The theoretical analysis of the possible application of Harrington’s desirability function for the state of natural ecosystems assessment was made. It was shown that the desirability function with double limiting should be used for water quality objects assessment. The function makes it possible to eliminate arbitrary rule while the number of quality classes at different ways of optimization (typification, classification etc.) of information was determined. This resulted in mathematically valid limitation to the maximum 5 types of the first level. It was underlined that if even one particular response or a particular desirability function does not satisfy the requirements, the general estimation should be considered unsatisfactory, in spite of the fact that the other parameters (properties) of the studied system were satisfied. The tables were made which gave the possibility to assess the up-to-date state of the Rybinsk Reservoir ecosystem according to the content of nutrients and organic matters on the basis of future mesurments. The seasonal pattern of hydrophysical, hydrochemical and hydrobiological processes should be taken into account in order to make more accurate the water quality assessment at the concrete time in the definite site in the Rybinsk Reservoir area.

Hydroxylamine concentration and localization in the Volga reservoir cascade and in the lakes of Yaroslavl province and Vladimir province were studied. It was found that, in microaerophilic zones of the most productive lakes, the rate of hydroxylamine formation is higher and its localization does not depend on the type of the limnological object (a reservoir or a lake), but is determined by the oxygen regime of the water body in question.

The data on the content of total and some forms (suspended, labile, and carbohydrate-like) of organic substances in inflowing rivers of the Rybinsk Reservoir are presented. The greater portion of organic substances enters the Rybinsk Reservoir in the form of difficulty decomposable dissolved compounds. The labile fraction accounts for no more than 4.6% of the total amount. The distribution of the total organic substance runoff within a year correlates with the water runoff volume. The quantity of organic substance inflow with each river depends both on the water runoff volume and on the value of mean weighted concentrations of organic substances in water of these rivers. It is suggested that the obtained values should be considered as regional geochemical constants, which may be useful in preparing the total balance.

The results of studies of spatial and seasonal variations in hydrocarbon concentrations in hypertrophic Lake Nero are used to discuss the problem of division of hydrocarbons into anthropogenic and natural components. It is shown that the use of bitumoids as a component of oil pollution instead of total organic matter can introduce errors in the results. Seasonal variations in the activity of hydrobionts in an undisturbed area are shown to have a notable effect on the assessment of the proportion of anthropogenic hydrocarbons in their total concentration, that is, on the assessment of the extent of pollution of water bodies. The natural component is found to dominate over the anthropogenic component only in the periods of algae blooming. In spring and autumn, the total concentration of hydrocarbons was found to exceed the MAC and to be due mostly to natural hydrobiological factors.

The processes of co-oxidation of natural humic substances (HS) at the presence of glucose in concentrations of 10, 25, 100 mg C/L were studied. Is has been shown that for 112 days less than 10% of the initial HS was involved in bacterial metabolism.

Methods of evaluating the trophic status of water bodies based on nitrogen- and phosphorus-containing compounds are analyzed. It is suggested that the trophic status of water bodies be evaluated using the total average annual concentrations of mineral and labile organic forms of phosphorus or nitrogen. It is emphasized that additional account of regeneration potential of these elements does not modify the trophic status of the Rybinsk Reservoir--it remains mesotrophic.

Experiments were conducted to study the microbial co-oxidation of naturally structured humic substances in different water bodies at different glucose concentrations. At low buffer capacity of aquatic ecosystems, the initial stages of the process are shown to be accompanied by a considerable increase in the medium acidity, a decrease in its alkalinity, and accumulation of colored intermediate products of bacterial and enzymatic synthesis. It is found that the co-metabolism processes with highly labile compounds either do not play any significant role in the trophodynamics of aquatic ecosystems (<10% of the original humic material is co-oxidized) or are not detected, notwithstanding the sufficient sensitivity of the analytical techniques involved. The low degree of co-oxidation (or its virtual absence) are found to be attributable to the masking effect of intermediate products.

The results of studies of the Ivankovo, Uglich, Rybinsk, and Gorkii reservoirs are analyzed. The studies involved a wide range of hydrochemical (salt composition, biogenic elements, organic matter, oil products, and heavy metals) and hydrophysical characteristics in the summer low-water period. The concentrations of organic matter and major biogenic elements are shown to decrease downstream along the chain of reservoirs with a respective general decrease in the trophic status. Large built-up areas located downstream of the Rybinsk Hydroelectric Plant are shown to adversely affect the river water quality.