Structural Indices and Toxic Species of Cyanobacteria of Pskov Lake

—The cyanobacterial communities of Pskov Lake were studied in different seasons of 2021, and a comparison was made with data of 2018–2020. Information on the taxonomic composition and quantitative characteristics of cyanobacteria was obtained, and the species toxic to living organisms were demonstrated. A total of 196 species taxa of phytoplankton were registered at four permanent stations of the lake, 15.8% of which accounted for cyanobacteria. The cyanobacterial communities were most similar in terms of species in the summer and autumn periods. The quantitative indices of cyanobacteria, such as the abundance and biomass, were minimal in spring and maximal in the summer–autumn period. In each of the seasons of the year studied, toxic and potentially toxic species belonging to the genera Anabaena , Aphanizomenon , Lyngbya , Microcystis , Nodularia , and Nostoc were present among the detected cyanobacteria. The total contribution of the representatives of these genera to the total abundance of phytoplankton changed from 4.0% in spring to 65.0% in autumn. In summer, the portion of cyanobacteria from the above genera in the total abundance of microalgae accounted for about 40.0%. The largest role was noted for the algae Microcystis aeruginosa (Kützing) Kützing, which causes “blooming” of the reservoirs and produces a microcystin toxin with hepatotoxicity. The study of Pskov Lake requires constant monitoring of the content of cyanotoxins, especially in autumn, when the maximal biomass content of cyanobacteria (including toxic species) is observed, which can lead to serious consequences.