Phytoplankton functional diversity in the tropical Atlantic under ENSO influence

In this study, we investigated the spatial and temporal distribution of phytoplankton functional groups in the tropical Atlantic by analyzing temperature, salinity, and nutrient gradients under the influence of the Intertropical Convergence Zone (ITCZ, 5-10° N) and El Niño-Southern Oscillation (ENSO) events (La Niña/El Niño) of 2017 and 2018. Environmental gradients provide habitat-specific scenarios for phytoplankton functional diversity, which determine the composition of populations on both spatial and temporal scales. Using microscopy, we evaluated nano- and microphytoplankton assemblages through qualitative and quantitative analyses. Higher diazotrophic cyanobacteria abundance and higher functional richness and functional dispersion were observed in 2017 (La Niña). Our results show that nutrient availability associated with local hydrography, temperature, and salinity influenced the distribution of diazotrophic cyanobacteria. The El Niño event of 2018 was characterized by higher sea surface temperatures and weaker convergence, impacting nutrient availability and altering the N:P ratio in the euphotic zone. These alterations affected the functional traits and species richness in the ITCZ, leading to discrete gradients across sea surface temperature and nutrient availability. Conversely, the La Niña event of 2017 was associated with decreased sea surface temperature and lower influence of the Amazon river plume, which added diazotroph-related functional traits. This study highlights the interplay of environmental factors in shaping phytoplankton communities, emphasizing the importance of understanding their dynamics and biogeography in offshore tropical waters.