Effects of Wind Mixing in a Stratified Water Column on Toxic Cyanobacteria and Microcystin-LR Distribution in a Subtropical Reservoir

We analyzed the effects of stratification changes due to wind on the vertical cyanobacteria distribution and microcystin-LR concentrations in a reservoir and assessed the implications for water management. Under stratified conditions, the highest microcystin concentrations (up to 4.16 µg/L) and toxic cyanobacteria biovolume occurred in the epilimnion (~ 1 m). The lowest microcystin concentrations were between 0.02 and 1.28 µg/L and occurred in the hypolimnion (~ 20 m). A cold front passage associated with high wind velocities induced water column mixing, promoting the redistribution of microcystin-LR and cyanobacteria throughout the water column and increasing their concentrations in deeper zones. Microcystin-LR concentration was positively correlated with cyanobacteria biovolume (r = 0.747) and chlorophyll a concentration (r = 0.798). Changes in thermal profile due to wind would imply a greater challenge for drinking water treatment plants, since high cyanobacterial and microcystin concentrations could reach deep-water intakes.