LAUSR

This study reports the effects of the El Niño event of 2015–16 on abiotic and biotic variables in tropical reservoirs in semi-arid and wet regions. Samples were collected in control years (2014) and during the El Niño event (2015–16). Phytoplankton was identified and its biomass quantified. Physicochemical and climatic variables were evaluated. Canonical analyses were performed to determine the effects of abiotic variables on phytoplankton. In semi-arid ecosystems, there was a decrease in diversity during the El Niño event, as well as an increase in the relative biomass of centric diatoms and a decrease in filamentous cyanobacteria. In wet ecosystems, changes in the community were not observed during the El Niño event. The synergistic contribution between physiochemical and climatic variables was most pronounced during the El Niño event in all ecosystems. Filamentous cyanobacteria were associated with the control year and favoured by higher pH values and sulfate concentrations. Centric diatoms were related to higher air temperatures and lower sulfate concentrations during the El Niño event. El Niño caused morphofunctional modifications to the phytoplankton in a distinct way in semi-arid and wet ecosystems. We show that in semi-arid reservoirs the El Niño event decreased the relative biomass of filamentous cyanobacteria and favoured central diatoms because of changes to sulfate concentrations.