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The research assesses the state of different types of lakes in specially protected natural areas of the Nizhny Novgorod Region and the anthropogenically disturbed territory of Nizhny Novgorod. The trophic status of water bodies was determined using the trophic status index ( TSISD ). It has been established that most of the lakes have eutrophic and mesotrophic status. The analysis of the species structure of zooplankton in water bodies has been carried out. The species richness of both lakes of the protected areas and urban lakes was high. Along with eurybiontic species, alien and rare species of zooplankton were identified. Significant differences in the indicators of the species structure were established, however, no critical values of the abundance, biomass, Shannon and Pielou indices were recorded. Most of the identified zooplankton species were indicative. On the basis of the Pantle-Bukk saprobity index in the Sladecek modification, the water quality class was established. The reservoirs were characterized by quality class II-III (clean-moderately polluted water). In some lakes an excess of the maximum permissible concentrations of ammonium, nitrite, iron, manganese, copper, zinc, lead has been established. However, in general, in terms of a set of indicators, water bodies are in a satisfactory condition. In order to objectively assess the ecological state of aquatic communities, to predict possible changes and develop recommendations for the conservation of species diversity, it is important to carry out regular monitoring studies.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution 4.0 LicenseModern methodological approaches were applied to analyze the zooplankton community spatial distribution (with the example of the middle river part of the Cheboksary Reservoir). Zooplankton communities were sampled in the middle river part of the Cheboksary Reservoir (from the city of Nizhny Novgorod to Vasilsursk town) in the summer low-water period in 2018. The boundaries between the communities in the Cheboksary Reservoir were gradually changing during the history of the Cheboksary Reservoir from the time of its construction to present. In the middle river part of the Cheboksary Reservoir there are two distinct spatially stable zooplankton communities associated to the Oka and Volga streams. The distinction between these two zooplankton communities was demonstrated by hierarchical cluster analysis. Redundancy analysis has shown that chlorophyll-a and pH were the main factors influencing the specific zooplankton structure.Chlorophyll-a concentration reflects the meso-scale heterogeneity of the horizontal phytoplankton distribution and hence the distribution of the zooplankton's food sources. The relation of zooplankton to pH level reflects the high sensitivity of the species of the genus Brachionus Pallas, 1766 to high acidity. The influence of pH as an environmental factor was less evident. However, this variable is well known as one of the leading factors determining the structure of zooplankton communities. Its role in zooplankton community assembly of lowland reservoirs deserves further investigation.

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The paper presents the results of zooplankton research in June, July and September 2017 in eight floodplain lakes of the middle reaches of the Kerzhenets River located in the Kerzhinski Nature Reserve. The studied water bodies are shallow lakes with weakly acidic pH (5,40-6,23) and low mineralized water (46,90-68,40 mg/l), with increased chromaticity (202,00-1048,20 deg.) and content of suspended substances (4,40-22,90 mg/l). As a result of a carried out hydrobiological research, 106 species of zooplankton were identified. According to the zoogeographical characteristics, the zooplankton fauna of floodplain water bodies is typical for the European part of Russia, with the exception of two invasive species: the North American rotifers Kellicottia bostoniensis (Rousselet, 1908) and the Caspian branching diocese of southern origin Diaphanosoma orghidani (Negrea, 1982). In addition, in a number of lakes, the Arctic relict crustacean Holopedium gibberum (Zaddach, 1855) was found. The crustacean is listed in the Red Data Book of the Nizhny Novgorod Region. Alien rotifers in a number of floodplain reservoirs were the dominant species and had a high abundance. The Caspian southern crustacean was found only in two reservoirs, its number was not high, and it did not play a significant role in the zooplankton communities of these reservoirs. New findings of the Arctic relict species Holopedium gibberum (Zaddach, 1855) make it possible to recommend the introduction of a number of floodplain reservoirs into the Red Book of the Nizhni Novgorod Region as habitats of this rare species. Statistical redundancy analysis (RDA) has revealed four factors that related with development of zooplankton in a number of floodplains, such as water temperature, hydrogen index, dissolved oxygen content and suspended solids.

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