Frequent upwelling intrusions and rainfall events drive shifts in plankton community in a highly eutrophic estuary

Abstract Seasonal upwelling and rainfall events influence estuarine communities, but their interaction is not well understood, particularly in estuaries impacted by coastal eutrophication. Here we evaluated the responses of the plankton community to multiple drivers in Guanabara Bay, a eutrophic estuary in the western Atlantic influenced by coastal upwelling. Plankton taxonomy and biomass were determined through weekly sampling over 1-year. The presence of the South Atlantic Central Water (SACW) signature was detected in the bay in half of our samples during the wet season. Importantly, SACW intrusions derived from coastal upwelling had a significant impact on plankton community composition. For microplankton, taxonomic shifts reflected in different cell sizes, with larger mixotrophic and heterotrophic taxa characterizing SACW intrusions. Larger species of copepods, including indicators of SACW, were also associated to the upwelling intrusion. Overall, plankton biomass was not explained by the SACW presence or absence alone; instead, the interaction between rainfall and upwelling was key to determine temporal variations in biomass. Our findings suggest that SACW intrusions can impact food web functioning through shifts in community composition and size depending on the relative intensity and frequency of upwelling events and rainfall. Frequent SACW intrusions might also dilute the highly eutrophic waters, with the potential to shift the metabolism of an estuary heavily impacted by eutrophication.