Influence of the strong 2015/16 El Niño and extreme rainfall events on micro- and mesozooplankton in a coastal pelagic ecosystem of the western tropical Atlantic

In this study, we analyzed the effects of the record-strength 2015/16 “Godzilla” El Niño (EN), and subsequent extreme rainfall events (in 2017 and 2019) on local zooplankton communities in Tamandaré Bay, Brazil. Micro- and mesozooplankton (>64 μm) samples and abiotic data were obtained in, approximately, four campaigns per year, from June 2013 to August 2019. Micro- and mesozooplankton abundance, composition, and environmental conditions (chlorophyll a, salinity, temperature, winds, river discharge, and water transparency), and the Tropical Southern Atlantic SST index (TSA) were analyzed in a seven-year time series study. Local rainfall in the rainy seasons of 2015 and 2016 was far below the seasonal average, leading to lower Una River discharge and more transparent water in Tamandaré Bay during the EN period. Conversely, there were extreme rainfall events in 2017 and 2019, which had conspicuous effects on several abiotic factors. The copepod Euterpina acutifrons, the ciliate Favella ehrenbergii and “other ciliates” displayed high abundances in the rainy season. The 2015/16 EN led to a community change in the study area. We observed high abundances during the Post-EN period for many zooplankton taxa. The abundance of ciliate F. ehrenbergii was high in the Post-EN period and showed strong positive correlations with Una River discharge and chlorophyll a, showing that this species was strongly affected by nutrient inputs due to the peak in Una River discharge. This approach permitted new insights into processes and drivers that shape the pelagic ecosystem in a tropical coastal pelagic ecosystem.