Hydro-chemical basis of biological productivity in the closing reservoirs of the Volga cascade

Abstract

Long-term dynamics of organic matter and nutrients in the closing-reservoirs of the Volga cascade in the period 2004–2014 were studied. This period was characterized by changes in climatic conditions: reduce of rainfall, increase of summer temperatures, the negative trend in water discharge of the Volga River with the pattern of decline in indicators of water availability and floods. Organic matter and nutrients can be considered as the basic characteristics of the state of the aquatic ecosystems that determine the productivity and patterns of development. Organic matter (OM) in water of the Saratov and Volgograd reservoirs was evaluated with permanganate (PO) and dichromate (COD) oxidation, BOD5. Concentrations of three forms of mineral nitrogen, mineral phosphorus, dissolved iron and silicon were analyzed. During the study period negative trends of the allochthonous organic matter, easy oxidized organic matter and major nutrients (nitrogen and phosphorus) were recorded. It was shown that a long-term decline in water runoff was the possible reason of changes in quantitative values of these hydrochemical components in the studied reservoirs. Correlation between water runoff, permanganate oxidation and nitrogen concentration indicates a role of terrigenous sources in the balance of nitrogen mineral forms. Average seasonal fluctuations of the total OM concentration, maintaining synchronicity, in the Volgograd reservoir occur at a higher level than in Saratov reservoir. We have not find changes in total OM concentrations during our study period. In the Volgograd reservoir, flow velocity deceleration and more intense heating of the water mass provide higher potential of biological production. The values of autochthonous organic matter content in the Volgograd reservoir confirm this assumption.