Picoplankton and nanoplankton communities in a tropical estuary (Ilha Grande, RJ, Brazil)

Small plankton fractions have great importance for the functioning of aquatic trophic web. The high surface/volume ratio and remineralization capacity of the dissolved organic matter by bacterioplankton are capable of sustaining the primary biomass in environments nutrient limited, mainly due to high production rates and rapid cycling of carbon in the mixed layer. This study aims to determine the density of picoplankton and nanoplankton organisms in a tropical estuary (Ilha Grande - RJ - Brazil), analyze their relationship with tide variations and estimate the amount of dissolved organic carbon (DOC) associated with the local picoplanktonic fraction. Four campaigns were conducted in seasonal scale (in 2012) and fixed station, in the estuary of the Barra Grande River (23°11'S and 44°12'W) over two spring tide cycles (25 hours). Accordingly, water samples were collected at the surface and bottom, using a Van Dorn bottle, and subsequently analyzed by flow cytometry with BDFACS- Calibur apparatus. Thus, was verified the presence of Synechococcus acclimated cells and ecotypic, coccoid cyanobacteria, small cyanobacteria filaments, between 2 and 4μm, and cyanobacteria filamentous chain (> 4μm). Furthermore, a vertical variation exists, where the highest density of all organisms were on the surface layer, having Synechococcus type cells the maximum density of 10 9 cel.L -1 . Seasonally, the greatest change was observed during the winter, probably due to formation of a sandy bar preventing communication among estuary and sea. Thus, with a smaller hydrodynamic, the establishment of filamentous chains was possible. In addition, there has been a positive correlation of the variation of cell density with tide, which is verified by finding different characteristic types of Synechococcus cells of ocean waters. Although total carbon biomass associated with picoplanktonic fractions was smaller (10 3 μgC.L -1 ) than associated to larger phytoplankton fractions (nano and microphytoplankton) (10 14 μgC.L -1 ), the minor plankton, focus of this study, have high refresh rates of biomass. Finally, was observed that these organisms present with great relevance for this estuary and therefore should be studied under this approach, as well as its taxonomic composition should be evaluated from molecular techniques that allow the identification of species.