How Extreme Droughts Change the Impact of Eutrophic Reservoir on Its Outflow, with Special References to Planktonic Cyanobacteria and Their Secondary Metabolites?
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Increasingly frequent weather extremes induce changes in the quantity and quality of surface waters, complicating their use and resource management. These challenges are particularly relevant to dam reservoirs, designed to provide high-quality water for various recipients. The impact of extreme drought on lowland eutrophic reservoir–river systems remains poorly understood. Our research showed that the effects of extreme droughts, resulting in a decrease in the water level in a lowland reservoir and its outflow, are more severe than those of floods. During extreme droughts, reservoir pressure increases because the large load of cyanobacteria released from the reservoir, in conditions of low river discharge, is not diluted. unlike during floods. The increase in the total biomass of potamoplankton and, especially, cyanobacteria responsible for the production of toxic microcystins was positively correlated with reservoir outflow. Additionally, a shift in the dominant cyanobacteria species was observed, from Planktothrix agardhii to Microcystis spp., leading to changes in the oligopeptide profile, including microcystins.