Influence of Municipal Wastewaters on the Zooplankton Communities of a Small River from the Industrial Zone of the Middle Volga Region Based on the Results of Long-Term Observations

Abstract—The anthropogenic impact on various components of aquatic ecosystems and, in particular, on the structure of river zooplankton communities requires comprehensive study in modern conditions of intensive environmental natural resource management. The aim of our study is to assess the impact of urban wastewaters on the chemical composition of water and structural parameters of river zooplankton of the River Stepnoy Zay, a small river from the industrial zone of the Middle Volga region and left tributary of the Kama River. Sampling was carried out from 2008 to 2019 upstream and downstream of municipal wastewater discharges of the four cities located in the upper, middle, and lower reaches of the river. Our results have shown that the main chemical compounds that significantly differed in their concentrations in the areas above and below urban discharges are ammonium, nitrates, phosphates, and organic substances (BOD₅). These pollutants are also the most significant variables explaining variations in the species composition and quantitative indicators of zooplankton. In relation to the leading environmental factors, three groups of species are distinguished: indicators of organic pollution, species tolerating increased mineralization, and species sensitive to oxygen content. A total of 114 zooplankton taxa have been identified in the river. The abundance and biomass of zooplankton varied significantly at stations above and below cities. Zooplankton communities below cities in the upper and middle reaches of the river (Bugulma, Leninogorsk, and Almetyevsk) are influenced by the influx of planktonic organisms from biological treatment facilities with wastewaters. The stations in the lower reaches of the river belong to the mouth section, and zooplankton communities here are formed under the influence of the Kama River. Observations of the development of zooplankton communities will continue.