Oxygen Depletion in Lower Serpentine River

Transcription

1 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

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3 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

4 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 ii

5 Kate Roehner increased respiration, therefore higher oxygen uptake creating anoxic conditions within the system, causing the death of many fish species within the river. iii

6 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. iv

7 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. v

8 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 LITERATURE REVIEW Dissolved Oxygen in Water Bodies Importance of Dissolved Oxygen for Ecology Hypoxia and Anoxia in Water Bodies Respiration Effects of an aquatic environment on respiration Photosynthesis Effects of a water environment on photosynthesis Dissolved Organic Carbon Oxidation and Reduction Depletion of Oxygen in Estuaries Increased Temperature Estuaries and Stratification Phosphorus Cycling Phosphorus in Estuaries and turbidity maximum Release of Phosphorus from Sediments Sediment Oxygen Demand Eutrophication and Algal Blooms in Waterways BACKGROUND Peel-Harvey Estuary and the Lower Serpentine River METHODOLOGY Selection of Sampling Sites...28 vi

9 Kate Roehner 4.2 Water Sampling Analysis of Water Samples Chlorophyll a Unfiltered and Filtered Phosphorus Total Organic Carbon (TOC) Sediment Sampling Analysis of Sediment Samples Particle Size Analysis Soluble Phosphorus Desorption Sediment Uptake of Phosphorus Carbon RESULTS Historical Data Temperature Rainfall Flow Serpentine Dam Water Quality Parameters Physical Parameters Water Temperature Total Dissolved Salts (Salinity) ph Light Attenuation Dissolved Oxygen Biological Parameters Chlorophyll a Chemical Parameters Total Phosphorus Soluble Phosphorus Total Organic Carbon Sediment Samples Particle Size Soluble Phosphorus Desorption Uptake of Soluble Phosphorus by Sediment Carbon DISCUSSION Land Use and Nutrients Temperature Algal Blooms Rainfall and River Flow Water Characteristics Light Limitations Phosphorus Stratification Chlorophyll a...63 vii

10 Oxygen Depletion in the Lower Serpentine River 6.5 Sediment Characteristics Size Distribution of Sediment Carbon in Sediment P in Sediment CONCLUSIONS FUTURE RECOMMENDATIONS REFERENCES APPENDICES...72 viii

11 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 ix

12 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 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 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 FIGURE 7: DAWESVILLE CHANNEL VIEWED FROM THE ESTUARY SIDE (DEPARTMENT OF EDUCATION AND TRAINING 2003) 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) 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) FIGURE 13: SAMPLING LOCATIONS IN THE LOWER SERPENTINE RIVER FROM JUNE AND JULY (GOOGLE EARTH 2005) 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) FIGURE 21: LOCATION OF FLOW GAUGE AT KARNUP BRIDGE IN THE SERPENTINE RIVER (DEPARTMENT OF ENVIRONMENT 2005C) x

13 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 FIGURE 23: AVERAGE MONTHLY DISCHARGE FOR THE LOWER SERPENTINE RIVER FOR SELECTED YEARS BETWEEN 1980 AND 2004 (DEPARTMENT OF ENVIRONMENT 2005B) 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 21 ST 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 FIGURE 28: CHLOROPHYLL A CONCENTRATIONS IN THE SERPENTINE RIVER MEASURED IN SAMPLES TAKEN ON THE 29TH JULY FIGURE 29: TOTAL PHOSPHORUS CONCENTRATIONS IN THE SERPENTINE RIVER MEASURED IN SAMPLES TAKEN ON THE 29TH JULY 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 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 FIGURE 33: WATER SAMPLE FROM THE LOWER SERPENTINE RIVER ON 29 TH JULY 2005 SHOWING THE COLOUR OF THE WATER DUE TO HIGH LEVELS OF DISSOLVED ORGANIC CARBON 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 xi

14 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 TABLE 3: CARBON CONTENT BY PERCENTAGE OF WEIGHT IN THE SEDIMENT SAMPLES FROM THE SERPENTINE RIVER FOUND BY LOSS-ON-IGNITION...58 xii

15 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. 1

16 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

17 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 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

18 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 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

19 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): C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + 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) 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

20 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) 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): CO 2 + H 2 O CH 2 O (carbohydrate) + O 2 Equation 2 The carbohydrate most often produced is glucose, this reaction can be seen in Equation 3: 6

21 6CO 2 + 6H 2 O C 6 H 12 O 6 (glucose) + 6O 2 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) 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

22 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

23 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 µm (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 mg/L (Findlay & Sinsabaugh 2003). The flow path of water through a soil profile is an important determinant for the final 9

24 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

25 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 (PO 3-4 ), 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 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

26 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 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

27 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

28 Oxygen Depletion in the Lower Serpentine River nutrients for bacteria further enhancing respiration, which can lead to extended periods of anoxia 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 (PO 3-4 ) (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.) 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/1000cm 3 ) 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

29 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 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

30 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

31 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 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

32 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

33 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

34 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

35 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 136km 2 (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 1600km 2 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 Peel- Harvey 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

36 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

37 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

38 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

39 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 Phosphorus input (tonnes) Year Figure 8: Phosphorus input into the Serpentine River from the surrounding catchments (Department of Conservation and Environment 1984). 25

40 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 23 rd 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

41 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

42 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 21 st March, 10 th June and 29 th July 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 10 th June 2005 and 29 th 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

43 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

44 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

45 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

46 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 WP- 80D 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 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

47 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

48 Oxygen Depletion in the Lower Serpentine River 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 (PO 3-4 ). 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

49 3- produce an equation to relate the absorbance to the PO 4 concentration. The equation used to convert the absorbance to a PO 3-4 concentration is: y = x Equation 7 Where y is the absorbance at 625nm and x is the PO 4 3- concentration. Therefore, to find the concentration (in µg/l) the equation can be rearranged: x = (y )/ Equation 8 3- Once the absorbance readings were obtained, these values were converted to PO 4 concentrations using the above equation 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 29 th July 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

50 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 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

51 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 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 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

52 Oxygen Depletion in the Lower Serpentine River 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 (%) = (Weight b - Weight a ) 100 Equation 9 Weight b Where Weight b is the weight of the sample before it is placed in the oven at 400ºC and Weight a 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

53 5 Results 5.1 Historical Data 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 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 33 Temperature (degrees celcius) Temperature Average (5 years) 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

54 Oxygen Depletion in the Lower Serpentine River January Minimum Temperatures Temperature (degrees celcius) Temperature Average (5yrs) Linear (Temperature) 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 20 Temperature (degrees celcius) Temperature Average (5yrs) Linear (Temperature) 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

55 July Minimum Temperatures 12 Temperature (degrees celcius) Temperature Average (5yrs) Linear (Temperature) 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 The graph shows the number of days in each year that have reached the temperature of 37.8 C or higher since 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

56 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) 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

57 Annual Rainfall for Mandurah Region Rainfall (mm) Year Annual rainfall 5 yr running avg Linear (Annual rainfall) Figure 20: Annual rainfall for the Mandurah region from 1988 to 1992 (Peel Centre for Water Excellence 2005) 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 x10 6 m 3 per year since This is a significant decrease considering the annual flow in the Serpentine has averaged around 50 x10 6 m 3 for the past few years; it is a decrease of nearly half. 43

58 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 Flow ( 10^6 m^3) 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

59 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 Average discharge (10m3) Month Figure 23: Average monthly discharge for the lower Serpentine River for selected years between 1980 and 2004 (Department of Environment 2005b) 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 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

60 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

61 5.2 Water Quality Parameters Physical Parameters 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 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 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

62 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 Light Attenuation Within the Serpentine River light levels are consistently low. Measurements of light taken on the 10 th June 2005 showed that just below the surface of the water the light was 32 W/m 2, while just 25cm below the surface the light was reduced to 2 W/m 2 and any depth below this the light was reduced to 0 W/m 2. 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 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

63 Figure 26: Photo taken on sampling day on March 21 st 2005 showing fish kill in the Serpentine River. Date of Sampling Water Temperature (degrees celcius) Total Dissolved Solids (ppm) 21/03/ C % 10/06/ C % 29/07/ C % ph Dissolved 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 Biological Parameters Chlorophyll a Measurements for chlorophyll a concentrations were taken for the water samples collected on the 10 th June and 29 th 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

64 Oxygen Depletion in the Lower Serpentine River Chlorophyll a Concentrations (10/06/2005) Chlorophyll a (ug/l) Station number from upstream to downstream Figure 27: Chlorophyll a concentrations in the Serpentine River measured in samples taken on the 10th June Chlorophyll a Concentrations (29/07/2005) 10 8 Chlorophyll a (ug/l) Station from furthest upstream to downstream Figure 28: Chlorophyll a concentrations in the Serpentine River measured in samples taken on the 29th July

65 5.2.3 Chemical Parameters 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 29 th 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. 0.3 Total Phosphorus Concentration Total Phosphorus Concentrations of phosphorus considered eutrophic Phosphate Concentration (mg/l) Site Figure 29: Total phosphorus concentrations in the Serpentine River measured in samples taken on the 29th July 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

66 Oxygen Depletion in the Lower Serpentine River Total phosphorus concentration in the Serpentine 0.3 Median Total Phosphorus Concentration (mg/l) 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 Phosphorus Input into the Serpentine River 50 Phosphorus Input (tonnes) Year Figure 31: Total phosphorus input into the Serpentine River from the surrounding catchments from the years 1990 to 1995 (Lord 1998). 52

67 Soluble Phosphorus The soluble phosphorus concentration in the Serpentine River was measured using water samples collected on 29 th July 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 156 Soluble Phosphorus Concentration (mg/l) Figure 32: Soluble phosphorus concentration in the Serpentine River from samples taken 29th July Site 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 29 th July 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

68 Oxygen Depletion in the Lower Serpentine River Figure 33: Water sample from the lower Serpentine River on 29 th July 2005 showing the colour of the water due to high levels of dissolved organic carbon. 5.3 Sediment Samples 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

69 Sample % 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 1b(1) (top) b(1) (bottom) b(2) (top) b(2) (bottom) b (top) b (bottom) b (top) b (bottom) b (top) b (bottom) Table 2: Particle size distribution within the sediments from the Serpentine River. Sediment from top of cores Percentage Sediment Site 1b Site 2b Site 3b Site 4b Particle size sediment is under (um) Figure 34: Sediment particle size distribution from the top of cores taken in the lower Serpentine River. 55

70 Oxygen Depletion in the Lower Serpentine River Sediment from bottom of cores Percentage Sediment Site 1b Site 2b Site 3b Site 4b Particle size sediment is under (um) Figure 35: Sediment particle size distribution from the bottom of cores taken in the lower Serpentine River Soluble Phosphorus Desorption It was found that there was no soluble phosphorus desorption by the sediments using the method described above 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

71 Percentage Uptake of Soluble Phosphorus by Sediments Percentage Uptake Top of soil cores Bottom of soil cores Site Figure 36: Percentage uptake of soluble phosphorus from the river water samples by the sediments in the laboratory 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

72 Oxygen Depletion in the Lower Serpentine River Soil Sample Weight before Weight after Mass lost 1b top b bottom b top b bottom b (1) top b (1) bottom b (2) top b (2) bottom b top b bottom LOI (carbon) (%) Table 3: Carbon content by percentage of weight in the sediment samples from the Serpentine River found by loss-on-ignition. 58

73 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

74 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 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

75 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 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

76 Oxygen Depletion in the Lower Serpentine River photosynthesis is the dominant source of oxygen supply to the lower Serpentine River lakes system 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

77 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 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

78 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 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 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 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

79 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

80 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

81 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

82 Oxygen Depletion in the Lower Serpentine River 9 References Arar, E. & Collins, G. 1997, In Vitro Determination of Chlorophyll a and Pheophytin 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 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, no. 2, pp de Lestang, S., Hall, N. & Potter, I. C. 2003, 'Influence of a deep artificial entrance channel on the biological characteristics of the blue swimmer crab Portunus pelagicus in a large microtidal estuary', Journal of Experimental Marine Biology and Ecology, vol. 295, no. 1, pp Dejours, P. 1988, Respiration in Water and Air, Elsevier Science Publishers B.V., The Netherlands. Department of Conservation and Environment 1984, 'Report of research findings and options for management', Management of Peel Inlet and Harvey Estuary, vol Department of Conservation and Environment 1985, 'Peel-Harvey Estuarine System Study Management of the Estuary', in Peel-Harvey Estuarine System Study Management of the Estuary, University of Western Australia. Department of Conservation and Land Management, NatureBase [Online], Available: tine%20dam' [ ]. Department of Education and Training, Eutrophication [Online], Government of Western Australia, Available: m [23/6/2005]. Department of Environment 2005a, Algal Blooms in the Serpentine River, Perth. Department of Environment 2005b, Flow data for the Serpentine River, K. Smettem, Department of Environment, Perth. Department of Environment 2005c, Unpublished Report, Perth. Dirnberger, Ensign & Sutton, Physical Properties of Water [Online], Available: [ ]. 68

83 Ecological Study and Community Consultation 1996, Report on the Serpentine River, Chambers and Galloway Associates, Perth. Ekholm, P., Turtola, E., Gronroos, J., Seuri, P. & Ylivainio, K., 'Phosphorus loss from different farming systems estimated from soil surface phosphorus balance', Agriculture, Ecosystems & Environment, vol. In Press, Corrected Proof. Elliott, M. & McLusky, D. S. 2002, 'The Need for Definitions in Understanding Estuaries', Estuarine, Coastal and Shelf Science, vol. 55, no. 6, pp Felsing, M., Glencross, B. & Telfer, T. 2005, 'Preliminary study on the effects of exclusion of wild fauna from aquaculture cages in a shallow marine environment', Aquaculture, vol. 243, no. 1-4, pp Findlay, S. & Sinsabaugh, R. 2003, Aquatic Ecosystems: Interactivity of Dissolved Organic Matter, Elsevier Science, USA. Gardolinski, P. C. F. C., Worsfold, P. J. & McKelvie, I. D. 2004, 'Seawater induced release and transformation of organic and inorganic phosphorus from river sediments', Water Research, vol. 38, no. 3, pp Gerritse, R. G., Wallbrink, P. J. & Murray, A. S. 1998, 'Accumulation of Phosphorus and Heavy Metals in the Peel-Harvey Estuary in Western Australia: Results of a Preliminary Study', Estuarine, Coastal and Shelf Science, vol. 47, no. 6, pp Giffin, D. & Corbett, D. R. 2003, 'Evaluation of sediment dynamics in coastal systems via short-lived radioisotopes', Journal of Marine Systems, vol. 42, no. 3-4, pp Goni, M. A., Cathey, M. W., Kim, Y. H. & Voulgaris, G. 2005, 'Fluxes and sources of suspended organic matter in an estuarine turbidity maximum region during low discharge conditions', Estuarine, Coastal and Shelf Science, vol. 63, no. 4, pp Google Earth, Google Earth [Online], Available: [ ]. Gwinn, R. 1987, Encyclopaedia Britannica, International Copyright Union by Encyclopaedia Britannica Inc., United States of America. Hall, D. O. & Rao, K. K. 1987, Photosynthesis, D.O. Hall and K.K. Rao, Great Britain. Heathwaite, A. L., Dils, R. M., Liu, S., Carvalho, L., Brazier, R. E., Pope, L., Hughes, M., Phillips, G. & May, L. 2005, 'A tiered risk-based approach for predicting diffuse and point source phosphorus losses in agricultural areas', Science of The Total Environment, vol. 344, no. 1-3, pp Hoeting, W. A. G. 1982, 'Seawater reverse osmosis with energy recovery', Desalination, vol. 40, no. 3, pp House, W. A., Jickells, T., Edwards, A., Praska, K. & Denison, F. H. 1998, 'Reactions of phosphorus with sediments in fresh and marine environments', Soil Use and Management, vol. 14, pp Hu, W. F., Lo, W., Chua, H., Sin, S. N. & Yu, P. H. F. 2001, 'Nutrient release and sediment oxygen demand in a eutrophic land-locked embayment in Hong Kong', Environment International, vol. 26, no. 5-6, pp Jarvie, H. P., Jurgens, M. D., Williams, R. J., Neal, C., Davies, J. J. L., Barrett, C. & White, J. 2005, 'Role of river bed sediments as sources and sinks of phosphorus across two major eutrophic UK river basins: the Hampshire Avon and Herefordshire Wye', Journal of Hydrology, vol. 304, no. 1-4, pp Kalff, J. 2002, Limnology, Prentice-Hall, United States of America. 69

84 Oxygen Depletion in the Lower Serpentine River Kobliz, M., Ston-Egiert, J., Sagan, S. & Kolber, Z. S. 2005, 'Diel changes in bacteriochlorophyll a concentration suggest rapid bacterioplankton cycling in the Baltic Sea', FEMS Microbiology Ecology, vol. 51, no. 3, pp Kostoglidis, A., Pattiaratchi, C. B. & Hamilton, D. P. 2005, 'CDOM and its contribution to the underwater light climate of a shallow, microtidal estuary in south-western Australia', Estuarine, Coastal and Shelf Science, vol. 63, no. 4, pp Kowald, B. 2005, Bureau of Meteorology, Perth. Lawlor, D. W. 1987, Photosynthesis: metabolism, control and physiology, Longman Group Limited, England. Long, S. P. & Baker, N. R. 1986, Photosynthesis in Contrasting Environements, Elsevier Science Publishers B.V., The Netherlands. Lord, D. A. 1998, Dawesville Channel Monitoring Programme, Water and Rivers Commission, Perth. Motomizu, S. & Li, Z.-H. 2005, 'Trace and ultratrace analysis methods for the determination of phosphorus by flow-injection techniques', Talanta, vol. 66, no. 2, pp Muylaert, K., Dasseville, R., De Brabandere, L., Dehairs, F. & Vyverman, W. 2005, 'Dissolved organic carbon in the freshwater tidal reaches of the Schelde estuary', Estuarine, Coastal and Shelf Science, vol. 64, no. 4, pp Nelson & Sommers 1996, 'Carbon and Organic Matter', in SSSA Book Series Number 5, ed. D. L. Sparks, Soil Science of America, USA. Paresys, G., Rigart, C., Rousseau, B., Wong, A. W. M., Fan, F., Barbier, J.-P. & Lavaud, J. 2005, 'Quantitative and qualitative evaluation of phytoplankton communities by trichromatic chlorophyll fluorescence excitation with special focus on cyanobacteria', Water Research, vol. 39, no. 5, pp Parr, L. B. & Mason, C. F. 2004, 'Causes of low oxygen in a lowland, regulated eutrophic river in Eastern England', Science of The Total Environment, vol. 321, no. 1-3, pp Peel Centre for Water Excellence, Peel Region Data Sets [Online], Available: [29/4/05]. Peuhkuri, T. 2002, 'Knowledge and interpretation in environmental conflict: Fish farming and eutrophication in the Archipelago Sea, SW Finland', Landscape and Urban Planning, vol. 61, no. 2-4, pp Smith, K., Allison, R., Hammond, M. & Weir, K. 2004, The impact on fish species in the Serpentine River by a fish kill event on the 23 February 2004, Department of Fisheries, Mandurah. Stoddart, J. & Simpson, C. 1996, Issues in the Western Australian Marine Environment, Environmental Protection Authority, Perth. Summers, R. N., Van Gool, D., Guise, N. R., Heady, G. J. & Allen, T. 1999, 'The phosphorus content in the run-off from the coastal catchment of the Peel Inlet and Harvey Estuary and its associations with land characteristics', Agriculture, Ecosystems & Environment, vol. 73, no. 3, pp Tissue, B. M., Infrared Absorption Spectroscopy (IR) [Online], Available: [ ]. University of Neuchatel, Natural Oxygen Cycle [Online], Chimie Generale, Available: [ ]. Water and Rivers Commission and Peel Inlet Management Authority 1998, 'Serpentine River Management Plan, Stage 1 - Goegrup Lake to Barragup Bridge', Water Resource Management Series, vol

85 Water Corporation 2004, Water Corporation, K. Smettem, Perth. Water Corporation, Serpentine Dam [Online], Water Corporation, Available: [ ]. Waterwatch, Salinity - Dissolved Solids [Online], Available: [ ]. Wetzel, R. 2001, Limnology - Lake and River Ecosystems, Academic Press, California. Wikipedia, Wikipedia Encyclopedia [Online], Available: [5/5/05]. Yin, K., Lin, Z. & Ke, Z. 2004, 'Temporal and spatial distribution of dissolved oxygen in the Pearl River Estuary and adjacent coastal waters', Continental Shelf Research, vol. 24, no. 16, pp Zammit, C., Sivapalan, M., Kelsey, P. & Viney, N. R., 'Modelling the effects of landuse modifications to control nutrient loads from an agricultural catchment in Western Australia', Ecological Modelling, vol. In Press, Corrected Proof. Zeng, X., Rasmussen, T. C., Beck, M. B., Parker, A. K. & Lin, Z., 'A biogeochemical model for metabolism and nutrient cycling in a Southeastern Piedmont impoundment', Environmental Modelling & Software, vol. In Press, Corrected Proof. 71

86 Oxygen Depletion in the Lower Serpentine River 10 APPENDICES Appendix A: Temperature, electrical conductivity, water depth, and ph data Sampling Date Site Temperature ( C) Electrical Conductivity (ms/cm) TDS (ppm) ph Water Depth (m) 21/03/ /06/2005 1a a a a a a /07/2005 1b b b b

87 Appendix B: Light attenuation and dissolved oxygen concentrations Sampling Date Site Light Attenuation Licor (W/m 2 ) Secci Depth (cm) Dissolved Oxygen (%) 21/03/ /06/2005 1a Surface 32 Lower: 40cm 41 25cm deep 2 50cm deep - 0 Upper: 35cm 2a Lower: 38cm 45 Upper: 30cm 3a Surface 30 Lower: 35cm 42 25cm deep 2 50cm deep 0 Upper: 28cm 4a Surface 31 25cm deep cm deep a Surface Lower: 32cm 25cm deep 1.8 Upper: 25cm 50cm deep a 42 29/07/2005 1b Lower: 40cm 46.5 Upper: 32cm 2b b Surface 35 Lower: 40cm cm deep cm deep 0 Upper: 30cm 4b

88 Oxygen Depletion in the Lower Serpentine River Appendix C: Chlorophyll a and pheaopigments concentrations 74