How Extreme Droughts Change the Impact of Eutrophic Reservoir on Its Outflow, with Special References to Planktonic Cyanobacteria and Their Secondary Metabolites?

Increasingly frequent weather extremes induce changes in the quantity and quality of surface waters, which significantly complicates their use and management of their resources. The above problems concern in particular dam reservoirs, created to ensure good quality water for numerous recipients. Impact of extreme drought in lowland eutrophic reservoir-river system is still poorly understood. Our studies were focused on average and high water levels in the reservoir and outflowing river. These studies conducted during three extreme drought periods showed the dominance of toxic cyanobacteria in the phytoplankton of the reservoir and its outflow. During the drought periods, a change in the dominant cyanobacteria was observed from Planktothrix agardhii to Microcystis spp., which resulted in a change in oligopeptides profile, including toxic microcystins. The negative pressure of the strongly eutrophic reservoir on downstream river, due to release of a large biomass of cyanobacteria, was positively correlated with reservoir outflow.