Dynamics of phytoplankton functional groups and carbon concentrations in an estuary on the Brazilian equatorial margin

The study of phytoplankton functional groups is essential for understanding the ecological dynamics of hypertrophic environments. The Anil River Estuary (ARE) is located on the Brazilian equatorial margin on the northern coast of Brazil and is influenced by semidiurnal macrotides from the South Atlantic and the river waters of the Anil River, which are highly urbanized and have many anthropic pressures. Between 2022 and 2023, six sampling campaigns were conducted at eight points across two sectors (SI - eutrophic; SII - hypertrophic) during the dry and rainy seasons. The functional diversity (FD) of the community was assessed using indices FEve, FDiv, and FDis, based on a matrix of functional traits and species density. The ARE phytoplankton community comprised 234taxa in six taxonomic groups, with diatoms being the most abundant (57.69 %), followed by Cyanobacteria (14 %) and Myozoa (8.5 %). Twenty-five Reynolds functional groups (FGs) were identified, significantly influenced by salinity and pH, with groups M and X1 being the most representative. The FGs varied between sectors and seasons, with SII recording 85 % of the species within 25 groups during the rainy season, while SI showed 76 % in 23 groups. Functional traits allowed the formation of three major groups: (I) species from hypertrophic environments (<20 μm), dominated by cyanobacteria and charophytes; (II) marine diatoms from eutrophic environments (>20 μm); and (III) species from estuarine waters, including euglenophytes and chlorophytes (>20 μm). The study highlights the relationship between the functional composition of phytoplankton and environmental conditions in an estuary impacted by urbanization and eutrophication.