Short-term effects of very heavy rainfall events on the water quality of a shallow coastal lagoon

Global climate change is expected to affect precipitation patterns over Southeastern South America, increasing the number and intensity of heavy rainfalls. We aimed to identify short-term effects of very heavy rainfall events (VHREs) on abiotic and biological indicators of water quality in Rodrigo de Freitas Lagoon (RFL; Rio de Janeiro, Brazil). Our hypothesis is that VHREs alter water parameters reducing the overall quality of lagoon. A before-and-after effect design was applied by compiling mean weekly data of four abiotic and three biological indicators for 1st and 2nd weeks before and 1st and 2nd weeks after VHRE. One extreme and 15 VHREs were detected from 2018 to 2019, directly interfering on the stormwater input into RFL. Particular water conditions after VHREs were mainly related to increased Escherichia coli density and decreased dissolved oxygen and phytoplankton abundance. Moreover, shifts in relative contribution of phytoplanktonic groups were significantly detected. We successfully identified short-term effects of VHREs on biological and hydrological indicators of water quality in RFL, which have partially supported our hypothesis. Predictions of increased rainfall intensity and frequency of extreme events may lead to harsher and unpredictable changes in ecosystem functioning and lagoon conditions that may impair aquatic biodiversity and fisheries.