Abstract
The method of ecological modifications was used to determine the invariant state of aquatic ecosystems in the Middle Volga basin. Based on the analysis of long-term regime hydrochemical and hydrobiological information using the methods of mathematical statistics (cluster, hierarchical cluster analysis, ordination methods), the studied water bodies were grouped according to the chemical composition of water, highlighting the main indicators of natural and anthropogenic genesis. We highlighted the general indicators of the chemical composition of water (temperature, salinity, hardness, content of oil products, chromium ions) that affect the change in the qualitative and quantitative characteristics of hydrobiocenoses, as well as specific indicators (nitrogen-containing compounds, phosphates, chlorides, sulfates, copper and iron ions), negatively affecting the quantitative and qualitative characteristics of hydrobiocenoses. A group of ciliates – cilioplankton (qualitative and quantitative characteristics) was used to assess the invariant state. Cilioplankton in the ecosystem of the Kuibyshev reservoir is characterized by high species diversity, which indicates anthropogenic stress. The quantitative characteristics of cilioplankton in rivers are low, and the community changes are ecological modulations. The share of rotifers development in zooplankton indicates the elements of ecological regress in water bodies, as well as anthropogenic eutrophication with elements of ecological regress (in most of the Kuibyshev reservoir). The level of zoobenthos development and its size, as well as the oligochaetes relative proportion indicate anthropogenic stress in the ecosystem of most parts of the Kuibyshev reservoir. The elements of ecological regress are traced only in the area of Chistopol. The water ecosystem in Tetyusha area is characterized as a transition from background to anthropogenic stress. The Vyatka and Mesha rivers are characterized as background rivers with the transition to anthropogenic stress. Anthropogenic eutrophication with elements of ecological regress can be traced in the Kazanka River. The Stepnoy Zai, Noksa and Ilet rivers has elements of ecological regress. Hydrobiocenoses indicators statistically depend on hydrochemical composition.