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This paper presents the results of the hydrochemical, hydrobiological, and paleolimnological studies of 17 Arctic lakes in the Pechora River delta. Water samples and surface sediments were collected during the 2020 summer expedition to the Nenets State Nature Reserve. Based on the chemical composition of their waters, the studied lakes belong to the chloride class, sodium group II. Overall, they are characterized by shallow depths typical of this region, low mineralization (below 200 mg/L) and water hardness, as well as high dissolved oxygen levels. To assess the ecological status of the lakes, cladocerans and their subfossil remains, which have been widely recognized as reliable indicators for tracking long‐term changes in the development of freshwater ecosystems, were analyzed. A total of 23 cladoceran taxa were identified. The dominant species were Bosmina longispina (Leydig, 1860) and Chydorus cf. sphaericus (Müller, 1776) – either of these species, often displacing each other, prevailed in the lakes. Other cladoceran species were low-abundant and found sporadically: Sida crystallina (Müller, 1776), Pleuroxus uncinatus (Baird, 1850), Coronatella rectangula (Sars, 1862), and Bythotrephes longimanus (Leydig, 1860). The study also reports the first finding of Rhynchotalona latens (Sarmaja-Korjonen, Hakojarvi and Korhola, 2000), a rare glacial relict species with a narrow distribution range, in this region of Russia. The zoogeographical analysis of the data obtained on the identified taxa showed that species typical of the Palearctic and Holarctic zones prevail in the cladoceran communities under study.

The qualitative and quantitative characteristics of benthic communities in the shore area and shallow waters of the Volga reach of the Kuibyshev Reservoir near the village of Staroe Arakchino (Kazan) were studied during the growing season of 2015. A total of 27 hydrobiont taxa belonging to three types of invertebrate organisms were registered. The taxonomic composition of benthic communities was compared for the year of the study and previous years. A decrease in the values of the quantitative characteristics of macrozoobenthos was revealed: a decline in the average abundance and biomass was observed in 2010–2015. The main groups of invertebrates that mostly contribute to the abundance of the population were dominant invasive species, such as Dreissena polymorpha (Pallas, 1771) and Lithoglyphus naticoides (Pfeiffer, 1828). The biomass was determined by large bivalve and gastropod mollusks: Unio longirostris (Rossmaessler, 1836), Pseudanodonta complanata (Rossmaessler, 1835), Viviparus viviparus (L., 1758), V. contectus (Millet, 1813), and L. auricularia (L., 1758). Soft zoobenthos played an important role in terms of the abundance and biomass only in the area of shallow waters and was represented by chironomids (Chironomus sp. (Meigen, 1803)) and heteropterans (I. cimicoides (L., 1758) and Micronecta sp. (Kirkaldy, 1897)).

The structural and functional characteristics of zooplankton communities of two thermokarst lakes on Samoylov Island (Lena River delta, Yakutia) have been studied. A total of 76 taxa of hydrobionts have been found in the zooplankton community of the studied lakes. The taxonomic lists of the spring-summer zooplankton compositions have been provided. The complex of species determining the community structure has been defined. Rotifers determined the species diversity and zooplankton abundance in the lakes. The biomass values were defined by large copepods. Cold-water-dwelling northern species made the greatest contribution to the quantitative indicators of zooplankton. The seasonal dynamics of quantitative indicators of zooplankton has been analyzed. The current state of the lakes has been assessed using the saprobity index and species diversity.

The purpose of our study is to reconstruct the climatic and environmental changes that took place over the Holocene in Northwest Russia. The results of the palaeobiological analysis of cladoceran community of Lake Medvedevskoe (Karelian Isthmus, located between the Gulf of Finland of the Baltic Sea and Lake Ladoga, Northwest Russia) have been discussed. In the sediments of Lake Medvedevskoe, we have identified 38 cladoceran taxa that belong to 7 families: Bosminidae, Holopedidae, Chydoridae, Daphnidae, Polyphemidae, Macrotricidae, and Sididae. It has been revealed that Bosmina (Eubosmina) longispina and Alonella nana are the most common for subfossil Cladocera community of the lake. Palearctic and Holarctic species are dominant. Both pelagic and littoral taxa are well represented in the lake. The down-core changes in cladoceran community allowed to identify five statistically significant zones. It has been discovered that the taxonomic richness of biological communities is low at the bottom of the core with the dominance of typical northern species and increases towards the sediment surface alongside with the rise of organic content in sediments. Based on the shifts in the taxonomic composition of cladoceran community, we have concluded upon the trophic status of the lake and climate changes. The obtained data have been compared with the results of the chironomid analysis that was performed earlier.

The paper considers the qualitative characteristics (chemical composition of waters and sediments, biological diversity) of the Lena runoff and their transformation as a result of the biogeochemical pro-cesses taking place in the delta region. The hydrological, hydrochemical, geochemical (chemical com-position of river sediments), and hydrobiological description of the delta branches at the current stage has been provided. The Lena delta branches are characterized by a wide variety of water discharge values and hydro-dynamic conditions. It has been revealed that the hydrochemical composition of the delta is specific due to the dominance of hydrocarbonates and calcium ions, pH 6.9–7.9, high oxygen saturation conditions (more than 100 % sat.) throughout the water column because of wind activity, high concentrations of dissolved silica (1.1–2.7 mg/L) and iron (0.01–0.07 mg/L) if compared to the world’s mean values in natural freshwater bodies, and low concentrations of trace elements (mainly below the detection limits). The concentrations of nitrates and phosphates have been estimated in the range of 2.6–34.7 µg/L and 3.1–26.2 µg/L, respectively. The bottom and suspended sediments have been assigned to the siliceous type (Si/Al varies from 4.2 to 18 µg/L). The results of the analysis of hydrodynamic conditions, hydrobiological features, geochemical indi-ces, as well as data of the statistical factor analysis, have shown chemical elements redistribution dependencies. Several factors determining the chemical composition of water and sediments in the Lena River delta have been distinguished. The interpretation of these factors reflects the influence of river catchment area, cryogenic processes in the river delta, and biogeochemical processes. Based on the modern multidisciplinary research performed in the Lena delta, it has been demon-strated that processes leading to an inhomogeneous distribution of chemical elements between water (dissolved fraction), suspended matter, and bottom sediments occur in the delta of the river.

Seasonal fluctuations in the species composition and quantitative indices of benthic communities in the littoral zone of the Kuybyshev Reservoir were investigated near the village of Staroe Arakchino in the summer–autumn season of 2014. On the whole, 40 taxa of macrozoobenthos were identified during the period of investigation. These taxa belong to the following groups of invertebrates: the phyllum of Mollusca (the classes of Bivalvia and Gastropoda), the class of Insecta (the orders of Diptera, Ephemer-optera, Odonata), and the phyllum of Annelida (the classes of Oligochaeta and Hirudinea). The highest species diversity was observed for insects and mollusks species. There was a reduction in the species diversity compared to the data of a similar research performed in 2010–2013. Zoobenthos samples were characterized by the absence of some orders of insects (Trichoptera, Coleoptera larvae and adults) and Maxillopoda. The abundance and biomass indices of macrozoobenthos in 2014 were lower than the same indices in 2010–2013. Two classes of mollusks (Gastropoda and Bivalvia) formed the dominant group in abundance and biomass. The significant influence on the quantitative indices was produced by Viviparus viviparus (L., 1758), as well as by such invasive species as Dreissena polymorpha (Pallas, 1771) and Lithoglyphus naticoides (Pfeiffer, 1828).

The current taxonomic composition, structure, and quantitative characteristics of zooplankton in channels of the mouth region of the Lena River were considered. The community of summer zooplankton in the Lena channels included 34 zooplanktonic species belonging to 18 families and 29 genera. Rotifers, as the most diverse and abundant group, provided a basis for the species diversity. It was found that the zooplanktonic community of the Lena channels is characterized by the low values of species diversity and quantitative characteristics.