Structure of microphytoplankton community and environmental variables in a macrotidal estuarine complex, São Marcos Bay, Maranhão - Brazil

São Marcos Bay is an estuarine complex with semidiurnal tides that can reach more than 7 m during equinoctial spring tides. It is situated in the second largest Brazilian coastline and is subjected to continuous human activities holding an important port complex of Latin America. In order to contribute to the knowledge about the structure of the phytoplankton in a macrotidal systems, this study aims to evaluate the phytoplankton community and its relationship with environmental conditions in the São Marcos Bay. Five surveys in 2010 and 2011 were carried out on four sampling points during the rainy and dry seasons. Samples were taken during the flood and ebb phases at neap tides. Hydrological parameters were correlated with biological data (phytoplankton composition, abundance and biomass) using statistical analysis. The phytoplankton biomass (chlorophyll a) characterized the bay as quite productive (10.43±7.62 mg m-3) with the nanophytoplankton as the dominant fraction. Seasonal variation was observed in phytoplankton abundance with higher values (34,262±18,422 cells L-1) in the rainy season. Diatoms were the most important phytoplankton group, pointing out Nitzschia sp., Diploneis weissflogii and Synedra sp. as the dominant species. This study revealed that the composition of phytoplankton community was mainly influenced by the local dynamics, governed by macrotides, and precipitation regime that mostly contributed to the seasonal fluctuations of the environmental conditions, such as salinity, dissolved oxygen, and nutrients. AbstrAct Lisana Furtado Cavalcanti1, Andrea Christina Gomes Azevedo-Cutrim2, Amanda Lorena Lima Oliveira1, Jordana Adorno Furtado1, Bethânia de Oliveira Araújo1, Ana Karoline Duarte-dosSantos Sá1, Francinara Santos Ferreira1, Nágela Gardênia Rodrigues Santos1, Francisco José Silva Dias1, Marco Valério Jansen Cutrim1*