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Changes in the water quality of Lake Pskovskoe in 2000–2018 are considered. The water quality assessment employed a new ecotoxicological approach to comprehensive assessment of surface water pollution based on the risk of death from exposure to a wide range of concentrations (risk of combined exposure) in Daphnia. The new approach was applied to assess the among-year variation of the pollution of the lake with inorganic and organic harmful substances. A noticeable “negative trend” was detected in the risk posed by a combined action of pollutants and, hence in the water quality of Lake Pskovskoe, indicating a decrease in the concentration of pollutants from 2000 to 2018.

The quality of water in Lake Peipus (Pskov-Chudskoye) is described according to data from long-term monitoring. The lake tends to be of the eutrophic type, with turbid water and oxygen deficiency in near-bottom water layers during winter and summer stagnation. Permissible nutrient loadings on the lake were defined according to its assimilating capacity. Current phosphorus and nitrogen loadings are above the permissible levels, and P and N supply from point and non-point sources needs to be reduced.

The aim of the study was to develop a technique for evaluation of transboundary water pollution. A method for evaluation of transboundary water quality was worked out. The method is based on calculation of the chemical index and classification of the water quality. It includes the measurement of several chemical parameters in water samples. The combination of the results is then represented as the chemical index CJ, which stands for the generalized water quality. A relative weight W is determined for each parameter. This weight can be considered as the priority level of each parameter. Then the value of the sub-index q is determined for each parameter with the help of digitized calibration graphs. The chemical index of water pollution CJ is the multiplicative function of sub-indexes q in the degree equal to the relative weight of each parameter. Also, a classification system was produced for the pollution assessment of transboundary water bodies based on the ‘broken stick’ model. Water quantity was grouped into five classes. The suggested technique was applied to reveal changes in the water quality of the transboundary Lake Peipus from 2000 to 2015 year. The water in this period is generally classified as “low polluted” (quality class 2). The chemical index CJ varied from the minimal 89.9 in 2003 to the maximal 96.8 in 2013

A method for predicting the lake trophic status relying on wintertime monitoring data was developed. The technique is based on A. G. Volokhonskii’s approach, according to which the values of the minimum energy costs calculated for the virtual model of the process of involving mineral nutrients in phytoplankton cells are universal criteria of these nutrients’ availability to phytoplankton. Statistically significant one-parameter linear dependence between the average annual concentration of total phosphorus (Pt) and the values of minimum energy costs (Ep), which characterize mineral phosphorus levels in February, was detected. A classification of the trophic status of lakes ofKazakhstan depending on the values of the minimum energy costs of mineral phosphorus in February was developed.