Critical flow velocity thresholds for preventing persistent thermal stratification and cyanobacterial blooms in rivers

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 a highly variable flow regime, often experiences persistent thermal stratification (PTS), i.e., continuous stratification for days or weeks, during periods of low discharge, commonly resulting in cHABs. This study evaluated whether a critical flow velocity threshold can prevent or disrupt the formation of PTS and subsequent cHABs. The relationships between gauged discharge and flow velocity were determined within six weir pools along the river using acoustic Doppler current profiling. These relationships were used to interpret 20 years of historic daily river discharge data as mean cross-sectional flow velocities which were then compared to cyanobacterial biovolumes over the same period, focusing predominantly on the potentially toxic genera Dolichospermum. An upper limiting relationship between flow velocity and Dolichospermum biovolume was found and almost all blooms > 4 mm³/L (exceeding the Australian recreational safety guidelines for toxic cyanobacteria) occurred when maximum antecedent 7-day flow velocities were less than 0.05 m/s. Quantile regression estimates suggested maintaining flow velocities of 0.05 m/s can limit blooms to 3.96 mm³/L while increasing flow velocities will reduce this limit. These findings support the premise of using river flow velocity thresholds for cHAB management when blooms are related to PTS. These flow velocities should be applicable to other rivers in similar climates while the approach could be utilised more widely.