Long-Term Changes Reconstruction in Two Upper Volga Reservoirs Based on the Bottom Sediments Comprehensive Analysis Data

The paper presents a new research program for assessing long-term transformations occurring in reservoirs. To assess the prospects of the proposed studies, the Ivankovo and Rybinsk reservoirs were selected due to their advanced age, the power of accumulated deposits, and the degree of study. The novelty of the research consisted of the joint analysis of the results of phytoplankton complexes biomonitoring and the analysis of the diatom complexes species composition from the bottom sediments columns. At the same time, the analysis of the waters and samples chemical composition from the bottom sediments columns was carried out. The absence of redeposition processes during the formation of sediments at the sampling points was confirmed by the method of taxonomic proportions graphical analysis in diatom complexes. This determines the reliability of the diatom and chemical analysis results’ comparison of the bottom sediments. The results of the water quality assessment by two indexes are considered according to the monitoring data: the saprobity index S and the integral water quality index QI. The expected seasonal repeatability of the numerical values of S and QI is stated. Similar calculations were carried out for indicator species from diatom complexes in the bottom sediments columns samples. The processes of water quality deterioration according to the above-mentioned indexes are noted. As a result of the research conducted, the optimal location of bottom sediments column sampling points in the Rybinsk and Ivankovo reservoirs was established. During the formation of reservoirs, the lakes located on their territory were f looded. The resulting morphometric bottom relief determined those relict lakes’ locations as zones of stable sedimentation. According to the comprehensive studies results, longterm negative processes caused by anthropogenic load have been confirmed in the investigated reservoirs.

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