El Niño and La Niña events shape distinct phytoplankton dynamics in a subtropical shallow lake

Aim This study aimed to evaluate the effects of El Niño Southern Oscillation (ENSO) on the structure and dynamics of the phytoplankton community of an extensive subtropical shallow lake in southern Brasil (Lagoa Mangueira, RS). Methods Phytoplankton and environmental variables were collected twice per year at three different sites in the pelagic region of the lake, between 2001 and 2013, totaling 6 samples per year. Meteorological variables such as rainfall, radiation, wind speed and direction, as well as limnological variables such as total phosphorus, total nitrogen, dissolved total nitrogen, among others, were also used to compose the analyses. To assess the phytoplankton community’s response to environmental variability during El Niño and La Niña periods, and the regular year, we analyzed the species richness, diversity, evenness, dominance, species composition, and biomass. The correlation among meteorological and limnological data was verified using a Principal Component Analysis (PCA). To verify the relationship between the descriptor species and the environmental data, a Redundancy Analysis (RDA) was performed. Results We identified 188 taxa distributed in 9 classes, of which Chlorophyceae, Cyanobacteria and Bacillariophyceae were the most representative. Among the analyzed groups, Cyanobacteria and Raphidophyceae showed significant variations between ENSO periods. The results showed statistically different environmental scenarios between ENSO periods, mainly with regard to total suspended solids, reactive soluble silica, total phosphorus, total nitrogen, total dissolved nitrogen, conductivity, dissolved oxygen, alkalinity, ammonia and wind speed. Phytoplanktonic biomass was significantly higher in La Niña years. No differences were observed in richness, dominance, diversity and evenness between the periods studied. However the descriptor species reflected the environmental differences of each observed scenario. Conclusions The study demonstrated the occurrence of different ecological species grouping of phytoplanktonic species in the El Niño and La Niña periods, associated with the different environmental scenarios of each ENSO event.