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This study aimed to test the hypothesis of the year-round presence of toxigenic Microcystis and cyanotoxins in the water and ice of the shallow eutrophic Lake Ytyk-Kyuyol located in the continuous permafrost zone. Three independent approaches—mass-spectrometry, molecular methods and light microscopy—were applied in the study. The cyanobacterial biomass ranged from 1.0 × 10−4 to 4.8 mg L−1. Microcystis flos-aquae and M. aeruginosa were the dominant morphospecies in plankton throughout the observation. In environmental DNA, the presence of M. aeruginosa was supported and mcy gene regions responsible for microcystin biosynthesis were detected through a BLAST (Basic Local Alignment Search Tool) search and phylogenetic estimation based on newly obtained 16S rRNA, 16S–23S ITS rRNA, mcyA and mcyE nucleotide sequences. The intracellular microcystin concentration ranged from −1, and the microcystin quota in the Microcystis biomass was extremely low. For the first time, it was shown that Microcystis cells containing mcy genes and microcystins presented permanently in the water column, both during the ice-free period and under ice, as well as inside thick ice covers within 7 months of severe winter. We hypothesized that minor pelagic and ice populations of Microcystis could participate in increasing cell density in the spring. However, further studies are needed to confirm the viability of the overwintering Microcystis colonies in the water and inside the ice of Lake Ytyk-Kyuyol.

The relation between pigmentary and structural characteristics of phytoplankton is analyzed by the example of a high-eutrophic lake (Lake Nero, Yaroslavl area) where grey-blue algae of planktotrichetic type prevail in phytoplankton. Strong positive correlation between chlorophyll “a” content in seston and phytoplankton biomass is noticed in concrete years. The regression lines with different tilt angles have been obtained at the comparative analysis of a chlorophyll - biomass relation with the previous researches (1987-1989) and it does not allow to determine a transitive factor between the total aglae biomass and chlorophyll amount in a separately taken reservoir even on the basis of a long-term supervision. Wide variability of the specific chlorophyll “a”content in a unit of raw phytoplankton biomass (0.05-3.97 %) is marked. The reasons of chlorophyll abundance fluctuations in phytoplankton are discussed. Of all the phytoplankton pigmentary parameters considered in this work (chlorophylls “b” and “c”, indexes Е480/Е664, Е430/Е664, Е450/Е480, carotenoids, pheopigments) the close and significant links of phytoplankton structural characteristics are found out only with pigmentary index Е450/Е480.This index is proposed to be used as a ratio parameter of certain phytoplankton taxonomic groups.