Fish communities undergo climate-induced shifts; it is crucial to study the trophic interactions of various fish species in order to understand the extent to which fish trophic niches overlap and the degree of competition between them. We investigated the food web structure, feeding habits, and trophic positions of common fish in the subarctic Lake Imandra. Two methods were used: SCA (stomach content analysis) and SIA (stable isotope analysis). Perch (Perca fluviatilis) and whitefish (Coregonus lavaretus, large sparsely rakered morph) had similar trophic positions (TP = 3.69 ± 0.55 and 3.67 ± 0.55, respectively; p > 0.05); both species were generalists. The diet similarity (the index of relative importance of food items in stomach contents) of perch and whitefish was 48%, with aquatic insects (Trichoptera) being common items in both fish. According to carbon isotope values (δ13C), vendace (Coregonus albula), smelt (Osmerus eperlanus), and burbot (Lota lota) were more closely related with pelagic food sources (δ13C ranged from −27 to −25‰), whereas perch, whitefish, and ruffe (Gymnocephalus cernua) were more fuelled by benthic food web compartments (δ13C ranged from −24 to −21‰). The highest average nitrogen values (δ15N) were found in smelt and ruffe, 15.0 ± 0.7‰ and 14.2 ± 1.9‰, respectively. Perch and whitefish overlap significantly in their isotopic composition (δ13C and δ15N), demonstrating 36% overlap in the combined 40% ellipses (according to Stable Isotope Bayesian Ellipses in R) of the isotopic space. This study confirms the existence of distinct food competition between these two species in a subarctic lake.

The majority of research on food webs has focused on temperate lakes, and little is known about the food web of lakes in polar regions. Subarctic lakes are particularly sensitive to climate change, which affects their stability. Therefore, the trophic structure of the food web in such lakes was considered as the object of this study. We studied a clear-water oligotrophic lake located in the subarctic region of Eurasia, specifically in northern Karelia and the White Sea coast of Russia. The study examined both open water periods (summer–autumn) and ice-covered periods (winter–spring) in this lake. Stable isotope analysis of carbon (13C/12C ratio or δ13C value) and nitrogen (15N/14N, δ15N) in producers and consumers was applied and revealed significant seasonal variations in the structure of the food web. The results indicate the presence of both pelagic and littoral/benthic food web compartments, with a notable contribution of autochthonous carbon derived from benthic sources. Omnivorous fish (perch, Perca fluviatilis; vendace, Coregonus albula; nine-spined sticklebacks, Pungitius pungitius) and some benthic invertebrates (mayfly, Ephemera vulgata; bivalves, Sphaerium corneum) had intermediate δ13C values, integrating these compartments by obtaining resources from both. Planktonic invertebrates had significantly depleted 13C, with the lowest δ13C value reaching −41.7‰, indicating an important contribution of methane-derived carbon. The study also revealed close trophic relationships between lake invertebrates and cyanobacteria, namely with planktonic Dolichospermum lemmermannii and benthic Phormidium sp. Seasonal changes in δ15N values and in trophic position have been observed among predacious omnivorous fish and crustaceans (amphipods, Gammaracanthus loricatus, and copepods, Cyclops scutifer), which are capable of a generalist feeding strategy depending on food availability. Using the example of this lake, it can be concluded that polar lake ecosystems are characterized by different seasonal intakes of allochthonous organic carbon from wetland catchment (humic compounds) and nitrogen because of nitrogen fixation in the air by cyanoprocaryotes. Alternative energy sources, such as carbon derived from methane, can also contribute to the energy balance of lake ecosystems. This study contributes to our understanding of energy flow and connectivity between producers and consumers in high-latitude lakes.

This research focuses on the physiological indicators of zebra and quagga mussels, such as metabolic activity (oxygen consumption level), body mass index, and cardiac resistance (restoration of the heart rhythm after exercise), when the water is polluted with copper and tributyltin compounds. The results revealed differences in the physiological responses of two dreissenid mussel species (Dreissena polymorpha (Pallas, 1771) and D. bugensis (Andrusov, 1897)) to pollution of the aquatic environment with these substances. Quagga mussels were more vulnerable than zebra mussels to copper and tributyltin in terms of mortality, heart rate biomarkers, and metabolic activity. Exposure to these compounds also led to a decrease in body mass index in zebra mussels in some cases. Both dreissenid species exposed to copper exhibited similar levels of respiratory activity, with a dose-dependent bell-shaped response. Mollusks from the treatment with the lowest copper concentration (50 μg/L) and the control did not differ in the level of respiratory activity. However, at average copper concentrations of 100 and 500 μg/L, zebra mussels showed a statistically significant increase in respiration intensity, and at high copper levels (1 and 5 mg/L), they showed a significant decrease. Exposure to organotin compounds also significantly affected the respiratory activity of both dreissenid mussel species, leading to an increase in oxygen consumption by 1.5–1.7 times compared to the control. Quagga mussels were more sensitive to the presence of tributyltin than zebra mussels. As a result of the study, zebra mussel heart rate recovery time increased with increasing copper concentration (from 100 μg/L to 5 mg/L), such that mussels required 10–25 min longer to recover heart rate than the controls. In the case of quagga mussels, the time for recovery of heart rate at high copper concentrations (1000 μg/L) almost doubled (up to 103 min) compared to the control (56 min). Both dreissenid species showed significantly longer heart rate recovery times when exposed to tributyltin (10–100 ng/L) than the controls. The largest differences were observed at 100 ng/L tributyltin, with D. bugensis taking 27–35 min longer to recover heart rate than D. polymorpha. Our data showed that the better resistance of the zebra mussels to toxicants in water can contribute to its further expansion and successful competition with quagga mussels. However, quagga mussels may be more effective as an indicator species at monitoring of water pollution with copper and organotin compounds.

Macrozoobenthos plays an important role in biogeochemical processes. In the eastern part of the Gulf of Finland, macrozoobenthos has been changing rapidly in recent decades due to the massive development of alien species of annelids. The current state of benthic communities in the Gulf of Finland is considered based on data at 24 stations in 2019–2021. In open areas of the bay, the biomass of the bivalves Macoma balthica increased as compared to the beginning of the 2010s. The populations of glacial relict crustaceans, which had greatly decreased in the early 2000s due to benthic hypoxia, have practically recovered. The abundance of oligochaetes has also increased due to the spread of the alien species Tubificoides pseudogaster. This led to a decrease in the proportion of alien polychaetes Marenzelleria spp. in the abundance and biomass of macrozoobenthos. Due to differences between species in living and bioturbation activity, the changes that have occurred in the benthos should affect the processes at the water–bottom interface. There is likely to be an increase in the supply of phosphorus from bottom sediments under modern conditions compared to the period of total dominance of polychaetes in benthic communities in the early 2010s.

The structure of phytoplankton during the under-ice period in northern oligotrophic lakes (Lake Krivoe and Lake Krugloe) has been considered and its level of development during ice melt is assessed. In winter 2019–2021, cyanobacteria, diatoms, and cryptophyte algae were noted in phytoplankton. The phytoplankton spring increase in Lake Krivoe was due to the development of the dinophyte Peridinium aciculiferum and, in Lake Krugloe, to the diatom Aulacoseira subarctica. Biomass varied from 0.003 to 0.083 mg/L in winter and up to 0.65 mg/L in spring. The spring peak (April–May) of phytoplankton is associated with the development of dinophyte algae and exceeded summer values. The average Chl a concentration in plankton reached 0.3–0.4 μg/L. The cryoflora contained algae frozen into the ice in an inactive state (0.003–0.04 mg/L). The development of phytoplankton under the ice makes an important contribution to the productivity of northern lakes.

Alkylphenols, which are industrial chemicals, are xenoestrogens; however, their sublethal effects on aquatic animals are poorly studied. The aim of this work is to study possible metabolic and reproductive disorders in amphipods using the example of Gmelinoides fasciatus from the Gulf of Finland (Baltic Sea) after experimental exposure to 4-tert-octylphenol (4t-OP) from the group of alkylphenols. The mortality of G. fasciatus amphipods in 4t-OP concentration ranges from 0.5 to 1000 μg/L after 96 h and 7 days increases with increasing concentration. At 4t-OP concentrations of 0.5 and 5 μg/L, the mortality of G. fasciatus has not been observed. At a concentration of 20 μg/L, after 7 days, the mortality of crustaceans is 20

One of the top ecological priorities is to find sensitive indicators for pollution monitoring. This study focuses on the bioconcentration and responses (condition index, survival, oxygen consumption, heart rates, and oxidative stress and neurotoxic effect biomarkers) of mussels from the Volga River basin, Dreissena polymorpha and Dreissena bugensis, to long-term exposure to toxic chemicals such as tributyltin (TBT, 25 and 100 ng/L) and copper (Cu, 100 and 1000 μg/L). We found that TBT was present in the tissues of zebra and quagga mussels in comparable amounts, whereas the bioconcentration factor of Cu varied depending on its concentration in water. Differences in responses between the two species were revealed. When exposed to high Cu concentrations or a Cu-TBT mixture, quagga mussels had a lower survival rate and a longer heart rate recovery time than zebra mussels. TBT treatment caused neurotoxicity (decreased acetylcholinesterase activity) and oxidative stress (increased levels of thiobarbituric acid reactive substances) in both species. TBT and Cu levels in mussel tissues correlated positively with the condition index, but correlated with the level of acetylcholinesterase in the mussel gills. The principal component analysis revealed three main components: the first consists of linear combinations of 14 variables reflecting TBT water pollution, TBT and Cu levels in mussel tissues, and biochemical indicators; the second includes Cu water concentration, cardiac tolerance, and mussel size; and the third combines weight, metabolic rate, and heart rates. Quagga mussels are less tolerable to contaminants than zebra mussels, so they may be used as a sensitive indicator.

Changes in the macrozoobenthos of two northern lakes differing in degree of humification of water over the 50-year period that have passed since the first studies were considered. In the lake with water uncolored by humus, the benthos biomass increased, and in the humified one, it decreased. The changes that have occurred are due to climate warming, especially pronounced in winter. It was concluded that even closely located bodies of water can respond differently to climate change. Depending on the nature of the catchment area and the morphology of the lakes, either an increase in their productivity due to the supply of nutrients or a decrease due to humification can be observed.

Промышленные химикаты, алкилфенолы, относятся к ксеноэстрогенам, однако до сих пор сублетальные эффекты этих веществ на физиологические показатели водных животных мало изучены. Целью данной работы явилось изучение возможных метаболических и репродуктивных нарушений у амфипод (на примере вида Gmelinoides fasciatusиз Финского залива Балтийского моря) после воздействия в эксперименте 4- трет -октилфенола (4t-OP) из группы алкилфенолов. Смертность амфипод G. fasciatus в диапазоне концентраций 4t-OP от 0,5 до 1000 мкг/л через 96 ч и 7 сут возрастала с увеличением концентрации 4t-OP. При концентрации 20 мкг/л через 7 сут смертность рачков составила 20 %, а при его концентрации 100 мкг/л - 60 %. 100%-я выживаемость рачков была при его концентрациях 0,5 и 5 мкг/л. При долговременном воздействии нелетальных концентраций 4t-OP (0,5 мкг/л) на этих амфипод отмечено снижение их половой активности через одну неделю: только 25 % амфипод сохраняли прекопуляторные пары, в то время как их было 100 % в контроле. При хроническом воздействии (28 сут) наименьших концентраций выявлены признаки демаскуланизации самцов (уменьшение ширины гнатопод и увеличение глубины коксальных пластин), снижение общей плодовитости самок и необратимые нарушения в развитии эмбрионов, а именно высокая доля (>50 %) эмбрионов, остановивших развитие на разных стадиях органогенеза. Проведенное исследование показывает значительные функциональные нарушения метаболических процессов и репродукции у рачков даже при воздействии нелетальных концентраций октилфенола. Эти результаты подтверждают высокую токсичность этого промышленного загрязнителя, присутствие которого в водной среде может привести к необратимым изменениям биоты. The industrial chemicals alkylphenols are xenoestrogens, but the sublethal effects of these substances on aquatic animals are poorly studied. The aim of this work was to study possible metabolic and reproductive disorders in amphipods, on the example of the species Gmelinoides fasciatus from the Gulf of Finland of the Baltic Sea, after experimental exposure to 4- tert -octylphenol (4t-OP) from the group of alkylphenols. The mortality of G. fasciatus amphipods in the 4t-OP concentration range from 0.5 to 1000 μg/L after 96 hours and 7 days increased with increasing concentration. It was absent at 4t-OP concentrations of 0.5 and 5 μg/L. At a concentration of 4t-OP of 20 μg/L, after 7 days, the mortality of crustaceans was 20 %, and at its concentration of 100 μg/L, it was 60 %. When exposed to sublethal concentrations of 4t-OP (0.5 μg/L), these amphipods showed a decrease in sexual activity after one week of amphipods: only 25 % of amphipods retained precopulatory pairs versus 100 % in control. Under chronic exposure (28 days) to the lowest concentration, males showed signs of demasculinization (decrease in the width of the gnathopods and an increase in the depth of the coxal plates); females showed a decrease in the overall fecundity and irreversible disturbances in state of embryos, namely, a high proportion (>50 %) of embryos that stopped their development at different stages of organogenesis. The conducted study shows significant functional disorders of metabolic processes and reproduction in crustaceans even when exposed to non-lethal concentrations of octylphenol. These results confirm the high toxicity of this industrial pollutant, the presence of which in the aquatic environment can lead to irreversible changes in the biota.

The growing impact of human activities on the biosphere requires research into the effects of exposure to hazardous toxic substances on aquatic ecosystems. The ecophysiological parameters of the amphipod Gammarus oceanicus Segerstråle, 1947, such as rates of food consumption, oxygen consumption, and phosphate excretion (excreted in the urine), were studied in order to determine the responses of these crustaceans to the influence of a little-studied micropollutant of anthropogenic origin, xenoextrogen, 4-tert-octylphenol (4t-OP). After 28 days of exposure to the lowest concentrations of 4t-OP (0.25 μg/L), a decrease in the feeding and excretory activity of crustaceans was detected, while the level of oxygen consumption was similar to the values in control individuals. All the parameters of the tested animals studied changed significantly after the same time of exposure to 2.5 μg/L 4t-OP, showing an adverse effect on the functioning of the crustacean organism. The data obtained can be used to develop criteria for quality control of the aquatic environment.

The Gulf of Finland (with the estuary of the Neva River) is located in the northeastern part of the Baltic Sea and plays a key role in the formation of biological resources and the quality of the natural environment of the entire Baltic Sea. Along with eutrophication, its ecosystem is subject to anthropogenic pollution with toxic elements, including heavy metals. In this work, we analyzed the content of cadmium and copper in bottom sediments of the Gulf of Finland and in the tissues of the dominant species of benthic macrofauna — the deposit-feeder polychaete Marenzelleria arctia and the predatory isopod Saduria entomon. Significant variability was found in the spatial distribution of these elements in both environments. The average concentrations of cadmium (0.67±0.1 mg/kg of dry matter) in bottom sediments were correspondingly two times higher, and copper (34.4±4.0 mg/ kg) 1.5 times lower than the threshold levels established for the Baltic Sea. A significantly higher content of these metals was found in the tissues of isopods than polychaetes; in some locations it is an order of magnitude higher in isopods than in polychaetes. The factor of trophic transfer of metals in the Gulf food web showed biomagnification, i. e. accumulation of metals during the transition from low-level consumers (M. arctia) to predatory consumers (S. entomon), cadmium by 3.7 times and copper by 8.7 times. Thus, both species of benthic animals have high accumulative activity in relation to cadmium and copper, which contributes to the active transport of both metals from bottom sediments to marine biota, and, ultimately, transfer to the higher levels of the food web (fish, birds and mammals).

The seasonal feeding patterns of the cold-adapted fish, Coregonus albula, are poorly studied in high-latitude lakes but could provide insight for predicting the effects of global warming. We examined vendace’s diet composition, traced the carbon and nitrogen isotope ratios from producers to consumers in the food web, and estimated vendace’s trophic position in a subarctic lake (the White Sea basin). Results showed the vendace to be a typical euryphagous fish, but clear seasonal differences were found in the relative importance of plankton and benthos in the diet. The vendace consumed primarily benthic amphipods in the summer, planktonic cladocerans in the autumn, and copepods in the winter–spring (under ice); larvae of aquatic insects were the second-most important food items throughout the year. Because of the substantial proportion of fish embryos in its diet, the vendace had a trophic position similar to that of a predatory fish (perch). The Bayesian food source-mixing model revealed that the majority of vendace energy derives from planktonic copepods. The dominant Cyclops had the lowest carbon isotope values, suggesting a carbon-depleted diet typical for methanotrophic bacteria, as its probable food source was in a lake under ice. Understanding the feeding patterns of vendace provides information to better predict the potential biotic effects of environmental change on lake ecosystems.

The faunal composition of Amphipoda crustaceans of continental water bodies in northwestern Russia is analyzed. To compile a checklist of amphipod species, data from original collections made in 1998–2021, the collections of the Zoological Institute, and published information are used. To date, 29 species of amphipods have been recorded from the continental waters of the region. An arealogical analysis of the faunal composition reveals six groups or distribution patterns: Holarctic, West Palearctic, and North Atlantic estuaries, and further three groups of emigrants (settlers) from the Baikal–Siberian, Ponto-Caspian, and Arctic regions. The greatest species richness is recorded in estuarine sections of Baltic Sea rivers on the territories of Leningrad and Kaliningrad oblasts, as well as freshwater bodies of the Republic of Karelia. In terms of species richness, estuarine species amphi-Atlantic in distribution and representatives of allochthonous elements of fauna (invasive species) from the Ponto-Caspian basin dominate in the region. The most common in the region are representatives of Holarctic and Palearctic complexes, as well as species of Arctic and Baikal–Siberian origins. Among them, the following species are most often be recorded in lakes: Gammarus lacustris, Monoporeia affinis, Palaseopsis quadrispinosa, and Gmelinoides fasciatus, compared to G. zaddachi, G. oceanicus, G. tigrinus, and Pontogammarus robustoides in estuarine brackish waters. Human-mediated introductions, both intentional and accidental, must have facilitated the appearance of invasive species in the fauna of northwestern Russia. In the future, an increase in species richness in the region can be expected to occur in two ways: through the introduction of marine species into freshwater and due to speciation.

The narrow-clawed crayfish Pontastacus leptodactylus is a large native European astacid crayfish species distributed in the drainage area of the Azov, Black and Caspian Seas. The species shows immense morphological diversity across its range. Various species and subspecies have been described and, recently, eight species and two subspecies were confirmed in the updated classification of freshwater crayfishes. However, genetic diversity studies of the species are scarce, mostly from limited geographical areas. Therefore, we aimed to analyse the genetic diversity of this species using mtDNA and nuDNA from across their distributional range to verify the presence of described species. Analyses of individuals from 65 populations and 14 countries confirmed the existence of three divergent mtDNA phylogroups corresponding to the geographical area of Central and Southern Europe, Eastern Europe and Asia, and Turkey. Analysis of ancestral ranges revealed the Black Sea basin as the most likely area of origin. The nuDNA analysis showed low diversity partially corresponding to the mtDNA pattern. All analyses showed mitonuclear discordance indicating the existence of a single species. Therefore, we point out the possibility of taxonomic inaccuracy with the current number of valid species within the P. leptodactylus species complex.

The question of the possibility of using meiobenthos to monitor the state of bottom sediments (BS) in the Baltic Sea, where bottom macrofauna are unstable due to hypoxia, is of interest. The parameters of meiobenthos and the physicochemical characteristics of BS and bottom water were studied, and the toxicity of BS was determined by the survival of the amphipod Gmelinoides fasciatus in 10-day tests for 11 stations in the brackish part of the Gulf of Finland (depths of 10–52 m). Principal component analysis revealed two factors explaining 67% of the variability of the variables, and groups of stations similar in terms of conditions. Factor 1 combined nine related variables (depth, metal concentrations and fraction of sand in BS, ratio of nematodes and copepods (N/C index) and survival of amphipods), and factor 2 combined four variables (organic matter and polyaromatic hydrocarbons in BS, phosphates in water and the number of meiobenthos). Thus, bioindication of the environment by meiobenthos is promising, but requires an integrated approach to be applied in monitoring.

Cold waters and low productivity of the reservoir create energetically special conditions for the life of aquatic organisms, which are exacerbated by the current climate change. The temperature-dependent indicators of energy metabolism (active metabolic rate) have been studied for the representatives of different ecological–iogeographic groups of amphipods: arctic ( Gammaracanthus loricatus ), palearctic ( Gammarus zaddachi ), and holarctic ( G. lacustris ), from the coastal lake and the littoral of the White Sea basin. Interspecific differences in the standard energy metabolism rate and its 1.5–2.0-fold increase for predating amphipods are found.

The life of lakes in winter is still insufficiently studied. The purpose of this article is a comparative analysis of the quantitative development of planktonic and benthic communities and the abiotic conditions of their existence during the ice-covered period in two small (<1 km2) northern lakes that differ greatly in morphometry and the degree of water humification. A significant difference was revealed in the seasonal dynamics of plankton communities in the studied lakes. In a deeper lake with water uncolored by humus, the quantitative development of phytoplankton at the end of the ice-covered period was almost as significant as during the open-water period. It is necessary to note the rapidity of changes in under-ice phytoplankton in early spring: in April 2020, the content of chlorophyll a in water changed more than six times during a week. There was no mass development of phytoplankton under the ice in a shallow highly humified lake. The zooplankton of the studied lakes was characterized by the opposite trend. In the deep-water lake, the winter zooplankton was noticeably inferior to the summer one in terms of the level of quantitative development; in the shallow lake, a high biomass (~1 g/m2) of zooplankton was noted throughout the entire study period. The macrozoobenthos of lakes in winter, on the whole, retained the same high level of quantitative development as in the open-water period. Seasonal differences were local and were caused by the redistribution of bottom animals due to changes in the oxygen and, possibly, temperature regime of bottom waters. In particular, due to hypoxic-anoxic conditions at the end of the ice-covered period, benthic animals migrated from the profundal zone of the deep-water lake. After the ice melted and the near-bottom waters were saturated with oxygen, the profundal community quickly recovered. Thus, the quantitative development of biological communities in winter can vary greatly even in closely located lakes, in particular, it depends on the degree of humification of the water body. Given the high dynamism of phytoplankton in early spring, the assessment of the role of the under-ice period in the life of lakes, apparently, requires long-term research, including detailed observations at the end of ice-covered period.

Macroalgae are widely used for bioindication and assessment; however, in the case of pollutants of different origin, it is still unclear which contaminants in thalli can be regarded as indicative because too many factors influence the ability of algae to uptake them. The present study is a part of an international HAZLESS project and was conducted in the eastern Gulf of Finland (GoF). The main goal of our study was the application of metal concentrations in macroalgae as a tool for environmental quality assessment. To achieve this goal, we calculated the threshold metal concentrations in macroalgae (Cladophora glomerata) and compared our obtained values with actual concentrations. We found significant Spearman correlations in May between metals in sediments and pore water (−0.73 for Zn, −0.62 for Cd, 0.85 for Pb) and also between metals in algae and metals in pore water (1 for Cu and Cd, 0.98 for Zn and Pb). In July, Pb in algae were significantly correlated with Pb in pore water (0.88). The application of the calculated environmental quality standard (EQSMPC) for macroalgae has shown moderate pollution by Cu and Pb in the coastal zone of the eastern GoF. This was confirmed by an assessment based on the comparisons of metal concentrations in water with Environmental Quality Standards for water (EQSw). However, differences in the bioaccumulation factor and EQSMPC between May and July have shown that it is necessary to compare samples taken during the same period every year for adequate results in long-term monitoring. Considering the sensitivity of accumulating processes to the surrounding environment, we believe that in the case of habitats with diverse conditions, even for the same species of algae, threshold values should be calculated and used individually for every habitat. Our results have shown that this approach can be widely used for an assessment of environmental quality via metal concentrations in opportunistic macroalgae and can be recommended for further use.

The development of sensitive indicators reflecting the state of the environment is an important issue for the monitoring of marine ecosystems. The spionid polychaete Marenzelleria arctia and pontoporeiid amphipod Monoporeia affinis are common macrobenthic species in the brackish Gulf of Finland (the easternmost Baltic Sea). This paper aims to apply the Benthic Opportunistic Polychaetes Amphipods (BOPA) and Benthic Opportunistic Annelid Amphipods (BO2A) indices based on the polychaete/amphipod ratio as indicators of the environmental state in this region. We analyzed the relationships between environmental variables and benthic indices based on samples from two benthic surveys in 2019 (10 sites) and 2020 (9 sites). The coastal sites were characterized by worse water quality (i.e., the higher concentration of hydrocarbons, total phosphorus and chlorophyll-a), but cleaner sediments (i.e., total phosphorus, organic carbon, polyaromatic hydrocarbons and trace metals) than offshore sites. The BOPA and BO2A correlated positively with the level of water pollution and negatively with sediment pollution. The activity of the benthic organisms seems to strongly influence the concentration of contaminants in sediments, so this factor hinders the use of BOPA and BO2A indices for the assessment of bottom sediments quality in the eastern Baltic Sea. At the same time, this study shows that BOPA and BO2A indices can be used in assessing water quality.

The faunal composition of Amphipoda crustaceans of mainland water bodies in northwestern Russia is analyzed. To compile a checklist of amphipod species, data from original collections made in 1998–2021, the collections of the Zoological Institute, and literary information are used. To date, 29 species of amphipods have been recorded from the continental waters of the region. An arealogical analysis of the faunal composition reveals six groups or distribution patterns: Holarctic, West Palearctic, and North Atlantic estuaries, and further three groups of emigrants (settlers) from the Baikal-Siberian, Ponto-Caspian, and Arctic regions. The greatest species richness is recorded in estuarine sections of Baltic Sea rivers in the territories of the Leningrad and Kaliningrad regions, as well as fresh water bodies of the Republic of Karelia. In terms of species richness, estuarine species amphi-Atlantic in distribution and representatives of allochthonous faunas (invasive species) from the Ponto-Caspian basin dominate the region. The most common in the region are representatives of Holarctic and Palearctic patterns, as well as species of Arctic and Baikal-Siberian origins. Among them, the following species are most often to be recorded in lakes: Gammarus lacustris, Monoporeia affinis, Palaseopsis quadrispinosa and Gmelinoides fasciatus, vs G. zaddachi, G. oceanicus, G. tigrinus and Pontogammarus robustoides in estuarine brackish waters. Human-mediated introductions, both intentional and accidental, must have facilitated the appearance of invasive species in the fauna of Russia’s northwestern region. In the future, an increase in species richness in the region can be expected to occur in two ways: through the introduction of marine species into fresh waters and due to speciation.