 The Samara River has a diverse community of phytoplankton, consisting of 160 species and intra-specific taxa from 8 divisions. The most common species are chlorophyte algae, followed by bacillaria phytolgy. The degree of species composition similarity differs between the river reaches, ranging from 15% in the upper part to 59% in the ecotone zone where the river meets the Serratif reservoir. The abundance and biomass of phytoplankton increases significantly along the river, with the highest levels occurring in the ecotone zone. Diatoms, green algae, and large cell phytoflagellates are the most abundant species, but their relative importance varies depending on location. Flood control, tributary influx, anthropogenic eutrophication, and backwater effect have all had an impact on the dominant species composition in the river's mouth area. This article was authored by O. G. Korakova.