Phytoplankton Communities of the Vychegda River in the Area of Treated-Wastewater Intake from the Pulp and Paper Industry

The results are presented for a study of summer phytoplankton (July 2018–2020) in the middle reaches of the Vychegda River at the sites where treated wastewater is discharged from the Mondi Syktyvkar JSC pulp and paper mill. In the phytoplankton of the studied section of the river, 263 species with algal varieties from seven divisions were identified: Cyanoptokaryota, 8; Ochrophyta, 4; Rhodophyta 1; Euglenophyta, 1; Bacillariophyta, 232; Chlorophyta, 15; Charophyta, 2. Widespread, eurytopic species predominate in terms of diversity. In 3 years of research, phytoplankton species communities have demonstrated stability of the complex of dominant and structural diversity. The basis of the dominant complexes is formed by species from the genera Aulacoseira, Dolichospermum, Pandorina, Eudorina, Closterium, etc. The composition of dominants is similar for conditionally background points and the zone of wastewater discharge. A massive development of the thermophilic species of diatom Diadesmis confervacea, an indicator of thermal pollution, was noted below the wastewater-discharge site. The total recorded number of phytoplankton in the Vychegda river at the studied stations was 1 150 000–18 000 000 cells/dm³, and the biomass was 0.01–0.29 mg/dm³. The ratio of the total abundance and biomass of phytoplankton algae and the indicator species forming the complex of dominants reflect an increase in the trophic status of the waters of the surveyed areas and a high degree of anthropogenic load on the ecosystem of the Vychegda River. Based on algo-indication, the waters of the conditional background and the receiving wastewaters at observation points were assigned to quality class III (with the quality category 3b, slightly polluted).