Phytoplankton functioning in the Kondopogskaya Bay of Lake Onego under conditions of cage trout farming

The Kondopogskaya Bay of Lake Onego is under the influence of anthropogenic pressure due to the inflow of wastewater from a pulp and paper mill (the upper part) and trout farms (the central part). The primary production characteristics (photosynthesis, chlorophyll a, specific rate of photosynthesis, assimilation numbers) of the bay ecosystem during the “outbreak” of phytoplankton number and biomass in August 2021 are assessed. It is proved that the photosynthesis rate in the bay was determined by small diatoms during the research period. Diatom plankton cells with a volume of less than 600 µm accounted for 80-90% of the number of Bacillariophyta, the dominant group of the community. The photosynthesis values (150-330 µg/l•day) did not exceed the limits of oligo-mesotrophic ecosystems. The specific rate of photosynthesis (0.4-1.2 day–1) and daily assimilation numbers (22-40 µg C/µg Chl) were also temperate and did not indicate strong biogenic water pollution. It was revealed that the low content of chlorophyll a in phytoplankton biomass (0.13-0.39%) was mainly determined by a small number of productive green algae. The share of this phytoplankton group in the community biomass did not exceed 26%, in the number – 11%. At the same time, phytoplankton biomass (2.1-5.7 mg/l) and chlorophyll a concentrations (5.1-9.8 µg/l) reached values typical of meso-eutrophic ecosystems. Comparing the results of the study with long-term photosynthesis data showed that the trophic state of the central part of the bay, where large trout farms are located, has not changed. Long-term data on phytoplankton biomass indicate an increase in the trophic status of the central part of the bay at the present time. The inconsistency of the trophy level in terms of quantitative and functional indicators of phytoplankton may indicate the initial stage of eutrophication of the central part of the Kondopogskaya Bay as a result of the activities of trout farms.