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The species composition, taxonomic structure, and the dominant complex of algae, and the distribution of phytoplankton abundance in the studied watercourse were identified based on data obtained for phytoplankton from the Ob River (from Tomsk to Salekhard) in summer 2019. Green algae (division Chlorophyta) make up the basis of the phytoplankton abundance in the river. The dominant complex is represented mainly by centric diatoms (genera Aulaсoseira, Cyclotella, Stephanodiscus) and non-heterocyst forms of cyanoprokaryotes (genus Aphanocapsa). The numbers and biomass of phytoplankton gradually decrease downstream of the Ob River; below the confluence of the Irtysh River, the edge effect occurs: increase in the diversity and density of organisms at the boundaries of ecosystems. Compared to the previous studies, the proportion of green and euglena algae, and cyanoprokaryotes in the taxonomic spectrum of phytoplankton increased, the composition of the dominant complex enriched, including due to non-heterocyst forms of cyanoprokaryotes, and the trophic status of the river increased to the category of eutrophic waters.

According to the study of summer phytoplankton of the Ob River, 153 new species and intraspecific taxa(SIT) of algae from 8 divisions were found, including Cyanoprokaryota – 14, Dinophyta – 2, Chrysophyta – 17, Xantophyta –6, Euglenophyta – 34, Bacillariophyta – 2, Chlorophyta – 72, Charophyta – 6. New taxa are dominated by small-cell greenalgae, phytoflagellates and non-heterocyst cyanoprokaryotes. The intensive succession of the phytoplanktocenosis speciescomposition and the taxonomic structure of the new SIT algae are signs of anthropogenic eutrophication.