analyses.
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Although Total Nitrogen (TN) analysis was performed for only approximately half of the
locations, preliminary analysis found a high correlation between TOC and TN for benthic
material throughout the lake system. To extrapolate the relationship to locations that did not
have samples analysed for TN a second order polynomial equation (R2 = 0.9558) was fitted to
the data, and values for TN at other locations were calculated. Low correlations between TOC
and, Total Phosphorus (TP) and Total Sulphur (TS) were found which precluded performing
this extrapolation on these analytes.
Table 6.1. Number of samples analysed in each region of Myall Lakes.
Region
Number of samples - Analytes
TOC
TN
TP
TS
Grainsize
Myall Lake
28
12
12
12
28+
Mid Lakes
20
8
8
8
20+
Broadwater *
21*
21*
21*
21*
21*
Myall River and Nerong Creek
6
6
6
6
6+
* - Data from 2000, provided by Geosciences Australia
+
- Analysis provided by Geosciences Australia, 2002. Data not presented.
6.3. RESULTS.
6.3.1. Vegetation
Figure 6.1 to Figure 6.9 present a summary of each survey’s findings. The maps are intended
as a guide to the approximate distribution of each taxa based on:
1. The depth to which each type of plant was observed in each location,
2. The known depth contours which were mapped by DIPNR in 2002.
As mentioned, the maps are generalisations about the distributions of macrophytes, but are a
good indication of the different characters of different locations.
Taxa that occurred in discrete beds (the large macrophytes), were quite easily mapped by the
method described above, while the algae and ephemeral plants presented a more challenging
scenario. The distribution of many of these (especially Chara spp. and Nitella spp.) were
more or less ubiquitous throughout the lakes, occurring in low density in most shallow
locations. While the maps presented are intended to show a presence/absence of each type of
macrophyte, the distribution of the charophytes is restricted to locations where these taxa
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formed a substantial component of the benthic flora. While this was not formally quantified in
the field, the presence of charophytes was not plotted unless they composed at least
approximately 5% of the total cover, estimated in the field visually.
6.3.2. Types of aquatic plants found in Myall Lakes
Table 6.2 presents a summary of the characteristics of the dominant taxa in Myall Lakes that
are presented in the maps.
Table 6.2. Summary of dominant plants found in Myall Lakes.
Taxon
Vallisneria gigantea
Life-strategy - Type
Perennial Angiosperm
Ruppia megacarpa
Perennial Angiosperm
Potamogeton perfoliatus
Perennial Angiosperm
Myriophyllum
salsugineum
Najas marina
Charophytes
? Eg. Nitella hyalina
and others.
? Chara spp.
Macroalgal Assemblage
? Filamentous green.
Perennial Angiosperm
Annual - Angiosperm
Ephemeral - green
algae
Ephemeral – various
algae.
Notes on Occurrence
Experienced winter die-back in the Broadwater in
study period. Confined to shallow areas of
Broadwater and Two-Mile lake and fringes of other
areas.
Confined to shallow areas of Broadwater and TwoMile lake and fringes of other areas. Noted to be
undergoing significant recruitment in deeper areas of
Broadwater in study period.
Although not found in 2001, this taxa recruited and
colonised bare shallow areas in early 2002, and
became quite abundant by late 2002 especially in the
Broadwater.
Present in all lakes except the Broadwater. Did not
appear to change distribution in study period.
Underwent great increase in biomass and distribution
in all lakes except Broadwater over summer, and
suffered dramatic die-back over winter.
Present all year in all lakes, but in far greater
abundance in summer in Myall Lake.
Present only in Broadwater on otherwise bare
sediment and in conjunction with macrophytes in this
lake. Distribution did not appear to change over the
study period.
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Figure 6.1. Spring macrophyte distribution in the Broadwater.
Figure 6.2. Summer macrophyte distribution in the Broadwater.
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Figure 6.3. Autumn macrophyte distribution in the Broadwater.
Figure 6.4. Winter macrophyte distribution in the Broadwater.
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Figure 6.5. Spring macrophyte distribution in the Mid-Lakes.
Figure 6.6. Summer macrophyte distribution in the Mid-Lakes.
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Figure 6.7. Autumn macrophyte distribution in the Mid-Lakes.
Figure 6.8. Winter macrophyte distribution in the Mid-Lakes.
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Figure 6.9. Spring macrophyte distribution in Myall Lake.
Figure 6.10. Summer macrophyte distribution in Myall Lake.
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Figure 6.11. Autumn macrophyte distribution in Myall Lake.
Figure 6.12. Winter macrophyte distribution in Myall Lake.
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6.3.3. Spatial Distribution of plants in Myall Lakes.
Broadwater - Perennials.
In the Broadwater large areas of perennial macrophytes (V. gigantea to approximately 1.0m,
and R. megacarpa to approximately 1.5m depth) occur on the sandy benches on the northern
and south eastern shores. While a few individuals may be found in deeper water the steep
drop-off found near the channel areas restrict the distribution of these taxa. Deeper areas of
the R. megacarpa beds contained mainly immature plants which formed a dense turf.
In the western Broadwater the distribution of V. gigantea is further limited to approximately
0.8m, presumably because of the low light penetration in this area (see Chapter 5) and forms a
narrow band along the shore. Similarly, R. megacarpa in this area is found to only
approximately 1.0 m depth, and is restricted to the shore line areas, with a sparse small bed
being found on the bar south of the entrance of the upper Myall River. P. perfoliatus was
found to grow in similar locations to the other two species, but was found to depths of
approximately 2.0m.
Broadwater - Ephemerals.
Large areas of assemblages of filamentous green macroalgae were present on all sampling
occasions on the northern, eastern, and southern shores of the Broadwater. These were
composed of a variety of estuarine and marine genera mainly Gracillaria, Enteromorpha, and
others. These assemblages occurred mainly in conjunction with R. megacarpa beds, but were
also found to occupy bare sediment in some areas. The macroalgal assemblage was observed
to be present to approximately 2.0 m depth in some locations, and formed a dense mat on the
northern Broadwater shallows approximately 2 km in length.
Much of the sediment mapped as being ostensibly bare in the Broadwater, did support low
densities of charophytes on all occasions although these did not form dense beds as were
found elsewhere in the system. Stunted charophytes (~5 cm in height) were observed growing
in depths to approximately 2.5 m, in some locations.
Mid-Lakes – Perennials
The sand bars of Two-Mile Lake and Korsmans Landing, have a similar macrophyte
community composition to the bars of the Broadwater, with the addition of Myriophyllum
salsugineum which grows in conjunction with the other taxa. Stable beds of V. gigantea and
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R. megacarpa are present all year to a depth of approximately 1.5m. However apart from this
southern portion of Two-Mile lake, and east of the mouth of Boolambayte Creek, there are no
large beds of perennial macrophytes in the mid-lakes. Although Boolambayte Lake does
contain the same macrophyte taxa as Two-Mile Lake these plants are not present other than as
narrow beds along the shore and around Goat and Sheep Islands.
Some of the cause of the lack of perennial macrophytes might be attributed to the steep bed
edges in ‘The Narrows’ of northern Two-Mile Lake, which does not contain shallow areas
that might support macrophyte communities. However, Boolambayte Lake which does have
large areas of shallow bed that might be expected to hold perennial macrophytes is also
without large beds, so presumably the absence of perennial plants is due to some other factor.
Mid-Lakes – Ephemerals / Annuals
During summer the mid lakes support an abundance of charophytes and N. marina which
occupy all parts of these lakes to approximately 3.5 m depth. Even the deepest parts of
Boolambayte Lake are thus able to support these plants, which form very dense beds which
reach the water’s surface in many locations. Two-Mile lake also was found to support large
areas of N. marina, but did not contain beds of charophytes as extensive as those found in
Boolambayte Lake.
Myall Lake – Perennials
Myall Lake does not contain any substantial areas of perennial macrophytes, although nearly
all the rocky fringing shoreline areas do support a narrow band of V. gigantea, R. megacarpa,
and M. salsugineum, on the northern and southern shores. Although a formal assessment of
plant health was not undertaken, during the study period the macrophytes appeared to be
healthy (not fouled with epiphytes) and produced flowers (V. gigantea in summer, M.
salsugineum in winter). No noticeable change in the density or depth to which the plants were
growing was seen during the study period.
Myall Lake – Ephemerals / Annuals
As was found for the Mid-Lakes, almost the entire bed of Myall Lake is shallow enough to
allow the growth of charophytes and N. marina for some of the year. Consequently, at the
peak of the growing season dense shoals of these plants were found in most parts of the lake.
In places that were less than 3 m deep N. marina was observed forming mono-specific shoals
over 2 km long that reached the surface of the lake. There did not appear to be any spatial
differences in the abundance of these plants within Myall Lake other than a low abundance of
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these taxa in shallow sandy areas, which contained small scattered plants. The sandy areas of
western Myall Lake (Neranie Sands) and the south western corner (Palmers Bay) did not
support extensive beds of these plants, and were mainly bare sediment on all sampling
occasions.
6.3.4. Temporal Change in the Distribution of plants in Myall Lakes.
A number of changes in the abundance and distribution of macrophytes in the lakes were
observed in the study period. Table 6.3 summarises the changes through time within the study
period of the area covered by each taxa.
Table 6.3. Summary of extent of cover of each taxa.
Taxa
V. gigantea
R. megacarpa
M. salsugineum
P. perfoliatus
Charophytes
N. marina
Macroalgal Assemblage
Approximate Total Area of each Taxon in Myall Lakes.
(Hectares)
Spring
Summer
Autumn
Winter
Nov. 2001
Feb. 2002
June 2002
Sept. 2002
377
377
377
377
710
710
710
710
140
140
140
140
0
191
382
568
2310
4159
4159
2186
1412
5443
5443
526
210
210
210
210
6.3.4.1. Najas marina and Charophytes – Annual growth cycles
The most obvious changes over the 4 surveys is the large annual change in area covered by N.
marina, which underwent a large summer bloom and winter die-back in all areas except for
the Broadwater. This species was observed to grow vigorously from seed (presumably) in
early spring to mature plants in summer, which then broke up and decayed over winter. Much
of the areas formerly occupied by living Najas was found to contain decaying fragments in
Autumn 2002. Drifting fragments containing seeds is a mechanism by which this species is
distributed (Sainty & Jacobs, 1994). Sediment samples taken from most areas of Myall Lakes
in September 2002 often contained large numbers of seeds.
Although plants over 4 m in length were found in the lakes, the roots system of this species
consists of relatively short (~40 cm) simple roots that arise from a creeping underwater stem,
and is densely covered with short (<10 mm) fine hairs. It would appear that this species is
able to extract nutrients from the benthos, but may also be reliant on absorbing water borne
nutrients through its leaves like many aquatic plants (Sainty & Jacobs, 1994).
In conjunction with the summer growth of Najas, Charophytes also underwent a large
summer increase in area covered by living plants, and winter die-back in the study period. An
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increase of approximately double the area occupied in winter was observed, with roughly 4,
100 hectares of lakebed containing these plants in summer.
All lakes of the system except the Broadwater supported a summer bloom of these taxa.
Although individual Najas plants were found scattered in many locations in the Broadwater,
only a small area demonstrated a significant abundance of these in summer. In parts of the
Broadwater small charophytes were found to be fairly abundant as small clumps, but not in
large monospecific stands that were observed elsewhere.
6.3.4.2. Vallisneria gigantea – Broadwater die-off
Although a detailed assessment of the standing stock and productivity of this taxa was not
performed there seemed to be a significant die-off of adult plants in the winter of 2002 in the
Broadwater. Other areas did not appear to be affected. Overall, the areal distribution of plants
did not change appreciably following the decline, but a noticeable thinning of the beds was
found in the September 2002 survey, which followed the die-off. The northern shore and
shoal to the north of the Lower Myall River entrance was observed to contain virtually no
mature plants, in the winter of 2002. Large numbers of small V. gigantea were found in these
locations that lost adult plants, so this decline did not cause a complete loss of this taxa from
these areas.
Curiously, locations elsewhere that contained this taxa did not display a loss of adult plants. A
dense bed near the entrance of the Upper Myall River was found to contain both adult and
juvenile plants, as did Two-Mile Lake, which was not found to suffer the winter die-off.
6.3.4.3. Potamogeton perfoliatus – Widespread colonisation.
This taxa was not recorded in the November 2001 survey, but was found in many locations by
February 2002 and became widespread later in 2002. Potamogeton colonised bare substratum
(mainly sandy locations) in many areas of the Broadwater, Two-Mile Lake, and Violet Hill in
early 2002, and was well established in the Broadwater in Autumn 2002. In particular the
northern and southern shores of the eastern Broadwater became densely populated with this
species, perhaps utilising free space created by the decline in the density of mature Vallisneria
plants at around the same time. In late 2002 this species was present in approximately 570
hectares of lakebed, perhaps growing from seeds which had been dormant.
6.3.5. Benthic Survey - Results and Discussion.
The results of the benthic surveys were plotted in a GIS application and the contours of
nutrient gradient were extrapolated to form the maps shown in Figure 6.13 to Figure 6.16. The
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data used combines the 2000 Geosciences Australia survey of the Broadwater, as well as the
recent DIPNR 2002 work in other parts of the lake. To check for temporal differences that
might make combining the datasets unwise, samples were taken close to the 2000 survey
locations and were analysed in this study. No significant differences were shown for the
grainsize or nutrient content of sediments taken in 2002 when compared to 2000.
6.3.5.1. Percent Total Organic Carbon (TOC).
There was a general gradient of higher percent TOC with increasing distance from the
Broadwater. While TOC was generally below 10% of dry weight of sediment in the
Broadwater, very high percent TOC was found in Myall Lake, with some locations exceeding
25%. However, sandy areas of Myall Lake (eg. Neranie Sands and Palmers Bay) had similar
values to the sandy benches of the Broadwater. The Mid-Lakes had TOC of between 10% and
15%.
As was mentioned in Chapter 5, this distribution is probably a result of a low load of
suspended inorganic clay and silt being transported into Myall Lake and a high amount of
deposition of plant material on the lake bed following the Najas and Charophyte blooms that
occur each summer.
6.3.5.2. Total Nitrogen (TN).
The concentration of TN in sediments was very highly correlated with TOC and consequently
the distribution of TN mirrors that for TOC. Again, far higher concentrations of TN were
reported in the benthic material from Myall Lake, than were found in the Broadwater.
Sediments from the Broadwater and Nerong / Upper Myall River contained less than 10
gN/kg dry sediment, as did the southern portion of Two-Mile Lake. In contrast ‘The Narrows’
(northern Two-Mile Lake) Boolambayte Lake and Myall Lake had concentrations as high as
30 gN/kg, except in sandy areas which were less than 5 gN/kg.
6.3.5.3. Total Phosphorus (TP).
In contrast to Total N, Phosphorus concentration in benthic material was poorly correlated
with sediment TOC and was similar throughout the lake system. Generally, the greatest
concentrations were found in the deeper parts of each lake system (> 1 gP/kg dry sediment),
and shallow sandy areas had the lowest concentrations (<0.01 gP/kg dry sediment), but there
was considerable variability in the concentration found in similar areas. For example two
nearby locations in Myall Lake of similar depth had respective concentrations of 0.26 gP/kg,
and 0.82 gP/kg, which is more than a three-fold difference.
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6.3.5.4.Total Sulphur (TS).
While sulphur is an important macronutrient for plant growth, it is less common for it to be
present in limiting concentrations in estuarine waters because of the high concentration of this
nutrient in seawater (Waite, 1984). The concentration of this element in the benthic material
of this system was the converse of what might be expected. With the lowest concentrations
being present in areas closest to the source of seawater (ie. the Broadwater) and much higher
concentrations being found in all other parts of the system. Equally high (>2 gS/kg dry
sediment) concentrations were present in Nerong Creek, the Upper Myall River, and many
parts of Myall Lake, while concentrations of <0.01 gS/kg dry sediment were typical of sandy
locations in all areas.
6.3.5.5. Grainsize Analysis.
No data is presented for this analyte. The method utilised in this study to perform grainsize
analysis, using the dispersal of laser light in a slurry of suspended sediment sample in liquid,
returned results that were in conflict with simple observations of sediment properties. It is
thought that because of the physical properties of the benthic material (containing mucous and
algal colonies) an unrepresentative value of each size category was reported.
A small recent survey of grainsize using conventional methods found significantly different
results to those returned by the laser scattering method (Pers. Comm. A. Kubiak, Masters
candidate The University of Sydney). Consequently the grainsize data are not being presented
for this survey.
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Figure 6.13. Benthic percent TOC – September 2002.
Figure 6.14. Benthic Total Nitrogen – September 2002.
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mg/kg)
>1000
Figure 6.15. Benthic Total Phosphorus – September 2002.
Figure 6.16. Benthic Total Sulphur – September 2002.
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6.3.5.6. Benthic Microcystis sp.
In mid 2001, bathometric surveyors reported the presence of an unconsolidated organic
benthic material in much of Myall Lake. It was noted because it created difficulties in
mapping the lakebed, due to its ability to absorb sonar signals. The Myall Lakes vegetation
surveys that were began in November 2001 also noted in shallow bays the occurrence of a
highly organic, algal-like material that resembled decomposing plant material. It was also
noted that large amounts of gas which smelt strongly of Hydrogen Sulphide (H2 S) were
liberated by the movement of boats in these shallow bays.
Some preliminary investigations revealed that the material was composed of high densities of
algal cells which were grouped into colonies and bound in a mucous matrix. Some of the cells
were also thought to resemble various blue-green algal taxa including Aphanothece and
Microcystis although the exact identity of the cells was unclear. Samples that were
subsequently identified by DIPNR were initially thought to be Microcystis, although the
material was unlike samples encountered before. In particular the fact that the material was
benthic and sheathed in mucous did not conform to known descriptions of this taxon.
Although the discovery of the benthic microbial material was incidental to the vegetation and
sediment surveys, a small survey was undertaken to map the distribution of the benthic
microbial layer in Myall Lakes, and commence some investigations into the identity of the
organism. Because of the initial finding of the similarity of the organism to Microcystis, a
mouse bioassay was conducted on samples taken from shallow bays. This analysis found that
4 of the 5 samples submitted were mildly hepatotoxic.
In early 2002 numerous samples were dispatched to laboratories in Australia and one in the
U.K. and toxicity testing was conducted to search for microcystin and cylindrospermopsin
toxins. The results of these analyses were inconclusive and in conflict between laboratories.
In mid 2002 DIPNR initiated formal investigations into the genetic sequencing of the
organism to identify the similarity of the alga to known blue-greens, and to identify the
presence of genes that coded for the production of microcystin toxin. The work being
performed by Brett Nielan of the School of Biotechnology and Biomolecular Sciences, at The
University of NSW.
In late 2002 it was found that the main organism in the benthic microbial layer was a
previously undescribed species that had a high (96% similar) genetic similarity to Microcystis
flos-aquae, a taxon known to be able to produce microcystin under some circumstances.
Further investigations found that genes that enable the organism to produce microcystin toxin
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were also present, but that gas vacuoles, that might enable the cells to become buoyant, were
lacking.
In conjunction with the benthic nutrient content assessment in late 2002 (section 6.3.5),
samples were also qualitatively assessed for the presence of the new Microcystis organism in
all samples by examining a small sample under compound microscope. It was found that the
organism was present in variable densities in nearly all samples except those taken in the
Broadwater, Nerong Creek, and Myall River. Although the numbers of colonies per unit
volume was not quantified, it seemed that sandy areas of Myall and the Mid Lakes had little
or none of the organism, while the shallow bays where the organism was first observed had
the highest density of colonies. Based on the observed distribution there is approximately
7,500 hectares of lakebed which contains these cells in variable densities.
The distribution of the benthic Microcystis sp. corresponds well with initial field observations
of benthic material quality which noted (before the organism was identified) that the benthos
in some areas had a cohesive quality that was unlike usual estuarine or lacustrine muds. This
was distinct to the shallow bays, which appeared like composting vegetation. In particular
cores taken from the bed of the upstream lakes have a consistency similar to jelly, which can
be ‘fractured’ along lines into smaller pieces which retain a firm character. It would seem then
that the jelly-like character may be a consequence of much microbial material on the lakebed,
and that, the Microcystis sp. may play a significant role in the ecology of these areas and the
lake as a whole.
6.4. DISCUSSION.
Discussing the findings of these surveys is extremely difficult because of the unique benthic
microbial layer which appears to play a very important role in determining the character of
much of the system. Interpreting the significance of both the distribution of vegetation types
and benthic nutrient concentrations are confused by the presence of the benthic microbial
layer which is probably an important link in understanding the ecology of the lakes. In the
absence of definite knowledge of the role of the microbial layer, the discussion following
attempts to draw some conclusions about the vegetation and benthic nutrient surveys.
6.4.1. Vegetation
Vascular plants.
A clear distinction between the character of the Broadwater and Two-Mile Lake (the
downstream lakes), and that of Myall and Boolambayte Lakes (the upstream lakes) was
found. In brief, while the downstream lakes contain large areas of perennial angiosperm
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macrophyte beds which persist throughout the year, the upstream lakes are dominated mainly
by annual / ephemeral plants and macroalgae, which demonstrate a more seasonal cycle.
Generally, it would be assumed that the absence of these perennial plants is a sure sign of
poor aquatic health and that the waterway is in decline. Adams et al. (1999), for example
describe the loss of perennial macrophytes as one of the steps that occur in the progression of
a eutrophied waterway degrading into a disturbed, algal-dominated system. It is confusing
then that the highest water nutrient concentrations and the least light penetration (Chapter 5)
were usually found in the Broadwater than other parts of the system, and then largest beds of
macrophytes are also present in this waterway.
Similarly, finding that ephemeral plants are dominant in Myall Lake would normally lead to
the conclusion that this waterway was in poor health, and that excess nutrients were likely to
be present in this waterway. Likewise, the high concentration of benthic nutrients (except
total phosphorus) and organic carbon in Myall Lake seems also to indicate that this waterway
is suffering from an overload of nutrient inputs and is in poor condition.
An alternative, more probable, model however could lie in the finding of large amounts of
benthic microbial material which seems to play a role in excluding the occurrence of
perennial macrophyte bed development. Much of the gently sloping or flat lakebed of Myall
Lake which would be expected to hold macrophyte beds is covered with the benthic
Microcystis sp. instead, while steeply sloping rocky areas nearby (which don’t allow the
retention of Microcystis sp) supports small healthy stands of these plants. While correlation
does not necessarily indicate causation, this anomaly implies that some mechanism may be
preventing the growth of perennial plants in areas that contain the benthic microbial layer.
It seems unlikely that competitive interactions (eg. shading or space monopolisation by
Najas) are excluding perennial macrophytes because of the complete absence of even juvenile
plants in areas containing Microcystis sp. and the fact that such plants are present in adjacent
rocky areas.
Whatever (if any) mechanisms are responsible for the exclusion of perennial macrophytes
from areas containing large amounts of benthic Microcystis sp. is presently unknown.
However, it is likely that it may be a combination of the lack of benthic attachment provided
by the material (all the perennial macrophytes in Myall Lakes are quite buoyant) and the
smothering or decomposition of propagules by microbial action, which contribute to the
prevention of macrophyte establishment.
Charophytes.
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Although the distribution of perennial macrophytes in the lake system is counter to an
intuitive assessment of aquatic health (ie. there are few of these plants in the healthiest part of
the lakes), the distribution and abundance of charophytes is more in line with the perceived
and measured gradient of eutrophication. Many charophytes are only able to grow in low
nutrient (particularly phosphorus) conditions (Auderset-Joye et al., 2002), and have been lost
where nutrient enrichment of freshwaters has occurred. Some taxa are known to only occur in
groundwater-fed wetlands which are particularly low in phosphorus (Bornette et al., 1996).
Auderset-Joye (2002) described changes in the abundance in distribution of charophytes in
Swiss lakes since 1800 to 2000 using herbarium collections and contemporary sampling.
Much of the change found was centred around sites of nutrient enrichment of waterways. One
of the taxa found in that study showed that Nitella hyalina, which is abundant in Myall Lakes,
was adversely affected by increases in nutrients. Other studies in Australia on shorter time
scale have established correlations, if not causation, between elevated nutrients and declines
in charophyte abundance (eg. Casanova and Brock, 1999).
It is apparent then that fact that these plants which have been demonstrated to be sensitive to
eutrophication are abundant in Myall Lake and less so in the Broadwater is circumstantial
evidence of differences in nutrient loads. Other physical factors such as the widely different
salinity regime of each of these lakes, and different benthos, however clearly confound the
direct interpretation of this pattern of distribution. It is possible though, that these plants could
be a useful bioindicator of future environmental health in this system.
Although further study is required, if Nitella or Chara could be utilised as indicators of
environmental decline due to nutrient enrichment they could be a very valuable asset in
managing the future health of the lakes. To monitor changes in the presence of these taxa it
would be necessary to quantify the abundance (standing crop or productivity) of the plants.
The plants are distributed throughout the lakes, but are present only as small plants in the
Broadwater.
The absence of these plants in locations known to be high in nutrients, such as in the western
Broadwater, and the presence in the eastern Broadwater (noting physico-chemical water
quality is similar at each location), suggests that there may be a good correlation between
elevated nutrient concentrations and lack of plants in this system at present. Consequently, the
potential for charophytes to be used as a bioindicator in this system seems likely.
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6.4.2. Benthic Nutrient Content.
It is clear that (apart from the sandy areas) the material on the bed of Two-Mile,
Boolambayte, and Myall Lakes is very high in nitrogen and, sulphur and organic carbon in
comparison with the Broadwater, and that there is a gradient of increasing concentration in
the Mid lakes region. As previously mentioned, the presence of high benthic nutrient content
is not necessarily an indication that there is significant nutrient being added to the water
column from the sediments, and this might only occur under conditions of benthic anoxia.
It is possible that the presence of photosynthetic Microcystis sp. and the addition of much
organic material following Najas and charophyte blooms in these areas, which presumably is
broken down by benthic microbes, has created a biologically active benthic layer which
contains much organic matter, both living and decomposing. As well, the main source of silt
and clay which might lead to a ‘dilution’ of benthic nutrient concentrations by weight, is
located in the Broadwater and the precipitation of particles before they reach the upstream
lakes means that little inorganic material is deposited.
The fact that Total Phosphorus is low in most samples in the upstream lakes suggests strongly
that this could be the limiting nutrient in these locations, although further studies would be
needed to confirm this.
6.4.3. General Discussion
The overall implications for the presence of a highly organic, and possibly biologically active
lakebed in managing the health of Myall Lakes is presently unknown. Apart from seemingly
influencing the types of plants that can grow in these areas a variety of more subtle ecological
affects might also exist that require further investigation.
Because the lakes are shallow and easily turned over, the respiratory oxygen demand of such
material is unlikely to cause deoxygenation of benthic waters, and the physico-chemical
measurements made have not found anoxia in bottom waters in these lakes. However, if
Myall Lake were to become stratified for an extended period it is possible low dissolved
oxygen could eventuate, which might have severe consequences for lake health.
Perhaps the greatest role the material may play in lake health is how it cycles nutrients within
the system. The benthic microbial material may at times serve as a scavenger of water-borne
nutrients which could be part of the reason that water column nutrients are typically lowest in
Myall Lake. As well, the material may play a role in cycling nutrients and releasing them to
fuel plant growth. In Chapter 5 a small study is reported into the concentration of nutrients
present in benthic waters of Two-Mile Lake. A high concentration of NOx was found over an
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area known to contain the benthic microbial layer. As well, in the shallow bays where the
decomposing plant material and abundant Microcystis sp. are found the interstitial-water
ammonium concentration was found to be extremely high (~15 mg/L - Pers. Comm, Andrew
Sampaklis, Honours candidate, University of Newcastle, 2003).
It is clear that the benthic microbial material, because of its widespread distribution and
demonstrated high nutrient content, has the potential to play a significant role in mediating
and regulating the concentration of water column nutrients and physical water quality. Several
studies are presently underway to examine various aspects of this material’s qualities, which
will assist in understanding its importance for managing the lake’s ecology.
Although a full understanding of the influence of the benthos over the macrophytes of the
lakes is not possible at present, clear differences between the character of the two major
waterbodies (Myall Lake and the Broadwater) of this system are apparent. This disparity
presumably developed historically because of differences in the quality of water delivered to
the receiving waters of each lake, and the maintenance of each is reliant on the continuation
of those conditions.
Although water quality would have declined in recent years, the Broadwater probably always
has received intermittent fresh, turbid and somewhat nutrient-laden waters from the Myall
Catchment, and will continue to do so. Myall Lake on the other hand which is has clearly an
oligotrophic character, could be very badly harmed by declining water clarity and increased
nutrient loads.
As detailed in Chapter 5 upper Myall Rivermouth and Broadwater which are most susceptible
to reduced water clarity have the shallowest compensation depth. This means that any
decrease in water clarity in these areas due to increased turbidity in river inputs may decrease
the depth to which aquatic plants are able to grow. Depending on the bathometry of the
region, this may substantially reduce the area available for plant growth.
At present,
macrophyte beds are restricted to the shallow areas of the Broadwater, indicating a response
to lower light levels
An increase in phosphorus and declines in water clarity in Myall Lake would undoubtedly
disturb the annual cycle of charophyte and Najas growth, which occupies the entire bed of the
lake including the deepest areas of the waterway. These changes would probably be noticed
first in the deeper areas of the lake, as lakebed illumination in these areas would be first to fall
below the required intensity required for plant growth. Whether or not the benthic Microcystis
sp. would be affected is unknown.
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7. Synthesis of the data.
The 'Monitoring Blue Green Algal Blooms in Myall Lakes' project was designed to answer a
range of specific questions posed by the community and project sponsors regarding bluegreen algae in Myall Lakes and the overall health of the lakes system. The preceding chapters
in this report provide an introduction to the lakes system; background to the problem; and the
results of the various studies undertaken during the course of the project. This chapter
attempts to answer those questions, drawing on the information in the previous chapters.
Rather than repeating the information provided in the previous chapters, reference is made to
the relevant sections where appropriate.
7.1. WHAT ARE THE PHYSICAL AND BIOLOGICAL CONDITIONS / PROCESSES
WHICH CONTRIBUTE TO BLUE-GREEN ALGAL BLOOMS IN MYALL
LAKES?
Blue-green algae blooms in Myall Lakes were first confirmed in Bombah Broadwater in
April, 1999. These blooms were composed of large, toxigenic, scum forming species such as
Anabaena, and later Microcystis. These species were not recorded in bloom concentrations
after July 2000. However, in 2001 and 2002, small non-toxigenic species such as
Chroococcus and Merismopedia were recorded in Myall Lake and Mid Lake areas at levels
which caused a high alert status to be instated1 . These two types of blue-green algae have
quite different characteristics and physiology including size, buoyancy and nutrient
requirements. Consequently, different taxa of blue-green algae will bloom in response to
different environmental conditions within the lakes at any given point in time.
7.1.1 Anabaena/Microcystis blooms in the Broadwater.
The environmental conditions which allow Anabaena and Microcystis to gain competitive
advantage over other phytoplankton species in waterways generally include warm water, low
salinity, elevated nutrient inputs and increased turbidity (see Chapter 4 for more detail). All
these conditions occurred during the period from April 1999 to July 2000 when blue-green
algal blooms, dominated by Mycrocystis and Anabaena, were recorded in the Broadwater.
Water temperatures range from 10 to 28?C in Myall Lakes annually and ranged between 11 to
23?C at the time of the blooms. Summer temperatures are favourable for blue-green algal
growth. During the period of the bloom, the salinity in the Broadwater was very low
(<2mS/cm). This was caused by moderate rainfall in the catchment in late 1998, followed by
large rainfall events in March and April 1999 (see section 2.2.1 for details for details on
1
High Alert for recreational use is 15,000 cells/mL for potentially toxic species or 2mm3 /L for contact
irritant species.
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climate). It is likely that these inflows were large enough to cause the entire Broadwater to
become essentially fresh until July 2000. As salinity increased after this time, toxigenic bluegreen algae concentrations declined rapidly, and have not been recorded in bloom
concentrations since. Section 4.3.3 describes the relationship between the various algae taxa
and salinity concentrations. In Fig. 4.7 it can be seen that Anabaena and Microcystis
concentrations both decline rapidly after salinity concentrations exceed the 2 to 4 mS/cm
range.
The rain events of late 1998 and early 1999, would have also washed large quantities of
nutrients into the Broadwater. Studies in 2001/2002 have shown that the catchment of the
Upper Myall River can produce up to 45.1 tonnes of nitrogen (N) and 9.4 tonnes of
phosphorus (P) in a single rain event (Chapter 3). Modelling of nutrient generation for the
Myall Lakes has shown that contributions from the catchment are a major source of nutrients.
Phosphorus concentrations in the water column were often high at the mouth of the Upper
Myall River, and in the Broadwater throughout this study, also indicating the Upper Myall
River is the main nutrient source to the Broadwater. Delivery of Phosphorous through
sediment recycling is also a potential source, however, preliminary studies (AGSO, 2000) on
this were inconclusive.
It is also likely that light penetration in the Broadwater decreases during large flow events due
to large loads of sediment washed in from the catchment. Even in dry periods, the Broadwater
has relatively lower clarity because of tannins in the water, and wind driven re-suspension of
fine particles from the bottom (this is discussed in more detail in Chapter 5). . Because both
Anabaena and Microcystis have the ability to control their buoyancy, and hence where they
are positioned within the water column, they are able to outcompete other species when light
penetration into the water column is low.
7.1.2 Chroococcus dominated assemblages in Myall Lake .
Although the Anabaena and Microcystis blooms occurred primarily in the Broadwater and
Two Mile Lake, extensive sampling throughout the Myall Lakes was undertaken to determine
the spatial extent of the bloom. In October 2000, high biovolumes of blue-green algae were
detected in Myall Lake and continued sporadically during the life of the project. These
blooms were composed of a mixed assemblage of small species usually dominated by
Chroococcus and occasionally Merismopedia. These species produce compounds (ie. toxins)
which cause skin irritation. They do not produce the neurotoxins and hepatotoxins that are
typical of Anabaena and Microcystis respectively and therefore are less harmful to human
health. These algae are not as visible as the Anabaena/Microcystis blooms as the cells are not
buoyant, and therefore remain well mixed through the water column.
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It is impossible to know for certain whether or not these species were present in Myall Lake
in large numbers prior to April 2000 due to infrequent sampling and the use of laboratory
techniques that were mainly intended to assess human health risks from large-celled
(toxigenic) blue-green taxa. After April 2000 a different laboratory method was employed,
which found small-celled taxa in low concentrations in Myall Lake and Violet Hill. After
April 2000 monthly sampling (except for August and September 2000) at these locations
found low concentrations at most sites until December 11th , 2000, when all samples from the
upper system were found to contain elevated concentrations of small-celled blue-green algae.
Although the general pattern of low abundances of small-celled taxa in winter, and a bloom in
Spring that was observed in Myall Lake in 2001 and 2002 was the same in 2000 (Chapter 4,
Figure 4.4) there was a significant difference in the composition of the blue-green algae.
While the spring blooms of 2001 and 2002 were dominated by Chroococcus, with low
numbers of Merismopedia being present, the opposite was true in 2000, Merismopedia being
dominant and Chroococcus being either absent or scarce until January 2001. This distinct
difference between 2000 and the subsequent 2 years could indicate a succession of these two
taxa, and that the environmental disturbance shown in the Broadwater in 1999/2000 was
being reflected in the ecology of the phytoplankton in other parts of the lake system.
Conditions that are likely to favour these small species over other algal groups, such as
diatoms or green algae, are nitrogen enrichment, silica limitation (disadvantaging
competitively superior diatoms), clear water and a well mixed water body. Also, if the bloom
and succession of Merismopedia / Chroococcus was a result of disturbance elsewhere in the
lake system, subtle ecological changes may also have had an influence on the appearance of
these taxa.
During the study, there was some evidence of nutrient enrichment in Myall Lake with
extremely high values of ammonium (NH4 +) recorded at times. Small blue-green algal species
do not fix nitrogen and thus thrive in conditions of high nitrogen availability, particularly
NH4 +. As there is no major riverine inflow into Myall Lake, the NH4 + is likely to be
regenerated from the sediments. The floor of Myall Lake is covered in a highly organic layer,
which may be involved in the release of NH 4 + into the water column from the sediment under
certain conditions.
Silica is required by diatoms for incorporation in their cell walls. In low silica conditions
(<0.5 mg/L), they are unable to compete effectively with non-siliceous algae (Wetzel and
Likens, 2000). Creeks and rivers are the main source of reactive silica in coastal waters.
Given the large distance from major riverine inputs to Myall Lake, silica limitation in Myall
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Lake is a possibility. Low concentrations of soluble silica (<0.3 - 1.8 mg/L) were recorded in
Myall Lake in 1972-1973 by Johnson (unpublished manuscript). In September 2000 reactive
silica from all locations throughout the lake system reported values below 0.1mg/L.
The clarity of the water in Myall Lake was consistently higher than in other parts of the lake
throughout this study. In addition, the water body is relatively stable as it has a long residence
time and is not greatly influenced by riverine inflows. It is also well mixed by wind. As small,
blue-green algal taxa are not able to regulate their position in the water column, a well-mixed
water body with adequate light throughout for photosynthesis would create favourable
conditions for their growth. In addition to this, the smaller celled species appear to have a
greater tolerance of increased salinity levels. This gives them an advantage over larger celled
species (Microcystis and Anabaena) in conditions of high water clarity, low P / high N, and
relatively increased conductivity.
The differing characteristics of the Myall Lake from the Broadwater appear to influence the
type of algae taxa present at any given time. It is only after prolonged rainfall, and hence
displacement of saline water by turbid freshwater in the Broadwater, that toxic blooms of
Anabaena and Microcystis are likely to occur. The conceptual diagrams (Figures 7.1 and
7.2), provide a pictorial representation of the factors which lead to the development of algal
blooms on Myall Lakes.
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7.2. WHAT ARE THE RELATIVE CONTRIBUTIONS OF NUTRIENT SOURCES
TO ALGAL GROWTH IN THE LAKES?
7.2.1. The role of nutrients in algal growth
Nitrogen (N) and phosphorus (P) are the nutrients that most often limit algal growth in aquatic
and estuarine environments. These nutrients must be in a dissolved inorganic form – nitrogen
as ammonium (NH4 +) and nitrate (NOx) and phosphorus as phosphate (PO 4 -) – to enable
uptake by algae. The amount and availability of nutrients in a water body will depend on the
form in which they are delivered, and cycling of nutrients and organic matter within the lake.
7.2.2. Sources of nutrients to Myall Lakes
Nutrients enter Myall Lakes through a number of sources including rainfall, groundwater,
surface runoff and riverine inflows. The relative contribution of each source is summarised
below (further detail can be found in Chapter 3). Once nutrients enter the lake, they are cycled
through aquatic plants, algae (including macroalgae and phytoplankton) and sediments. These
processes are important in a very enclosed system like Myall Lakes, as there is little export of
nutrients from the system through flushing. It is important to make the distinction between
external nutrient sources, and internal nutrient cycling. Under certain conditions, internal
nutrient cycling may return nutrients to the water column to stimulate algal blooms. However,
they do not represent a new source of nutrients to the lake.
7.2.2.1 Rainfall
Rain contains small amounts of nutrients often associated with dust particles. The
contribution of rainfall directly to coastal waters is generally considered small or negligible.
However, Myall Lakes has a large surface area (>100km2 ) and although the amount is only a
small proportion of the annual budget, the contribution of direct rainfall is significant. Direct
rainfall was calculated to contribute 1.3 tonnes of P and 10 tonnes of N per year, which is
approximately 10% of total annual load from the catchment, in an average rainfall year.
7.2.2.2 Groundwater
There are no accurate quantitative estimates of groundwater discharge to Myall Lakes (MHL
1998) (see Chapter 3). Therefore nutrient concentrations of ground water cannot be used to
calculate an accurate nutrient load to the lake. Results of groundwater testing showed that
nitrogen concentrations were naturally high even in areas remote from human activity. Some
contamination of groundwater with nutrients, especially during summer was detected near
camping grounds at White Tree Bay and Myall Shores Resort. Most nutrients generated by
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anthropogenic activities undertaken on the foreshores are delivered to the lakes system via
groundwater. Nevertheless, as discussed in the following section, these do not represent a
major contribution to overall nutrient loads.
7.2.2.3 Catchment Inflows
Riverine inflows during wet weather events were shown to be the largest source of nutrients
to Myall Lakes. These flows carry nutrients and sediment generated in the catchment through
activities such as soil erosion (sheet, rill, gully, and streambank), sewage disposal, intensive
agriculture, fertiliser application and clearing of vegetation. The main riverine input into
Myall Lakes is the Upper Myall River, which flows into the western edge of the Broadwater.
A much smaller riverine input is made by Boolambayte Creek, which enters at Two Mile
Lake.
Estimates of the nutrient loads delivered to the lake from each catchment, and from foreshore
areas of Myall Lakes were modelled using the Catchment Management Support System
(CMSS). This modelling identified the catchment of the Upper Myall River as the major
contributor of nutrients to Myall Lakes, and highlighted dairy farming and poultry farming
(assuming the litter is spread within the catchment) activities as significant contributors,
particularly, given the relative area of land they occupy. The annual load of nutrients from the
catchment of the Upper Myall River was estimated to be 96.5 tonnes of N and 12.8 tonnes of
P per year which represents 70% and 80% of the annual load of N and P respectively, from all
catchment sources.
Results of sampling during rain events by an autosampler stationed at Bulahdelah were in
close agreement with estimates of nutrient loads obtained by modelling. Up to 45.1 tonnes of
N and 9.4 tonnes of P were calculated to be delivered to Myall Lakes in a single rain event of
270 mm.
Modelling (using the CSIRO developed SEDNET model) is currently being
undertaken in the Myall Lakes catchment to allow better pinpointing of diffuse nutrient
sources.
7.2.2.4 Nutrient recycling within the lake
Myall Lakes acts as an effective trap for nutrients and sediment due to its naturally limited
capacity to export nutrients and sediment to the ocean through tidal exchange. Therefore the
internal cycling of nutrients within the lake is likely to be very important in nutrient dynamics
and algal growth. Inorganic (bioavailable) nutrients that enter the lakes from the catchment
are likely to be rapidly consumed by phytoplankton and macroalgae. When these algal cells
die, they sink to the bottom of the lake and decompose, and under certain conditions the
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nutrients can be released back into the water column. Significant amounts of nutrients are also
delivered from the catchment attached to sediment particles and organic matter, which sinks
to the lake bed. Nutrients that reach the lake bed are recycled through complex biogeochemical cycles. These cycles do not represent a new source of nutrients to the lake
system, rather they are pathways by which nutrients delivered from external sources are
processed. These processes may either remove nutrients from the lake through burial in lake
sediments, remove nitrogen through denitrification, or, under certain conditions, release
inorganic (bioavailable) nutrients to the water column that are then available for algal growth.
In Myall Lakes, two preliminary investigations have been undertaken to examine the potential
of nutrient release from the sediments (the AGSO study described in Chapter 2 and the nearbenthic and benthic sampling described in Chapters 5 and 6 respectively). The former study,
although restricted to the Broadwater, showed that denitrification efficiency was low in
muddy sediments, and that significant quantities of ammonium (NH4 +) were returned to the
water column, which could potentially fuel algal blooms. The amount of inorganic
phosphorus (PO4 ) released from the sediments was small. However, phosphorus release
usually requires low oxygen conditions, which did not occur at the time of the study, but have
been recorded in the Broadwater at other times.
The denitrification efficiency recorded in muddy sediments in the Broadwater at the time of
the study is comparable to other estuarine sites in Australia which are considered eutrophied.
Low denitrification, and release of ammonium and phosphorus, is increased in conditions of
high organic loading, low oxygen, and high turbidity which decreases light penetration to the
sediment surface. These conditions occur with increases in nutrient and organic matter loads
from the catchment. In contrast, reducing the nutrient and organic carbon load will create
conditions which are suitable for increasing denitrification and a reduction in the flux of
nutrients from the sediment. While internal nutrient cycling may play an ongoing and
important role in algal growth for some time, the loads entering the system from the
catchment are the main area where action to reduce nutrient inputs will be most effective.
Reducing the nutrient loads entering the lake is likely to reduce the intensity or frequency of
algal blooms.
The studies described in Chapters 5 and 6 also support sediment nutrient release as a mode of
fuelling algal blooms in the lake system. In particular, the role of the Microcystis sp. layer in
nutrient recycling (particularly N) requires further investigation in Myall Lake, where it is
hypothesized that the majority of N that fuels algal blooms is derived from benthic sources.
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7.2.3. Summary of nutrient source data:
One of the key issues, which this project has attempted to understand, is the source and role of
nutrients in the establishment of blue-green algal blooms in the Myall Lakes system. From the
assessment of nutrient sources to the lake system it has been shown that there is significant
delivery of nutrients to the lake system in freshwater inflows. However it is the cumulative
impacts of the freshwater inflows over time which is the most significant issue or factor in the
development of blue-green algal blooms in the Myall Lakes system.
The assessment of sources and contributions of nutrient to the Myall Lakes (which is
summarised in Table 7. 1) identifies nutrient inflow from the catchment as the major
contributor of nutrients to the lake system. The inclusion of estimates describing rainwater
and groundwater contributions provides contextual data reflecting the ‘natural’ nutrient
cycling in the landscape.
7.3. WHAT IS THE NUTRIENT STATUS OF MYALL LAKES?
Determining the nutrient status of a coastal water body is a difficult task primarily due to the
non-linear responses of aquatic systems to nutrient loads, and intermittent nature of nutrient
delivery. Aquatic ecosystems that are oligotrophic may ‘switch’ to eutrophic conditions
relatively quickly. Other factors that complicate the task of assessing nutrient status in coastal
waters include:
?
High natural variability of many parameters used to assess nutrient status (nutrient
concentration and loads, chlorophyll - ? , phytoplankton composition),
?
Complex, long term changes in parameters due to factors other than nutrient loading eg.
water quality (other than nutrients), seasonal effects, or species specific responses,
?
Differences in the pathways for nutrient cycling between fresh and estuarine systems,
?
The effect of water body residence times on nutrient cycling and susceptibility to nutrient
loads,
?
The influence of drought and flood conditions on timing of delivery of nutrients, and
changes to water quality parameters.
While the occurrence of toxigenic blue-green algal blooms in the Broadwater is a likely
symptom of eutrophication, an assessment of the nutrient status of Myall Lakes should
incorporate a range of characteristics of the system.
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7.3.1. Techniques to assess nutrient status
An assessment of nutrient status is particularly difficult in Myall Lakes given its unusual
characteristics. These include changes in nutrient chemistry and water quality parameters
associated with long term changes in salinity; considerable differences in character and
ecology between interconnected lakes, and little previous information on the water quality
and phytoplankton communities. However, there are a number of ways in which the
information collected during this project can be used to make a preliminary assessment of the
nutrient status of Myall Lakes. These include:
?
comparing the characteristics of Myall Lakes with known signs of eutrophication based
on research in other coastal lakes; and
?
sustainable loads assessment - comparing nutrient loads to Myall Lakes with other NSW
coastal lagoons.
7.3.1.1 General signs of eutrophication
Evidence from a large number of studies from Australia and around the world have shown
that the general characteristics of eutrophication of coastal lakes include:
?
changes in the composition of the phytoplankton community with dominance by bluegreen species;
?
an increase in frequency of filamentous and phytoplankton algal blooms, and decline in
macrophyte and seagrass communities;
?
an increase in plant and animal biomass;
?
a decrease in species diversity;
?
a decrease in water clarity;
?
an increase in sedimentation;
?
the development of anoxia; and
?
high organic loads in the sediment.
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It is important to note that short term trends in these parameters may simply be responses of
the system to natural variations, and that there is little information for Myall Lakes to enable
assessment of long term trends.
7.3.1.2 Sustainable loads of nutrients
Scanes et al. (1997) applied a nutrient assessment to a range of NSW coastal lakes, by
comparing catchment nutrient loads with a subjective index of degradation based on
characteristics of each system. From this, a risk assessment for coastal lakes from catchment
loads of nitrogen and phosphorus was derived and estimates of sustainable loads of nutrients
to coastal lakes developed. This assessment was applied to Myall Lake by comparing loads of
nutrients to the Broadwater, Mid Lakes and Myall Lakes to these risk categories (Scanes et
al.1997).
7.3.2. Assessing the nutrient status of Myall Lakes
Due to the large differences in physical and biological characteristics among the different
lakes, the nutrient status of the Broadwater, Mid-Lakes and Myall Lake will be assessed
separately.
Table 7. 1 Nutrient status of Myall Lakes compared to sustainable nutrient load
assessment of Scanes et al. (1997).
Lake
Broadwater
Mid-Lakes
Myall Lake
Nutrient
Nitrogen
Phosphorus
Nitrogen
Phosphorus
Nitrogen
Phosphorus
Average
weather Load
(Tonnes/km2 /yr)
4.6
0.6
1.0
0.1
0.3
0.02
Low
Risk of Eutrophication
Moderate
High
?
?
?
?
?
?
Note that loads are calculated for average rainfall for nutrient runoff from the catchment and may underestimate actual loads as
this assessment does not account for movement of nutrients between lakes or contributions from regenerated nutrients in
sediments.
7.3.2.1 Broadwater
Of the three lakes, the Broadwater receives the highest nutrient load from the catchment, has
experienced toxigenic algal blooms, has relatively lower water clarity, low denitrification
efficiency of muddy sediments, and some areas of filamentous macroalgae. These
characteristics would indicate a eutrophic status. However, there are large areas of healthy
macrophytes, no evidence of anoxia, and a relatively diverse fish community that forms the
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basis of an ongoing commercial fishery. These characteristics would indicate an oligotrophic
or mesotrophic status.
The conditions which trigger toxigenic blue-green algal blooms in the Broadwater are
associated with high rainfall events – fresh water, high nutrient loads, and turbid conditions
(Chapter 4). During periods of below average rainfall, the Broadwater does not exhibit
sustained algal blooms. If this area were in a highly eutrophic status, it would be expected that
macrophyte communities would decline. Phytoplankton blooms would persist during low
rainfall periods, with the dominant species shifting from blue-greens to dinoflagellates with
increasing salinity. There may also be reports of bottom water anoxia and associated fish kills.
It appears then, that the Broadwater does experience periods of eutrophication associated with
high rainfall events, but is able to ‘recover’ to a mesotrophic status during periods of low
rainfall (see Figures 7.1 and 7.2).
According to the model of Scanes et al. (1997), the estimated nutrient load to the Broadwater
under average rainfall conditions places the Broadwater in Moderate Risk category for
eutrophication. However, studies in March 2001 showed that up to 43% and 67% of the
annual loads of nitrogen and phosphorus respectively were delivered to the Broadwater in a
single rain event. The risk of eutrophication would then be significantly increased during
these wet weather events.
This is an early warning sign that unsustainable loads of nutrients are reaching the Broadwater
and there is potential for the system to switch to a eutrophic status if nutrient loads continue.
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Figure 7. 1 Conceptual diagram showing nutrient loading to the Broadwater under
conditions of large freshwater inflows.
Figure 7. 2. Conceptual diagram showing system functioning in dry weather.
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7.3.2.2 Myall Lake
Myall Lake is the largest of the three lakes, has no major riverine input and is furthest from
tidal exchange. Consequently it has an extremely long flushing time. Its phytoplankton
assemblages are diverse and are dominated by non-toxigenic blue-green species and green
species, some of which are typical of eutrophic lakes; the sediments have an extremely high
organic carbon and total nitrogen content (see Chapter 6); and the water column concentration
of ammonium is sometimes extremely high (see Chapter 5). These conditions would indicate
a mesotrophic or eutrophic status. However, it is also characterised by low estimated nutrient
loads, large seasonal blooms of macroalgae, particularly charophytes; clear water; low
phosphorus concentrations in the water column and sediments, and has not experienced
blooms of toxigenic blue-green algae. These characteristics would indicate an oligotrophic
status for this lake, with characteristics indicating it is limited by phosphorus.
There are a number of factors that may influence the particular characteristics of Myall Lake.
The absence of large blooms of toxigenic blue-green algae may be due to phosphorus
limitation. The main source of phosphorus to Myall Lakes system is the Upper Myall River,
which would be diluted and absorbed in the Broadwater and Mid-Lakes over the 10-15 km
distance to Myall Lake. The dominance of species other than diatoms may be due to very low
levels of silica in Myall Lake. The presence of Najas, rather than perennial macrophytes may
be due to a lack of suitable substrate due to the flocculant, organic nature of the lake bed (see
the description of the Mycrocystis layer in Chapter 6). Rooted macrophytes such as
Vallisneria occur in Myall Lake, but only in rocky areas. The charophytes that occur in Myall
Lake are typical of freshwater systems with low phosphorus inputs. However, these are very
sensitive to nutrient sources and cannot physiologically tolerate elevated nutrient
concentrations.
Estimates of nutrient load (Chapter 3) would place this area in a low risk category for
eutrophication according to Scanes et al. (1997). However, given its unique characteristics
and extremely long flushing time, the susceptibility of this lake to any increase in phosphorus
loads is extremely high.
7.3.2.3 Mid-Lakes
Two Mile and Boolambayte Lakes lie between the Broadwater and Myall Lake and have
intermediate characteristics between the two. The Mid Lakes has a moderate nutrient load,
has experienced some toxigenic algal blooms, has moderately low water clarity (compared to
Myall Lake), moderate to high organic carbon and nitrogen in sediment. Again, these
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characteristics would indicate a mesotrophic to eutrophic status. However, macrophyte
communities are diverse and composed of both rooted and floating species and the water is
relatively clear. These characteristics would indicate an oligotrophic to mesotrophic status.
According to the sustainable nutrient model (Scanes et al, 1997) estimates of nutrient load to
the mid lakes would place it at the upper end of the low risk category for eutrophication in
average rainfall conditions, but would probably be in the Moderate Risk category during rain
events.
7.3.3. Risks due to increasing nutrients
The Broadwater is the lake that is most susceptible to existing nutrient loads carried in by the
upper Myall River from the catchment. It exhibits characteristics that favour blue-green algae
under high rainfall conditions. At present it is able to recover from these events during periods
of low rainfall. However, if the nutrient load is not reduced, the Broadwater is likely to show
signs of eutrophication (algal blooms, low oxygen, and loss of diversity) even in periods of
low rainfall.
Myall Lake is presently oligotrophic to mesotrophic but there is an extremely high risk of
eutrophication from any increase in phosphorus loads to the lake from catchment sources. If
phosphorus load increases and clarity decreases, it is likely that the Najas will be lost and the
lakes may switch to a phytoplankton dominated system, with toxigenic blue-green algal
species occurring. Its low level of connectedness from direct catchment inputs, however,
provide it with some level of protection. High organic Carbon levels in the sediment are
speculated to be the result of decay, over long time periods, of the annual (prolific) growth of
Najas and also reflect very little delivery to the system of sediment-laden, turbid waters from
the catchment. The role of the Microcystis layer in recycling N and its potential contribution
to growth of Chroococcus requires further study.
The Mid Lakes has characteristics which are intermediate between these two systems. It has
lower nutrient loads than the Broadwater, but the presence of charophytes indicates it is also
at risk from increases in nutrients, particularly phosphorus.
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7.4. SUPPORTING THE DEVELOPMENT OF EFFECTIVE MANAGEMENT
STRATEGIES TO REDUCE CATCHMENT LOADS
Information gained from studies to investigate the ecology and nutrient status of the Myall
Lakes system has been used to formulate broad catchment based strategies and to develop
priority actions to reduce the risk of future blue-green algal blooms. The key theme from this
work is the need to adopt nutrient management activities in the catchment. The focus of these
are reducing nutrients at the source and promoting strategies to intercept and reduce
movement of nutrients into waterways.
Information from the Myall Lakes investigations suggests that there may be a long time frame
before management actions will deliver an outcome due to the potentially significant internal
nutrient loading in the system. The challenge for the community, resource management
agencies and local authorities is to be able to understand the implications of these findings
and to ensure that realistic expectations can be established.
7.4.1. Regional Strategic Focus: Lower North Coast Catchment Blueprint.
In early 2000, Catchment Management Boards (CMBs)2 were established by the NSW
government to strengthen the community-government partnerships in managing natural
resources. These catchment management boards are comprised of membership from rural
landholders, the Aboriginal community, nature conservationists, local government and state
government representatives.
The Lower North Coast Catchment Management Board (LNCCMB) area covers approx 13
200 km2 and includes the Manning, Wallis Lake, Smith Lake, Myall and Karuah catchments.
The catchment board has developed an Integrated Catchment Management Plan for the Lower
North Coast which has been "designed to provide direction and priorities for the investment
of funds and activities by our community, industry; local and state governments; to manage
and improve our natural resources" (DLWC, 2003).
The Catchment Board’s Integrated Catchment Management Plan has identified a number of
targets with regard to managing the areas terrestrial biodiversity, soil health, aquatic health,
and water quality. The Board has recognised blue-green algal blooms in the Myall lakes as a
significant natural resource management issue for the Lower North Coast and has endorsed
targeted actions to deal with the issue of nutrient management in the Myall Catchment.
2
Catchment Management Boards have now been replaced by Catchment Management Authorities.
The former LNCCMB has now been incorporated into the Hunter / Central Rivers Catchment
Management Authority (HCRCMA). While specifics of the LNCCMB Blueprint may change, the
intent will remain the same.
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A Lower North Coast Catchment Board water quality target has been established with the
goal of reducing total phosphorus levels in the Myall River (a high priority river) 3 by 10 % by
the year 2012. In establishing this target the catchment board recognised the importance of
reduction of all nutrients entering the waterways and was conversant with the data showing
correlations between total P levels and Total N levels for the Myall River. Hence the decision
for the use of total P as an indicator for nutrient management success. Monitoring programs
established to report on this target include monthly nutrient and storm event sampling to
report on catchment loads together with proposals to further develop catchment nutrient loads
as a co-operative project with CSIRO using SedNet catchment modelling programs.
Management targets, among others, identified / endorsed by the LNCCMB targeted at
improving water quality in the Myall River include;
?
100% adoption by landholders of BMPs for chicken litter use and diary effluent
management of farming lands by 2005;
?
development of off stream watering / riparian fencing / restricted access to waterways;
and
?
promotion of strategies to maintain / enhance / net gain of effective and functioning
riparian / littoral vegetation.
The LNCCMB has used the knowledge gained from the investigations into the causes of the
Myall Lakes algal blooms to endorse a general catchment wide approach for the reduction of
nutrients entering coastal waterways in all river systems in the catchment board area. The
LNCCMB has also recognised that co-operative community action is required to address the
issue of nutrient management at a catchment scale.
As such, it has endorsed priority
management actions that support the development and capacity building of community
groups and funding for implementation of Community Catchment Plans.
7.4.2. Local catchment Level : Myall Community Catchment Plan
A cooperative project was developed between the community and DIPNR to oversee the
development of a community catchment plan to tackle land degradation and water quality
issues in the Myall Lakes' catchment. The outbreak of the blue-green algal blooms in the
Myall Lake system providing the catalyst for the Plans’ development.
3
High priority rivers are those that have been determined by DIPNR and the CMB to have the most
elevated levels of phosphorus and nitrogen in the Lower North Coast catchments.
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To oversee the development of the plan a 'Catchment Planning Group' was formed, with
representatives from government agencies and a broad cross-section of the community
including cattle, dairy/chicken farmers, environmental, land-care, chamber of commerce,
tourism and the Myall Lakes Blue-green Algae Action committee representation.
The Myall Lakes Community Catchment Management Plan identifies a number of objectives
that describe long term goals for the management and/or control of the land management
issues in the catchment. In order to achieve these objectives, the Community Catchment Plan
details a number of strategies and actions.
The C&CS Monitoring Blue-green Algae project was developed with reference to the issues
identified as part of the Catchment Plans' development, and specific actions recommended by
the Plan. These include actions to support community understanding of algal issues and
importance of catchment nutrient management. The C&CS project facilitated community
information days and produced community newsletters with the focus on explaining lakes
system ecology, and factors influencing algal blooms.
7.4.3. Specific initiatives: Dairy Industry trials to investigate effectiveness of buffer
strips
Nutrient movement from improved pasture to waterways has been identified as one of the
targeted management issues as part of the Myall Lakes Community Catchment Plan and the
LNCCMB (Lower North Coast Catchment Management Board) Integrated Catchment
Strategy. Data collected as part of the C&CS project has been used in educational material
produced to support adoption of management actions.
In the Myall River Valley, dairy production is often undertaken in conjunction with poultry
production, with manure from chicken sheds spread on pastures as fertiliser. The regular
application of manure and/or fertiliser has the potential to result in movement of nutrients
from the landscape to waterways and hence contribute to eutrophication. A number of studies
have identified the benefits to catchment water quality where vegetated buffer strips, and/or
application free buffer strips are used as part of the property management regime.
The Dungog-Gloucester Dairy Development Team has commenced a study to develop local
data to report on the effectiveness of poultry manure/fertiliser application free buffer strips.
The trial has been developed to provide benchmark data that substantiates the industry
endorsed "best management practices" to reduce potential run-off from fertiliser applications.
Run-off data collected from paddocks with/without vegetated (non application) buffer strips
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identified a 25 % reduction in nutrient loading where buffer strips are used. This reiterates
that the use of buffer strips may provide a significant decrease in catchment nutrient loads.
A field day was held on one of the trial sites with summary information distributed providing
further information about the role of nutrients in estuary system health.An extract from the
information sheet produced for the Poultry Litter Field Day, organised by NSW Agriculture
and Dungog Gloucester Dairy Development team, is shown in Figure 7-3.It shows the effects
of increased nutrient loads on the functioning of a conceptualised estuary.
In healthy river and estuary systems nutrient inflows are in balanced with that of the
natural rate of processing.
Nutrient cycling in a healthy estuary system.
Increased nutrient movement from catchments can result in severe stress for rivers
and estuary systems with excess nutrients creating algal (plankton) blooms.
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Nutrient cycling in an unhealthy estuary system.
Figure 7-3 Extract from Poultry Litter Field Day Handout (Source: DIPNR,
unpublished)
7.4.4. Specific Initiatives : Upgrading of effluent management systems on the foreshores
of Myall Lakes
Myall Lakes National Park is one of the most frequently visited national parks in NSW.
Visitation generally peaks during holidays, particularly the summer and Easter holiday
periods with camping probably the most popular recreational activity within the park. For this
reason NPWS Northern Directorate developed a recreation management strategy aimed at
reserving the parks' values and attributes while providing an appropriate range of recreational
opportunities (NPWS, 2002). A major component of this strategy is the provision of minimal
facilities essential for public safety and environmental protection.
As part of the strategy toilet facilitates are provided at all major camping and picnic areas
within the park as a whole. The Myall Lakes Plan of Management (NPWS, 2002) states that
toilet facilities need to be located at camping areas with a capacity of more than five sites to
ensure that they do not impact upon groundwaters and subsequently, surface waters. The Plan
of Management also summarises the actions and guidelines for the management of the whole
park and highlights as a high priority the need to provide non-polluting toilet systems for
camping areas with a capacity of more than five sites (NPWS 2002). It goes on to also state
that visitors to small camping areas with less than five sites will be required to provide their
own self-contained toilet systems. This is to be monitored and if deemed to have unacceptable
environmental impacts toilets will be constructed.
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As a result, NPWS initiated a program of replacing all existing pit toilets along the Myall
Lakes foreshore. To date facilities have been upgraded at all major camping and picnic areas
around the Lakes and have been replaced with new Gough Hybrid toilet systems (Turner
2004). This system is a wet composting toilet and is completely sealed so no effluent can
escape into the groundwater. The system consists of a series of three holding tanks, the third
of which is pumped out as required to ensure that the system is completely isolated from the
surrounding groundwater.
As these toilets have intermittent usage throughout the year a regular maintenance program
has been implemented to ensure that the tanks are operating effectively and are pumped out
when required to prevent overflow.
In addition to the facilities managed by NPWS, a commercially operated camping area exists
within the park operating under lease between the NSW Government and the lessee for the
'Myall Shores Resort Camping/Caravan Park at Bombah Point and Ferry Service'. Under the
lease Myall Shores Eco Resort operates 132 camping /caravan sites having a maximum
occupancy of 600 people on a 14 hectares site at Bombah Point.
In accordance with detailed development controls prepared by NPWS Myall Shores Eco
Resort was required to prepare a Master Plan for Myall Shores Eco Resort, which was
adopted by NPWS in 2002. The Master Plan includes comprehensive performance objectives
and detailed standards relating to operation of the park, which includes effluent disposal.
Myall Shores Eco Resort is currently implementing the Master Plan, which includes an
upgrading of toilet facilities and effluent disposal. As part of the Myall Lakes Plan of
Management NPWS have highlighted, as a high priority, the need to ensure that the
redevelopment of the Myall Shores Eco Resort complies with the approved Master Plan and
achieves the outcomes sought by NPWS for the park as a whole.
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7.5. DEVELOPING PRO-ACTIVE ASSESSMENT PROTOCOLS TO ASSIST IN THE
MANAGEMENT OF FUTURE ALGAL BLOOMS
An early warning of the likelihood of impending blue-green algal blooms would be very
helpful in the management of human health concerns in this system. Before this is possible a
good understanding of the conditions that allow the formation and persistence of blooms is
necessary. Sampling of parameters that have been identified as being precursors of excessive
phytoplankton growth in this system could thus provide a means of predicting the likelihood
of a bloom occurring.
As was mentioned in Section 7.1 above , and described in some detail in Chapter 4, two
distinctly different types of blue-green algal blooms have occurred in the system which
were/are initiated and moderated by quite different environmental conditions.
It is clear that the 1999 – 2000 blooms of Anabaena and Microcystis that occurred in the
Broadwater, Two-Mile and Boolambayte Lakes, was enabled by a sequence of rainfall events
that rendered the Broadwater and mid lakes very fresh. The runoff events also would have
produced nutrient laden turbid water that favoured these buoyancy regulating taxa. Under
normal (average rainfall) conditions, it appears that the water nutrient concentrations are not
elevated enough, the water clarity is reasonable, and the salinity is too high for these taxa. It
seems reasonable to conclude then, that a discrete change in conditions produced a change in
the algal community to one that favoured blue-greens over algal taxa that would otherwise be
competitively superior (Chapter 4).
It appears that this change was brought about by a period of high rainfall, but rainfall events
of similar magnitude have occurred previously. As such, this alone was not responsible for a
shift in algal community towards blue-greens. It is also highly likely that the long-term
accumulation of nutrients in the sediment may also assist in fuelling blooms following an
influx of freshwater. Subsequent monitoring of runoff events has also found that heavy
downpours facilitate the transport of disproportionately large nutrient loads in comparison to
events that deliver the same total amount of rain over a longer period. So the intensity of
rainfall could also have played a part in the 1999 bloom.
Once the toxigenic bloom was established it persisted until salinity gradually became high
enough to preclude these taxa from the phytoplankton assemblage. Apart from the catchment
rehabilitation measures mentioned in Section 7.4 there are no strategies that can prevent a
bloom of this nature becoming established, nor measures that can be taken to hasten the
decline of these blooms.
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There is sound evidence that understanding the trends of changing salinity can provide an
early warning for the risk of toxigenic algal blooms which might provide some notice to the
local community of the potential loss of recreational amenity. It is therefore recommended
that long-term real time monitoring of salinity be undertaken to assist as an early warning for
the potential of a bloom occurring in the broadwater. An initial trigger of 2mS/cm or lower
conductivity would be useful initial warning system.
7.6. MYALL LAKES – WHERE TO FROM HERE?
The results of the project have identified the key characteristics, and ecological and physical
processes that contribute to the occurrence of blue-green algal blooms in Myall Lakes. This
study has provided information on the levels of temporal and spatial variation of different
parameters including water quality, nutrient concentrations, phytoplankton blooms,
macrophyte growth, and sediment characteristics. Because the project was undertaken over an
eighteen month timeframe, it effectively only represents a “snap-shot” of conditions within
the Myall Lakes. In addition, time and resource limitations has dictated that not all aspects of
the lakes biological, chemical, and physical processes could be studied in detail. As a result,
the project has also highlighted knowledge gaps that could be addressed to improve future
management of the lake and its catchment. The findings have stimulated considerable
scientific interest from other agencies, and from a number of universities.
7.6.1. Continued Monitoring in Myall Lakes and its Catchment
NPWS are committed to continue monitoring to determine the types and amount of bluegreen algae from a public health risk perspective.
The project has identified the conditions under which toxigenic algae might occur in the
lakes, and highlighted the times of year, and locations that small-celled blue-green algae
predominate. These findings have made it possible to focus monitoring to times and places
that are most likely to detect risks to public health from blue-green algae, thus ensuring a
cost-effective monitoring program. This monitoring program will consist of sampling on
reduced spatial and temporal scales to determine the composition of the phytoplankton
community, including blue-green species, and measure water quality parameters, nutrient
concentrations and water clarity.
To examine nutrient and phytoplankton responses to a rainfall event, a short-term sampling
program was initiated in December 2002 following a rainfall event of 253 mm at Bulahdelah
on the December 10th - 11th , 2002. From 12 December 2002 – 14 January 2003, an intensive
program of sampling was conducted throughout the Myall Lakes at intervals of 1, 2, 4, 6, 8,
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12, 22 and 34 days after the rain event. The results of this study will be analysed to determine
the response of phytoplankton to the event.
Sampling of nutrient loads to the Broadwater during rain events will continue to be collected
by the autosampler located at Bulahdelah. In addition, DIPNR will continue to monitor
nutrient concentrations on a monthly basis in the Upper Myall River at Bulahdelah.
7.6.2. Additional ongoing investigations
Benthic Microcystis layer in Myall and Mid-Lakes: Sediment surveys during the project
revealed the presence of a highly organic layer containing a previously unknown benthic
blue-green algal species, that is closely related to the toxin-producer Microcystis. A series of
investigations additional to the project were commissioned by DIPNR to determine the
properties, toxicity and taxonomic status of this material. Ongoing studies by the University
of Newcastle, will investigate the role of this material in releasing or absorbing nutrients to, or
from, the water column. This would assist in determining its role in fuelling algal blooms,
particularly during low rainfall periods. A number of other studies by the University of NSW
and Australian Nuclear Science and Technology Organisation (ANSTO) are directed at
investigating the age and composition of the Microcystis layer and uptake of benthic
Microcystis material into the food chain through grazing. Additional work is occuring to
identify sediment biomarkers to determine whether toxigenic algal blooms have occurred
previously in the lakes.
Phytoplankton growth and nutrient limitation: It is possible that the type of nutrient (nitrogen
or phosphorus species) which stimulates phytoplankton blooms in Myall Lakes will vary
among the three lakes, and at different times depending on the prevailing environmental
conditions. Determining the type of nutrients that stimulate phytoplankton blooms is going to
be crucial to support the nutrient reduction programs in the catchment. Studies to address this
issue will be undertaken by The University of Newcastle.
Hydrodynamics of Myall Lakes: Although there is ample evidence of the strong currents
between each lake at Bombah Point and Violet Hill, there is no quantitative information on
the amount or direction of water movement between the lakes, especially after rain events.
DIPNR has collected data that will assist in determining the dynamics of water movement
between the lakes at these locations. This information will be used to support a more detailed
project by The University of Newcastle to model the hydrodynamics of the Myall Lakes.
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7.6.3. Future Studies
There is still relatively little known about many aspects of physical and ecological processes
and characteristics important in maintaining the health of Myall Lakes. The topics below are a
suggested list of future studies that are likely to be of direct relevance to assessing the nutrient
status and risk of algal blooms in Myall Lakes in the short to medium term.
Determining sustainable loads of nutrients:
While the project has identified excess nutrients from the catchment as a cause of
eutrophication in Myall Lakes, and the Catchment Blueprint has identified nutrient reduction
strategies as a priority action, a quantitative target which will be effective to reduce algal
blooms has not been determined. Applying the “Sustainable Loads of Nutrients to Estuaries
Model” to Myall Lakes using information collected during the project would assist in
quantifying the load of nitrogen and phosphorus which can be delivered to Myall Lakes to
minimise the risk of damage to the health of the system. The information from this study
could be used to revise the nutrient target for Phosphrous established by the LNCCMB.
Health of Macrophytes and Seagrass:
One of the consequences of eutrophication in aquatic ecosystems is often a decline in the
distribution, biomass and health of macrophytes and seagrass. There is little quantitative
information on the growth and biomass of aquatic macrophytes in Myall Lakes. Due to the
high nutrient loads entering the Broadwater, this area is likely to be the most susceptible to
nutrient enrichment. A program to monitor the growth, biomass and condition of macrophyte
communities is required to assess the long-term health status of Myall Lakes and determine if
nutrient reduction strategies are effective.
Salinity modelling:
The risk of toxigenic algal blooms occurring in the Broadwater appears to be greatly
increased when above average rainfall delivers nutrients and reduces the salinity in the
Broadwater to less than 2mS/cm. Information on the volume of riverine inflow required to
cause these conditions, and how riverine inflow disperses throughout the lakes is needed to
develop a predictive model to determine the conditions which will increase the risk of future
blue-green algal blooms in Myall Lakes.
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Sediment / nutrient fluxes:
The work undertaken by the project, as well as the earlier work undertaken by AGSO, has
highlighted the potential of nutrient fluxes from the sediment as a major internal source of
nutrients, which may also fuel blooms under certain conditions. Denitrification efficiency has
been identified as a useful tool in determining the nutrient status of a number of coastal
estuary systems. Further work on sediment / nutrient fluxes within the Myall Lakes system is
required to determine the ability of the lake’s sediment to process nutrients. Such a study
would compliment the work already underway by the University of Newcastle (see earlier).
Studies of other nutrients:
Chapter 4 provided information on algal succession in Myall Lakes, with reference to other
similar studies.
Of particular interest in Myall Lakes is the role of Silica limitation in
determining algal succession and its influence on diatoms.
Groundwater:
Groundwater studies undertaken by DIPNR to determine the potential of contamination due to
onsite effluent disposal highlighted high natural background levels of nutrients in the
groundwater. At present, there is no accurate information on nutrient fluxes into and out of
the lake as a result of interactions with groundwater.
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NSW Department of Infrastructure, Planning and Natural Resources
175
9.1. APPENDIX 1. Groundwater quality data in Myall Lakes National Park. Concentrations in mg/L.
Sample ID
Site
Type
pH
42980
1
The Big Gibber Quarry
Location
Jan-77
Date
Na-Cl
Class
Groundwater
6.1
47057
2
The Big Gibber Quarry
Jan-77
Na-Cl
Groundwater
6.4
78017
3
Mungo Brush
Aug-99
Na-Cl
Groundwater
5.4
78018
4
Mungo Brush
Aug-99
Na-Mg-HCO3-Cl
Groundwater
5.4
78019
5
White Tree Bay
Aug-99
Na-Mg-Cl-HCO3
Groundwater
5.9
78020
6
Broadwater
Aug-99
Na-Cl
Groundwater
4.1
78021
7
Bombah Point
Aug-99
Na-Cl
Groundwater
78022
8
Myall Shores
Aug-99
Na-Mg-Cl
78023
9
Myall Shores
Aug-99
Na-Ca-Mg-HCO3-Cl
78024
10
Korsmans Landing
Aug-99
78036
11
Yagon
May-00
79756
12
Mungo Point
79757
13
Bombah Point
80300
14
80301
15
MD1
MD4
Water
Temperature
Conductivity
? S/cm
Na
K
Mg
Ca
200
27
1.2
3.2
1
Mn
Fe
NH 4
F
Cl
Br
180
26
1.2
3.5
1.6
357
51
2.7
7
2.3
0.25
0.14
89
0.3
17
39
5.4
0.6
1.1
0.5
0.2
0.11
7.4
16.3
416
56
3.4
12
5.3
1
0.68
95
17
121
14
0.6
1.6
0.8
0.45
0.08
4
14.6
112
13
0.3
1.2
0.9
0.4
0.09
Groundwater
5
14.9
128
17
0.7
3.8
1.4
4.9
Groundwater
6.3
17.7
97
12
1
2.8
5
2.1
Na-Cl
Groundwater
5.8
17
858
120
2.3
14
18
8.3
Na-Cl-HCO3
Groundwater
6.9
18.8
766
106
5.7
14.4
10.8
Oct-99
Na-Cl
Groundwater
5.8
1575
240
8.2
31
17
0.1
4.6
0.07
Oct-99
Na-Cl
Groundwater
6.1
195
27
1.5
3.6
3.1
0
0.55
0.21
Old Gibber Rd
Jun-00
Na-Mg-Cl
Groundwater
4
19.4
106
11
1.6
2
0.6
0.65
River Mouth
Jun-00
Na-Mg-Cl
Groundwater
3.8
19
131
11
1.6
2
0.6
0.99
16
Myall Lake area
Jul-75
Na-Cl
Groundwater
6.3
200
30
1.2
3.2
0.8
17
Myall Lake area
Jul-75
Na-Cl
Groundwater
6.8
150
20
1
2.1
0.5
MD7
18
Myall Lake area
Jul-75
Na-Cl
Groundwater
6.5
160
22
1.1
2.4
SR1
19
Seal Rocks
Sep-74
Na-Cl
Groundwater
7.2
352
48
2
A
20
Upper Myall River
Apr-78
Mg-Ca-Na-Cl-HCO3
River
7.7
20.8
270
15
B1
21
Upper Myall River
Apr-78
Ca-Na-Mg-HCO3-Cl
River
7.4
19
230
C1
22
Upper Myall River
Apr-78
Na-Ca-Cl-HCO3
River
7.2
17.8
C2
23
Little Myall River
Apr-78
Na-Ca-Cl-HCO3
River
7
L2
24
Myall River
Apr-78
Na-Cl
River
44
SO4
NO 3
NO 2
HCO 3
CO 3
P
SiO2
O2
12
0.28
13
0
0
16
0
0.015
< .2
0.8
0
0
15
0
0.025
0.5
0.3
3.4
0
0
58
0
0.05
1.1
25
< .2
0
0
0
0
0
0.055
0.8
19
< .2
0
0
0
0
0
0.07
0.85
0.1
29
< .2
0
0
0
11
0
0.035
1.4
0.1
7.6
0
4.3
0
0
36
0
0.015
0.45
0.24
220
0.6
57
0
0
54
0
0.015
55
0.01
180
0.7
28
0.3
0.24
86
0
0
0.24
0
460
1.4
0
0.02
17
0
0.015
0.5
0
47
0.2
1.4
0
0.02
17
0
0.015
0.3
0.14
1
19
0.054
0.95
0.18
0
25
0
0.6
0
0
0
0.051
0.96
52
6.7
6.03
12
48
7
0.09
10
0.8
47
6.2
0.05
10
6
2.3
82
14
0.4
19
0.7
8.5
13.8
39
13.5
0.38
60
13.7
12.5
0.5
6.3
10.9
32
12
0.04
68
15.5
230
25
3.7
5
9.2
42
10.7
53
16.7
18.2
160
18.6
3.2
2.3
11.3
31
10.8
43
19.2
6.2
21.2
5200
963
38.5
125
39.5
1849
250
5.9
21.2
510
80
8.4
10.5
5.2
135
19
35
2
6.7
2.5
65
7.5
18
43
0
0.02
0.01
0.02
Ls
25
Myall River
Apr-78
Na-Cl
River
River 1
26
Crawford R.
Feb-75
Na-Mg-Cl
River
River 2
27
Upper Myall
Feb-75
Na-Mg-Cl-HCO3
River
32
2
6.8
5
57
12.5
M
28
Myall River Broadwater
Apr-78
Na-Cl
Lake
5.7
21.6
2900
550
37.8
65
19
960
105
0.15
N
29
Lower Myall Broadwater
Apr-78
Na-Cl
Lake
6.3
21.5
5400
1080
39
126
42
1917
260
0.26
R
30
The Broadwater
Apr-78
Na-Cl
Lake
6.5
22.3
5300
1120
43
130
42.5
1967
284
0.22
21.6
34
4
34
5.2
28
40
34
0.12
7.3
34
9
0.12
39
8.4
8.3
S
31
Bombah Point
Apr-78
Na-Cl
Lake
5.3
5400
1080
44
137
43.8
1988
273
0.04
34
Mungo
32
Mungo
Jul-75
Na-Cl
Swamp
4.7
120
18
0.7
3
1
46
10
0.18
15
Neranie
33
Neranie
1974
Na-Cl
Swamp
4.5
440
70
2.5
10
2
140
24
1
15
Scientific
34
Scientific
Dec-75
Na-Cl
Swamp
4.4
100
17
0.3
2.5
0.5
40
4
0.16
10
Seal Rocks
35
Seal Rocks
Jul-74
Na-Cl
Swamp
4.8
300
38
3.5
5
2.5
90
15
0.6
18
Smith Lake
36
Smith Lake
Dec-75
Na-Cl
Swamp
4.5
140
23
1.3
2.1
1
50
3.8
0.03
7
Wooli
37
Wooli
Dec-75
Na-Cl
Swamp
4.8
190
34
0.8
3.5
1.3
60
6.6
0.02
10
X2
38
Smith's Lake
Apr-78
Na-Cl
Swamp
6.6
21.6
140
21
2.4
1.8
0.1
33
8.8
0.18
15
2.7
Z
39
Bombah Point
Apr-78
Na-Cl-SO4
Swamp
4.5
21.6
140
27
3.4
2.7
0.1
50
30
0.22
12
4
CSR1
40
Myall Lakes National
Park
Jul-75
Na-Cl
Runoff
6.7
100
16
2.8
1.7
2.5
33
0
2.1
11
Jul-75
CSR4
41
Myall Lakes National
Park
Upper Myal
42
Upper Myall
B2
43
Tank 1
N2
14.4
Na-Cl
Runoff
6.5
290
33
7
0.6
0
63
15
12.4
17
1972
Na-Cl-HCO3
Runoff
7.3
84
10
3.6
1.5
1.5
43
0
5
20
Upper Myall River
Apr-78
Na-HCO3-Cl
Rain
6.4
50
2.5
1.4
0.5
0.5
5
2.2
1.3
21
44
Smith's Lake
Feb-75
Na-Cl-HCO3
Rain
6.1
0.5
0.7
0.5
11.4
2.2
Tank 2
45
Upper Myall
Feb-75
Na-HCO3-Cl
Rain
2.6
0.5
0.45
0.8
4.7
2.4
X1
46
Smith's Lake
Apr-78
Na-Cl-HCO3
Rain
6.9
0
0.5
10
2.6
0.16
Sea
47
World's Average
Na-Cl
Sea
8.1
1283.7
412.1
2712.4
0
60
25
NSW Department of Infrastructure, Planning and Natural Resources
5.6
10783.8
399.1
0
6.8
19352.9
67.2
0.8
18
17
11
0
107
16.1
176
9.2. APPENDIX 2
9.2.1. Theoretical assessment of toilet facilities nutrient contribution
In the assessment of the potential for tourism facilities to contribute nutrient loads to the
system, two scenarios were considered. The first is a ‘worse case’ scenario, which assumes
that all human-generated effluent, is disposed directly to the lakes. All effluent produced at
Myall Shores resort and additional waste produced by visitors to the national park were
incorporated in a direct waste input scenario. The second is a more realistic assessment based
on consideration of septic system design and incorporating a ground water contribution.
9.2.2. Assessment of tourist facilities contribution (‘worse case’).
9.2.1.1. Myall Lake National Park.
Nutrient contribution from human waste as per ‘worse case’ scenario.
Annual Park Visitors numbers 52 195 adults (average 143 day ),
Waste produced - every adult visitor’s waste directly in lake = 454 kg/N/ year; 62
kg/P/year.
With estimates derived from US EPA data. Toilet waste per adult 8.7 g N; 1.2 g P.
9.2.1.2. Myall Shores ‘Ecotourism’ resort.
Annual Occupancy assuming 400 person a day * (40 % occupancy),
146 000 visitors year, plus washing, showering et c. if directly released into lake,
Estimated nutrient generation per person of 11.2 g/N; 4 g/P per day (US EPA, 1980). The
visitors to Myall Shores are counted here as additional visitors to the National park area.
Table 9.1. ‘Worse case’ sewage input and waste-water discharge scenario.
Site
Myall Shores
Myall Lakes National Park
Total direct sewage
Nitrogen kg / yr
1635
454
2089
Phosphorus kg/yr
584
62
646
Table 9.2. Comparison of ‘worse case’ input estimates with catchment loads.
NSW Department of Infrastructure, Planning and Natural Resources
177
Site
Total direct waste input
Estimate of catchment load
Nitrogen kg / yr
2089
138400
Phosphorus kg/yr
646
16040
9.2.1.3.More realistic assessment of potential tourist facilities contribution
Investigation of ground water around NPWS pit toilet sites and the Myall Shores transpiration
trenches provide a more representative estimate of nutrient contribution from these sources.
These estimates were developed using high estimates of grey water production per site.
Upper estimates of grey-water produced per park visitor per day for campsites were
calculated at 51.7 L/person/day and 88 L/person/day at Myall Shores Resort area (US EPA,
1980). The following table provides an estimate of the nutrient load from facilities use in the
Myall Shores resort and Myall Lakes National Park.
Table 9.3. Estimation of potential annual nutrient load from tourism facilities.
Facilities Location
Myall Shores
Total Nitrogen (kg)
Total Phosphorus (kg )
Ave. Daily
Total Peak
Annual
Ave. Daily Total Peak
6.8
91.52
2345.1*
0.23
3.1
Annual
79.1
NPWS Facilities
Broadwater
0.6
81.8
278.2
0.02
2.7
9
Mungo Brush
0.2
53.2
128.5
0.02
1.6
11.3
White Tree Bay
0.3
83.1
164.7
0.03
4.9
15.8
Korsmans Landing
0.1
30.8
73.9
0.02
4.3
10.3
Total NPWS
?
?
?
Total Tourist
Facilities
645.3
340.42
2990.4
46.4
16.6
125.5
Note 1. The Groundwater system is naturally high in Nitrogen and it is considered that measured nitrogen is a
combination of groundwater plus nutrient seepage from trench (Chapter 2).
Note 2. No faecal coliforms were detected in any ground water samples (Chapter 2).
Note 3. The AGSO Biomarker studies also found no trace of human sewerage contamination in lake
sediments.
NSW Department of Infrastructure, Planning and Natural Resources
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