Microphytoplankton in a tropical oligotrophic estuarine system: spatial variations and tidal cycles

Camamu Bay is a shallow estuarine system, and its circulation pattern is governed by tidal forcing. The system is formed by four sectors including the main channel and three hydrodynamic regions, delimited by the influence of the five tributaries. Water samples were collected in two different pluviometric periods (dry and rainy), at nine sampling points over the three hydrodynamic regions, and at a mooring (13°52′27.42″S, 38°57′46.19″W), in the main channel, where samples were collected every 3 h over cycles of spring tides. At each sampling station, physicochemical variables were measured and water samples were collected for analysis of dissolved inorganic nutrients and chlorophyll a, composition and cell density studies of microphytoplankton. A total of 201 taxa were identified, and the great majority of the taxa were from the marine environment. The taxonomic composition varied between the hydrodynamic regions, with greater chain-forming diatom richness, in the two study periods. Although the highest concentration of dissolved inorganic nutrients was observed in the rainy period, microphytoplankton cell density did not increase in this period. The patterns of the estuarine phytoplankton community in tropical oligotrophic systems are still little known when compared to the temperate regions. Camamu Bay is one of the last known areas in the tropical South Atlantic, and this study confirms its oligotrophic characteristics, based on abiotic and biotic conditions. We highlighted the importance of knowledge of pristine coastal systems as a tool for the evaluation of anthropogenic changes in these areas.