Assessing the use of critical flow velocity thresholds for preventing persistent thermal stratification and suppressing cyanobacterial blooms

Toxic cyanobacterial harmful algal blooms (cHABs) are a major worldwide issue in freshwater environments, exacerbated by climate change with rising water temperatures and prolonged and intensified periods of thermal stratification. The Barwon-Darling River in Australia, a dryland river with unpredictable rainfall and highly variable flow regime, often experiences persistent thermal stratification when hotter periods coincide with low discharge, commonly resulting in cHABs. Here we assess a previously proposed management strategy that involves maintaining flows above a critical velocity threshold (0.03-0.05 m/s) to prevent or disrupt stratification and cHABs with a 20-year analysis of Dolichospermum cell counts and river discharge from 6 weir pools on the Barwon-Darling River. Acoustic doppler current profiling was used to find the relationship between discharge and velocity at each site. A negative relationship between flow and cell counts was found and quantile regression estimates show Dolichospermum blooms >15,000 cells/ml can likely be suppressed when a flow velocity of 0.052 m/s (96th quantile, 95% CI: 0.034, 0.104) is maintained. The data also show that blooms frequently occurred below this threshold. Water column temperature measurements between 2020 and 2023 showed that persistent stratification (>1 degree for 7 days) was absent at velocities exceeding 0.05 m/s with a maximum antecedent flow of 0.05 m/s in the prior 7 days. These findings support the use of river flow thresholds for cHAB management and should be applicable to other rivers with similar climates. 

Primary Presenter: Donald Davis, University of Technology, Sydney (donald.c.davis@student.uts.edu.au)

Authors:

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

Andrew Brooks, NSW Department of Planning and Environment  (andrew.brooks@dpie.nsw.gov.au)

Doug Westhorpe, NSW Department of Planning and Environment  (doug.westhorpe@dpie.nsw.gov.au)

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