Trophic state in Brazilian semiarid reservoirs after reflooding

Prolonged drought extreme events can intensify eutrophication symptoms. This study aimed to analyze the trophic state of two semiarid reservoirs (Cruzeta and Dourado) that experience regular periods of drying and reflooding, verifying limnological changes. Two distinct periods were defined: Period I -before water depletion and Period II - after water renewal. To assess the changes in the limnological variables and the trophic state following the reflooding was conducted Wilcoxon test, Pearson's Correlation, and cluster analysis. Despite similar hydro-meteorological conditions, the reservoirs exhibited different outcomes in limnological variables. Due to the prolonged drought, Cruzeta maintained the eutrophic state in both periods, with the reservoir retaining only 2% of its water volume. In the Dourado reservoir, the trophic state change was observed, shifting from mesotrophic to eutrophic. The acceleration of eutrophication may have occurred due to the existence of organic matter in the sediment, coming from plantations inside the reservoir in addition to phosphorus carried from the drainage basin and by internal loading. Thus, reflooding did not improve water quality conditions and may intensify eutrophication. Therefore, it is recommended that effective reservoir and drainage basin management practices be implemented to mitigate eutrophication and aim for improvements in water quality.