Pollutants such as nutrients and sediment enter waterways through urban stormwater runoff and effluent disposal. The increase in dissolved nutrients frequently promotes microalgal growth which can have many detrimental impacts, including a decrease in the available dissolved oxygen in the water column, water discolouration and odour problems. This can be problematic for both aquatic and terrestrial organisms, including humans. Two important processes affect the development and maintenance of high microalgal biomass: stratification of the water column and grazing losses via herbivores. With increasing pressure on our water supplies through population growth and the predicted increase in frequency and intensity of storm events associated with climate change, catchment condition is critical for maintaining water quality in receiving waters. Microalgal blooms have historically been a problem in the Lower Hawkesbury-Nepean River System but in recent years algal biomass has been relatively low despite large nutrient inputs. This may be attributed to the high levels of suspended sediment within the water column that limit the amount of light available for growth.
With a focus on the Berowra Creek sub catchment, this project aims to:
1. establish the physico-chemical conditions affecting microalgal growth within the water column;
2. determine the importance of grazing as a loss process for microalgae;
3. assess how stratification affects microalgal biomass and species composition.
By identifying nutrient concentrations within the system and how they change during flow events as well as determining light availability and the response of microalgae to light increases, I will determine whether microalgal growth is limited.
The second part of project considers the two important processes of stratification and grazing pressure.
To determine the magnitude of these processes on microalgal growth and species composition, water will be incubated under stratified and non-stratified conditions and grazing will be quantified.