Many arid and semiarid regions are likely to become warmer and drier by the end of this century, due to human-induced climate change. We hypothesize that a reduction in water level caused by droughts will aggravate eutrophication, leading to higher cyanobacteria biomass and dominance in tropical regions. To test this hypothesis, we analyzed physical and chemical variables and plankton communities of 40 man-made lakes in warm semiarid northeastern Brazil at the end of the wet and dry seasons. We also constructed a predictive model of cyanobacteria biovolume in these lakes. The lakes had significantly lower water volume, transparency, and CO 2 concentrations but higher water temperature, water column stability, electrical conductivity, pH, suspended solids, ammonium, total nitrogen concentrations, bacteria biomass, phytoplankton biomass, and cyanobacteria biomass and dominance in the dry than in the wet season. Our regression model suggested that cyanobacteria biovolume was positively related to water column stability, pH, and total nitrogen and negatively related to water transparency and concentrations of inorganic suspended solids. These results suggest that the projected warmer and drier climate in the future will reduce water quantity and quality of man-made lakes in the region, increasing the risks of salinization, anoxia, eutrophication, and cyanobacteria blooms.