Phytoplankton and ecological state of Dolgoe Lake (Pskov Region)

The paper deals with the planktonic algoflora of Dolgoe Lake in the Pskov Region. The studies were carried out in the summers of 2017–2020. The taxonomic composition of the phytoplankton of the lake was revealed, including 156 species and intraspecific taxa from 8 phylums: Chlorophyta, Bacillariophyta, Cyanobacteria, Charophyta, Ochrophyta, Dinophyta, Euglenophyta, Cryptophyta. The basis of the floral complex was made up of representatives of the phylums Chlorophyta and Bacillariophyta, as well as Cyanobacteria. The greatest similarity in the species composition of phytoplankton communities was noted in 2017 and 2019 (the Sierensen-Chekanovsky coefficient was 60%), the lowest – in 2018 and 2019, as well as in 2018 and 2020 (39%). The maximum quantitative development of microalgae was noted in 2019 (32,9 million cells/l), with 90,7% of the total number accounted for cyanobacteria Merismopedia minima G. Beck. The smallest number of planktonic algae was observed in 2020 – 1,3 million cells/l. The total biomass of phytoplankton was the highest in July 2019 – 0,45 mg/l, and the lowest in 2020 – 0,15 mg/l. According to the Milius trophic index, the waters of the lake are oligotrophic. According to the ecological and geographical characteristics, widespread freshwater species of phytoplankton prevailed in the lake, preferring slightly alkaline waters. Saprobiological analysis showed that the waters of Dolgoe Lake are moderately polluted and belong to the 3^rd class of quality. The saprobity index of the Pantle–Buck, depending on the year of the study, varied from 1,80 to 1,98.