Matos, J.B.; Oliveira, A.R.G.; Trindade, W.N.; Leite, N.R.; Koening, M.L.; Pereira, L.C.C., and Costa, R.M. da., 2016. Phytoplankton dynamics in three metropolitan beaches of the Amazon littoral (São Luís-Maranhão). In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 413–417. Coconut Creek (Florida), ISSN 0749-0208.The present study was based on a spatial-temporal analysis of the phytoplankton of three beaches in São Luís (Maranhão) with the aim of understanding the influence of the factors on the phytoplankton community. Samples were obtained in the dry season (December 2008 and October 2009) and the rainy season (March and June, 2009), and were collected using internationally standardized methods. Salinity was significantly higher during the dry season, while temperature, dissolved oxygen, pH, turbidity and dissolved nutrient concentrations (nitrate) were all significantly higher during the rainy season. Diatoms contributed to the high taxonomic richness observed at all the beaches. Total phytoplankton density and chlorophyll-a concentrations (20.9 mg.m−3; B1) were highest during the rainy months, with significantly higher densities being observed at B3 (1274.3 x 103 cell.L−1). The cluster analysis revealed the formation of two groups influenced by seasonality, reflecting the relative homogeneity of the beaches. The results indicate that the variation in the composition and density of the phytoplankton is determined by the high degree of interaction between the highly hydrodynamic characteristics typical of Amazonian environments and the local rainfall cycle, which modulates the other environmental variables, such as salinity and dissolved nutrient concentrations, thus affecting the dynamics of coastal phytoplankton communities. The strong antropogenic influence observed in these urban beaches (sewage effluent discharges) also interfered within the dynamics of local phytoplankton.