Environmental controls on the phytoplankton community structure in the Equatorial Atlantic Ocean

Abstract

This study analyzes how environmental conditions structure the distribution of phytoplankton in the Equatorial Atlantic, a region marked by strong seasonality in primary production, influenced by wind variability and the position of the Intertropical Convergence Zone (ITCZ). Based on data collected during the M181 expedition in 2022, two distinct phytoplankton habitats were identified along a zonal transect through statistical analyses and oceanographic variables. Habitat 1, located in the eastern sector, was characterized by a high abundance of diatoms of the genus Hemiaulus, presumably associated with nitrogen-fixing symbionts, in waters with lower salinity, shallower nutriclines, and stronger stratification. These conditions suggest a favorable environment for diatom diazotroph associations in the absence of nitrate. In contrast, Habitat 2, in the western sector, exhibited oligotrophic, with dominance of the cyanobacterium Trichodesmium, favored by phosphate availability and possibly enhanced iron input via wet deposition. Other groups such as Synechococcus and Prochlorococcus were also present, especially Synechococcus, which dominated in several stations. The comparison of techniques such as flow cytometry, optical imaging (PlanktoScope), and pigment analysis (CHEMTAX) helped validate these community patterns. Although the RDA statistical model explained only 21 % of the variation in community structure, potential ecological relationships were identified, such as an association between Synechococcus and silicate, which may reflect a physiological function not yet fully understood. Overall, the results highlight the importance of spatial variability in the analysis of marine biological processes and emphasize the value of integrated methodological approaches to understand the interaction between phytoplankton functional groups and their physicochemical environment in understudied oceanic regions