Assessment of the ecological state of small rivers of St. Petersburg according to the benthic macroinvertebrates indicators in 2019-2021

The work is devoted to the topical issue of assessing the state and changes of river ecosystems located within the boundaries of large cities and industrial agglomerations. The paper presents the results obtained during hydrobiological studies on small rivers of St. Petersburg (Russian Federation) in 2019-2021. The application of bioindication methods is substantiated in order to assess long-term changes in aquatic ecosystems under conditions of increasing anthropogenic load. Various characteristics of the community of benthic invertebrates are used as biological indicators of the state of hydroecosystems. Benthos samples were taken on eight small watercourses of the city, which are both tributaries of the Neva River of various orders (the Okhta, Slavyanka, Chernaya Rechka, Lubya, Okkervil, Volkovka rivers) and delta watercourses (the Smolenka and Karpovka rivers). The paper provides a brief description of the studied water bodies and sampling sites. A number of biotic metrics and biotic indices used for bioindication purposes both in Russia and abroad have been identified. It is noted that the unstable hydrochemical regime and low flow velocity in the small watercourses of the Neva River delta contribute to the intensification of the accumulation of sediments with high concentrations of ecotoxicants, which negatively affects both quantitative and qualitative characteristics of biocenoses. In some watercourses re is a complete absence of bottom invertebrates, which can be assessed as an ecological disaster. When assessing water quality by quantitative and qualitative characteristics of communities of benthic invertebrates (Mayer index, BMWP index, some biotic metrics), it can be concluded that Okkervil River, Chernaya Rechka River and Volkovka River should be attributed to watercourses whose ecological state is extremely unfavorable. The ecological state of the rivers Karpovka River, Slavyanka River, as well as the estuaries of Okhta River and Smolenka River should be considered unfavorable. And finally, the situation in Lubya River, the source of Smolenka River and in the middle reaches of the Okhta River can be considered close to satisfactory (low quality according to BMWP or moderately polluted water according to IM).