Long-term changes of water quality sites in the Lower Volga, which differ in hydrological regime

It is considered a change to water quality in the closing part of the Volgograd reservoir and unregulated part of the Lower Volga in temporary aspect. Decrease in concentration of organic substance and compounds of mineral nitrogen in a dam reach of the Volgograd reservoir and increase in concentration of these substances in the river section of the Volga River in 2015 in comparison with 1989 is registered. It is shown that possible reasons for the change of the considered indicators can be a long-term decrease in water flow, successional processes and reduce anthropogenic pressure in the last 26 years in connection with the decline in industrial production. It is proposed to use the indicator of plasticity ecosystems (averaged module of correlation coefficients of hydrochemical parameters) as a criterion of the “health” of aquatic ecosystems.