Effects of extreme inflows on the water quality and phytoplankton of seven reservoirs in subtropical Australia

Differentiating the relative importance of catchments and reservoirs in driving water quality in impounded rivers is challenging because water quality reflects the integration of peak events (e.g., rainfall-driven inflows) and longer-term processes (e.g., sediment remineralization of nutrients). We examined water quality in several subtropical reservoirs following extreme summer rainfall and associated inflows to determine short-and longer-term implications in terms of reservoir vulnerability to poor water quality and blooms of cyanobacteria, as reflected by the Vulnerability Index (VI), a previously described index validated under low-flow conditions. The reservoir with the highest VI, Wivenhoe, had the highest concentrations of total suspended solids, phosphorus (total and dissolved inorganic fractions), and nitrate/nitrite following the event, whereas the reservoir with the lowest VI, Cooloolabin, had the lowest concentrations. This suggests that vulnerability to poor water quality during major inflows is driven by suspended solids and nutrients, with the VI being a robust measure irrespective of inflow conditions. By contrast, phytoplankton-related measures, including cyanobacteria proportions, in Wivenhoe were not higher than in Cooloolabin, possibly due to reduced water residence times and increased light attenuation (shallower Secchi depth) preventing bloom establishment. This finding contrasts with low-flow summers when reservoirs with the highest phytoplankton biomass align with those with the highest VI and suggests that short-term vulnerability to poor water quality associated with peak events is not driven directly by phytoplankton. While inflows increase nutrients and suspended solids, biotic responses appear delayed, highlighting the challenges of linking catchment-derived nutrient loads to phytoplankton responses in event-driven systems.