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In ichthyological publications from both Russia as a whole, and from the study region, lack of data indicating the actual results of observations in a specific place all result in publication of a generalised analysis. Although our publications contain such data, they are, however, not convenient for users performing global analysis. The main purpose of publishing a database is to make our data available in the global biodiversity system to a wide range of users. Dataset represents a significant addition to the distribution of species in this area. The data can be used to analyse future changes in ichthyofauna, as well as to help the authorities to manage their territory more efficiently. This publication describes a dataset that contains information on fish encounters in the Upper Don basin and the middle Volga (centre of the European part of Russia) over a 30-year period (1990-2020). The dataset contains information on 6400 occurrences of 394341 specimens of 56 species, 99.9% of specimens being identified to the species level. A total of 883 sites were studied, of which 253 were in lentic biotopes (lakes - 121, ponds - 123, backwater - 5, reservoir - 3, pit - 1), 630 were in lotic (rivers - 628, stream - 1, channel - 1). One collecting site has an average of 7.2 species (from 1-21 species per location). Only those species that form self-reproducing populations are given. The dataset is a compilation of data from several working author groups. All observations have precise geo-referencing with the names of water bodies (rivers, lakes etc.). All presented data are published in the form of a database for the first time. Some data form the basis of previously-published works (3998 observations, 62%) and some are published for the first time (2402 observations, 38%). A large amount of data comes from small water bodies that have been neglected by previous researchers.

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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.