The University of Western Australia
Department of Environmental Engineering
Oxygen Depletion in
Lower Serpentine River
Kate Roehner
Honours Dissertation
Supervised by:
Keith Smettem, Anas Ghadouani
2005
Acknowledgements
I would like to thank the following people for all their support and efforts in helping to
make this study possible. Firstly, thankyou to both Associate Professor Keith
Smettem and Dr. Anas Ghadouani, my supervisors at the Centre for Water Research
(CWR). I would like to thank both of you for you time and effort, and enthusiasm in
helping me with this project.
I would like to thank Thelma Crooke, James Mackintosh, and Jeanette Bray from the
Department of Environment (DoE) for the data that they provided and their
willingness to help. I would also like to thank Brian Kowald from the Bureau of
Meterology with his help in providing data.
I would like to express my appreciation to Dianne Krikke from CWR and Michael
Smirk from the Department of Agriculture for their help with laboratory work and
analysing samples.
Lastly, I would like to acknowledge and express my great appreciation to my family
and friends for their love and support throughout the duration of this project. Without
you encouragement this would never have been possible, thankyou.
i
Oxygen Depletion in the Lower Serpentine River
Abstract
The Serpentine River is one of the three major rivers that drain into the Peel-Harvey
estuary, which is located about 75km south of Perth. The lower Serpentine River can
be regarded as an estuary; it consists of freshwater inflow from the upper Serpentine
River mixing with inflowing ocean water from the Peel-Harvey estuary. The upper
Serpentine River does not flow perennially, therefore the water in the lower
Serpentine is fresh to brackish during the winter months and has salinity close to that
of seawater during the summer months. The lower Serpentine River has experienced
a problem with fish kills since the late 1990’s. The cause of these fish kills is a low
concentration of dissolved oxygen in the water due to a number of factors. This
project is aimed at determining the factors affecting the dissolved oxygen
concentrations in the lower Serpentine River, and determining how these factors
affect the oxygen levels.
Historical data was collected for temperature, phosphorus input, rainfall and flow
within the lower Serpentine. Sampling was also conducted in the river to determine
the physical, chemical and biological water quality parameters. The current status of
the river was compared with the past data to find the most probable cause for the low
dissolved oxygen levels.
The results indicated that it is a combination of factors affecting the dissolved oxygen
levels within the Serpentine River. As the river is shallow, the temperature of the
water increases in summer, which both reduces the solubility of oxygen in the water
and increases respiration. Due to the construction of the Dawesville Channel in 1994,
which opened the southern section of the Peel-Harvey estuary to the ocean,
stratification in the lower Serpentine now appears earlier in summer and remains for
a longer period of time. The water is dark in colour, due to the high concentration of
dissolved organic carbon, therefore limiting photosynthesis to the upper layer.
Respiration occurring within the sediments causes an uptake of dissolved oxygen
and due to stratification, no vertical mixing occurs to re-oxygenate these bottom
waters. Phosphate concentrations within the river water and sediment were found to
be high and the anoxic conditions within the water cause the release of phosphorus
from the sediments. This phosphorus can be used as a food source for bacteria
within the sediments and for marine species of non-photosynthetic algae causing
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Kate Roehner
increased respiration, therefore higher oxygen uptake creating anoxic conditions
within the system, causing the death of many fish species within the river.
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Oxygen Depletion in the Lower Serpentine River
Glossary of Terms
Anoxia: An absence of dissolved oxygen within a water body.
Autochthonous: Derived from within a system, such as organic matter in a stream
resulting from photosynthesis by aquatic plants.
Autotroph: An organism that is capable of synthesising its own food from inorganic
substances, using light in the case of photosynthesis.
Chromophore: The segment of a molecule responsible for its colour.
Decomposition: Break down or decay of organic materials.
Epilimnion: The water overlying the thermocline of a lake.
Estuary: A section of water where a river meets the ocean, therefore the fresh river
water interacts with the saline ocean currents.
Euphotic Zone: Surface layer where sufficient light is available for photosynthesis.
Eutrophic: Well-nourished, excess of nutrients.
Eutrophication: The process where a water body enriched in nutrients which
promotes excessive growth of aquatic plants, particularly algae.
Heterotroph: An organism that requires carbon compounds from other plant or
animal sources and cannot synthesise them itself.
Hypoxia: A low level of dissolved oxygen within a water body, usually less than
2mg/L.
Mesotrophic: Having a moderate amount of dissolved nutrients.
Microtidal: A small tidal range.
Oligotrophic: Deficient in plant nutrients, having an abundance of dissolved oxygen.
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Kate Roehner
Phototrophs: Organisms that use photosynthesis to produce energy.
Saturation or Equilibrium Concentration: Amount of dissolved oxygen that can be
held by water in equilibrium with the atmosphere at a particular temperature,
pressure and salinity.
Soluble Reactive Phosphorus: The phosphorus that is dissolved within the water
column and is reactive.
Super-saturation: An observed concentration higher than saturation concentration.
Thermocline: The layer in a body of water in which there is a sharp temperature
gradient.
Total Dissolved Phosphorus: The amount of inorganic and organic phosphorus
dissolved in the water body. Dissolved fractions are taken as those that pass through
standard filters that have a 0.45µm cut-off.
Total phosphorus: Total dissolved phosphorus plus the particulate phosphorus
present in the water.
Turbidity: A measure of the sediment or foreign particles that has been stirred up
and reduces the clarity of the water and causes light limitation.
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Oxygen Depletion in the Lower Serpentine River
Table of Contents
ACKNOWLEDGEMENTS ................................................................................I
ABSTRACT .....................................................................................................II
GLOSSARY OF TERMS ............................................................................... IV
TABLE OF CONTENTS ................................................................................ VI
LIST OF APPENDICES................................................................................. IX
LIST OF FIGURES ......................................................................................... X
LIST OF TABLES......................................................................................... XII
1
INTRODUCTION ......................................................................................1
2
LITERATURE REVIEW ............................................................................3
2.1
Dissolved Oxygen in Water Bodies .........................................................................................3
2.1.1
Importance of Dissolved Oxygen for Ecology ......................................................................3
2.1.2
Hypoxia and Anoxia in Water Bodies ...................................................................................4
2.1.3
Respiration ............................................................................................................................5
2.1.3.1
Effects of an aquatic environment on respiration.........................................................5
2.1.4
Photosynthesis .......................................................................................................................6
2.1.4.1
Effects of a water environment on photosynthesis.......................................................7
2.2
Dissolved Organic Carbon.......................................................................................................9
2.3
Oxidation and Reduction.......................................................................................................10
2.4
Depletion of Oxygen in Estuaries..........................................................................................11
2.4.1
Increased Temperature ........................................................................................................11
2.4.2
Estuaries and Stratification..................................................................................................12
2.4.3
Phosphorus Cycling.............................................................................................................14
2.4.4
Phosphorus in Estuaries and turbidity maximum ................................................................14
2.4.4.1
Release of Phosphorus from Sediments.....................................................................15
2.4.5
Sediment Oxygen Demand..................................................................................................17
2.5
3
3.1
4
4.1
Eutrophication and Algal Blooms in Waterways ................................................................18
BACKGROUND .....................................................................................21
Peel-Harvey Estuary and the Lower Serpentine River.......................................................21
METHODOLOGY ...................................................................................28
Selection of Sampling Sites ....................................................................................................28
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Kate Roehner
4.2
Water Sampling......................................................................................................................28
4.3
Analysis of Water Samples ....................................................................................................32
4.3.1
Chlorophyll a.......................................................................................................................32
4.3.2
Unfiltered and Filtered Phosphorus.....................................................................................34
4.3.3
Total Organic Carbon (TOC) ..............................................................................................35
4.4
Sediment Sampling.................................................................................................................35
4.5
Analysis of Sediment Samples ...............................................................................................36
4.5.1
Particle Size Analysis ..........................................................................................................36
4.5.2
Soluble Phosphorus Desorption ..........................................................................................37
4.5.3
Sediment Uptake of Phosphorus..........................................................................................37
4.5.4
Carbon .................................................................................................................................38
5
RESULTS ...............................................................................................39
5.1
Historical Data........................................................................................................................39
5.1.1
Temperature ........................................................................................................................39
5.1.2
Rainfall ................................................................................................................................42
5.1.3
Flow.....................................................................................................................................43
5.1.4
Serpentine Dam ...................................................................................................................45
5.2
Water Quality Parameters ....................................................................................................47
5.2.1
Physical Parameters.............................................................................................................47
5.2.1.1
Water Temperature ....................................................................................................47
5.2.1.2
Total Dissolved Salts (Salinity) .................................................................................47
5.2.1.3
pH ..............................................................................................................................47
5.2.1.4
Light Attenuation.......................................................................................................48
5.2.1.5
Dissolved Oxygen......................................................................................................48
5.2.2
Biological Parameters..........................................................................................................49
5.2.2.1
Chlorophyll a .............................................................................................................49
5.2.3
Chemical Parameters ...........................................................................................................51
5.2.3.1
Total Phosphorus .......................................................................................................51
5.2.3.2
Soluble Phosphorus....................................................................................................53
5.2.3.3
Total Organic Carbon ................................................................................................53
5.3
Sediment Samples...................................................................................................................54
5.3.1
Particle Size.........................................................................................................................54
5.3.2
Soluble Phosphorus Desorption ..........................................................................................56
5.3.3
Uptake of Soluble Phosphorus by Sediment .......................................................................56
5.3.4
Carbon .................................................................................................................................57
6
6.1
DISCUSSION .........................................................................................59
Land Use and Nutrients.........................................................................................................59
6.2
Temperature ...........................................................................................................................60
6.2.1
Algal Blooms.......................................................................................................................60
6.3
Rainfall and River Flow.........................................................................................................61
6.4
Water Characteristics ............................................................................................................61
6.4.1
Light Limitations.................................................................................................................61
6.4.2
Phosphorus ..........................................................................................................................62
6.4.3
Stratification ........................................................................................................................63
6.4.4
Chlorophyll a.......................................................................................................................63
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Oxygen Depletion in the Lower Serpentine River
6.5
Sediment Characteristics.......................................................................................................64
6.5.1
Size Distribution of Sediment .............................................................................................64
6.5.2
Carbon in Sediment .............................................................................................................64
6.5.3
P in Sediment ......................................................................................................................64
7
CONCLUSIONS .....................................................................................66
8
FUTURE RECOMMENDATIONS...........................................................67
9
REFERENCES .......................................................................................68
10
APPENDICES.....................................................................................72
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Kate Roehner
List of Appendices
APPENDIX A – Temperature, Electrical Conductivity, Water Depth, and pH Data
APPENDIX B – Light Attenuation and Dissolved Oxygen Concentrations
APPENDIX C – Chlorophyll a and Pheaopigments Concentrations
APPENDIX D – Total and Soluble Phosphorus Concentrations
APPENDIX E – Soil Particle Distribution
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Oxygen Depletion in the Lower Serpentine River
List of Figures
FIGURE 1: CHEMICAL REACTANTS AND PRODUCTS OF AEROBIC RESPIRATION AND PHOTOSYNTHESIS
(UNIVERSITY OF NEUCHATEL 2004). ...............................................................................................6
FIGURE 2: REFLECTION, SCATTERING AND ABSORPTION OF LIGHT ENTERING AND AQUATIC ECOSYSTEM.
OF THE LIGHT THAT ENTERS A WATER BODY, NOT ALL WILL PENETRATE THROUGH THE WATER
COLUMN, MUCH OF IT WILL BE REFLECTED, SCATTERED OR ABSORBED BY EITHER THE WATER
ITSELF, PARTICULATE MATTER IN THE WATER AND DISSOLVED MATERIALS SUCH AS DISSOLVED
ORGANIC CARBON............................................................................................................................8
FIGURE 3: OXYGEN BUDGET FOR AN AQUATIC SYSTEM. OXYGEN IS INPUT INTO AN AQUATIC SYSTEM
THROUGH PHOTOSYNTHESIS AND INPUT THROUGH THE WATER SURFACE FROM THE ATMOSPHERE.
OXYGEN IS REMOVED FROM AN AQUATIC SYSTEM BY RESPIRATION AND THROUGH EXCHANGE
ACROSS THE WATER SURFACE..........................................................................................................9
FIGURE 4: DYNAMICS OF AN ESTUARY. WITHIN AN ESTUARY, FRESHWATER FROM THE RIVER FLOWING
TOWARDS THE OCEAN LIES OVER SALINE OCEAN WATER MOVING TOWARDS THE LAND FORMING
STRATIFICATION. ...........................................................................................................................13
FIGURE 5: STRATIFICATION, OXYGEN DEPLETION AND INTERNAL LOADING. STRATIFICATION OCCURS
DUE TO THE DENSITY DIFFERENCE BETWEEN LAYERS IN AN ESTUARY. THERE IS NO VERTICAL
MIXING DUE TO THIS STRATIFICATION, THEREFORE OXYGEN PRODUCED FROM PHOTOSYNTHESIS IN
THE UPPER LAYER IS NOT TRANSFERRED TO THE LOWER LAYER. OXYGEN IS CONSUMED IN THE
LOWER LAYER DUE TO RESPIRATION AND WITH NO REPLENISHMENT OF OXYGEN THE LOWER LAYER
BECOMES ANOXIC. ANOXIC CONDITIONS CAUSE THE RELEASE OF PHOSPHORUS FROM THE
SEDIMENTS. ...................................................................................................................................17
FIGURE 6: MAP OF PEEL-HARVEY ESTUARY AND SERPENTINE RIVER (GERRITSE ET AL. 1998). THE
SERPENTINE RIVER FLOWS INTO THE NORTHERN END OF THE PEEL ESTUARY. ..............................22
FIGURE 7: DAWESVILLE CHANNEL VIEWED FROM THE ESTUARY SIDE (DEPARTMENT OF EDUCATION AND
TRAINING 2003). ...........................................................................................................................23
FIGURE 8: PHOSPHORUS INPUT INTO THE SERPENTINE RIVER FROM THE SURROUNDING CATCHMENTS
(DEPARTMENT OF CONSERVATION AND ENVIRONMENT 1984)......................................................25
FIGURE 9: SERPENTINE RIVER SHOWING LOCATION OF KARNUP BRIDGE (DEPARTMENT OF
ENVIRONMENT 2005C). .................................................................................................................26
FIGURE 10: SECCI DISC USED FOR MEASURING LIGHT PENETRATION INTO THE WATER (DIRNBERGER ET
AL. 2005). ......................................................................................................................................29
FIGURE 11: MAP SHOWING LOCATION OF SAMPLE SITES FOR JUNE AND JULY (DEPARTMENT OF
ENVIRONMENT 2005C). .................................................................................................................30
FIGURE 13: SAMPLING LOCATIONS IN THE LOWER SERPENTINE RIVER FROM JUNE AND JULY (GOOGLE
EARTH 2005). ................................................................................................................................31
FIGURE 14: SEDIMENT CORE EXTRACTED FROM THE LOWER SERPENTINE RIVER....................................36
FIGURE 15: AVERAGE JANUARY MAXIMUM TEMPERATURES FOR EACH YEAR FROM 1950 TO 2004
(KOWALD 2005). THE RED LINE IS THE TREND IN TEMPERATURE OVER THE TIME PERIOD AND THE
GREEN LINE IS THE 5-YEAR RUNNING AVERAGE FOR THE TEMPERATURE DATA..............................39
FIGURE 16: AVERAGE JANUARY MINIMUM TEMPERATURE FOR EACH YEAR FROM 1950 TO 2004
(KOWALD 2005). THE RED LINE IS THE TREND IN TEMPERATURE OVER THE TIME PERIOD AND THE
GREEN LINE IS THE 5-YEAR RUNNING AVERAGE FOR THE TEMPERATURE DATA..............................40
FIGURE 17: AVERAGE JULY MAXIMUM TEMPERATURE FOR EACH YEAR FROM 1950 TO 2004 (KOWALD
2005). THE RED LINE IS THE TREND IN TEMPERATURE OVER THE TIME PERIOD AND THE GREEN LINE
IS THE 5-YEAR RUNNING AVERAGE FOR THE TEMPERATURE DATA. ................................................40
FIGURE 18: AVERAGE JULY MINIMUM TEMPERATURE FOR EACH YEAR FROM 1950 TO 2004 (KOWALD
2005). THE RED LINE IS THE TREND IN TEMPERATURE OVER THE TIME PERIOD AND THE GREEN LINE
IS THE 5-YEAR RUNNING AVERAGE FOR THE TEMPERATURE DATA. ................................................41
FIGURE 19: NUMBER OF DAYS IN EACH YEAR WITH A TEMPERATURE GREATER THAN 37.8°C IN THE
PERTH METROPOLITAN REGION (WATER CORPORATION 2004).....................................................42
FIGURE 20: ANNUAL RAINFALL FOR THE MANDURAH REGION FROM 1988 TO 1992 (PEEL CENTRE FOR
WATER EXCELLENCE 2005). .........................................................................................................43
FIGURE 21: LOCATION OF FLOW GAUGE AT KARNUP BRIDGE IN THE SERPENTINE RIVER (DEPARTMENT
OF ENVIRONMENT 2005C). ............................................................................................................44
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Kate Roehner
FIGURE 22: ANNUAL FLOW FROM THE FLOW GAUGE AT KARNUP BRIDGE IN THE LOWER SERPENTINE
RIVER (DEPARTMENT OF ENVIRONMENT 2005B). THE RED GRAPH SHOWS THE TREND IN FLOW
WITHIN THE RIVER. ........................................................................................................................44
FIGURE 23: AVERAGE MONTHLY DISCHARGE FOR THE LOWER SERPENTINE RIVER FOR SELECTED YEARS
BETWEEN 1980 AND 2004 (DEPARTMENT OF ENVIRONMENT 2005B). ...........................................45
FIGURE 24: LOCATION OF SERPENTINE DAM WITH RESPECT TO THE LOWER SERPENTINE RIVER
(DEPARTMENT OF CONSERVATION AND LAND MANAGEMENT 2005). ...........................................46
FIGURE 25: FLOW IN SERPENTINE RIVER SHOWING THE EFFECT OF THE SERPENTINE DAM WHICH WAS
BUILT IN 1961 (ECOLOGICAL STUDY AND COMMUNITY CONSULTATION 1996).............................46
FIGURE 26: PHOTO TAKEN ON SAMPLING DAY ON MARCH 21ST 2005 SHOWING FISH KILL IN THE
SERPENTINE RIVER. .......................................................................................................................49
FIGURE 27: CHLOROPHYLL A CONCENTRATIONS IN THE SERPENTINE RIVER MEASURED IN SAMPLES
TAKEN ON THE 10TH JUNE 2005.....................................................................................................50
FIGURE 28: CHLOROPHYLL A CONCENTRATIONS IN THE SERPENTINE RIVER MEASURED IN SAMPLES
TAKEN ON THE 29TH JULY 2005.....................................................................................................50
FIGURE 29: TOTAL PHOSPHORUS CONCENTRATIONS IN THE SERPENTINE RIVER MEASURED IN SAMPLES
TAKEN ON THE 29TH JULY 2005.....................................................................................................51
FIGURE 30: TOTAL PHOSPHORUS CONCENTRATION IN THE LOWER SERPENTINE RIVER MEASURED BEFORE
THE RIVER JOINS TO THE PEEL-HARVEY ESTUARY (LORD 1998). THE RED GRAPH INDICATES THE
OVERALL TREND IN PHOSPHORUS CONCENTRATION FROM 1990 TO 1995. .....................................52
FIGURE 31: TOTAL PHOSPHORUS INPUT INTO THE SERPENTINE RIVER FROM THE SURROUNDING
CATCHMENTS FROM THE YEARS 1990 TO 1995 (LORD 1998). ........................................................52
FIGURE 32: SOLUBLE PHOSPHORUS CONCENTRATION IN THE SERPENTINE RIVER FROM SAMPLES TAKEN
29TH JULY 2005. ...........................................................................................................................53
FIGURE 33: WATER SAMPLE FROM THE LOWER SERPENTINE RIVER ON 29TH JULY 2005 SHOWING THE
COLOUR OF THE WATER DUE TO HIGH LEVELS OF DISSOLVED ORGANIC CARBON. ..........................54
FIGURE 34: SEDIMENT PARTICLE SIZE DISTRIBUTION FROM THE TOP OF CORES TAKEN IN THE LOWER
SERPENTINE RIVER. .......................................................................................................................55
FIGURE 35: SEDIMENT PARTICLE SIZE DISTRIBUTION FROM THE BOTTOM OF CORES TAKEN IN THE LOWER
SERPENTINE RIVER. .......................................................................................................................56
FIGURE 36: PERCENTAGE UPTAKE OF SOLUBLE PHOSPHORUS FROM THE RIVER WATER SAMPLES BY THE
SEDIMENTS IN THE LABORATORY...................................................................................................57
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Oxygen Depletion in the Lower Serpentine River
List of Tables
TABLE 1: MEASUREMENTS TAKEN ON SAMPLING DATES FOR WATER TEMPERATURE, TOTAL DISSOLVED
SOLIDS AND DISSOLVED OXYGEN. THE VALUES FOR THESE PARAMETERS WERE AVERAGED OVER
ALL THE SITES THAT WERE TESTED AS THERE WAS NO TREND BETWEEN SAMPLING POINTS...........49
TABLE 2: PARTICLE SIZE DISTRIBUTION WITHIN THE SEDIMENTS FROM THE SERPENTINE RIVER. ...........55
TABLE 3: CARBON CONTENT BY PERCENTAGE OF WEIGHT IN THE SEDIMENT SAMPLES FROM THE
SERPENTINE RIVER FOUND BY LOSS-ON-IGNITION. ........................................................................58
xii
1 Introduction
Fish kills are an important environmental issue in many lakes, rivers, and estuaries
around the world. A fish kill is an event in which dead fish are observed, usually in
large numbers. Fish kills commonly occur in lakes, rivers and estuaries and can be
damaging to the fish population within the aquatic environment. There are several
reasons for a fish kill to occur, however the most common cause for the death of fish
of a number of different species is depletion of oxygen within the water body. Other
causes of fish kills include pollution of the water from anthropogenic influences and
toxic algal blooms. Therefore, water quality is an important factor in determining the
event of a fish kill.
The water quality within a lake, river or estuary can be influenced by many factors
including both environmental and anthropogenic influences. One of the most
common anthropogenic activities that will influence the quality of water within a water
body is the land use of the surrounding catchment. Within Australia, a large
proportion of the land has been cleared for agricultural use. Clearing of land for
agriculture can often result in changes in the water quality such as an increase in
nutrients, particularly nitrogen and phosphorus. Agricultural land use is an important
factor contributing to the increased nutrient load in many rivers and estuaries. Some
agricultural land uses include crop growing to produce food for human consumption,
and raising livestock to produce either food for human consumption or other products
such as wool. It is necessary to clear land of native vegetation for agriculture leaving
the land exposed. Therefore, when there is rain the upper layer of soil is washed
overland and into the nearest waterway. Agricultural land can be a diffuse source of
nutrients, such as nitrogen and phosphorus, to nearby water bodies. This excess
nutrient loading into the waterways is important as it can lead to eutrophication, poor
water quality and algal blooms.
The surrounding vegetation also influences the quality of the water. Vegetation
surrounding a water body has the role of filtering runoff from surrounding catchments
before it enters the water, as well as preventing erosion of the soil. Surrounding
vegetation also contributes large quantities of organic matter to a water body. Other
environmental influences on water quality include the surrounding climate, with
rainfall and temperature being important factors, and processes occurring within the
aquatic environment.
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Oxygen Depletion in the Lower Serpentine River
The lower Serpentine River is located south of Perth and flows into the northern end
of the Peel-Harvey estuary. The lower Serpentine River can be regarded as an
estuary; it consists of freshwater inflow from the upper Serpentine River mixing with
inflowing ocean water from the Peel-Harvey estuary. The lower Serpentine River has
had a problem with fish kills since the late 1990’s. These fish kills are caused by a
low dissolved oxygen concentration in the water and usually occur during the warmer
months of the year. The aim of this project is to determine the causes of low
dissolved oxygen in the river to help manage the fish kill events.
2
2 Literature Review
This literature review provides background on estuarine processes, the importance of
dissolved oxygen for the ecology of a system, environmental and anthropogenic
influences on dissolved oxygen concentrations, and nutrient concentrations in
estuaries. The dominant processes that effect the concentration of dissolved oxygen
within water bodies, and the processes effecting nutrient concentrations in waterways
will be outlined as this can often lead to a depletion of oxygen in the water. The
negative environmental effects of a low dissolved oxygen concentration within a
waterway will be discussed. This literature review also emphasizes the lack of
literature on estuarine processes within Australia.
2.1 Dissolved Oxygen in Water Bodies
2.1.1 Importance of Dissolved Oxygen for Ecology
Oxygen is essential for the metabolic activity of all aerobic aquatic organisms. An
‘aerobic aquatic organism’ is described as an organism living in water that has an
oxygen based metabolism (Wikipedia 2001). Oxygen is one of the most fundamental
parameters of rivers and estuaries (Wetzel 2001). Kalff (2002) described dissolved
oxygen measurements as having the potential to reveal more about the nature of a
water body than any other form of chemical data. While dissolved oxygen is
important for respiration to occur in an aquatic environment, there are other factors in
the system affecting the distribution of aquatic organisms within the environment
such as heavy metals, which are toxic to many species. However, the level of
dissolved oxygen within a water body helps to determine the types of organisms that
can survive within the dissolved oxygen bounds and is one of the factors in
determining the health of the surrounding ecosystem. Aerobic organisms use
oxygen, combined with the food molecules (such as glucose) they consume, to
obtain the energy that is needed for their survival. The concentration of dissolved
oxygen contained in a water body reflects the balance between the oxygen input
from the atmosphere and through photosynthesis, and of the metabolic processes
that consume oxygen from the system, such as respiration (Kalff 2002). As the
contribution of oxygen from the atmosphere is relatively small, photosynthesis and
3
Oxygen Depletion in the Lower Serpentine River
respiration are the dominant processes that determine the concentration of dissolved
oxygen contained within a water body.
Unlike in a terrestrial environment, where there is usually unlimited oxygen available,
the concentration of oxygen in an aquatic environment is limited by the amount that
can be dissolved in the water. It is necessary for oxygen in waterways to be
dissolved in the water before it can be taken up by aquatic organisms; this is not an
issue for terrestrial organisms. The concentration of dissolved oxygen in water is
dependent on its solubility; therefore concentrations of dissolved oxygen in a water
body can vary. Generally, molecular oxygen has a low solubility in water, however
the solubility is dependent on many factors including temperature, salinity and to a
lesser extent pressure (Brune et al. 2000). The higher the solubility, the more oxygen
that will be dissolved in the water, and the more that will be available for uptake by
aquatic organisms.
2.1.2 Hypoxia and Anoxia in Water Bodies
Hypoxia and anoxia are conditions associated with the concentration of dissolved
oxygen in water. Hypoxia is a condition in which the dissolved oxygen concentration
in a water body is reduced below 2mg/L (Yin et al. 2004). Anoxia is a condition in
which no dissolved oxygen exists in the water (Yin et al. 2004). The development of
hypoxia or anoxia in an aquatic system can lead to death of many aerobic aquatic
organisms, and can also decrease the habitats carrying capacity. This implies that
the abundance of organisms capable of existing in a certain aquatic environment can
be reduced due to the development of hypoxic or anoxic conditions (Yin et al. 2004).
Low dissolved oxygen concentrations, usually below 2mg/L, also affect the
distribution of fish and invertebrates within a water body (Kalff 2002). While most
species cannot exist in areas where there are low levels of dissolved oxygen, there
are some species that are more tolerant than others. The concentration of dissolved
oxygen can also effect the concentrations of other inorganic nutrients and toxic
metals in an aquatic system by altering the redox potentials, this will be discussed
later in more detail (Kalff 2002).
4
2.1.3 Respiration
Cellular respiration is the process in which chemical bonds of energy rich molecules,
such as glucose, are broken down and converted into energy that is utilised by
organisms to sustain their life processes (Wikipedia 2001). Respiration occurs as an
exothermic oxidation reaction. The importance of respiration being an exothermic
reaction is that it releases a large amount of energy quickly; this energy is then free
to be utilised by the organism to sustain its life processes. All plants and animals
carry out the process of respiration, where oxygen is extracted from the atmosphere
by living tissues to assist in the conversion of carbohydrates into carbon dioxide and
water (Hall & Rao 1987).
The equation below represents the oxidation of glucose (respiration) (Kalff 2002):
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
Equation 1
Within an aquatic environment, the dissolved oxygen needed for respiration to occur
is obtained from the surrounding water. The most common method of aquatic
organisms obtaining oxygen is via diffusion across the surface of the skin or in
specialised structures, such as the gills of fish (Kalff 2002).
2.1.3.1 Effects of an aquatic environment on respiration
The major factor that differentiates respiration in the terrestrial environment from
respiration in the aquatic environment is that the amount of oxygen available for
uptake in an aquatic system can vary (Dejours 1988). Oxygen must be dissolved in
the water column to be available for uptake by organisms. The amount of dissolved
oxygen within an aquatic environment is dependent on its solubility. In aquatic
environments, the solubility of both carbon dioxide and oxygen decrease with an
increase in temperature, however the energy and oxygen demand of aerobic aquatic
organisms will increase with an increase in temperature (Dejours 1988). With the
solubility of oxygen being reduced, it is important that the levels of dissolved oxygen
in the water remain high enough for respiration to occur. The concentration of
dissolved oxygen in the water will determine the abundance and type of species that
can exist in the environment. The cycling of chemicals between photosynthesis and
respiration as well as the product of each process is shown in Figure 1. From this
figure it can be seen that photosynthesis uses the chemicals that are produced from
5
Oxygen Depletion in the Lower Serpentine River
respiration as its reactants. A healthy aquatic environment will maintain a balance
between the photosynthetic and respiration processes.
Figure 1: Chemical reactants and products of aerobic respiration and photosynthesis (University
of Neuchatel 2004).
2.1.4 Photosynthesis
Photosynthesis is a biochemical process in which plants, algae and some bacteria
use energy from sunlight to produce their own food, which provides them with energy
for growth and maintenance (Wikipedia 2001). During photosynthesis, plants
synthesise organic compounds from inorganic materials in the presence of sunlight
(Hall & Rao 1987). These organic compounds are used to maintain the life processes
within an organism. It is the process of photosynthesis that is responsible for the
production of oxygen (Wikipedia 2001). During photosynthesis carbon dioxide is
consumed in order to produce oxygen and carbohydrates, as can be seen in the
equation below. Organisms that use photosynthesis to produce their own energy are
called phototrophs (or photoautotrophs) (Kobliz et al. 2005). Photosynthesis occurs
according to the following reaction, in the presence of sunlight (Lawlor 1987):
CO2 + H2O Æ CH2O (carbohydrate) + O2
Equation 2
The carbohydrate most often produced is glucose, this reaction can be seen in
Equation 3:
6
6CO2 + 6H2O Æ C6H12O6 (glucose) + 6O2
Equation 3
Photosynthesis, as indicated by the above equation, is the conversion of energy poor
compounds, carbon dioxide and water, to the energy rich compounds, carbohydrates
and oxygen to provide fuel for an organism to survive (Hall & Rao 1987).
Photosynthesis is a highly important process in regulating the oxygen within any
environment, including the aquatic environment. Photosynthesis is an oxidationreduction process; these are described in more detail below (Lawlor 1987).
Photosynthesis within a water body is carried out by phytoplankton, benthic algae,
bacteria and submerged macrophytes (Kalff 2002). While algae and bacteria are not
as complex as terrestrial plants, they perform photosynthesis in the same way as
terrestrial plants, to produce energy to function (Wikipedia 2001). Algae contain
chloroplasts, similar to terrestrial plants, which assist in the process of
photosynthesis. Photosynthetic bacteria do not contain chloroplasts therefore
photosynthesis takes place directly within the cell (Wikipedia 2001).
2.1.4.1 Effects of a water environment on photosynthesis
Unlike in terrestrial environments, an aquatic environment contains dissolved and
particulate colouring matter, and absorbs light to an extent that drives plants to
compete for solar radiation not only between each other but also with all other light
absorbing components present in the water (Long & Baker 1986). Solar radiation that
penetrates a water surface is scattered and/or absorbed by three major components,
these being the water itself, particulate material, including living and nonliving
suspended matter, and dissolved materials, including dissolved organic carbon
(DOC) (Kostoglidis et al. 2005). Within an aquatic system, more than half of the light
that enters is absorbed by the water itself (Long & Baker 1986) and therefore
reducing the availability of solar energy to autotrophic communities. This light
limitation can inhibit photosynthetic processes therefore contributing to oxygen
depletion, as photosynthesis is unable to occur without sufficient availability of
sunlight. It is for this reason that the deeper layers of water within rivers and lakes
often become oxygen depleted. Another factor affecting the distribution of sunlight in
the water is turbidity. Turbidity results in re-suspension of sediment from the benthos,
causing light limitation within water bodies by increasing the amount of soil particles
7
Oxygen Depletion in the Lower Serpentine River
in the water column which potentially absorb the light and prevent it from penetrating
deeper layers (Cabello-Pasini et al. 2002).
Figure 2: Reflection, scattering and absorption of light entering and aquatic ecosystem. Of the
light that enters a water body, not all will penetrate through the water column, much of it will be
reflected, scattered or absorbed by either the water itself, particulate matter in the water and dissolved
materials such as dissolved organic carbon.
Diffusion of carbon dioxide is another factor that restricts photosynthetic processes
within an aquatic environment. The diffusion of carbon dioxide is several orders of
magnitude slower in liquid than in the gaseous phase (air), therefore localised
depletion of this inorganic carbon source can limit the rate of photosynthesis (Long &
Baker 1986). Sunlight is essential for the process of photosynthesis to occur,
therefore in the dark, photosynthesis cannot occur due to a lack of incoming energy
from the sun. Therefore, at night, aquatic plants respire exhibiting a net consumption
of oxygen and net production of carbon dioxide (Long & Baker 1986). Although
aquatic plants primarily respire during dark hours, the production of oxygen through
photosynthesis during the day outweighs this small consumption of oxygen due to
respiration at night.
8
Oxygen exchange from surface
Photosynthesis
Water
Respiration
Figure 3: Oxygen budget for an aquatic system. Oxygen is input into an aquatic system through
photosynthesis and input through the water surface from the atmosphere. Oxygen is removed from an
aquatic system by respiration and through exchange across the water surface.
2.2 Dissolved Organic Carbon
Dissolved organic matter (DOM) is the major form of organic matter in most aquatic
systems and has the potential to alter the optical properties of a water body (Findlay
& Sinsabaugh 2003). A large proportion of DOM in a water body is derived from leaf
litter and organic matter in the soil that has been exported into the waterway (Kalff
2002). In most aquatic systems, the primary constituent of DOM is dissolved organic
carbon (DOC). DOC is defined as the organic carbon that will pass through a filter
with a pore size of 0.2-0.7µm (Kalff). The carbon that does not pass through this filter
is known as particulate organic carbon (POC). POC and DOC combine to give the
total organic carbon (TOC) concentration in a waterway. In rivers DOC accounts for
approximately 60% of the total organic carbon (TOC) load (Findlay & Sinsabaugh
2003). DOM is also the primary substrate supporting bacterial growth (Findlay &
Sinsabaugh 2003). Therefore, the higher the DOM concentration, the greater the
number of bacteria that will be able to survive in the water and the higher the
respiration will be due to the larger number of bacteria. The concentration of DOC
within rivers typically ranges from 0.5-50mg/L (Findlay & Sinsabaugh 2003). The flow
path of water through a soil profile is an important determinant for the final
9
Oxygen Depletion in the Lower Serpentine River
concentration of DOC that reaches the water body, therefore water that flows through
sandy soils or soils with poor nutrient binding capacity is more likely to wash excess
nutrients, such as carbon, into the waterway.
As mentioned above, light that enters a water body can be scattered or absorbed by
DOC. In many estuaries where the suspended matter is low and DOC is
comparatively high, DOC may be the major influence on light limitation within the
water column. DOC within estuaries can be either of terrestrial origin, originating from
plants and trees on the banks of the estuary, or there can be internal sources of DOC
from within the estuary itself (Muylaert et al. 2005). The internal sources can be
linked to phytoplankton blooms or to the vegetation from surrounding marshes
(Muylaert et al. 2005). High chromophoric dissolved organic matter (CDOM), which
includes DOC, is a common feature of the estuaries in the south-west of Western
Australia (Kostoglidis et al. 2005). Elevated CDOM levels can be a consequence of
catchment characteristics, including vegetation. Eucalyptus and Melaleuca trees are
both high in humic substances, contributing to the CDOM. It is more likely for these
humic substances to enter the waterway if the catchment contains porous sandy soils
as they will be transported by the flow of water either on the surface of through the
ground (Kostoglidis et al. 2005).
2.3 Oxidation and Reduction
Photosynthesis is an oxidation-reduction (redox) reaction in which aquatic plants use
solar energy for the reduction of carbon dioxide and the oxidation of water to produce
oxygen and glucose. Oxidation is the process in which electrons are removed from
an atom, while reduction is the process in which an atom gains electrons, therefore
becoming more negative (Kalff 2002). The redox potential of a system shows the
tendency of the environment to receive or supply electrons and indicates the degree
of balance between oxidising and reducing processes within a system. As redox
reactions involve inorganic plant nutrients, including phosphorus, oxidation and
reduction will affect the concentrations and the form of these nutrients in the water
column (Kalff 2002). When organic matter is oxidised within a water body, dissolved
oxygen is the electron acceptor (Kalff 2002). Therefore the concentration of dissolved
oxygen within a water column will be affected by the redox potential of the system
and the oxidation of organic matter.
10
The oxidation state of redox elements that come from the surrounding catchment,
including dissolved oxygen, organic carbon, and nitrogen, will determine the form that
they will be found in the water (Kalff 2002). These elements will be either dissolved in
the water or be insoluble which makes it possible for them to be removed by
sedimentation. For nutrients to be available for uptake by aquatic plants they need to
be dissolved in the water. The redox potential of a system influences the form of
phosphorus in water, for example it is possible for phosphorus bound to the
sediments to be remobilised during periods with low redox potential. The redox
potential will also be a factor in determining whether phosphorus is available for
uptake by aquatic organisms. The presence of inorganic phosphorus (PO43-), one of
the main forms of phosphorus, within a waterway is dependent on the oxidation state
and abundance of other inorganic substances such as iron and aluminium, although
it is not a redox element itself (Kalff 2002).
2.4 Depletion of Oxygen in Estuaries
2.4.1 Increased Temperature
The solubility of oxygen in water is largely dependent on the temperature of the
water, as the temperature increases the solubility of oxygen decreases (Kalff 2002).
In pure water (fresh water), the maximum solubility of oxygen occurs at a
temperature of 0°C (Kalff 2002). The water temperature is an important factor in
determining the amount of oxygen that can be dissolved within a water column.
Studies have shown that the solubility of oxygen at 0°C is approximately twice the
solubility at 30°C (Kalff 2002).
The temperature of the water also has an effect on the amount of oxygen actually
required by aquatic organisms for respiration. As the water temperature increases,
the biological respiration rate also increases (Hu et al. 2001). It has been shown in
previous studies that for each 10°C rise in temperature, until an optimum temperature
is reached, biological processes double (Hu et al. 2001). Therefore, although the
amount of dissolved oxygen within the water decreases with increasing temperature,
the amount of oxygen needed for respiration increases. This can cause a decrease in
the amount of dissolved oxygen to an extent that causes the water to become
hypoxic or even anoxic. Bacteria within the sediments have a large contribution
11
Oxygen Depletion in the Lower Serpentine River
towards increasing the biological respiration. With this increased respiration more
oxygen will be consumed within the bottom layers of the water column. It is for this
reason that anoxia is often initiated in the lower layers of the water body.
2.4.2 Estuaries and Stratification
An estuary is a partially enclosed body of water that forms at the outlet of a river
system, where fresh and seawater mix, hence the sea water is diluted (Elliott &
McLusky 2002). The forcing (river or ocean currents) more dominant, and the
location and movement of the interface of fresh and sea water, vary seasonally. In a
Mediterranean climate, such as that which exists in the south-west of Western
Australia, there is more freshwater runoff in winter due to periods of increased
rainfall; therefore the estuary will become less saline, whilst in summer there is little
or no runoff so the estuary can be equally as saline, or even exceed the salinity of
the ocean water. It is possible for an estuary to become more saline than sea water
due to evaporation.
An estuary generally has two-way flow, the flow of fresh river water towards the
ocean and the flow of sea water towards the land (Figure 4). As the water flowing
from the land is fresher and less dense than ocean water, it forms a layer above the
denser, saline water (Gwinn 1987). The flow in an estuary is affected by tidal
currents, however it has a net seaward flow equal to the discharge of its tributary
rivers (Gwinn 1987). Therefore, the extent of the saline water varies depending on
the time of the tidal cycle. When the river water has considerable velocity, it tends to
drag the upper section of the seawater wedge along with it (Gwinn 1987). During
times when there is little rainfall and the river is not flowing, the only forcing affecting
the flow in the estuary is the tidal influx of seawater.
12
Figure 4: Dynamics of an estuary. Within an estuary, freshwater from the river flowing towards the
ocean lies over saline ocean water moving towards the land forming stratification.
Stratification occurs within estuaries, caused primarily by the density difference
between the lower saline layer and the upper freshwater layer from the river, as can
be seen in Figure 4. It is possible for this stratification to be enhanced during summer
by the temperature difference between these two layers. The sun heats the upper
layer of water resulting in a formation of a temperature gradient between the upper
and lower layer. The density of the fresh surface layer is further decreased and
stratification is intensified. Stratification causes vertical mixing to be inhibited.
Stratification is more likely to occur when the water is stagnant or moving with small
velocities (Parr & Mason 2004). When the water is moving at high velocities mixing
dominates the water column and stratification will not be present. It is common for
shallow estuaries to develop persistent stratification due to less frequent flushing in
comparison to deeper estuaries (Buzzelli et al. 2001).
It is most common for oxygen to be produced in the upper layer of a water body by
photosynthesis due to greater light penetration. With limited vertical mixing occurring
due to stratification, oxygen will not be transferred to the deeper waters, however
respiration within the sediment will continue (Buzzelli et al. 2001). Therefore, oxygen
is being consumed in the bottom layer but is not being replaced which can lead to
anoxic conditions. Anoxic conditions in the water can cause the release of inorganic
nutrients that are trapped within the sediments, particularly phosphorus, this is
discussed in more detail below. This release of phosphorus provides excess
13
Oxygen Depletion in the Lower Serpentine River
nutrients for bacteria further enhancing respiration, which can lead to extended
periods of anoxia.
2.4.3 Phosphorus Cycling
Phosphorus is a nutrient that exists naturally in rivers and estuaries. It is the least
abundant and therefore the limiting nutrient to biological activity in most Australian
fresh waters (Wetzel 2001). It can enter these systems either via diffusion through
the soil profile or by direct deposition on the water surface from the atmosphere,
however the contribution from the surrounding catchment (land) tends to dominate
(Kalff 2002). In poorly vegetated areas, or areas containing plants with shallow root
systems, phosphorus is usually released into the waterway while sorbed to soil
particles (Kalff 2002). This release occurs due to the lack of deep rooted vegetation
to inhibit erosion, resulting from the clearing of land for agricultural purposes. The
most common form of soluble phosphorus in waterways is orthophosphate (PO43-)
(Wetzel). However, in fresh waters, phosphorus mostly occurs as organic
phosphates and as cellular constituents adsorbed to inorganic, and dead particulate
organic material, and in the biota (Wetzel).
Once phosphorus has entered the water column, it is either taken up by plants in the
water if it is in the dissolved form, or it can become trapped within the sediments
(Ekholm et al.). Phosphorus can also be released into the water column from the
sediments; this is called internal loading (Kalff 2002). Internal loading occurs when
the conditions within the water are altered and causes phosphorus from sediments to
be released. Primarily, anoxia promotes the release of phosphorus from sediments,
however an increase in salinity under aerobic conditions can also result in the
discharge of phosphorus from a system (Ekholm et al.).
2.4.4 Phosphorus in Estuaries and turbidity maximum
There is a sudden change in ionic strength in an estuary where freshwater mixes with
seawater. Typical stream waters are about 1mM (millimoles/1000cm3) in ionic
strength while seawater has an ionic strength of about 500mM (House et al. 1998).
This sudden shift in ionic strength causes colloidal particles to flocculate and they will
become trapped at the seawater-freshwater interface. Saline ocean water is pushed
14
underneath the flowing river water due to the strong tidal forces as can be seen in
Figure 4 (Goni et al. 2005). This tidal forcing and non uni-directional flow results in
the re-suspension of sediment and other particulate material that is present on the
river bed. Flocculation causes the smaller particles to combine and form larger
particles which then become heavy enough to sink to the sediments. The turbidity
maximum is created at the seawater-freshwater interface; it is here that suspended
solids concentrations are greatly increased. The suspended solids concentrations are
elevated at this point due to the turbulence created by the mixing of the seawater and
freshwater. The turbidity maximum usually moves upstream with the salt wedge
during a flood tide and moves downstream during an ebb tide. Due to the continuous
movement of the estuarine turbidity maximum, particulate material is deposited and
resuspended at different locations within the estuary. The turbidity maximum, with
suspended sediment, offers a site for the conversion of dissolved phosphorus to
particulate forms (House et al. 1998). The estuarine turbidity maximum is a trap for
these particulate forms of phosphorus and hence many estuaries act as a long-term
sink for phosphorus.
Within many south-west catchments, the major source of phosphorus within the
water column is superphosphate, which is the fertiliser applied for agricultural
purposes in the surrounding catchments. The phosphorus in the soil attaches itself to
clay particles and is transported by overland flow. The sandy soils have a poor
phosphorus binding capacity and therefore discharge phosphorus in a soluble form
or bound to low molecular organics (Summers et al. 1999). Due to the hot dry
summers in the south-west region, the phosphorus component of organic matter is
rapidly mineralised making it available for runoff once the first winter rains occur
(Summers et al. 1999). Therefore, with the first large winter rains of the year, it is
common for a large concentration of phosphorus to be washed into the rivers and
estuaries.
2.4.4.1 Release of Phosphorus from Sediments
River and estuarine systems have the capacity to remove or release phosphorus to
and from the water column, they also have the ability to transform phosphorus into
different forms, such as organic, inorganic, particulate and dissolved forms (Jarvie et
al. 2005). The phosphorus removed from the water column is taken up by the
sediments which is the reason that a large proportion of the phosphorus within
aquatic systems exists within the sediments. The sediments have the ability to buffer
15
Oxygen Depletion in the Lower Serpentine River
concentrations of soluble reactive phosphorus within the water, particularly under
reduced flow conditions when there is a longer contact time between the sediments
and overlying waters (House et al. 1998). The ability of the sediments to buffer
concentrations of phosphorus is also increased with an increase in the sediment
surface area to water volume ratio, such as in shallow water bodies (House et al.
1998).
Phosphorus entering into the sediments becomes trapped, however under the right
conditions, it is possible for this phosphorus to be released back into the water
column (Hu et al. 2001). As mentioned above, this is called internal loading, and can
be the cause of large amounts of phosphorus within systems even after external
loading (such as input from agricultural land) has been greatly reduced. Therefore,
although the external nutrient load may have been reduced, nutrients are gradually
released back into the water column from the sediments, which will delay
improvement in water quality conditions. The phosphorus sorbs to the aerobic layer
of the surface sediments, which is several millimetres thick on the top of the
sediments. When the conditions of the bottom waters and the top layer of the
benthos become anaerobic, the sorption capacity of the sediments is greatly
reduced. The reduction in sorption capacity results in the release of dissolved
substances, including nutrients, into the overlying waters (Hu et al. 2001). Another
condition which can cause phosphorus to be released from the sediments is an
increase in salinity under aerobic conditions (Gardolinski et al. 2004). Studies have
shown that a substantial amount of phosphorus can be released from sediments
when the salinity is increased by at least 10-20%. The two conditions that will cause
a release of phosphorus from the sediments are anoxic conditions or an increase in
salinity.
Another method by which phosphorus can be re-released back into the water column
is by either diffusive or advective transport (Giffin & Corbett 2003). The phosphorus
that is released during these processes is the dissolved phosphorus that exists in the
pore-waters within the sediments, therefore it is not actually sorbed to the soil
particles. Diffusion occurs when the sediments are stationary and the nutrients move
into the water column due to the concentration difference between the sediments and
the overlying water. Advective transport of nutrients occurs with the re-suspension of
sediment, the nutrients trapped within the pore-waters of the sediment will be
advected into the water column.
16
Within aquatic systems, the abundance of bacteria is closely linked with the inorganic
nutrient levels, particularly total phosphorus. A positive correlation between total
phosphorus and abundance of bacteria within an aquatic system has been shown
(Kalff 2002). Therefore in most systems, the higher the total phosphorus
concentration, the higher the abundance of bacteria.
Figure 5: Stratification, oxygen depletion and internal loading. Stratification occurs due to the
density difference between layers in an estuary. There is no vertical mixing due to this stratification,
therefore oxygen produced from photosynthesis in the upper layer is not transferred to the lower layer.
Oxygen is consumed in the lower layer due to respiration and with no replenishment of oxygen the
lower layer becomes anoxic. Anoxic conditions cause the release of phosphorus from the sediments.
2.4.5 Sediment Oxygen Demand
Within rivers and estuaries, sediments exert an oxygen demand upon the overlying
waters as a result of biological respiration of living organisms within the sediment,
and the chemical oxidation of reduced substances that occurs within the sediment,
such as sulphide (Hu et al. 2001). The sediment oxygen demand can represent a
significant percentage of the total oxygen uptake in estuaries. It is possible for the
17
Oxygen Depletion in the Lower Serpentine River
sediment oxygen demand to account for up to 90% of the total oxygen demand in
aquatic systems (Parr & Mason 2004). The sediment oxygen demand is influenced
by temperature, flow rate of the water, and the vegetative cover (Parr & Mason
2004).
The effect of temperature on sediment oxygen demand has been examined in
studies to show that as the temperature of the water increases, the sediment oxygen
demand increases. One of the main reasons for this elevated sediment oxygen
demand is the increase in respiration rate of bacteria in the sediment. A study
conducted in a eutrophic land-locked embayment in Hong Kong investigated the
effect of temperature on sediment oxygen demand of sediment samples that were
collected and placed under controlled conditions in a laboratory. The experiment
showed that the increase in sediment oxygen demand values with an increase in
temperatures was more significant at lower temperatures compared to higher
temperatures. For a rise in temperature from 10°C to 20°C, the sediment oxygen
demand more than doubled, however for an increase in temperature from 20°C to
30°C the sediment oxygen demand increased by only 25% (Hu et al. 2001).
The type of sediment existent in a river or estuary has a large impact on the SOD as
well as on the ecology of the river or estuary as it determines the community
composition within the sediments. Muddy sediments, such as clay, can support a
higher number of bacteria than sandy sediments due to the higher surface area,
therefore they have a higher oxygen demand (Parr & Mason 2004). This oxygen
demand increases at higher temperatures due to greater respiration rates of the
bacterial communities.
2.5 Eutrophication and Algal Blooms in Waterways
Eutrophication is a major ecological problem that can occur in enclosed or semienclosed waterways such as lakes, estuaries and even slow moving rivers.
Eutrophication is an excess loading of nutrients in a waterway (Kalff 2002). These
excess nutrients have the potential to stimulate excessive plant growth usually called
an algal bloom, however for this to occur there must be sufficient light available (Kalff
2002). Within aquatic systems within Australia, phosphorus and nitrogen are most
commonly the limiting nutrients and an excess of these has the potential to cause an
algal bloom.
18
Eutrophication is often a naturally occurring process in estuaries resulting from a
leaching of nutrients through the soil that become concentrated in a confined
channel, where run-off enters the estuary (Wikipedia 2001). However, anthropogenic
activities often accelerate the rate at which nutrients enter an ecosystem. The flux of
both organic and inorganic nutrients into a system is accelerated by runoff from
agricultural land, pollution from sewers and septic systems and other anthropogenic
activities.
Western Australia’s coastal estuaries are highly oligotrophic systems; this means that
the natural level of nutrients within aquatic systems is very low compared to other
areas of the world (Felsing et al. 2005). Whilst Western Australia is the largest state
by its land area, it’s population is concentrated primarily in the south-west region,
with 80% of people living between Esperance and Geraldton (Stoddart & Simpson
1996). This has a large effect on the coastal waters and rivers within the south-west
of Western Australia and causes environmental stress within these highly populated
areas. It is the coastal waters adjacent to these highly populated regions in which
degradation has primarily occurred (Zammit et al.). Since human settlement,
intensive agriculture has become a major concern due to its effect in increasing the
concentration of nutrients in Western Australian waterways. Agricultural sources
have been identified as the primary cause of eutrophication in estuaries within the
south-west region (Stoddart & Simpson 1996). Agricultural land is a diffuse source of
contributing phosphorus into water, as they do not come from a specific point but are
derived from a large area. However, diffuse sources of nutrients have not always
been considered as important as they are now. Until recently, point sources such as
sewage plants and animal feedlots (including piggeries), were thought to be the main
sources of nutrients to the south-west waterways (Heathwaite et al. 2005).
The Peel-Harvey estuary, into which the Serpentine drains, has one of the most
severe cases of eutrophication in Western Australia (Stoddart & Simpson 1996).
Within most of the estuaries in the south-west of Western Australia, phosphorus has
been recognised as the limiting nutrient, therefore large anthropogenic inputs of
phosphorus are responsible for eutrophication of the water body resulting in algal
blooms at particular times of the year (Summers et al. 1999). The sandy soils of
catchments result in poor phosphorus binding capacity (Department of Conservation
and Environment 1985). As a result of this, phosphorus fertilisers that are applied
19
Oxygen Depletion in the Lower Serpentine River
within the catchments rapidly progress into drains and streams within the estuary’s
catchment as they are leached into the ground.
Algal blooms have the potential to limit sunlight reaching lower depths of the water
body. This prevents photosynthesis from occurring therefore no oxygen is produced
(Wikipedia 2001). Further oxygen depletion occurs when the dead plant material
decomposes and becomes a food source for micro-organisms. These microorganisms respire causing a greater uptake of oxygen. As oxygen is required by all
plants and animals that respire in an aquatic environment, a reduction in dissolved
oxygen levels will cause the death of many of these organisms (Peuhkuri 2002).
20
3 Background
3.1 Peel-Harvey Estuary and the Lower Serpentine River
The Serpentine River is located south of Perth and is one of the three major rivers
that discharge into the Peel-Harvey estuary; the Serpentine River discharges into the
northern section of the Peel Inlet (Figure 6). The Peel-Harvey estuary is located
approximately 75km south of Perth and occupies an area of 136km2 (de Lestang et
al. 2003). It is a shallow water body that is generally less than 2m deep and is
microtidal (de Lestang et al. 2003). The original entrance channel to the Peel-Harvey
Estuary is the Mandurah channel, which connects the northern part of the estuary to
the open ocean (Figure 6). The runoff discharging into the estuary comes from a
catchment area of about 1600km2 extending from Serpentine in the north to Harvey
in the south, and to the Darling Scarp in the east (Department of Conservation and
Environment 1984). Symptoms of an increasing phosphorus load within the PeelHarvey catchment were first observed in the late 1960’s, due to the large
anthropogenic inputs from the surrounding agricultural areas, and severe blooms of
Nodularia were first observed within the Peel-Harvey estuary in 1973, they now occur
regularly in late spring to early summer (Stoddart & Simpson 1996). These algal
blooms have lead to mortalities amongst many species within the system.
21
Oxygen Depletion in the Lower Serpentine River
Figure 6: Map of Peel-Harvey estuary and Serpentine River (Gerritse et al. 1998). The Serpentine
River flows into the northern end of the Peel Estuary.
22
The initial management response to the eutrophication problem in the Peel-Harvey
area was to take an engineering approach; hence the Dawesville channel was
created (Figure 7). The Dawesville channel, which is the man-made channel
connecting the Peel-Harvey estuary to the ocean, was opened in April 1994
(Department of Education and Training 2003). The intention of this channel was to
improve tidal flushing of the estuary therefore move nutrients offshore and out of the
estuary. The tidal exchange between the ocean and the estuary was improved, and
now a more constant saline environment exists in the estuary which has made
conditions unfavourable for the toxic phytoplankton, Nodularia, as it prefers fresh to
brackish water conditions (Department of Education and Training 2003). Therefore,
the Dawesville Channel has not only provided a solution for flushing nutrients from
the estuary, the more saline conditions are also unfavourable to toxic phytoplankton
growth. The Dawesville Channel has had an impact on the lower reaches of the
Serpentine River, with saline water now moving further upstream in the river.
Figure 7: Dawesville Channel viewed from the estuary side (Department of Education and
Training 2003).
Around the same time that the Dawesville Channel was constructed, management
plans for three of the major rivers discharging into the Peel-Harvey Estuary, the
Serpentine, Murray and Harvey Rivers, were implemented (Water and Rivers
Commission and Peel Inlet Management Authority 1998). The management plan for
the Serpentine River, the Rivers Environment Management Plan (REMP),
concentrated on the lower reaches of the Serpentine River (Water and Rivers
Commission and Peel Inlet Management Authority 1998). The lower reaches of the
Serpentine River comprise of a series of wide, shallow pools and lakes, and for a
23
Oxygen Depletion in the Lower Serpentine River
large proportion of the year this region is poorly circulated and highly stratified (Water
and Rivers Commission and Peel Inlet Management Authority 1998). The aim of the
management plan was to conserve and enhance the waterways, embankments and
foreshores of the river environment (Water and Rivers Commission and Peel Inlet
Management Authority 1998). The area concentrated on was the foreshore area of
the river as this area is important in the overall health of the river; the vegetation
within the foreshore reserves provides a buffer between the waterway and possible
sources of water pollution (Water and Rivers Commission and Peel Inlet
Management Authority 1998). For a healthy aquatic ecosystem it is necessary to
protect the river and foreshore areas from pollution.
The aim of this management plan for three of the major rivers, with respect to the
Peel-Harvey Estuary, was to reduce the amount of agricultural runoff entering the
estuary from the rivers, therefore reducing the quantity of nutrients within the system.
One of the ways in which the nutrients entering the system were reduced was by
using slow release fertilisers in the surrounding catchments. Slow release fertilisers
are not readily water soluble and release phosphorus at a slower rate, than the
majority of fertilisers, over a longer period of time (Department of Conservation and
Environment 1984). This increases plant uptake of phosphorus, and decreases the
loss of phosphorus to leaching and being washed into the waterways. Another
strategy to reduce the amount of nutrients within the catchment was to delay the
application of soluble fertilisers until the plants have developed sufficiently that they
are able to effectively take up the applied phosphorus, reducing the amount leached
into the soil (Department of Conservation and Environment 1984). Other methods
used are soil testing to estimate the amount of phosphorus required, and the
prediction of long term phosphorus requirements from past data (Department of
Conservation and Environment 1984). The purpose of the management plan was to
reduce fertiliser use and improve the nutrient retention by soils within the catchment.
Although the catchment plan was implemented, and due to this plan the total
phosphorus load entering the river has decreased (Figure 8), the Serpentine River
has shown an upward trend in phosphorus concentration. It is important to examine
the possible reasons for this.
The Serpentine River does not flow perennially; it only flows for a few months during
the wet season. When the Dawesville Channel was created, as mentioned above,
the tidal regime within the Serpentine River was also altered. This has led to a
greater tidal range in the lower Serpentine and its lakes and an increase in salinity.
24
Due to the larger tidal range, there is a density driven stratification that occurs for
most of the year (Water and Rivers Commission and Peel Inlet Management
Authority 1998). The stratification in the river has become stronger and has started to
develop earlier in the summer (Lord 1998).
The Serpentine region receives most of its rainfall in the winter months between May
and October (Water and Rivers Commission and Peel Inlet Management Authority
1998). The annual evaporation from the wetlands exceeds the annual rainfall for the
region, as is common for most regions within Western Australia (Water and Rivers
Commission and Peel Inlet Management Authority 1998). The floodplain of the lower
Serpentine River extends up to 500 metres from the main channel of the river (Water
and Rivers Commission and Peel Inlet Management Authority 1998). The tidal
exchange up the Serpentine, from the Peel Inlet, extends to about 2 kilometres
downstream of Karnup Road Bridge (Figure 9) (Water and Rivers Commission and
Peel Inlet Management Authority 1998). The salinity in the lower Serpentine varies
seasonally from 0mg/L in winter to around 55mg/L during summer, which is higher
than the salinity of sea water (around 36.5mg/L) (Water and Rivers Commission and
Peel Inlet Management Authority 1998).
Phosphorus Input into Serpentine River
45
Phosphorus input (tonnes)
40
35
30
25
20
15
10
5
0
1983
1985
1987
1989
1991
1993
1995
Year
Figure 8: Phosphorus input into the Serpentine River from the surrounding catchments
(Department of Conservation and Environment 1984).
25
Oxygen Depletion in the Lower Serpentine River
Figure 9: Serpentine River showing location of Karnup Bridge (Department of Environment
2005c).
Fish Ecology in the Serpentine
Within the Serpentine River there are many different species of fish. After the fish kill
that occurred on February 23rd 2003 a count was performed on the number and taxa
of dead fish species. Of the dead fish counted there were 17 taxa of fish and 1
species of crab. The fish counted ranged in size from less than 10mm to greater than
400mm (Smith et al. 2004). The 17 taxa of fish that were counted included: cardinal
fish, six-lined trumpeter, toadfish, black bream, cobbler, yellowtail grunter, blue
swimmer crab, yellow-eye mullet, roach, yellowfin whiting, western fortescue, Perth
herring, tailor, sea mullet, river garfish, hardyheads, and gobys (Smith et al. 2004).
26
From the count performed after the fish kill in February 2003 it can be seen that there
was a number of different species affected by the fish kill. This is evidence that it is
low dissolved oxygen levels causing fish kills as many different species were
affected.
27
Oxygen Depletion in the Lower Serpentine River
4 Methodology
In order to determine the factors within the lower Serpentine River that are affecting
dissolved oxygen levels in the water, it was necessary to first examine some
historical data on the area. The historical data that was looked at was phosphorus
input into the river, temperature in the region, annual rainfall and flow within the lower
Serpentine River. Once historical data was examined samples were collected from
the river and were analysed.
4.1 Selection of Sampling Sites
Prior to collection of data for this study, preparation was carried out to determine the
necessary locations for sampling within the lower Serpentine River. Water samples
and sediment cores were taken from the lakes system within the lower Serpentine
River. A number of sites were chosen to give an indication of what is happening in
the system. The water samples were analysed for chlorophyll a, total organic carbon,
total phosphorus and soluble phosphorus. The sediment cores were analysed for
particle size, carbon content and soluble phosphorus release and uptake.
Parameters that were measured in the field included pH, salinity, temperature, light,
and dissolved oxygen, these parameters were measured on 21st March, 10th June
and 29th July 2005.
4.2 Water Sampling
The water samples from the river were collected using a small boat as the river
widens out within the lakes section. Water samples were taken on 10th June 2005
and 29th July 2005, however samples were taken from different locations on each of
these sampling days. The locations of the samples are shown in Figure 13 below.
From each sampling location, two water samples (grab samples) were taken.
Samples were taken from the both the central channel of the river and from the
edges of the channel. The samples were collected from as close to the water surface
as possible for consistency.
28
The samples were collected using 1L clear plastic bottles that had been rinsed out
with distilled water in the laboratory. On site, the bottles and lids were rinsed twice
with river water before the sample was collected. The bottles were filled to the top
with river water to reduce the amount of air trapped with the sample so that there
would be little interference with the results. While in the field, the samples were
stored in an esky, to keep them cool and out of sunlight, until they could be stored in
a constant temperature room in the laboratory.
Measurements for pH, salinity, temperature, light and dissolved oxygen levels were
taken in situ on the sampling dates at most of the sampling sites; however salinity
and pH were tested again in the laboratory from the water samples taken. It was not
possible to re-measure temperature and dissolved oxygen from the water samples as
these readings are only accurate when taken in situ. Measurements for salinity and
pH were taken using a WP-80D dual pH-mV meter. A measurement of light
attenuation was taken using both a secci disc (Figure 10) and a licor light meter.
Light measurements were not taken at each sampling site, as it was not considered
necessary to do so, the colour of the water remained similar throughout the river and
there was very little turbidity.
Figure 10: Secci disc used for measuring light penetration into the water (Dirnberger et al. 2005).
When determining the salinity of the water the electrical conductivity was measured
and this was used to give an indication of the salinity. The purpose of measuring
electrical conductivity is that it is more easily measured than salinity, and the
conductivity of the water is dependent on the concentration of dissolved salt that is
present in the water (Kalff 2002). The more saline the water is, the higher the
29
Oxygen Depletion in the Lower Serpentine River
electrical conductivity will be. This electrical conductivity can then be converted to a
total dissolved solids concentration using the formula (Waterwatch 2004):
TDS mg/L (or ppm) = 0.64 x EC µS/cm
Equation 4
These dissolved concentrations can then be compared from the different samples
taken within the river and between the different sampling dates.
Figure 11: Map showing location of sample sites for June and July (Department of Environment
2005c).
30
Figure 12: Sampling locations in the lower Serpentine River from March (Google Earth 2005).
Figure 13: Sampling locations in the lower Serpentine River from June and July (Google Earth
2005).
31
Oxygen Depletion in the Lower Serpentine River
4.3 Analysis of Water Samples
The water samples collected were analysed for phosphate, chlorophyll a and total
organic carbon concentrations, and measurements for pH and salinity were taken in
the laboratory. The pH and salinity measurements were taken again using the WP80D dual pH-mV meter to compare with the measurements taken in the field.
Colorimetric analysis was used to analyse the samples for chlorophyll a, total
phosphorus and soluble phosphorus, and total organic carbon.
4.3.1 Chlorophyll a
Chlorophyll a was measured in the samples using a fluorometer. The samples were
filtered before they were analysed. To get results as accurate as possible it was
necessary to filter the samples within 48 hours of being collected. Each sample was
filtered to collect particulate matter, which was used for analysis, while the filtered
water was discarded. It was necessary to filter a sufficient volume of water to get an
accurate reading. The volume of water needed to be filtered can range from a couple
of hundred millilitres for productive river waters, to up to 3 litres for unproductive
ocean water. As the Serpentine River contains a large amount particulate matter, it
was only necessary to filter a maximum of 500mL, and with some samples as little as
200mL was filtered. The water was filtered using glass micro-fibre filters to remove
particles greater than 1.2µm. The volume of filtered water was recorded for use in
calculations. Light was kept to a minimum during the filtering and analysing process
to decrease the number of reactions occurring within the water.
After filtering the samples, the filter paper was placed into a test tube with 8mL of
90% acetone solution. The acetone solution is the extraction solvent which extracts
the pigments from the plankton in the sample. Parafilm was then placed over the top
of the test tube to seal it. The sample was placed in the freezer for 24 hours, and
shaken once during this time. After 24 hours, the filter was taken out and the acetone
was transferred into a clean test tube to measure the chlorophyll a concentration
using the fluorometer. It was necessary to first calibrate the fluorometer using a pure
acetone solution, then the concentration of chlorophyll a in the sample was
measured. After the first measurement was taken, 2 drops of 1M hydrochloric acid
was added to the test tube and the reading was taken again. Addition of acid
32
converts the chlorophyll a to phaeophytin a, and hydrochloric acid is used as the
conversion is more rapid and complete than with other acids.
These two readings were converted into chlorophyll a concentrations and
phaeopigment concentrations using the equations below:
Chlorophyll a (µg/L) = (r/(r-1))(Rb-Ra) v/V
Equation 5
Phaeophytin a (µg/L) = (r/r-1)(rRa-Rb) v/V
Equation 6
r = the before-to-after acidification ratio of a pure chlorophyll a solution (Rb/Ra=2.28)
Rb = fluorescence of the sample prior to acidification (189.1 for pure chlorophyll)
Ra = fluorescence of the sample after acidification (82.8 for pure chlorophyll)
v = volume (in mL) of the extract
V = volume of the filtered sample (in mL)
The fluorometer measures the concentration of chlorophyll a in the sample by
determining its absorbance (Paresys et al. 2005). Fluorescence is a process in which
particular
compounds
absorb
specific
wavelengths
of
light
and
almost
instantaneously emit longer wavelengths of light (Arar & Collins 1997). Chlorophyll a
naturally absorbs blue light and emits red light (Arar & Collins 1997). Optimum
sensitivity for chlorophyll a extract measurements is obtained at an excitation length
of 430nm and an emission wavelength of 663nm. The fluorometer detects chlorophyll
a in a sample by transmitting an excitation beam of light in the blue range, and by
detecting the light fluoresced by cells or chlorophyll in a sample at 663nm. In general,
the fluorescence is directly proportional to the concentration of chlorophyll a (Arar &
Collins 1997).
All green plants contain chlorophyll a and it constitutes approximately 1 to 2% of the
dry weight of algae. Chlorophyll a enables plants to perform photosynthesis by
capturing light. Chlorophyll a concentrations give an indication of the abundance of
phytoplankton in the water; the concentration is used to give a measure of primary
production. Therefore, the concentration of chlorophyll a will give an indication of the
amount of phytoplankton (including uni-cellular algae) that is present in the water
(Arar & Collins 1997).
33
Oxygen Depletion in the Lower Serpentine River
4.3.2 Unfiltered and Filtered Phosphorus
The water samples collected were analysed before and after filtering through a 0.45
micron glass fibre filter. The filtered sample gives the soluble orthophosphate fraction
(PO43-). Orthophosphate is the form of phosphorus that can be directly taken up by
algae and other organisms including bacteria (Zeng et al.). The unfiltered sample
provides an indication of total P but to obtain a true value of total P, any sediment in
the sample would have to be digested in acid and the solution analysed.
The concentration of total phosphorus and orthophosphate in the samples was
determined using malachite green as a reagent.
Phosphorus within the water
sample reacts with ammonium molybdate to form molybdophosphate compounds.
Most spectrophotometric methods are based on this reaction (Motomizu & Li 2005).
Malachite green reacts with molybdophosphate in an acidic medium to form a
coloured ion associate (Motomizu & Li 2005). This ion associate, in the presence of
poly vinyl alcohol, can dissolve in acidic aqueous solutions and shows strong light
absorption at 650nm (Motomizu & Li 2005). The molybdophosphate compounds in
the water samples, in the presence of malachite green and poly vinyl alcohol, will
form green molybdenum complexes that are detectable at 625nm. Therefore, using
the spectrophotometer, the concentration of orthophosphate and of total phosphorus
can be detected by finding the absorbance of the mixture at 625nm on a visible range
spectrophotometer.
As only small volumes of reagents and sample were used in this method, it was
important to avoid contamination, due to the sensitivity of the method. The same
method is followed for the detection of both total and soluble phosphorus however, to
analyse for soluble phosphorus the sample was filtered first using a 50mL syringe
filter with a 0.45µm membrane. To analyse for total phosphorus the sample was not
filtered. Of the sample to be analysed (either filtered or not filtered) 3mL was placed
into a 10mL vial using an acid washed glass pipette. Exactly 1mL of the reagent
(malachite green and ploy vinyl alcohol) was then added to the vial. The sample was
then mixed and left for 10 ten minutes to let the colour develop. Once the colour had
developed, the sample was transferred into a cuvette and placed into the
spectrophotometer and the absorbance was read at 625nm. When handling the
cuvette it was necessary not touch the sides that the reading will be taken from as
this would interfere with the final reading. Values for working standards were used to
34
produce an equation to relate the absorbance to the PO43- concentration. The
equation used to convert the absorbance to a PO43- concentration is:
y = 0.0018x + 0.0424
Equation 7
Where y is the absorbance at 625nm and x is the PO43- concentration.
Therefore, to find the concentration (in µg/L) the equation can be rearranged:
x = (y - 0.0424)/0.0018
Equation 8
Once the absorbance readings were obtained, these values were converted to PO43concentrations using the above equation.
4.3.3 Total Organic Carbon (TOC)
The total organic carbon in the water samples was measured using a
combustion/non-dispersive infrared gas analysis method. The instrument used was a
Shimadzu TOC 5000a. The first stage of the analysis is to oxidise all carbon in the
sample to carbon dioxide. This oxidation was achieved by catalysed combustion of
the sample at 680°C. Once oxidation had occurred, the carbon in the sample was
measured using infrared absorption. Infrared absorption is the measurement of the
wavelength and intensity of the absorption of mid-infrared light by a water sample
(Tissue 2000). The wavelength range of infrared light is from 2.5µm to 5µm (Tissue
2000). This reading is then measured against a calibration curve that has been
created by measuring solutions of known carbon content.
4.4 Sediment Sampling
Sediment cores were taken from the same sample sites that can be seen in Figure
13 on 29th July 2005. They were extracted using a length of Poly Vinyl Chloride pipe
about 1.5 metres long (see Figure 14) and were then pushed out, using a length of
wood, onto a plastic bag. The cores taken were about 30cm in length. In the field the
cores were stored in an esky until taken back to the laboratory where they were
stored in a freezer until they could be analysed.
35
Oxygen Depletion in the Lower Serpentine River
Figure 14: Sediment core extracted from the lower Serpentine River.
4.5 Analysis of Sediment Samples
After removing the cores from the freezer, a section was taken from the top and
bottom for analysis. About three centimetres was taken from both the top and the
bottom of the core, and these 3cm sections were then sliced vertically into halves.
One half was dried out to use for analysis for sediment uptake of soluble phosphorus,
release of phosphorus from the sediments and particle size, while the other half was
dried to be used for analysis of carbon. The samples were placed into a warm room
to dry completely.
4.5.1 Particle Size Analysis
The dried sediment sample was used to determine particle size fractionation within
the sediment. The sediment samples were placed through sieves of different sizes to
find the percentage of sample that passed through each sieve to give an indication of
what type of soil the sediment was composed of. The sieve sizes used were
36
1000µm, 500µm, 250µm, 100µm and 53µm. The soil was shaken through the sieves.
The mass of the sample was recorded before sieving, and the amount of soil that
passed through each sieve was recorded. From these masses, the percentage of the
soil sample that passed through each filter size was calculated.
4.5.2 Soluble Phosphorus Desorption
To determine the desorption that will occur from the sediment samples, 5g of
sediment from each sample was weighed out and placed into a container with 25mL
of deionised water. A lid was necessary for the container to prevent evaporation from
occurring. This mixture was then shaken and left for a minimum of 24 hours before
the analysis was performed. The soil was analysed for soluble phosphorus
concentration using the same method used to analyse the water samples, using
malachite green as a reactant. The samples were left to settle so that about 10mL of
water could be taken from the top of the sample to be filtered for the analysis. After
filtration of the water using a syringe filter with a 0.45µm membrane, the method
used for analysis of phosphorus in the water samples was followed.
4.5.3 Sediment Uptake of Phosphorus
The soil was analysed for uptake of phosphorus from the water samples collected.
The sediment was air dried and a known amount of sediment (about 2.5g) was
placed into a container. To the sediment, river water (with a known soluble
phosphorus concentration) was added at the ratio of 1:3, therefore 7.5mL was added
to most samples. A lid was then placed on the container and shaken to mix the water
and sediment. The sample was then left to sit for about 3 days before the water was
analysed for soluble phosphorus. To analyse for soluble phosphorus the water was
filtered using a 50mL syringe filter with a 0.45µm membrane and analysed for
phosphorus using the same method as described above.
37
Oxygen Depletion in the Lower Serpentine River
4.5.4 Carbon
The carbon content within the soils was determined using the method loss-onignition. This is one of the most commonly used methods for the destruction of
organic matter (Nelson & Sommers 1996). The high temperatures used for the
method loss-on-ignition causes oxidation of the organic matter present in the soil.
However, the high temperatures can also cause the inorganic constituents of the soil
to lose structural water and some hydrated salts are decomposed during heating,
although for this to occur the temperature needs to be around 750ºC or higher
(Nelson & Sommers 1996). Therefore, the temperature used to find carbon content of
the soil was 400ºC.
The crucibles were first heated in a furnace at 400ºC for 2 hours, they were then
cooled in a dessicator so they would not absorb water while cooling. The weight of
the crucible was determined and about 2g of air-dried sediment sample was added to
each. The crucibles were then weighed again with the sample. The samples were
then placed into a furnace at 400ºC for 16 hours. After 16 hours they were cooled
again in a dessicator and the weight of the crucible plus the sample was determined.
The loss-on-ignition of the sample was calculated using:
LOI (%) = (Weightb - Weighta) × 100
Weightb
Equation 9
Where Weightb is the weight of the sample before it is placed in the oven at 400ºC
and Weighta is the weight of the sample after it has been placed in the oven for 16
hours. This equation assumes that the organic matter content of the soil is assumed
to be equal to the loss of ignition.
38
5 Results
5.1 Historical Data
5.1.1 Temperature
Monthly temperature data for the Mandurah region from 1950 to 2004 was obtained
from the Bureau of Meteorology (2005). The temperature for the months of January
and July were examined. In choosing these months it was assumed that they would
represent the two extremes during any particular year, January representing the
highest temperatures and July representing the lowest. On the graphs below, the
blue line represents the average temperature, for the month of either January or July
for each year, the green line represents the 5-year running average and the red line
shows a linear trend in the average temperature. From the trendline it can be
observed that the temperature maximums and minimums for both July and January
show a slight increase over the years 1950 to 2004. This increase in temperature
ranges from 0.5°C to 1.5°C. Due to the shallow depth of most parts of the lower
Serpentine River, the air temperature will have an effect on the water temperature
within the river, causing the temperature in the river to also increase slightly.
January Maximum Temperatures
Temperature (degrees celcius)
33
32
31
30
Temperature
29
Average (5
years)
28
27
26
25
1950
1960
1970
1980
1990
2000
Year
Figure 15: Average January maximum temperatures for each year from 1950 to 2004 (Kowald
2005). The red line is the trend in temperature over the time period and the green line is the 5-year
running average for the temperature data.
39
Oxygen Depletion in the Lower Serpentine River
January Minimum Temperatures
20
Temperature (degrees celcius)
19.5
19
18.5
Temperature
18
Average
(5yrs)
Linear
(Temperature)
17.5
17
16.5
16
15.5
15
1950
1960
1970
1980
1990
2000
Year
Figure 16: Average January minimum temperature for each year from 1950 to 2004 (Kowald
2005). The red line is the trend in temperature over the time period and the green line is the 5-year
running average for the temperature data.
July Maximum Temperatures
Temperature (degrees celcius)
20
19.5
19
18.5
18
Temperature
Average
(5yrs)
Linear
(Temperature)
17.5
17
16.5
16
15.5
15
1950
1960
1970
1980
1990
2000
Year
Figure 17: Average July maximum temperature for each year from 1950 to 2004 (Kowald 2005).
The red line is the trend in temperature over the time period and the green line is the 5-year running
average for the temperature data.
40
July Minimum Temperatures
Temperature (degrees celcius)
12
11
10
Temperature
9
Average
(5yrs)
Linear
(Temperature)
8
7
6
1950
1960
1970
1980
1990
2000
Year
Figure 18: Average July minimum temperature for each year from 1950 to 2004 (Kowald 2005).
The red line is the trend in temperature over the time period and the green line is the 5-year running
average for the temperature data.
The graph below (Figure 19), which was obtained from the Water Corporation (2004),
also indicates that the temperature has increased since around 1950. The graph
shows the number of days in each year that have reached the temperature of 37.8°C
or higher since 1897. The 10 year moving average shows that up until around 1950
the number of days above 37.8°C has remained reasonably constant, however after
1950 the number of days above 37.8°C started to increase. This is an indication that
the average temperature is increasing.
41
Oxygen Depletion in the Lower Serpentine River
Figure 19: Number of days in each year with a temperature greater than 37.8°C in the Perth
Metropolitan region (Water Corporation 2004).
5.1.2 Rainfall
There are large variations in the annual rainfall over the 100-year period from 1890 to
1990; however from the linear trend line of the rainfall data, a slight decrease in the
annual rainfall in the Mandurah region can be observed. The graph also shows that
since around 1960 the peaks in annual rainfall have decreased. Annual rainfall has
an effect on flow within the Serpentine River, rainfall and flows are positively
correlated, the lower the rainfall, the lower the flow rate in the river.
42
Annual Rainfall for Mandurah Region
14000
Rainfall (mm)
12000
Annual
rainfall
10000
8000
5 yr
running
avg
6000
4000
Linear
(Annual
rainfall)
2000
0
1890
1910
1930
1950
1970
1990
Year
Figure 20: Annual rainfall for the Mandurah region from 1988 to 1992 (Peel Centre for Water
Excellence 2005).
5.1.3 Flow
As with the annual rainfall, it can be seen that the flow in the Serpentine River
fluctuates between years. Flow data was collected by the Department of Environment
using a flow gauge in the river slightly upstream of Karnup Bridge and can be seen in
Figure 21. Downstream of the flow gauge this branch of the river, the upper
Serpentine, joins with the Peel Drain and the joined branches become the lower
Serpentine River. While the flow gauge is only measuring flow from the one branch,
the upper Serpentine River, the contribution from this branch to flow in the lower
Serpentine is much greater than the contribution from the Peel Drain due to the
larger catchment size. Therefore, although the flow gauge does not measure the total
flow in the lower Serpentine it gives an accurate indication of how the flow has
changed over the years and any trends in the flow data. A linear trendline was added
to the annual flow in the river and it shows that the flow in the lower Serpentine River
has decreased by about 40 x106 m3 per year since 1980. This is a significant
decrease considering the annual flow in the Serpentine has averaged around 50 x106
m3 for the past few years; it is a decrease of nearly half.
43
Oxygen Depletion in the Lower Serpentine River
Figure 21: Location of flow gauge at Karnup bridge in the Serpentine River (Department of
Environment 2005c).
Flow in Serpentine River
140
Flow (×10^6 m^3)
120
100
80
60
40
20
0
1980
1985
1990
1995
2000
Year
Figure 22: Annual flow from the flow gauge at Karnup bridge in the lower Serpentine River
(Department of Environment 2005b). The red graph shows the trend in flow within the river.
44
From 1980 to 2004 the period of discharge in the lower Serpentine River has
decreased from about 6-7 months to only 3-4 months. Over the 25 year period the
peak discharge has reduced. The river now commences flowing later in the year,
around June instead of May, and the flow has decreased by around September
rather than November.
Average monthly discharge
900000
Average discharge (10m3)
800000
700000
600000
1980
500000
1990
1993
400000
2004
300000
200000
100000
0
1
3
5
7
9
11
Month
Figure 23: Average monthly discharge for the lower Serpentine River for selected years between
1980 and 2004 (Department of Environment 2005b).
5.1.4 Serpentine Dam
There are two dams on the Serpentine River situated in the Darling Scarp, Pipehead
Dam and Serpentine Main Dam (Figure 24). The Pipehead Dam, in which
construction finished in 1957, is located seven kilometres upstream from Serpentine
Falls. Construction on the Serpentine Main Dam finished in 1961 (Water Corporation
2005). These dams were constructed to be used as a water source for the fast
growing Kwinana industrial area during the 1950’s, located south of the city. As the
Serpentine River was seen as a major water supply source the dams were built. The
Serpentine Main Dam has a capacity of 137.7 million cubic metres and it is one of the
largest dams supplying the Perth region. The Serpentine Pipehead Dam has a
capacity of 3.14 million cubic metres (Water Corporation 2005). Due to the large size
45
Oxygen Depletion in the Lower Serpentine River
of the dams a considerable proportion of flow within the river is collected by the
dams. Therefore flow exiting the dams has been significantly reduced. As can be
seen in Figure 25, the opening of the Serpentine Dams has reduced the flow within
the lower Serpentine River by a factor of about 4 (Ecological Study and Community
Consultation 1996). Because this flow from the jarrah forest would have been low in
nutrients the diluting effects on nutrients entering from the land drains across the
coastal plain has been lost.
Figure 24: Location of Serpentine Dam with respect to the lower Serpentine River (Department
of Conservation and Land Management 2005).
Figure 25: Flow in Serpentine River showing the effect of the Serpentine Dam which was built in
1961 (Ecological Study and Community Consultation 1996). Note that the flow peaks in the late
1960s are due to flushing of the dam.
46
5.2 Water Quality Parameters
5.2.1 Physical Parameters
5.2.1.1 Water Temperature
The water temperature within the river fluctuates throughout the year and is
dependent on the season. The temperature was higher during the summer months
and lower during the winter months, which correlates with the air temperature. The
summer water temperature remained around 26°C, which is slightly lower than the
average maximum air temperature for summer, being about 29°C. The water
temperature in June was around 14°C and increased slightly in July to around 16°C.
Therefore, there is about a 10 degree decrease in temperature between the summer
and winter months. During winter there is freshwater inflow from the river which also
affects the temperature of the water cooling it down further. As there is no river inflow
in summer, the only influence on the water in the river is the tides, therefore the water
is more stagnant allowing it to be more easily heated by the sun.
5.2.1.2 Total Dissolved Salts (Salinity)
The major factors affecting salinity in the lower Serpentine River is the freshwater
runoff from rainfall, and the tidal influence from the ocean. As was predicted, due to
the Mediterranean climate, the salinity of the water was higher during the summer
and lower during winter. The average total dissolved salts (TDS) were around
46000ppm in March, 3000ppm in June and around 800ppm in July. The TDS can be
correlated with the flow in the river, the higher the flow, the more freshwater that is
flowing into the river. As freshwater flows into the river it dilutes the seawater and
causes a decrease in the TDS. The TDS value for seawater is around 35000ppm
(Hoeting 1982), therefore the TDS of the lower Serpentine River in summer is higher
than that of seawater. The TDS also showed an increasing trend from upstream to
downstream. Sampling points further from the ocean had lower TDS concentrations
as the tidal inflow from the ocean has less of an influence on upstream areas of the
river.
5.2.1.3 pH
The pH of the water in the lower Serpentine River remained reasonably constant
throughout the year. The pH fluctuated from around 6.7 to 7.7. According to the
47
Oxygen Depletion in the Lower Serpentine River
ANZECC (Australian New Zealand Environment Conservation Council) guidelines,
for a healthy aquatic ecosystem the pH should remain between 6.5 and 8 (Australian
and New Zealand Environment and Conservation Council 2000). The pH measured
in all areas of the lower Serpentine River remained within these guideline values; this
indicates that pH is not an issue of concern at present.
5.2.1.4 Light Attenuation
Within the Serpentine River light levels are consistently low. Measurements of light
taken on the 10th June 2005 showed that just below the surface of the water the light
was 32 W/m2, while just 25cm below the surface the light was reduced to 2 W/m2 and
any depth below this the light was reduced to 0 W/m2. As there was little turbidity in
the river, the main cause for the light limitation was the concentration of dissolved
organic matter in the water causing the water to be light brown in colour.
5.2.1.5 Dissolved Oxygen
The dissolved oxygen concentrations within the lower Serpentine River varied greatly
between the summer months and winter months. The measurements that were taken
during sampling in March showed a dissolved oxygen concentration of 0% saturation;
no dissolved oxygen was present in the water. Due to the lack of dissolved oxygen in
the water, a fish kill was observed on the same day that the measurement was taken.
The dissolved oxygen readings taken in June were around 40% saturation and the
readings in July were around 50% saturation. Therefore, the dissolved oxygen levels
in the river appear to correlate with flow in the river, the higher the flow the higher the
percentage saturation of dissolved oxygen.
48
Figure 26: Photo taken on sampling day on March 21st 2005 showing fish kill in the Serpentine
River.
Date
of Water Temperature Total Dissolved pH
Dissolved
Sampling
(degrees celcius)
Solids (ppm)
21/03/2005
26°C
46 000
-
0%
10/06/2005
14°C
3 000
7.3
40%
29/07/2005
16°C
800
6.9
50%
Oxygen
(% saturation)
Table 1: Measurements taken on sampling dates for water temperature, total dissolved solids and
dissolved oxygen. The values for these parameters were averaged over all the sites that were tested as
there was no trend between sampling points.
5.2.2 Biological Parameters
5.2.2.1 Chlorophyll a
Measurements for chlorophyll a concentrations were taken for the water samples
collected on the 10th June and 29th July. The concentrations of chlorophyll a in the
water in June were higher than the concentrations of chlorophyll a in July. The
concentrations in June ranged between 11µg/L and 25µg/L, while the concentrations
in July ranged between 1µg/L and 4µg/L. According to the ANZECC guidelines, the
concentration of chlorophyll an estuary should remain below 4µg/L for a healthy
aquatic ecosystem (Australian and New Zealand Environment and Conservation
Council 2000). Therefore the July concentrations are within guideline values however
the June values are higher than the guidelines recommend. The chlorophyll a
concentrations showed a decreasing trend from upstream to downstream.
49
Oxygen Depletion in the Lower Serpentine River
Chlorophyll a Concentrations (10/06/2005)
30
Chlorophyll a (ug/L)
25
20
15
10
5
0
1
2
3
4
5
Station number from upstream to downstream
Figure 27: Chlorophyll a concentrations in the Serpentine River measured in samples taken on
the 10th June 2005.
Chlorophyll a Concentrations (29/07/2005)
10
Chlorophyll a (ug/L)
8
6
4
2
0
1
2
3
4
Station from furthest upstream to downstream
Figure 28: Chlorophyll a concentrations in the Serpentine River measured in samples taken on
the 29th July 2005.
50
5.2.3 Chemical Parameters
5.2.3.1 Total Phosphorus
The total phosphorus concentrations in the Serpentine River were very high; however
the total phosphorus was measured only from the samples collected in July. There
were no trends within the river as the concentration remained similar at all sampling
locations. The concentrations were measured using samples taken on the 29th July
and averaged around 0.25mg/L. The phosphate concentration for a highly eutrophic
system is around 0.05mg/L, this line is plotted in Figure 29 with the total phosphorus
concentration and it can be seen that the concentrations in the river are much higher
than the concentration for highly eutrophic systems.
Total Phosphorus Concentration
Total Phosphorus
Concentrations of phosphorus
considered eutrophic
Phosphate Concentration (mg/L)
0.3
0.25
0.2
0.15
0.1
0.05
0
1
2
3
4
Site
Figure 29: Total phosphorus concentrations in the Serpentine River measured in samples taken
on the 29th July 2005.
From past data it can be observed that the total phosphorus concentration within the
lower Serpentine River increased during the period 1990 to 1995, while the total
phosphorus input into the river decreased over the same time period. The total
phosphorus concentration in the river from 1900 to 1995 averaged around 0.2mg/L
which is slightly lower than the concentrations measured in July 2005, indicating that
there still exists an increasing trend in phosphorus concentrations in the lower
Serpentine River.
51
Oxygen Depletion in the Lower Serpentine River
Total phosphorus concentration in the Serpentine
Median Total Phosphorus
Concentration (mg/L)
0.3
0.25
0.2
0.15
0.1
0.05
0
1990
1991
1992
1993
1994
1995
Year
Figure 30: Total phosphorus concentration in the lower Serpentine River measured before the
river joins to the Peel-Harvey estuary (Lord 1998). The red graph indicates the overall trend in
phosphorus concentration from 1990 to 1995.
Phosphorus Input into the Serpentine River
Phosphorus Input (tonnes)
50
40
30
20
10
0
1990
1991
1992
1993
1994
1995
Year
Figure 31: Total phosphorus input into the Serpentine River from the surrounding catchments
from the years 1990 to 1995 (Lord 1998).
52
5.2.3.2 Soluble Phosphorus
The soluble phosphorus concentration in the Serpentine River was measured using
water samples collected on 29th July 2005. Soluble phosphorus is the phosphorus
dissolved in the water and is available for uptake by organisms. The average soluble
phosphorus concentration in the river was 0.145mg/L. The results showed that the
total phosphorus was comprised of about 50-60% of soluble phosphorus
concentration.
Soluble Phosphorus Concentration
Soluble Phosphorus Concentration
(mg/L)
156
154
152
150
148
146
144
142
140
138
1
2
3
4
Site
Figure 32: Soluble phosphorus concentration in the Serpentine River from samples taken 29th
July 2005.
5.2.3.3 Total Organic Carbon
The total organic carbon (TOC) in the Serpentine River is relatively high being
around 38.4mg/L. This TOC concentration in the river was measured from the
samples taken on 29th July 2005. Figure 33 shows the colour of the river water,
predominantly due to the high dissolved organic carbon concentration, that was
collected in a 2L sample bottle. A high concentration of total organic carbon in the
river causes the water to become coloured and restricts light penetration into the
water.
53
Oxygen Depletion in the Lower Serpentine River
Figure 33: Water sample from the lower Serpentine River on 29th July 2005 showing the colour
of the water due to high levels of dissolved organic carbon.
5.3 Sediment Samples
5.3.1 Particle Size
The particle size distribution indicates that a large percentage of the sediment in the
Serpentine River is sand. Sites 3 and 4, the furthest downstream, had a higher
percentage of silt and clay on the bottom of the core and had a higher percentage of
sand on the top of the core. Sites 1 and 2 had a higher proportion of silt and clay on
the top of the cores.
54
Sample
1b(1) (top)
1b(1) (bottom)
1b(2) (top)
1b(2) (bottom)
2b (top)
2b (bottom)
3b (top)
3b (bottom)
4b (top)
4b (bottom)
% between
500µm
(0.5mm) and
1000µm
% between
250µm and
500µm
% between
100µm and
250µm
% between
53µm and
100µm
% less
than 53µm
(0.053mm)
Coarse sand
Medium
sand
Fine sand
Very fine
sand
Silt and
Clay
24.6
10.1
31.9
7.5
31.3
5.6
1.1
5.8
8.1
4.2
28.5
55.9
33.8
49.5
34.2
47.2
22.9
33.9
41.8
53.5
19.6
30.4
14.9
38.7
19.9
42.9
68.9
45.9
43.6
16.9
15.0
2.6
8.8
3.3
6.8
3.5
7.1
9.2
4.7
12.5
12.3
1.0
10.6
0.9
7.9
0.8
0.0
5.2
1.9
12.9
Table 2: Particle size distribution within the sediments from the Serpentine River.
Sediment from top of cores
100.0
90.0
Percentage Sediment
80.0
70.0
60.0
Site 1b
Site 2b
Site 3b
Site 4b
50.0
40.0
30.0
20.0
10.0
0.0
0
100
200
300
400
500
600
700
800
900
1000
Particle size sediment is under (um)
Figure 34: Sediment particle size distribution from the top of cores taken in the lower Serpentine
River.
55
Oxygen Depletion in the Lower Serpentine River
Sediment from bottom of cores
100.0
90.0
Percentage Sediment
80.0
70.0
60.0
Site 1b
Site 2b
Site 3b
Site 4b
50.0
40.0
30.0
20.0
10.0
0.0
0
100
200
300
400
500
600
700
800
900
1000
Particle size sediment is under (um)
Figure 35: Sediment particle size distribution from the bottom of cores taken in the lower
Serpentine River.
5.3.2 Soluble Phosphorus Desorption
It was found that there was no soluble phosphorus desorption by the sediments using
the method described above.
5.3.3 Uptake of Soluble Phosphorus by Sediment
The uptake of soluble phosphorus by the sediments was high. It can be seen in
Figure 36 that the uptake of phosphorus for sites 1 and 2 was higher for the top of
the soil cores while uptake was higher for the bottom of the soil cores for sites 3 and
4. The phosphorus uptake of the soil in milligrams per gram of sediment can be seen
in appendix D.
56
Percentage Uptake of Soluble Phosphorus by Sediments
100
90
Percentage Uptake
80
70
60
50
Top of soil
cores
40
Bottom of soil
cores
30
20
10
0
1
2
3
4
Site
Figure 36: Percentage uptake of soluble phosphorus from the river water samples by the
sediments in the laboratory.
5.3.4 Carbon
The carbon content of the soil was within the range 0.6% up to 17.2%. According to
Nelson and Sommers (1996), in most soils, inorganic material will make up 90% or
more of the weight of the soil, which means that in most soils the organic content will
be 10% or less of the weight. Most samples from the Serpentine River had carbon
content less than 10% with the two exceptions of the top of sample 2b and the
bottom of sample 4b, which had carbon contents of 17.2% and 16.8% respectively.
The samples with high carbon content were clay soils rather than sands.
57
Oxygen Depletion in the Lower Serpentine River
Weight Weight
Mass
LOI (carbon)
Soil Sample
before
after
lost
(%)
1b top
2.011
1.999
0.012
0.59
1b bottom
2.0093
1.86
0.1493
7.43
2b top
2.005
1.66
0.345
17.21
2b bottom
2.008
1.991
0.017
0.85
3b (1) top
2.005
1.845
0.16
7.98
3b (1) bottom
2.027
2.005
0.022
1.09
3b (2) top
2.01
1.886
0.124
6.17
3b (2) bottom
2.008
1.98
0.028
1.39
4b top
2.0528
2.0298
0.023
1.12
4b bottom
2.0195
1.68
0.3395
16.81
Table 3: Carbon content by percentage of weight in the sediment samples from the Serpentine
River found by loss-on-ignition.
58
6 Discussion
It has been found that the low concentrations of dissolved oxygen within the lower
Serpentine River appear to result from a combination of processes including
temperature, light, stratification, salinity, nutrients, respiration and photosynthesis.
While any one of these factors by itself would not be enough to cause oxygen
depletion in the lower Serpentine, it is the combination of factors that causes oxygen
depletion. Each of the processes that may contribute to reduced oxygen
concentrations is now discussed in detail, with reference to the Serpentine River.
6.1 Land Use and Nutrients
Within the Serpentine region the main land use is for broad scale agricultural
purposes; however there are other more intensive land uses including piggeries,
poultry farms, horticulture, stock holding yards, and industry. The major contributor of
nutrients to agricultural land is from fertiliser that is applied to promote crop
productivity. Fertilisers promote crop growth by adding nutrients. In Western
Australia, the bulk of fertiliser application is designed to overcome phosphorus
deficiency in the nutrient impoverished soils. However, in most cases only a small
proportion of this applied fertiliser is taken up by the plants. It is then possible for the
excess nutrients to be washed into nearby waterways. These nutrients can enter the
river in a number of ways. The nutrients can leach into the ground, entering the
groundwater and be transported to the river via a groundwater source. Due to
clearing of the land for agriculture, the nutrients can also be transported to the river
through soil erosion and will enter the river attached to soil particles. These are the
two most common ways in which phosphorus enters the Serpentine River.
Phosphorus originating from surrounding agricultural catchments is said to be a
‘diffuse source’ of phosphorus, as opposed to a ‘point source’ such as a pipe from an
industry outfall. Other land uses that are classified as point sources include piggeries,
stock holding yards, and poultry farms (Bradby 1997).
59
Oxygen Depletion in the Lower Serpentine River
6.2 Temperature
Over the past 50 years, the air temperature within the south-west region of Western
Australia has increased slightly. This increase in air temperature is likely to have had
an effect on the water temperature within the lower Serpentine River lakes system.
As the lower Serpentine is shallow in most parts, the increase in air temperature may
cause a subsequent increase in the water temperature within the lakes system. Any
increase in water temperature will cause a reduction in the solubility of oxygen within
the water and therefore lower the solubility of oxygen, decreasing the concentration
of dissolved oxygen in the water. With an increase in the water temperature, the
metabolic rate of most aquatic organisms will also increase, resulting in increased
respiration. Therefore, there is a combined impact in that the dissolved oxygen
concentration in the water decreases and at the same time, the oxygen demand
increases due to increased respiration. These combined effects cause oxygen
depletion in the water over the summer months. However, if this effect alone was
responsible for the fish kills then the kills might be expected to coincide with high
water temperatures and this does not appear to be the case.
6.2.1 Algal Blooms
Algal blooms usually occur during the warmer months of the year and are facilitated
by an increase in water temperatures. A temperature of 18°C is warm enough to
facilitate an algal bloom, therefore the high temperatures of around 26°C in the
Serpentine River lakes system in summer are conducive to algal growth (Department
of Conservation and Environment 1984). Conditions that facilitate the growth of algae
are calm, warm conditions with little turbulence in the water (Department of
Conservation and Environment 1984). These ideal conditions are found in the Lower
Serpentine River lakes during the warmer months of the year.
Algal blooms are known to occur within the lower Serpentine River lakes system
(Department of Environment 2005a), and they have the potential to increase oxygen
depletion within the water column. This occurs when the algae die and fall through
the water column to the sediments and decompose. The decomposition of the algae
caused by bacteria requires the consumption of oxygen, therefore contributing to
oxygen depletion within the water.
60
6.3 Rainfall and River Flow
The historical data shows that there has been a decrease in both rainfall and flow
within the Serpentine River over the past 100 years. The two main reasons for the
decreased flow within the river are construction of the Serpentine Dam and the
reduction in annual rainfall across the region since the mid 1970’s. These combined
effects have resulted in the river losing its perennial character and the lower
Serpentine lakes system no longer receives any river flow over the summer months.
Nutrient flushing throughout the system may also be reduced. A lower flushing rate
through the lakes system may also allow stratification to be prolonged as there is less
movement in the water to cause mixing of layers. As there is little or no flow within
the lower Serpentine River lakes system during the summer period the only inflow is
tidal. In consequence the salinity of the lakes system in summer can reach that of
seawater, or higher. High salinity water decreases the solubility of oxygen in the
water, contributing to oxygen depletion of the system.
6.4 Water Characteristics
6.4.1 Light Limitations
The light levels within the river are important in determining the rate of
photosynthesis. Within the lower Serpentine River lakes system, the light entering the
system is attenuated rapidly. From the results it can be observed that light is present
on the surface of the water but its intensity decreases rapidly with increasing depth.
Once the depth reaches about 25cm there is very little light available. This light
limitation is not caused by turbidity but by the high concentration of dissolved organic
carbon (DOC) within the system. This high DOC concentration can also be observed
by the colouring of the river as it is dark in colour.
Light limitation has a large impact on photosynthesis within the water body.
Photosynthesis cannot occur without sufficient light present. Therefore, within the
lower Serpentine lakes system it is only possible for photosynthesis to take place
close to the surface of the water. Below the surface of the water the light intensity is
not high enough for photosynthesis to occur. As with most water bodies,
61
Oxygen Depletion in the Lower Serpentine River
photosynthesis is the dominant source of oxygen supply to the lower Serpentine
River lakes system.
6.4.2 Phosphorus
From the results collected in 2005, it can be observed that there is a high
concentration of phosphorus present within the lower Serpentine River lakes system.
This phosphorus exists in both the water column and within the sediments. As
phosphorus is most often the limiting nutrient in this system, this excess phosphorus
within the river causes eutrophication. The phosphorus within the river is present in
different forms, including soluble and insoluble, and for phosphorus to be utilised by
organisms it must be dissolved within the water (soluble). Phosphorus present in the
sediments is sorbed to the soil particles and cannot be taken up by organisms until it
is released back into the water column. The soluble phosphorus present in the water
is used as a food source for bacteria, therefore the more phosphorus that is present,
the more bacteria that are able to exist. Due to respiration by bacteria, the higher the
numbers of bacteria present, the higher the oxygen depletion in the river. Therefore,
in most conditions, the greater the phosphorus concentration in the water, the higher
the number of bacteria and more oxygen is consumed.
With the presence of saline water as far up the Serpentine River as Karnup Bridge,
when the Serpentine starts to flow after the first heavy winter rains, a
freshwater/saltwater interface initially exists in the river. This interface is likely to
move downstream as the rainfall continues due to an increased freshwater flow from
the upper Serpentine. It is possible for a turbidity maximum to develop at this
interface due to the flocculation of clay resulting from the change in ionic strength
from fresh to saline water (House et al. 1998). With no flocculation (due to low salinity
conditions in the Serpentine Lakes system), this clay may have previously passed
through the system and moved out to sea. However, due to the larger tidal influence
from the ocean since the completion of the Dawesville Channel, there may be a
deposit of more clay-bound nutrients into the lakes system than prior to the
Dawesville Channel due to the settling out of flocculated clay. This has the potential
to increase nutrients within the lakes system and within the lower Serpentine River
due to settling out of adsorbed phosphorus. Due to this potentially large store of
sediment phosphorus, subsequent release from sediments has the potential to cause
eutrophication within the system and is also a food source for aerobic bacteria.
62
6.4.3 Stratification
All estuaries have the potential for stratification due to the density gradient caused by
the denser seawater flowing beneath and in the opposite direction to the less dense
freshwater. Stratification in the lower Serpentine River lakes system is reported to
have intensified since the opening of the Dawesville Channel which allowed seawater
to move further upstream in the river. Since the opening of the channel, stratification
in the lower Serpentine occurs earlier in the summer and is prolonged. Due to this
stratification, no vertical mixing is able to occur within the lower part of the river and
due to light limitation, photosynthesis can only occur in the very upper layer where
there is sufficient light. Therefore, although oxygen is being produced in the upper
layer this is not dispersed to the bottom layer due to stratification and lack of vertical
mixing. Respiration continues within the river and due to no oxygen entering the
lower layer, this layer becomes oxygen depleted. The lower layer can then become
hypoxic or in the worst case anoxic. Anoxic conditions within the water column have
the potential to cause the release of phosphorus within the sediments.
The increase in air temperature that has been observed from past data can intensify
stratification. Due to increased air temperatures the upper layer will become warmer,
therefore decreasing the density. The stratification is then intensified as there is a
warm, fresh layer of water overlying a cool, saline layer of water.
6.4.4 Chlorophyll a
The levels of chlorophyll a are representative of the quantity of algae present in the
river. As algal blooms usually occur during the warmer months the level of chlorophyll
a during winter was expected to be quite low due to increased flushing from higher
rainfall. Chlorophyll a concentrations in samples collected in June were higher than
the concentrations suggested by the ANZECC guidelines for a healthy aquatic
ecosystem. This indicates that algal blooms could be a problem in the lower
Serpentine River, however further water testing will need to be conducted during the
summer period. As the river has a high salinity during the summer months, the most
probable source of algae present in the river will be the Peel-Harvey estuary. Algae
entering the river from the Peel-Harvey estuary will enter on a flood tide therefore it is
63
Oxygen Depletion in the Lower Serpentine River
likely that a large amount of algae will enter the river at the one time causing
depletion of the oxygen supply.
6.5 Sediment Characteristics
6.5.1 Size Distribution of Sediment
The size distribution of the sediments indicated that the upstream sites had more silt
and clay while the downstream sites contained more sand. This supports the
proposition that clay and silt, present in the river flow from the surrounding
catchments, is being deposited at the upstream end of the Serpentine River lakes
system. This supports the proposition that deposition is driven by a turbidity
maximum at the freshwater/saltwater interface. As clay has a higher surface area
than sand, it has the potential to retain more phosphorus and support a higher
number of bacteria and organisms. Therefore, respiration is also likely to be higher
further upstream in the Serpentine River lakes system due to clay rich sediments
supporting a higher number of bacteria.
6.5.2 Carbon in Sediment
Sediments with a higher percentage of silt and clay also contained a higher carbon
content. The high carbon content of the sediment at certain sites indicates that
carbon is entering the river through an external source, most likely the surrounding
vegetation.
6.5.3 P in Sediment
The results indicated that under controlled conditions, almost all the soluble
phosphate from the river water was taken up by the sediments. However, the results
from sampling in the Serpentine lakes system show that phosphate was not taken up
by the sediment and that the water samples had high concentrations of dissolved
phosphate. From the laboratory experiment it was shown that phosphate was not
released from the sediment. These observations suggest that there is very little
phosphate within the sediment. Organic matter contents within the sediments are
64
high and it would require and acid digest to be performed in order to determine the
total P present in the sediments.
Results that were obtained during this project indicate that there is very little
phosphate within the sediments. One possible explanation for this is that a slime
layer (layer of bacteria) has formed on the very upper layer of the sediment. This
layer of bacteria may prevent a diffusive barrier to P uptake by the sediments.
However in this study, when the sediment cores were taken this slime layer was
disturbed. The sediment samples were then dried so that the top layer was mixed
with the remainder of the sediment. Therefore, due to a low concentration of P in this
sediment there was potential to adsorb much of the P from the river water. This
situation would also mean that there is no fresh sediment being deposited on the
river bed from the surrounding catchment. This could possibly be due to the low flow
conditions that now exist in the river as well as the higher tidal flow of water in the
river. As there is a high percentage of carbon within the sediment, carbon 14 dating
could be used to determine how old the sediment is and if the deposit is recent.
65
Oxygen Depletion in the Lower Serpentine River
7 Conclusions
Fish kills in the Serpentine River typically occur during the warmer months of the
year. These fish kills are a result of low dissolved oxygen levels in the river, which
also only occur during times when the temperatures are higher and there is little flow
within the river. The reduced dissolved oxygen level in the river is due to a
combination of factors including the high temperatures and low rainfall that occur
during the summer months.
During the summer months, there is little or no flow from the upstream catchment;
therefore the tidal influence from the Peel-Harvey estuary is the only influence on the
water in the lower Serpentine River. The inflowing ocean water has high salinity
reducing the solubility of dissolved oxygen in the water. The increased water
temperatures also decrease the solubility of oxygen in the water, further reducing the
concentration of dissolved oxygen. Stratification is more likely to occur during the
summer as the water is stagnant. Due to the stratification, vertical mixing between
the upper and lower layer is restricted, restricting the distribution of oxygen.
Light levels in the Serpentine River are low below the surface at all times of the year
due to the high concentrations of organic matter in the water. This restricts
photosynthesis to the very upper layer of the river; therefore oxygen is added to the
water column only in this upper layer of water. During winter there is a large amount
of mixing occurring in the river and the oxygen produced from photosynthesis is
distributed throughout the water column. However, in summer due to stratification,
the oxygen produced through photosynthesis is restricted to the upper layer causing
the lower layer to become further depleted of oxygen.
Respiration occurs throughout the water column causing oxygen to be consumed.
There is a high rate of oxygen consumption in the sediments due to both the
respiration of bacteria and the chemical processes occurring in the sediments. During
summer, oxygen depletion occurs in the lower layer, when the water becomes
stratified, and as no oxygen is being added to this layer it is possible for the water to
become hypoxic or even anoxic. These hypoxic or anoxic conditions are the cause of
fish kills in the lower Serpentine River.
66
8 Future Recommendations
Due to time constraints on this project, water sampling during the summer period was
limited. As oxygen depletion, therefore fish kills, occurs during the warmer months it
is important to find out what is happening within the system during this period. It is
suggested that extensive water sampling be performed during the summer period in
the future. The water samples should be analysed for total and soluble phosphorus,
chlorophyll a, total carbon as well as physical parameters such as pH, temperature,
dissolved oxygen, light attenuation, and salinity. It is also suggested that these
parameters be measured over the depth of the water column to examine how or if
they change from the surface of the water to the sediments.
As biological respiration is the major cause for oxygen depletion within the river, it
would be beneficial to test respiration rates throughout the water column if possible
but more importantly within the bottom sediments. This will give an indication of
where respiration is highest in the water column. This should be conducted at
different times throughout the year and during the different seasons to determine if
the season has an effect on the respiration rate.
Further investigation into the concentration of phosphorus present in the sediment is
recommended. Testing for phosphorus within the sediment by acid digestion is a
more accurate method than the methods used in this project. It is also possible for
the age of the sediment to be found using carbon 14 dating. As there is a high
concentration of carbon 14 in the sediment carbon dating is possible. This will
indicate how recent the sediment has been deposited in the Serpentine River.
67
Oxygen Depletion in the Lower Serpentine River
9 References
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a in Marine and Freshwater Algae by Fluorescence, U.S. Environmental
Protection Agency, Ohio.
Australian and New Zealand Environment and Conservation Council 2000,
'Australian and New Zealand Guidelines for Fresh and Marine Water Quality,
The Guidelines (Chapters 1-7)', in, vol. 1, Department of Environment and
Heritage.
Bradby, K. 1997, Peel-Harvey The Decline and Rescue of an Ecosystem, Greening
and Catchment Taskforce, Mandurah.
Brune, A., Frenzel, P. & Cypionka, H. 2000, 'Life at the oxic-anoxic interface:
microbial activities and adaptations', FEMS Microbiology Reviews, vol. 24,
no. 5, pp. 691-710.
Buzzelli, C., Luettich, R., Powers, S., Peterson, C., McNinch, J., Pinckney, J. & Paerl,
H. 2001, Estimating the spatial extent of bottom-water hypoxia and habitat
degradation in a shallow estuary, University of North Carolina at Chapel Hill,
Institute of Marine Sciences, North Carolina.
Cabello-Pasini, A., Lara-Turrent, C. & Zimmerman, R. C. 2002, 'Effect of storms on
photosynthesis, carbohydrate content and survival of eelgrass populations
from a coastal lagoon and the adjacent open ocean', Aquatic Botany, vol. 74,
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71
Oxygen Depletion in the Lower Serpentine River
10 APPENDICES
Appendix A: Temperature, electrical conductivity, water depth, and pH data
Sampling
Site
Date
Temperature
Electrical
TDS
(°C)
Conductivity
(ppm)
pH
Depth
(mS/cm)
(m)
21/03/2005 1
27.2
75.3
48 190
7.5
2
26.2
71.9
46 016
7.5
3
25.7
70
44 800
6.7
13.8
3.98
2547
7.4
2a
4.8
3072
7.67
3a
3.9
2496
7.5
4a
4.25
2720
7.2
5a
3.85
2464
6.9
4.4
2816
7.2
0.99
634
6.9
10/06/2005 1a
6a
14
29/07/2005 1b
2b
15.7
1.1
704
6.88
3b
16
1.4
896
6.7
4b
16.5
1.49
954
7.07
72
Water
1
1.5
0.65
Appendix B: Light attenuation and dissolved oxygen concentrations
Sampling
Site
Date
Light Attenuation
Secci Depth Dissolved
Licor (W/m2)
(cm)
Oxygen (%)
21/03/2005 1
0
2
0
3
0
10/06/2005 1a
Surface – 32
Lower: 40cm
25cm deep – 2
Upper: 35cm
41
50cm deep - 0
2a
Lower: 38cm
45
Upper: 30cm
3a
Surface – 30
Lower: 35cm
25cm deep– 2
Upper: 28cm
42
50cm deep– 0
4a
Surface – 31
39
25cm deep– 2.1
50cm deep– 0.1
5a
Surface – 34
25cm deep– 1.8
50cm deep– 0.12
Lower: 32cm
38
Upper: 25cm
6a
42
29/07/2005 1b
Lower: 40cm
46.5
Upper: 32cm
2b
3b
49.2
Surface – 35
Lower: 40cm
25cm deep– 1.6
Upper: 30cm
43.7
50cm deep– 0
4b
51.4
73
Oxygen Depletion in the Lower Serpentine River
Appendix C: Chlorophyll a and pheaopigments concentrations
74
Appendix D: Total and soluble phosphorus concentrations
Date
Site
Absorbance
Total
(at 625nm)
phosphorus
(µg/L)
29/07/2005 1b
0.505
257
2b(1)
0.512
261
2b(2)
0.468
236
3b(1)
0.532
272
3b(2)
0.54
276
4b
0.484
245
Filtered
Absorbance at
Soluble
sample
625nm
phosphorus
(µg/L)
1b (1)
0.3
143
1b (2)
0.303
145
2b (1)
0.297
141
2b (2)
0.289
137
3b (1)
0.313
150
3b (2)
0.306
146
4b (1)
0.327
158
4b (2)
0.316
152
Averaged
Filtered
Total
Soluble
Percentage soluble
sample
phosphorus phosphorus
phosphorus is of
(µg/L)
(µg/L)
total phosphorus
1b
257
144
52%
2b
249
139
55%
3b
274
148
57%
4b
245
155
63%
75
Oxygen Depletion in the Lower Serpentine River
Sediment release and uptake of phosphorus
Sediment
Concentration Release of
Sample
in water (µg/l)
phosphorus from
sediment (µg P/g soil)
1b (1) top
0
0
1b (1) bottom
0.33
1.65x10-3
1b (2) top
0
0
1b (2) bottom
0
0
2b top
0
0
2b bottom
5.33
2.665x10-2
3b top
0
0
3b bottom
0
0
4b top
0.085
4.25x10-4
4b bottom
0
0
Sediment
Water
Soluble
Soluble
Uptake of
%
Sample
Sample
phosphorus in
phosphorus in
soluble
uptake
water sample
water sample
phosphorus
before (mg/L)
after (mg/L)
by sediment
1b (1) top
1b
0.144
0.004
0.14
97.22
1b (1) bottom
1b
0.144
0.056
0.088
61.11
1b (2) top
1b
0.144
0
0.144
100.00
1b (2) bottom
1b
0.144
0.034
0.11
76.39
2b top
2b
0.139
0
0.139
100.00
2b bottom
2b
0.139
0.096
0.043
30.94
3b top
3b
0.148
0.028
0.12
81.08
3b bottom
3b
0.148
0.007
0.141
95.27
4b top
4b
0.155
0.04
0.115
74.19
4b bottom
4b
0.155
0
0.155
100.00
76
Soil Sample
Water
Sample
1b (1) top
1b (1) bottom
1b (2) top
1b (2) bottom
2b top
2b bottom
3b top
3b bottom
4b top
4b bottom
1b
1b
1b
1b
2b
2b
3b
3b
4b
4b
Uptake of
soluble
phosphorus
by sediment
Weight
of soil
(g)
0.14
0.088
0.144
0.11
0.139
0.043
0.12
0.141
0.115
0.155
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2
2.5
1.5
77
Vol of
water
(mL)
7.5
7.5
7.5
7.5
7.5
7.5
7.5
6
7.5
4.5
P uptake
in mg
1.05x10-2
6.6x10-3
1.08x10-2
8.25x10-3
1.04x10-3
3.23x10-3
9x10-3
8.46x10-3
8.63x10-3
6.98x10-3
Phosphorus
Uptake in mg
of
phosphorus/g
of sediment
4.2x10-3
2.64x10-3
4.32x10-3
3.3x10-3
4.17x10-3
1.29x10-3
3.6x10-3
4.23x10-3
3.45x10-3
4.65x10-3
Oxygen Depletion in the Lower Serpentine River
Appendix E: Soil Particle Distribution
Sample
Initial Mass
Mass less than
Mass less than
Mass after
(g)
500µm (g)
250µm (g)
Sieving (g)
23.935
17.923
11.153
23.605
1b(1)(bottom) 33.41
29.91
11.31
33.1
1b(2)(top)
23.163
11.66
34.02
1b(2)(bottom) 11.094
9.91
4.86
10.985
1b(2)(bottom) 21.96
20.54
9.28
21.82
2b(top)
6.285
4.193
2.205
6.225
2b(top)
4.42
3.106
1.465
4.31
2b(bottom)
17.412
16.51
7.75
17.36
2b(bottom)
10
9.3
5.148
9.913
3b(top)
11.52
11.39
8.925
11.52
3b(top)
15.45
15.27
11.555
15.4
3b(bottom)
3.49
3.11
1.967
3.454
3b(bottom)
5.58
5.383
3.47
5.5
4b(top)
34.62
31.815
17.335
34.6
4b(bottom)
8.944
8.477
3.744
8.75
1b(1)(top)
34.05
78
Sample
Initial Mass
Mass less
Mass less
(g)
than
than
100µm (g)
53µm (g)
1 (top)
21.59
1.53
0
1 (bottom)
4.02
0.58
0.21
2 (top)
5.59
0.82
0.44
2 (bottom)
19.93
0.86
0.16
3a (top)
18.55
5.07
2.28
3a (bottom)
28.05
1
0.28
3b (top)
28.93
5.604
3.07
3b (bottom)
27.696
1.166
0.256
4 (top)
29.89
1.95
0.56
4 (bottom)
3.756
0.956
0.486
79