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Observational studies and nutrient amendment experiments were conducted to better understand the nutrient and phytoplankton dynamics of the Hunter River estuary. Eutrophic conditions above ANZECC guidelines for estuaries dominate the Hunter River estuary. The upper Hunter estuary, upstream of its confluence with the Williams River, had the highest concentrations of nutrients and chlorophyll a. The major source of nutrients appears to be riverine discharge. Discharge from WWTP in the upper Hunter potentially contributes an important secondary source of phosphorus. Processes such as bank erosion and resuspension may also be important in explaining variation in nutrient concentrations. Light and turbidity were the main factors limiting phytoplankton growth in the upper estuary. The nutrient amendment experiments showed that when light limitation was alleviated, phytoplankton were either nitrogen limited or remained unlimited by nutrients (suggesting nutrients were in surplus for growth). The expression of nitrogen limitation is likely due to low N:P in the estuary. Organic nitrogen dominates the nitrogen pool within the Hunter estuary. The bioavailability of organic nitrogen in the estuary is unknown which may explain the lack of relationship between phytoplankton and nitrogen concentrations within the estuary. Diatoms and green algae dominated phytoplankton. There were occasions when toxic cyanobacteria was in high abundance in the upper estuary, however a longer data set of phytoplankton assemblage is needed to more adequately assess the risk of toxic cyanobacteria. Comparison of data from the monthly, twice-weekly, and hourly sampling intervals demonstrated the five-year monthly sampling data appeared to mostly capture the variability of nutrient and chlorophyll a concentrations in relation to their main explanatory factors (discharge and light). There were some examples of chlorophyll a and nitrogen concentrations that fell outside of predicted ranges. Overall the results suggest any increase in nitrogen loads to the estuary may lead to increased phytoplankton growth. Improved light climate may also lead to increased phytoplankton growth. Reducing inputs of both nitrogen and phosphorus to the upper Hunter estuary should be a priority action to increase ecosystem health.

University of Technology Sydney. Faculty of Science. Nutrient dynamics play a large role in structuring the phytoplankton community and regulating the severity of cyanobacterial blooms. While the importance of the macronutrients phosphorus and nitrogen for cyanobacterial growth is well established, little attention has been paid to the importance of micronutrient trace metals. This thesis examines how micronutrients effect cyanobacterial growth, bloom formation and toxin production. A combination of in-situ microcosm experiments, lab-based culture experiments, and a long-term monitoring study were conducted. Initially, in situ microcosms were performed at seven sites in South-Eastern Australia to determine how increased micronutrient availability impacts cyanobacterial growth and phytoplankton community structure, and to assess the prevalence of micronutrient growth limitation in Australian freshwater systems. These experiments indicated that micronutrients may be an important regulator of the severity of cyanobacterial blooms, and micronutrient limitation of cyanobacterial growth may be more prevalent than previously anticipated. An 18-month monitoring study on Mannus Lake assessed the role of micronutrients in the formation of recurring high-density cyanobacterial blooms. While persistent thermal stratification primarily drove cyanobacterial growth, some correlations between cyanobacterial biovolume and dissolved micronutrients were observed. Within-dam processes influenced micronutrient levels, particularly during periods of stratification and anoxia of the sediments, whereas inflowing creeks appeared to play a minor role as a micronutrient source. I investigated the specific micronutrient requirements of Microcystis aeruginosa under culture conditions. Low concentrations of Fe, Co and Mn limited M. aeruginosa growth. The minimum Co concentration required for optimal growth of M. aeruginosa was 0.06 μg/L. I compared this value to ten freshwater reservoirs with varying levels of cyanobacteria. Interestingly, all four sites that rarely undergo cyanobacterial blooms had cobalt concentrations below this value. This provides evidence for the capacity of Co to limit, or colimit, cyanobacterial growth in situ. There was some evidence that Co concentration influences microcystin-LR production in culture, but this requires further investigation. The individual studies forming this thesis all contribute new insights to their field. Combined, these studies provide strong evidence that micronutrients play an important role in the phytoplankton community and act as an important regulator of the severity of cyanobacterial blooms in some systems. The results provide a strong case for the increased consideration of micronutrient dynamics to aid the management of harmful algal blooms in freshwater systems.