Assessment of long-term changes in the amount of phytoplankton in the Petrozavodskaya Bay of Lake Onego

The ecosystem of the Petrozavodsk Bay of Lake Onega is currently under the influence of changing environmental factors. The load on the bay from wastewater of the Petrozavodsk industrial center has significantly decreased. At the same time, brownification occurs in the bay due to climate warming and an increase in river runoff containing humic organic substances and phosphorus. To assess the complex effect of these factors on the ecosystem of the Petrozavodskaya Bay, we studied long-term changes in phytoplankton and chlorophyll a, which determine the bioproductivity of natural waters. During the period 1992- 2018, we performed a statistical analysis of changes in the biomass of two groups of phytoplankton – diatom and non-diatom (green, chrysophyta, pyrrhophyta, euglenaphyta, xanthophyta, cyanobacteria) and chlorophyll a concentration. The analysis was carried out separately for spring and summer phytoplankton. Spearman's correlation coefficient revealed no significant changes in the biomass of diatom and non-diatom phytoplankton, as well as chlorophyll а in spring. The proportion of diatoms in the community was fairly stable throughout the study period (coefficient of variation of 60‒100 %). The variability of the proportion of non-diatoms in the community increased from 1‒10 % to 1‒44 %. During the summer period, there was a noticeable but insignificant tendency towards a decrease in the biomass of diatom plankton and an increase in the biomass of non-diatom algae. At the same time, a significant decrease in the concentration of chlorophyll a in water was detected in summer. This may be due to a decrease in the development of the summer diatom complex, which accounts for up to 80 % of the community biomass. The higher informative content of chlorophyll a values is determined by their lower variability (coefficient of variation 38 %). We believe that at this stage of the functioning of the ecosystem of the bay, the decrease in the amount of phytoplankton is mainly due to a decrease in anthropogenic pressure.