ENVIRONMENT REPORT
DROUGHT AND RIVER HEALTH IN VICTORIA
Publication 1171 December 2007
1.
INTRODUCTION
Drought is a natural feature of the Australian climate
and has been critical in shaping our unique aquatic
ecosystems1,2. Current climate change models forecast
that droughts will occur more frequently in southeastern Australia 3. Stream flows may be severely
reduced, particularly in Victoria’s western catchments
within the next 30 years. This will place further stress
on our already scarce freshwater resources.
Despite concern over drought impacts in Australia 4,5,
there have been few broad-scale investigations of the
effects of drought on aquatic ecosystems6. We need to
better understand the ways that drought affects river
health in Victoria. The current drought commenced in
1998 and provides an opportunity to see how river
health may be affected under the forecast drier
climate.
Since 1990, EPA Victoria has conducted biological
monitoring of aquatic macroinvertebrates at streams
statewide. This data, collected both prior to and during
the drought, enables a before–after–control–impact
(BACI) assessment of the effects of drought on river
health. This report examines:
•
•
•
how drought conditions have affected water
quality and river health in different biological
areas of Victoria from 1998 to 2004
which macroinvertebrate fauna respond to
drought conditions
whether the current rapid bioassesment (RBA)
methods are suitable for detecting drought
impacts.
Figure 1: The dry bed of Natimuk Creek in the
Wimmera Catchment, 2004
2.
DROUGHT IN VICTORIA
Victoria has experienced more than eight severe
droughts since rainfall records began in the late 1880s.
In some cases, severe droughts affected areas of
Victoria for more than 10 years (such as in 1958—
1968)7.
During the current drought, many parts of Victoria
have suffered severe rainfall deficiencies (as defined
by the Bureau of Meteorology, BOM8) at some point in
time, with certain areas affected as early as 1996. Most
areas, however, periodically moved in and out of
drought between 1996 and 2004. Severe rainfall
deficiencies were most extensive during 1999 and
2002, while 2000 and 2001 were slightly better
rainfall years. Areas under serious and severe rainfall
deficiency for one year or more from 1998 to 2004 are
shown in Figure 2. Dry stream sites that EPA visited
are also shown, to highlight the extent of the drought
across the state during this period.
While data from the recent years of the drought,
2005—07, are not included in this study, observations
from sampling indicate a significant increase in the
number of sites that were dry. In the 2006—07
assessment period about 25 to 30 per cent of the sites
visited were dry, compared to less than five per cent in
previous years. A dry site was one at which there was
insufficient water to gather a sample. Considerably
higher rates of dry sites were observed in some
individual catchments, particularly in the west and
north of the state.
3.
GENERAL IMPACTS OF DROUGHT ON
RIVER HEALTH
Factors associated with drought, such as reduced flow,
isolation of pools and fringing habitat, and degraded
water quality, are known to alter aquatic faunal
communities4. Under natural conditions, Australian
stream animals have adapted to become resistant
(able to survive dry periods) and/or are resilient (have
efficient recovery mechanisms) to drought. While the
effects of seasonal droughts on water quality and river
health have been documented, there have been few
investigations of macroinvertebrate response during
prolonged and widespread droughts6, and none carried
out at this scale. Macroinvertebrates are found in all
streams, occur under a range of conditions and are the
most commonly used biological indicator of river
health.
1
DROUGHT AND RIVER HEALTH IN VICTORIA
Figure 2: Drought-affected areas and dry stream sites across Victoria, 1998—2004
As streams dry up, aquatic linkages are lost. In the
early phase of a drought, the stream becomes laterally
disconnected from the streamside zone and important
habitats, such as edge water-plants, are lost (Figure 3).
Reduced flows and flow types (such as eddies and
cascades) also decrease habitat for some fauna that
require flow. Slight changes to water quality can also
be expected, including increased temperature and
reduced turbidity (suspended particles) due to reduced
water turbulence and run-off4.
Figure 3: Reduced flows in the Avon River in the
Wimmera Catchment, 2002
2
More abrupt ecological and water chemistry changes
occur as droughts persist and streams become
longitudinally fragmented into a chain of isolated pools
(Figure 4). Dissolved oxygen levels decrease due to
reduced mixing, and decomposing leaf litter causes the
pH to decrease as organic acids form4. Reduced water
volumes can cause large increases in the
concentration of nutrients (dissolved nitrogen and
phosphorus) and salts4. As riffles (rapid and turbulent
Figure 4: Pool isolation at Riley’s Creek in the
Moorabool Catchment, 2004
DROUGHT AND RIVER HEALTH IN VICTORIA
flow areas) dry, some macroinvertebrate fauna (such
as caddis flies and freshwater mussels) become
trapped and die, while others (like backswimmers)
migrate deeper into the sediment or fly to remaining
pools.
In the final stages of drought, surface water may dry
up completely (Figure 5). Contracting, shallow, warm
pools concentrate nutrients, leachate and organic
matter, providing suitable conditions for algal blooms.
These can kill all but the most tolerant
macroinvertebrates. Underground flow may cease and
trees and shrubs can invade the channel.
Macroinvertebrates then rely on adaptations, such as
drying-resistant egg masses and short life cycles, to
recolonise the stream once the drought breaks (for
example, flatworms, some types of caddisfly) 9.
study uses reference sites only, as these have been
repeatedly sampled since 1990, allowing for a paired
approach to data analyses. Use of reference sites alone
also attempts to separate drought impacts from other
land use impacts. Sites were assigned to two groups: ‘in
drought’ or ‘not in drought’, according to the BOM
classification. The ‘not in drought’ sites are the control
sites and the ‘in drought’ sites are the impacted sites.
Selected sites for this study are shown in Figure 6.
Stream sites were sampled using standard Rapid
Bioassessment (RBA) techniques (EPA publication
604.1). This involves collection, identification and
analysis of macroinvertebrates, as an indicator of
stream ecological health. Macroinvertebrates are
sampled from edge and riffle habitats in autumn and
spring, and identified to family level. In total, this study
used data from 532 combined-season edge samples
and 318 combined-season riffle samples from the
selected sites, collected between 1990 and 2004.
Victoria is divided into five biological regions (Figure
6), delineated primarily by having similar aquatic
macroinvertebrate communities. Each region has
established biological objectives based on up to four
indices. These vary between the regions due to:
•
Figure 5: The Avon River in the Wimmera Basin,
2002
4.
ASSESSING DROUGHT EFFECTS –
APPROACH
Drought can be defined in many ways, depending on
individual perceptions (for example, by stream flows,
soil moisture levels, reduced agricultural
productivity)10. For this investigation, stream sites
were defined as ‘in drought’ if they had a 12-month
period of serious or severe rainfall deficiency a
between 1998 and 2004, using GIS layers provided by
the BOM. Given that, in some areas, 1996 and 1997
were above-average rainfall years, 1998 was
conservatively defined as the ‘beginning of the
drought’.
Under the statewide biological monitoring program,
EPA samples two main types of stream sites — test sites
(often human-impacted and only sampled for one year)
and reference sites (those in essentially natural
conditions, minimally impacted or best available). This
a
The BOM defines ‘serious’ rainfall deficiency as being between the lowest 5
and 10% of rainfall on record for a three-month period, while ‘severe’
rainfall deficiencies are defined as being below the lowest 5% of rainfall
on record for a three month period.
natural variations expected in aquatic
ecosystems as a result of differences in factors
such as topography and geomorphology
• recognition of certain irreversible changes to
regions, such as clearing and draining for
agriculture.
Streams in the highlands region (B1) and Mallee region
(no bioregion) were not included in the analysis
because of low numbers of sites classified as ‘in
drought’ or sampled, respectively.
Biological indices are used to assess river health based
on the presence or absence of certain families for
combined autumn and spring samples. These indices
are as follows:
•
•
•
•
SIGNAL (Stream Invertebrate Grade Number —
Average Level): assigns a score to each family
present in a sample based on its tolerance to
water pollution. These scores are then
averaged for all families collected at a site.
EPT: the number of families of three sensitive
orders of macroinvertebrates; Ephemeroptera
(mayflies), Plecoptera (stoneflies) and
Trichoptera (caddisflies).
Number of families: the number of
macroinvertebrate families at a site, providing a
simple measure of diversity.
AUSRIVAS score: derived from a predictive
model, which compares the fauna at a site to
those from a similar site in reference condition.
The AUSRIVAS score (‘O/E’) is the ratio of
observed taxa (those actually collected) to
expected taxa (those predicted to occur by the
model).
DROUGHT AND RIVER HEALTH IN VICTORIA
Figure 6: Location of selected EPA control and impact biological monitoring sites
This study compares the rate of attainment of these
objectives before and during the drought, and the size
of the change for each biological index. It also looks at
which macroinvertebrate families are associated with
these changes by calculating how often they occur in
samples before and during the drought.
Spot water quality measurements (dissolved oxygen,
pH, alkalinity, water temperature, turbidity, electrical
conductivity, nitrate/nitrite, total nitrogen and total
phosphorus) were also collected during RBA sampling.
These measures were examined before and during
drought to characterise water quality changes
associated with low flows.
4
5.
RESULTS
5.1 Water quality
At the statewide scale, streams experienced
significant dissolved oxygen (DO) changes. DO
concentrations were distinctly more variable, and also
lower, during the drought (Figure 7). This suggests
that drought conditions may have led to increased
macrophyte or macro-algal growth, which can cause
large daily fluctuations in DO due to changes in
amounts of photosynthetic activity. These changes
may also have ramifications for the availability of food
sources and habitat for macroinvertebrate
communities. While electrical conductivity (EC), a
measure of salinity, was not found to be significantly
greater overall during drought, some streams did
experience markedly higher EC, possibly due to
increased contributions of saline groundwater and/or
increased evaporation of surface waters.
DROUGHT AND RIVER HEALTH IN VICTORIA
increases in alkalinity and electrical conductivity,
particularly in the Wimmera and Goulburn catchments.
13
12
5.2 Macroinvertebrates
DO (Mg/L)
11
10
9
8
7
6
5
Before
drought
During
drought
Figure 7: Box plot of DO concentrations before and
during drought
Turbidity levels were significantly lower and less
variable during drought years (Figure 8). This is most
likely due to decreased flow allowing particulate
matter to fall out of suspension, and fewer rainfall-runoff events that introduce sediment load to the stream.
These changes may provide increased clay-silt habitat
for the more tolerant macroinvertebrates, and
potentially fill spaces between stones in the stream
substrate, which are required by some sensitive
invertebrate families.
Edge habitat
Sites in the lower altitude forests (B3) and cleared hills
and coastal plains regions (B4) experienced the most
significant reductions across AUSRIVAS, SIGNAL and
EPT indices, with B4 sites also losing an average of 1.4
families per site (Table 1). In contrast, the upland
forests region (B2) experienced a small but significant
increase in EPT and family richness scores, while
AUSRIVAS and SIGNAL did not significantly differ due
to drought. The Murray and western plains region (B5)
sites did not show any significant changes for any
index.
Table 1: Mean differences between pre-drought and
drought index scores for the edge habitat. Values in
italics denote a statistically significant change
30
Turbidity (NTU)
No large drought-related changes to any biological
index, in either riffle or edge habitat, were detected at
impact sites that were not also detected in the control
sites. This indicates that ‘control’ sites (‘not in
drought’) were being affected by some factor, which
may have been reduced flow conditions, irrespective
of whether they were formally categorised as being ‘in
drought’. However, significant changes were detected
between the pre-drought and drought periods. As a
result of the inability to distinguish drought exclusively
from other disturbances at ‘control’ sites, the
remaining discussion focuses on ‘impact’ sites only.
R E G IO N
AU SR IV AS
S IG NAL
RIC H NE SS
EPT
B 2 - Forests A
-0.02
-0.01
0.87
0.3
B 3 - Forests B
-0.04
-0.08
0.3
-0.78
-0.02
-0.05
-1.41
-0.48
-0.01
-0.03
0.11
-0.19
20
10
0
Before
drought
During
drought
Figure 8: Box plot of turbidity levels before and
during drought
Water quality changes, as a response to low flow
conditions, appeared to differ between catchments. A
number of sites in the Kiewa catchment experienced
low DO in 2002 compared with background levels.
Similarly, some sites in the Otways and South
Gippsland catchments experienced similar reductions
in DO during the 1999 drought period. In some areas of
the state, low flow conditions led to considerable
B 4 - H ills and
coastal plains
B 5 - M urray
and W estern
P lains
Figure 9 shows the proportion of sites that did not
meet the State environment protection policy (Waters
of Victoria) (SEPP) biological objectives. SIGNAL and
AUSRIVAS indices had almost double the attainment
rate before the drought, compared with during the
drought. Attainment rates for family richness also
decreased for edge samples during drought.
Attainment against the EPT index was generally high
and remained relatively unchanged between predrought and drought periods. SEPP objectives for
EPTs are only specified for the Highlands and the two
forest regions, where drought impacts may be less
apparent than elsewhere. This may explain the lack of
response in this index.
No significant differences were detected in lower
altitude forests (B3).
40
35
30
Predrought
25
20
During
drought
15
10
5
0
RICHNESS
SIGNAL
EPT
AUSRIVAS
Figure 9: Percentage of edge habitat sites that did
not meet SEPP objectives before and during
drought.
In the edge habitat, fauna that require flow, including
Coloburiscidae mayflies and Simuliidae (filter feeding
black fly larvae) were encountered less frequently
during drought (by 51 and 28 per cent, respectively).
Some caddisflies (Odontoceridae, Helicophidae,
Hydropsychidae and Hydrobiosidae) were also less
prevalent (occurring at least 25 per cent less
frequently during drought). These relatively pollutionsensitive taxa were replaced by more tolerant taxa,
including Veliidae and Gerridae (predators that require
slow-moving or still water), Culicidae (mosquito larva
requiring stagnant or low flow conditions and tolerant
of low DO) and Stratiomyidae (fly larvae tolerant to
organic pollution). Drought conditions also favoured
detritovores including Dugesiidae (flatworms),
Atriplectididae (caddisfly larvae) and Corduliidae-like
dragonflies, as well as algal grazers such as
Planorbidae (snail) and Helicopsychidae (caddisfly
larvae). These families occurred more than 25 per cent
more frequently during drought.
Overall, taxa requiring flow were replaced with taxa
that prefer pool environments, and tolerant taxa
replaced pollution-sensitive taxa. While reduced water
quality (particularly lower DO) may have partly
contributed to this, changes were more likely a result
of altered habitat conditions during early phases of the
drought. Major changes can be explained by reduced
flow and a shift in the food sources available to
invertebrates (inputs from the streamside zone being
replaced by production from algae and water plants).
Riffle habitat
As with edge samples, the upland forests region (B2)
experienced a small but significant increase in number
of families (Table 2). SIGNAL and EPT scores were
significantly reduced during drought in the cleared
hills and coastal plains region (B4) although, similarly
to the edge samples, mean differences were not large.
6
Table 2. Mean differences between pre and during
drought index scores for the riffle habitat. Values in
italics denote a significant change.
R E G IO N
OE50
S IG N A L
R IC H N E S S
EPT
B 2 - F o re s ts
A
0 .0 0
-0 .0 1
0 .8 0
-0 .0 3
B 3 - F o re s ts
B
-0 .0 1
0 .0 0
-0 .1 4
0 .0 7
B 4 - H ills
a n d c o a s ta l
p la in s
-0 .0 3
-0 .0 3
-0 .7 9
-0 .3 7
There were few changes in attainment rates for riffle
habitat indices before and during drought for impact
sites (Figure 10). In fact, there were slight increases in
the percentage of sites meeting objectives for family
richness and AUSRIVAS scores during the drought.
% of sites that did not meet objectives
% of sites that did not meet objectives
DROUGHT AND RIVER HEALTH IN VICTORIA
40
35
30
Predrought
25
20
During
drought
15
10
5
0
RICHNESS
SIGNAL
EPT
AUSRIVAS
Figure 10: Percentage of riffle habitat sites that did
not meet SEPP objectives before and during
drought.
Only two families in the riffle habitat were less
frequently observed during drought. These were
Empididae (predators requiring fast-flowing waters)
and Helicophidae, a caddisfly family. These relatively
rare taxa would likely have little effect on changes to
overall community structure.
Many taxa benefited from reduced flows in riffles,
including caddisflies (Calamoceratidae,
Philorheithridae, Philopotamidae), lacewings
(Neurorthidae) and midge larvae in the subfamily
Podonominae. The caddisflies occurred more than 27
per cent more frequently during drought, while
Neurorthidae and Podonominae occurred 84 and 35
per cent more frequently, respectively.
DROUGHT AND RIVER HEALTH IN VICTORIA
While having preferences for lower flow conditions
these taxa are still quite pollution-sensitive. The
increased prevalence of these taxa, without significant
reductions in others, would explain the slight increases
observed for riffles in some indices during drought.
Increased available habitat created by macrophytes
and algal growth in riffle areas could potentially
outweigh effects of reduced flow during the early
phases of drought. Additionally, water chemistry
changes are likely to be minimal where streams still
maintain some flow and would have little influence on
macroinvertebrate communities, compared with
habitat changes.
Bioregional differences
Indices in the upland forests (B2) showed little effect
of the drought in edge or riffle habitats. These streams
are generally higher rainfall areas and at altitudes
greater than 400 m. Even during rainfall-deficient
years, total rainfall and snowmelt may be sufficient to
provide base flow to many of these stream types.
Furthermore, where riffles are still flowing, fast flow
may still be present due to the higher stream
gradients.
Indices for sites in the Murray and western plains (B5;
only assessed in the edge habitat) also experienced
minimal change due to the drought. This may be
because the taxa inhabiting these streams are better
adapted to more arid conditions. Also, many reference
sites in this region, particularly in large, lowland
streams, are subject to some human disturbances
(such as grazing impacts) and are not ideal reference
sites. As a result, some sensitive taxa may already be
absent from these systems, masking potential
drought-associated effects.
The lower-altitude forests and cleared hills, and
coastal plains bioregions (B3 and B4) experienced the
greatest declines in index values during drought.
These declines were most evident in edge habitats.
Drought and the Rapid Bioassessment Framework
The RBA methodology appears to be robust for the
riffle sampling of reference quality sites during
drought periods. The minimal change experienced by
biological indices in the riffle habitat indicates that
sites not meeting SEPP objectives during drought
periods should not be directly attributed to the
drought. However, the RBA protocol prevents
sampling of riffle habitats in the advanced stages of
drying out (sampling cannot be undertaken where
there is insufficient water), and prolonged drought
may have long-term negative effects on
macroinvertebrate community structure, system
resilience and recovery.
Where sufficient water exists to allow a riffle sample to
be taken, it’s likely a healthy macroinvertebrate
community will be found, assuming other
environmental conditions are suitable.
In contrast, edge habitat indices in reference streams
were depressed by drought conditions and there is
evidence that drought increases the likelihood of a site
not meeting SEPP objectives. This is particularly
important for AUSRIVAS and SIGNAL scores. If
reference sites are not meeting objectives more often
during drought, the possibility of drought impact
needs to be considered, in addition to human-induced
disturbances, in determining the cause where
objectives are not met at other sites.
Natural resource managers should interpret RBA
assessments during drought with caution, particularly
where summary indices of condition are developed for
large areas (such as at the catchment level). The
apparent lack of drought impact observed in some
cases in this study may simply be due to the focus on
those sites with water remaining.
DROUGHT AND RIVER HEALTH IN VICTORIA
6.
SUMMARY AND FUTURE DIRECTIONS
Overall, this study found a range of responses in river
health to drought. Decreases for some indices were
observed across sites ‘in drought’ as well as those
defined as ‘not in drought’. Where riffles persisted, the
macroinvertebrates indicated a healthy ecosystem but
this ignores the streams where riffles were dry due to
a lack of flowing water. Pools, although containing
many species tolerant of little or no flow, more
commonly indicated an impact due to drought. We
cannot exclude other stressors impacting some
streams, or natural variation contributing to these
results, but the likely major cause of the observed
changes was the reduced flow conditions prevailing
across the state during the drought period.
As streams become isolated pools, macroinvertebrate
diversity has been shown to decrease and
communities are eventually eliminated. However, little
is known about the recovery rate of these
communities in historically permanent streams that
dry up completely during drought6. Given this current
lack of knowledge, it is important that some refugia
(for example, deep pools, macrophyte beds) are
maintained throughout drought to aid dispersal and
recolonisation once the drought breaks.
The critical thresholds that exist before a stream
ecosystem changes irreversibly have not been well
studied4. An understanding of this would be essential
to inform managers how best to maintain refugia.
While this study has focused on reference sites, there
has been a little work done to understand the way that
drought affects streams that have been humanimpacted6,10. In such streams, poor water quality is
likely to play a more significant role in degrading
aquatic communities. EPA is currently working with
RMIT University to increase understanding of
sedimentation and salinity impacts on
macroinvertebrate communities, which may be
exacerbated under drought conditions.
The current RBA methods provide variable indications
of drought effects on river health and are biased to
those sites where water remains. There is potential
that family-level data, or the types of indices used, are
not sensitive enough to detect impacts of drought and
climate change, and further work is needed in this
area. This may involve the use of species-level or
quantitative approaches and/or development of
drought-specific indices based on drought-resistant
traits of certain macroinvertebrates.
8
7.
REFERENCES
1.
McMahon TC & Finlayson BL (2003). Droughts
and anti-droughts: the low flow hydrology of
Australian rivers, Freshwater Biology, 48: 1147—
1160.
2. Thoms M & Sheldon F (2006). Relationships
between flow variability and macroinvertebrate
assemblage composition: data from four
Australian Dryland rivers, 22: 219—238.
3. Jones RN & Durak PJ (2005). Estimating
Climate Change on Victoria’s Runoff Using a
Hydrological Sensitivity Model, Victorian
Department of Sustainability and Environment,
www.greenhouse.vic.gov.au/CSIRO%20Report
%20-%20Runoff.pdf (accessed 21/3/2006).
4. Lake PS (2003). Ecological effects of
perturbation by drought in flowing waters,
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5. Whetton P (1989). A synoptic climatological
analysis of rainfall variability in southeastern
Australia, Journal of Climatology, 8: 155-177.
6. Boulton AJ (2003). Parallels and contrasts in
the effects of drought on macroinvertebrate
assemblages, Freshwater Biology, 48: 1173-1185.
7. Nathan RJ & Weinmann PE (1993). Low Flows
Atlas for Victorian Streams, Department of
Conservation and Natural Resources, Victoria,
Australia.
8. Bureau of Meteorology (BOM) (2007). Drought
Statement,
www.bom.gov.au/announcements/media_relea
ses/climate/drought/20061103.shtml, (accessed
30/3/2007).
9. Gooderham J & Tsyrlin E (2002). The Waterbug
Book. A Guide to the Freshwater
Macroinvertebrates of Temperate Australia,
CSIRO Publishing, Victoria, Australia.
10. Humphries P & Baldwin DS (2003). Drought and
Aquatic ecosystems: an introduction,
Freshwater Biology, 48: 141-1146
11. EPA Victoria (2003). Guideline for
Environmental Management: Rapid
bioassessment methodology for rivers and
streams, EPA Victoria, Melbourne, Australia
(EPA publication 604).