Effect of Anthropogenic Factors on the Ecosystem of the Yenisei River Anabranch within the City of Krasnoyarsk

In recent decades, freshwater ecosystems have regularly experienced blooms of green algae (green tides), causing environmental and economic damage. Blooms of green alga Spirogyra sp. occur in the ‘Abakanskaya’ anabranch of the Yenisei River in summer months. The anabranch is influenced by several anthropogenic factors: construction of a dam in the upper reaches; heated water discharge from a thermal power plant; fish farming. The aim of this study was to assess the impact of anthropogenic factors on the ecosystem of the Abakanskaya anabranch and to identify the main factors favoring Spirogyra sp. blooms. Samples were taken in May-September 2018-2019 at four sites: 1 – upstream of the dam (control), 2 – downstream of the dam, 3 – near the point of heated water discharge, 4 – downstream of the fish farm. Physical and chemical parameters, biomass and species composition of phytoplankton, phytoperiphyton, zooplankton, zoobenthos, and higher water plants, gross primary production (GPP), nitrogen content in biomass of periphyton, higher plants, and zoobenthos were studied. The main factor that caused changes in the community was that there was no water flow in the anabranch because of the low capacity of the dam drainage pipes. While upstream of the dam most ecosystem features were within the limits typical for this part of the Yenisei River, downstream of the dam, the system seemed to change from lotic to lentic: percentage of motile forms of phytoplankton, biomass of zooplankton, percentage of true zooplankton species, taxon number and taxonomic diversity of zooplankton and zoobenthos became higher; phytoperiphyton primary production rose (because of Spirogyra sp. bloom); higher water plants thrived. The other investigated factors, namely, thermal, nutrient, and organic pollution evidently produced combined effect on the ecosystem because of the absence of water flow, and their specific effects were not obvious