Current state of spring phytoplankton of the Bratsk and Ust-Ilimsk reservoirs of the Angara Cascade

Continental reservoirs are sources of fresh water, but they are currently experiencing increasing anthropogenic stress. Reservoirs are among the most important water bodies for the needs of the population, and they require regular monitoring. However, some of these reservoirs have not been monitored for decades. The aim of this study was to assess the species richness, structure and density of spring phytoplankton of the Bratsk and Ust-Ilimsk reservoirs in the modern period. As a result of studies of these reservoirs, 62 taxa of planktonic algae were identified in phytoplankton in the spring season of 2024, of which diatoms and chrysophytes predominated in species richness. The dominant complex included the diatoms Aulacoseira islandica, Nitzschia graciliformis, Asterionella formosa, Fragilaria radians, and Stephanodiscus minutulus, the proportion of which in the total abundance and biomass varied between stations and reservoirs. The most significant predictors of phytoplankton community structure were nitrate and phosphate; conductivity, transparency and temperature were also important. Maximum values of total abundance and total biomass of phytoplankton were observed in the Bratsk Reservoir – 1119 thousand cells/L and 579 mg/m3, in the Ust-Ilimsk – 577 thousand cells/L and 274 mg/m3 respectively. These values do not exceed the indicators of previous years of observaions. The values of phytoplankton biomass characterize the studied areas of the Bratsk and Ust-Ilimsk reservoirs in June 2024 as oligotrophic.