This study describes and compares phytoplankton composition and richness at the mouths of the Gravataí, Sinos, Caí, and Jacuí rivers during the four seasons of the year. The investigation was based on bimonthly sampling of subsurface water, from December/2000 to December/2001. The phytoplankton community of the four river mouths was composed by 470 species (including those unidentified), 135 genera, and nine taxonomic classes. Chlorophyceae and Bacillariophyceae were the most representative classes in species number. High richness was probably associated with the presence of dams and swamps on the upper course of these rivers, and also with the wetlands of the Jacuí Delta floodplains. Temperature and fluviometric level were the main factors determining seasonal composition and richness variation of the phytoplankton community.
phytoplankton; richness; seasonal variation; lotic system
