Introduction. This paper is the first to address the state of phytoplankton in eutrophic Svyatozero Lake (61о32´ N, 33о35´ E.), used for trout farming in the Republic of Karelia, in the North-Western Region of Russia, during the period of water bloom. For northern reservoirs, water bloom is not a typical phenomenon. However, due to climate warming and the large-scale development of trout breeding in the region, it becomes more urgent to study blooming reservoirs and related consequences for northern aquatic ecosystems and humans. Methods. We processed phytoplankton samples and measured photosynthesis using conventional methods. The concentration of chlorophyll a in water was determined spectrophotometrically, and the content of cyanotoxins was estimated by liquid chromatography-mass spectrometry. Results. In September 2019, we studied the structural, quantitative and functional characteristics of phytoplankton in the bloom spot. The phytoplankton abundance was 198.712 mln cells/l, the biomass was 14.945 mg/l, and the concentration of chlorophyll a reached 215.3 μg/l, which corresponded to the β-eutrophic state of the ecosystem in the study area. It was revealed that cyanobacteria corresponded to 99.8% of the biomass and 96.7% of the abundance. The species of the genus Microcystis were dominant (42%). They are well-known potential producers of cyanobacterial hepatotoxins, in particular, microcystins. We established the presence of microcystins. Their intracellular and extracellular concentrations were equal and in total amounted to 12.56 μg/l. We also identified eight structural variants of microcystins; among those, [D-Asp3] MC-RR accounted for up to 90% of the total content. The most toxic MC-LR variant was present in trace amounts only in biomass. According to the WHO standards, the content of microcystins and the number of cyanobacterial cells in the Svyatozero Lake bloom spot corresponds to average danger in the case of recreational use and can pose a threat to human health. Conclusion. It is necessary to monitor the phytoplankton composition, the presence of potentially toxic cyanobacterial species and cyanotoxins, as well as the state of trout farmed in the waters of Svyatozero Lake, since cyanotoxins can accumulate in fish tissues.

Studying and forecasting the consequences of the climate change impact on aquatic ecosystems are among the important tasks of hydroecology and hydrobiology. Over the past 30 years, according to data of meteorological stations on the catchment area of Lake Onego-the largest reservoir of Northwestern Russia-the average annual air temperature is steadily increasing. In the winter of 2019-2020, an abnormally high air temperature was recorded, which exceeded the climatic norm by 5-9°С. According to satellite data, in winter 2020, for the first time in the 65-year history of observations, there was no ice cover on most of the open part of Onego. To assess the consequences of an abnormally warm winter for the ecosystem of Onego, its state in the spring-summer period was studied. In June 2020, the water temperature, the concentration of chlorophyll "a," and photosynthesis were measured, and the structural parameters of phytoplankton in Onego were studied using standard methods. Based on the data from the temperature sensors of the buoy station, it was found that the spring thermal bar passed 10-15 days earlier than the average long-term periods, the temperature stratification of water was established 15-20 days earlier. The temperature of the surface water layer in Onego in June 2020 was 5-8°C higher than the average long-term values for the period of spring mixing and corresponded to the period of summer heating of water. In the open part of the lake, a shift in the phenological phases of phytoplankton development was observed, in particular, its summer composition, the seasonal maximum of chlorophyll "a," and primary production were formed a month earlier. In the Kondopoga Bay of Onego, which was exposed to the highest anthropogenic phosphorus load, the level of phytoplankton development first reached the α-eutrophic state.

Phytoplankton photosynthesis in Lake Onego was measured in 1989-2006. An attempt was made to calculate photosynthesis by daily assimilation numbers and chlorophyll a concentration measured in 2007-2021 to supple-ment data on the primary production. Daily assimilation numbers were calculated by simultaneously measured photosynthesis and chlorophyll in 1989-2006 for different areas and seasons of a limnically complex water body. A comparative statistical analysis of the size-structural characteristics of phytoplankton in 1989-2006 and 2007-2021 was performed to assess the possibility of using the obtained assimilation numbers to restore photosynthesis values. Comparison of the average cell volume for the phytocenosis and the ratio of diatoms and smaller non-diatoms showed their similarity in both periods in the main water area of the lake. Only in the central sector of Kondopoga Bay, which has been influenced by biogenic loads from the trout farms since the 2000s, the average cell volume decreased significantly and the proportion of diatom plankton also decreased. Photosynthesis was not calculated for this sector. The calculated assimilation numbers were similar for almost the entire water area of the lake. The maximum daily values were observed in summer with medians from 11.8 to 18.4 µg C/µg Chl in different parts of the water body. In Kondopoga Bay, which during both study periods was experiencing a biogenic load of wastewater from the pulp and paper mill, their median values in summer reached 40.3 and 29.6 µg C/µg Chl day–1 in the top and open parts, respectively. It was possible to restore 197 values of photosynthesis by chlorophyll a measured in 2007-2021and by the calculated assimilation numbers in the surface water layer of different areas in Lake Onego in different seasons.

The paper presents the results of a study of the phytoplankton of small Lake Vendyurskoe (south of Karelia) in the spring ice period, in summer and autumn seasons of 2020. The dynamics of quantitative and structural indices of lake algae during the trout farm operation is considered in comparison with the indices of previous years. For the studied seasons of 2020 there were identified 122 taxa of algal flora belonging to 8 divisions. The structure of algocenoses can be characterized by rich species diversity and quantitative development as diatom green structure with golden algae insertions. In autumn the representatives of cyanobacteria were encountered en masse in some areas. Phytoplankton abundance in the spring ice period varied within 612 thousand cells/l - 2188 thousand cells/l, the biomass reached 2.9 mg/l with a minimum value of 0.1 mg/l. Phytoplankton abundance for the studied summer period of 2020 averaged 2216±796 thousand cells/l with a maximum value of 3986 thousand cells/l, the average biomass in the lake did not ex-ceed 1.325±0.504 mg/l. Phytoplankton abundance in autumn averaged 4934±1665 thousand cells/l with a maximum value of 7832 thousand cells/l; biomass averaged 5.8±1.5 mg/l with a maximum value of 7.9 mg/l. According to the eco-logical and geographical characteristics, most representatives of the algal flora found in the lake are widespread freshwater organisms belonging to the planktonic flora and preferring a neutral reaction of the environment. The Shannon index increased from 2.8 in the center of the lake to 3.3 in the trout farming area, which indicates good species diversity. The Sorensen index of 0.8 revealed the homogeneity between phytoplankton communities in different areas of the lake. Most of the identified types of saprobity indices belong to the representatives of the β-mesosaprobic and oligo-beta-mesosaprobic zones (up to 78% of the total number of identified indicator species). The saprobity index corresponded to the 2nd class of water purity (pure, α-oligosaprobic), which is due to the species composition of the dominant phytoplankton complex, however, the biomass indicates that the lake belongs to the 3rd class of water purity (satisfactorily clean, β-mesosaprobic).