Long-term study of phytoplankton dynamics in a supply reservoir reveals signs of trophic state shift linked to hydrodynamic changes associated with flow management and extreme events.

Over a decade of monitoring data (2009-2022) from Prospect Reservoir, a critical water supply for Sydney, was analysed to identify how hydrodynamics, influenced by flow management and inflow quality, impacts water quality and phytoplankton. Phytoplankton biovolume increased after a prolonged drawdown and subsequent refill event, mainly driven by dinoflagellates, and corresponded to increases in TP and water temperature. Following severe bushfires in 2019/2020 and subsequent heavy flooding, increased nutrient loading from connected reservoirs shifted phytoplankton functional group communities towards meso-eutrophic groups. This transition correlated with elevated nutrient levels and chlorophyll-a (Chl-a), and reduced Secchi depth and dissolved oxygen, providing evidence of eutrophication. Q index indicated good water quality post-bushfire, contrasting with a eutrophic status assessment using Chl-a. Our findings highlight the importance of analysing long-term datasets encompassing varied hydroclimatological conditions for a deeper understanding of reservoir behaviour. A comprehensive approach to water quality assessment is recommended, combining functional group classification, Q index and Chl-a measurements for effective reservoir health assessment. This research provides novel insights into the effects of disturbances such as bushfires, on water quality and phytoplankton dynamics in an underrepresented geographic region, offering valuable knowledge for managing water resources amidst growing climate variability.

Primary Presenter: Huy Luong, University of Technology Sydney (Huy.A.Luong@student.uts.edu.au)

Authors:

Ann-Marie Rohlfs, WaterNSW  (Ann-Marie.Rohlfs@waternsw.com.au)

Jordan Facey, University of Technology Sydney  (Jordan.Facey@uts.edu.au)

Anne Colville, University of Technology Sydney  (Anne.Colville@uts.edu.au)

Simon Mitrovic, University of Sydney  (Simon.Mitrovic@uts.edu.au)