In this paper we present the results of water quality studies in the Kudma River – a small tributary of the Cheboksary reservoir (Middle Volga). The water quality of the river is formed under the influence of karst processes, widespread in its catchment area, as well as heavily polluted wastewater discharge from municipal sewer systems without nutrient removal facilities. Geological features of the catchment cause a significant increase in total dissolved solids and changes in the major ions content from the headwaters to the confluence with the Volga. At the same time, there is a rapid increase in the phosphorus and especially nitrogen content in the water below the places of domestic sewage release into the river. As a result, the Kudma River contributes to the nutrient pollution of the Cheboksary reservoir – one of the most eutrophic lakes in the Volga basin.

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In this paper we present the results of field observations carried out at Lake Glubokoe (Moscow Region) in 2017–2019. Basic characteristics of the vertical distribution of main nutrient elements (phosphorus, nitrogen, and silicon) and organic matter (COD and chlorophyll-a) content are shown, along with their seasonal ranges. Due to the lack of surface runoff into the lake, the concentrations of total phosphorus and nitrogen are generally low throughout the year, with almost total depletion of inorganic phosphorus in the epilimnion during the growing season. Sufficient microbial activity allows most of the inorganic nitrogen to be presented in the nitrate form, except for late summer and fall, when plankton death rate exceeds bacterial nitrification capacity. During the period of hypolimnetic anoxia, significant amounts of mineral phosphorus and ammonia nitrogen are released from the bottom sediments. COD value shows a correlation with the intensity of both phyto- and zooplankton growth, which also affect the nutrient content.