During 2011 year were researched seasonal dynamics of phytoplankton species composition and biomass in lower course of the Velikaya River. Hydrochemical investigation of the water was performed and defined distribution of saprobity indexes on stations near city Pskov. In phytoplankton of lower course in the Velikaya River were identified 266 taxa from 8 devisions. Bacillariophyta were the most divers – 116 (43.6 %). Maximum of species quantity reached on station situated upper stream from Pskov. Downstream from the city species quantity decreased that can be connected with influence of Pskov on river waters. During researched period phytoplankton biomass changed from 0,02 to 1,2 mg/L and reached maximum at the downstream station in June. Average biomass on this station was higher than on other stations but species quantity was the lowest. Near city Pskov were registered 5 biomass peaks, while at other stations – only 3. Increase of quantity of biomass peaks can indicate eutrophication rise (Trifonova, 1990). In spring the most diverse were Bacillariophyta in phytoplankton composition, in summer period – Chlorophyta. In autumn dominated Bacillariophyta according to taxa amount, but Chlorophyta diversity also remained significant. The highest values of diatoms biomass were registered at the station near city. Role of green algae increased near city and at downstream station. Saprobity indexes increased in general on station situated downstream from the city and reached maximum – 2.19. According to nutrients concentration and saprobity indexes Velikaya River can be characterized as mesotrophic type and moderately polluted and based on biomass value – as oligo-mesotrophic type.

Keywords: Velikaya River, phytoplankton, saprobity indexes, eutrophication

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