Nutrients and their role in promoting cyanobacterial growth along the Murray River, Australia
- Publication Type:
- Thesis
- Issue Date:
- 2023
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Nutrients are important in structuring the phytoplankton community and regulating cyanobacterial blooms. In riverine environments, the prevalence of cyanobacterial blooms has increased, therefore the need to understand how nutrients are interacting with phytoplankton is significant. The macronutrients nitrogen (N) and phosphorus (P) are known drivers of cyanobacterial growth, and micronutrients such as sulfur and the trace metals iron, cobalt and manganese can also influence bloom formation. This thesis examined what macronutrients limit cyanobacterial and phytoplankton growth, spatial and temporal nutrient dynamics and sediment nutrient release within the Murray River, Australia. A combination of lab-based incubation experiments, in-situ microcosm experiments and long-term seasonal monitoring was conducted. Microcosm experiments were run in-situ over 2020–2021 to examine whether N or P or a combination of both, limited phytoplankton and cyanobacterial growth during warmer temperature bloom periods. These experiments indicated that the overall phytoplankton community may be limited by P or co-limited by N and P, whereas most N-fixing cyanobacteria were limited by P only. The control of P in the river may be a useful way to reduce nitrogen fixing cyanobacterial bloom concentrations. Monitoring of nutrient concentration including differences between surface and bottom water column concentration across the five sites was performed to see how they changed spatially and temporally. This was conducted at four weir pool and one riverine site across 24 months along the Murray River. There were only minor differences between top and bottom waters suggesting bottom waters and hence sediments are not a major source. Filtered (0.45 um) available nutrient concentrations were very low generally, suggesting nutrients are possibly locked up in phytoplankton biomass. However, NOₓ concentrations decreased downstream. Total nutrient concentrations were low for TP, however TN increased downstream. I also investigated experimentally the release of nutrients from riverine sediments from Kulnine and Mildura weir pools under oxic and anoxic conditions. Under anoxic treatments there were clear releases of ammonium, iron, cobalt, manganese, and sulfur. Similarly, ammonium, cobalt, manganese and sulfur were released under oxic conditions, while NOₓ was also released just at Kulnine. We found no release of P from sediments. These studies provide novel information for the understanding of nutrients and their role in phytoplankton and cyanobacterial growth in the Murray River. Results from this study may be useful in the management of cyanobacterial blooms as they provide a better understanding of what might be influencing these blooms.
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