The composition and distribution of diatom assemblages from surface sediments in the littoral zone of meso and macrotidal estuaries from Patagonia (Argentina) were studied with the aim to recover modern data on diatom distribution applicable to future paleoenvironmental reconstructions in coastal areas of Southern South America. Twenty-seven sites were selected to cover the maximum spatial distribution along the Negro, Chubut, Santa Cruz valleys and San Julián Bay. Salinity, water temperature, pH, sediment grain size and organic matter content were measured. Multivariate methods (cluster, CCA and NMDS analysis) were explored to analyse the relation between biotic and abiotic data. The results showed that the spatial distribution and species composition of diatom assemblages are strongly related to salinity and temperature. The diatom assemblages of riverine conditions (without salt-wedge influence) were characterized by high abundances of the freshwater diatom species Cocconeis placentula, C. euglypta and Aulacoseira granulata, meanwhile high abundances of marine and marine-brackish species such as Paralia sulcata, Rhaphoneis amphiceros, Delphineis surirella and Diploneis interrupta were characteristic of estuarine conditions (i.e. under salt-wedge influence). As temperature is directly related to latitude there are differences between northern and southern Patagonian communities. Different proportions or combinations of the taxa produce the distinction between assemblages. In southern estuaries with lower temperatures, the diatom assemblages are less diverse.Fil: Espinosa, Marcela Alcira. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Geología de Costas y del Cuaternario. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto de Geología de Costas y del Cuaternario; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; ArgentinaFil: Isla, Federico Ignacio. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Geología de Costas y del Cuaternario. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto de Geología de Costas y del Cuaternario; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentin
