The taxonomic structure and spatial distribution of the late-summer zooplankton in the Volga River from the upper Ivankovo reservoir to Volga Delta near Astrakhan during the period 2020–2021 were studied. Zooplankton biomass in the most of the Volga reservoirs was dominated by cladoceran Daphnia galeata and copepod Mesocyclops leuckarti (up to 45–84%). However different dominant species were observed in the Volgograd reservoir. Cladocerans Chydorus sphaericus and Bosmina cf. longispina formed there up to 43% of the total zooplankton biomass. Rotifers and Ponto-Caspian copepod Heteroscope caspian are dominated in the Volga River below the dam of the Volzhskaya hydropower plant (56% and 18% of the total zooplankton biomass respectively). Among the invasive species, most abundant are East Asian copepod Thermocyclops taihokuensis (up to 230 thsd. ind./m3 and locally 90% of the total biomass) and North American copepod Acanthocyclops americanus (up to 86 thsd. ind./m3 and 35% of the total biomass). The maximum abundance of the zooplankton (0.8–1.6 g/m3) was observed in the Upper Volga reservoirs while the minimum abundance (0.1–0.2 g/m3) – in the Lower Volga. The maximum abundance of zooplankton (0.8–3.3 g/m3) was recorded in the mouth areas of the reservoir tributaries, while the minimum abundance (0.1–1.4 g/m3) – in the pelagic zone of the reservoirs. A decrease in the zooplankton community biomass from Upper to Lower Volga was observed in all biotopes. A negative correlation between the community abundance and daily water inflow into the reservoirs and a positive correlation between the community abundance and water temperature were revealed. Long-term variations in zooplankton biomass and their relationship with water body trophic state, thermal and oxygen regimes were discussed.

В ходе гидробиологических исследований четырех малых озер Ярославской области в 2013–2015 и в 2018 гг. в фитопланктоне было выявлено 92 таксона цианобактерий и водорослей, в зоопланктоне – 76 видов планктонных животных, в макрозообентосе – 26 низших определяемых таксонов. Оз. Рюмниково по показателям фитопланктона и зоопланктона имело наиболее низкий трофический статус (мезотрофный), в нем отмечено максимальное разнообразие и количественное развитие макрозообентоса. Озера Вашутинское, Чашницкое и Заозерье по показателям фитопланктона и зоопланктона оценены как эвтрофные, а разнообразие и количественное развитие макрозообентоса в них было на низком уровне, в том числе в связи с влиянием «цветения» воды и накопления сероводорода. In total, 92 taxa of cyanobacteria and phytoplankton algae, 76 zooplankton species, and 26 lower definable taxa of macrozoobenthos were registered during hydrobiological studies at four small lakes of the Yaroslavl Oblast in 2013–2015 and 2018. According to indicators of phytoplankton and zooplankton, Lake Ryumnikovo had the lowest trophic status (mesotrophic), but both the maximum diversity and abundance of macrozoobenthos. Lakes Vashutinskoe, Chashnitskoe and Zaozer'e were eutrophic, the macrozoobenthos diversity and abundance there were low, partly due to the water bloom effect and hydrogen sulfide accumulation.

During the growing season of 2022, the studies were carried out in the water area of the Bratsk Reservoir (the Angara River) and in the near-dam site of the Bratsk high-level hydroelectric power station. The ecological effects of the hydroelectric dam on the zooplankton in the regulated section of the river were studied and the impact of the zooplankton downstream drift on the feeding conditions of the fish in the Bratsk Reservoir was assessed. It is established that the water column stratification is a significant factor in the ecological differentiation of plankton communities in the upper reaches of the hydroelectric power station. Unlike the warmed-up layer of the epilimnion, a specific, poor in species, low-abundant and low-productive plankton develops in the cold-water hypolimnion, which enters the dam intakes. A rigorous analysis does not reveal a transformation of the community composition, a statistically significant decrease in abundance and productivity, increased mortality of invertebrates during their passage through the dam, deterioration of the water ecological quality in the lower reaches of the hydroelectric power station. Both live and dead zooplankton drifting downstream from the reservoir makes up a significant proportion of the diet of fish, which during the feeding period come as close as possible to the hydroelectric power station and form aggregations exceeding those in the water area of the Bratsk Reservoir in density. The data obtained testify to the effective adaptation of lake-river planktocenosis to the specific natural and technogenic mode of operation of a large hydraulic structure. The local impact of the Bratsk hydroelectric power station has no ecologically significant negative effect on the plankton of the Bratsk Reservoir and the Angara River ecosystems and has a positive effect on the fish population in the lower reaches through the enrichment of its forage base.

When studying the ecological effects of the influence of a hydroelectric power station (HPS) dam on the zooplankton of the regulated section of the Angara River and the effect of the zooplankton downstream drift on the feeding conditions of fish of the Bratsk Reservoir during the vegetation period of 2022, it has been established that water column stratification is a significant factor in the ecological differentiation of plankton communities from the upper reaches of the Bratsk HPS. Relative to the heated epilimnion layer, a specific, poor in species, low-abundant and low-productive plankton, entering the dam water intakes, deve-lops in the cold-water hypolimnion. A rigorous analysis revealed no transformation in the composition of communities, a statistically significant decrease in abundance and productivity, and an increased mortality of invertebrates during the transition through the dam, as well as a deterioration in the ecological quality of water in the lower reaches of the HPS. Zooplankton (both alive and dead) drifting downstream from the re-servoir constitute a significant portion of the diet of fish, which get as close as possible to the HPS during feeding and form clusters that are denser than those in the water area of the Bratsk Reservoir. The data indicate the efficient adaptation of the lake–river planktocenosis to a specific natural and technogenic mode of operation of a large hydraulic structure. Locally, the Bratsk HPS has no ecologically significant negative effect on the plankton of the Bratsk Reservoir and Angara River and it has a positive effect on the fish population of the lower reaches through the enrichment of its forage base.

The composition and abundance of zooplankton were studied in 2012–2016 in the deepwater stratified mesotrophic Lake Pleshcheyevo, Yaroslavl oblast (Russia). During the study period, 130 species of rotifers and crustaceans were recorded in the lake. The plankton abundance and biomass during the study period were lower or close to the values indicated for 1979–1996. The open littoral of the lake was more often characterized by the low abundance of zooplankton compared to deeper areas (depths exceeding 4 m). Interannual variations in the total abundance and biomass of zooplankton in different seasons of the year were observed, caused by the climatic features of different years and different times of sampling. The composition of the dominant species of rotifers and crustaceans in 2012–2016 was close to that indicated earlier in 1979–1996. Synchaeta lakowitziana , S. kitina , Diaphanosoma mongolianum , and Thermocyclops oithonoides were noted for the first time as dominant zooplankton species in the lake.

Information on the species composition and distribution of zooplankton mycoparasites is summarized: 8 species of Chytridiomycota, 2 – Ascomycota, 7 – microsporidia, 19 – fungal-like organisms Oomycota. Known life cycles, dependence of infection spread on water temperature, density of host and parasite populations, host are given. The presence of saprotrophic species in plankton is indicated. They feed on dead inhabitants of plankton and introduced invertebrate corpses from terrestrial conditions: 30 species of Chytridiales, 4 – Mucorales and 35 species of fungus-like Saprolegniales. Information on the role of fungi in the nutrition of zooplankton is given, mainly zoospores and mitospore ascomycetes.

In the deep-water stratified Pleshcheyevo Lake, a cold-water species of rotifer belonging to the “northern invaders,” Synchaeta lakowitziana, was discovered for the first time. The rotifer was recorded in the winter–spring (March–May) and late autumn (November) periods at water temperatures of 1.9–12.9°С. The morphology of the trophi of the rotifer is described.

Details of the body structure of sexually mature copepods (females and males) of two morphologically similar species of the genus Thermocyclops are presented. One was the invader, T. taihokuensis from the Shat Reservoir, Tula oblast, while the other was the native T. oithonoides from Pleshcheyevo Lake, Yaroslavl oblast. Both were studied to clarify the known morphological features and to identify new ones in order to distinguish reliably between these species. The structure was analyzed using programs for visualization and measurement of objects observed under the AxioVision Rel. 4.8 and ScopePhoto 3.0 microscopes. Drawings of the fine structures were made from photographs. The distinctions between these two species are shown to be mainly based on qualitative characteristics (the shape of the element, its location, etc.). No taxonomically significant differences have been found in the measured parameters of the elements of the fourth, fifth, and sixth (in males) legs, the caudal rami, and their setae, all of which are commonly used to identify species of this genus. The shape of the seminal receptacle, the apical setae of the caudal rami, the apical spines of the endopodite of the fourth legs (P4), and the position of the lateral seta on the caudal rami have been found to be the most important for identifying the females of these species. Interspecies differences were revealed in the armature of the prominences of the intercoxal coupler, the ornamentation of the P4 coxopodite and its intercoxal coupler, and the armature of the 14th segment of the antennules of females. These characteristics are proposed as additional features for species identification. Thermocyclops taihokuensis males are shown to be significantly larger than those of T. oithonoides , and they also differ in the position of the lateral setae of the caudal rami, the P4 armature, and the length of the central apical setae. A key is proposed for identifying six species of the genus Thermocyclops common in European Russia.

В марте 2019 г. изучены таксономический состав, вертикальное и горизонтальное распределение зоопланктона в оз. Плещеево. Установлено, что его обилие невелико (численность 11.5–13.8 тыс. экз./м3, биомасса < 0.1 г/м3), значительно ниже, чем регистрировали в этот сезон в 1980–90-х гг. Среди ракообразных, как и прежде, преобладал холодолюбивый Cyclops kolensis. Отмечено возрастание доли представителей р. Synchaeta, которые составляли до 12–27% общей численности коловраток. В то же время наблюдалось снижение обилия Eudiaptomus graciloides, ранее многочисленного в подледный период. In March 2019, we studied the taxonomic composition, vertical and horizontal distribution of under-ice zooplankton in Lake Pleshcheyevo and revealed that its quantity (abundance 11.5–13.8 thous. ind./m3, biomass <0.1 g/m3) was much lower than in the same season of 1980–1990. The cryophilic species Cyclops kolensis dominated among crustaceans. An increase in the share of representatives of the genus Synchaeta (12–27% of the total abundance of rotifers) was also noted. At the same time, previously dominant during the under-ice period Eudiaptomus graciloides demonstrated the reduced abundance.

The magnetic environment may influence the functioning of the cardiovascular system. It was reported that low-frequency and static magnetic fields affect hemodynamics, heart rate, and heart rate variability in animals and humans. Moreover, recent data suggest that magnetic fields affect the circadian rhythms of physiological processes. The influence of the magnetic environment on heart functionating during early development has been studied insufficiently. We utilized transparent zebrafish embryos to evaluate the effect of the hypomagnetic field on the characteristics of cardiac function using a noninvasive optical approach based on photoplethysmographic microscopic imaging. The embryos were exposed to the geomagnetic and hypomagnetic fields from the second to the 116th hour post fertilization under a 16 h light/8 h dark cycle or constant illumination. The exposure of embryos to the hypomagnetic field in both lighting modes led to increased embryo mortality, the appearance of abnormal phenotypes, and a significant increase in the embryo’s heartbeat rate. The difference between maximal and minimal heartbeat intervals, maximal to minimal heartbeat intervals ratio, and the coefficient of variation of heartbeat rate were increased in the embryos exposed to the hypomagnetic field under constant illumination from 96 to 116 h post fertilization. The dynamics of heartbeat rate changes followed a circadian pattern in all studied groups except zebrafish exposed to the hypomagnetic field under constant illumination. The results demonstrate the importance of natural magnetic background for the early development of zebrafish. The possible mechanisms of observed effects are discussed.

Temperature adaptations of ectothermic species as well as the plasticity of their thermal strategies are important for survival during temperature fluctuations, in particular, caused by global warming. The critical thermal maximum (CTM)—the values of the water temperature at which heat shock was noted (loss of motor activity in case of copepods) was determined under laboratory conditions. The “chronic” method was used to identify the temperature preferences of the copepods in which a group of test organisms are placed into a thermogradient apparatus. The main result is that in the experiment for individuals of the summer generation C. bohater, the optimal thermal conditions (FTP) were within 6–11 °C. Summer generation of this copepod in natural water bodies develops at a temperature of 5–12 °C, which is close to FTP in the experiment. At the same time, the thermal resistance of C. bohater (CTM 31.5 °C) was found to be the lowest among the species of the genus Cyclops. The differences between the thermal preferences of the winter and summer generations expand the temperature of normal performance (TNP) range and indicate a high physiological plasticity of the C. bohater population. This property is likely to allow C. bohater to survive as the climate continues to warm.

In 2016–2019, the copepod Thermocyclops taihokuensis (Harada, 1931) was found in four areas of the Volga River basin, which are 270–1000 km apart from each other. The invader lives in the Lower Volga (river section from the village of Cherny Yar to the city of Astrakhan), in the two areas of the Middle Volga (lake-like part of the Cheboksary Reservoir, the Volga and Volga-Kama reaches of the Kuibyshev Reservoir), and the upper reaches of the Oka River basin (Shat Reservoir). The invader density varies from less than 1 to more than 100 000 ind./m3. To the north of European Russia, the species settled up to 56° N (mouth of the Sura River), to the west, up to 38° E (Shat Reservoir). In the Lower Volga, the invader was the most abundant representative of Cyclopoida and lived together with native Th. crassus. In the Cheboksary and Kuibyshev reservoirs, it coexisted with native Th. oithonoides and Th. crassus; native species dominated. The competitiveness and successful naturalization of Th. taihokuensis in European Russia is determined by its high fertility and long breeding period.

In 2008–2014, two new habitats of Cyclops bohater Kozminsky, 1933 were found in the European part of Russia, Lake Ferapontovskoe (Vologda oblast) and the Rybinsk Reservoir (Upper Volga River). The findings of this species were also confirmed for Lake Glubokoe (Moscow oblast). The morphological features and living conditions of C. bohater populations in different water bodies are described. C. bohater reproduces in the studied lakes in winter and summer (dicyclic) and, in the Rybinsk Reservoir, only in winter (monocyclic). The features that distinguish C. bohater from morphologically related species C. lacustris Sars, 1863 and C. abyssorum Sars, 1863 are discussed.

The distribution of the mycoparasite Aphanomyces patersonii and the level of infestation of cladocerans Daphnia cucullata in Lake Pleshcheyevo in summer 2020 are described. The high prevalence of mycoparasites (42%) is characteristic of the epilimnion in the deep part of the lake with a high abundance of D. cucullata.

Previous studies in Lake Sevan during summer stratification have shown that the highest zooplankton abundance and biomass was found at the lower boundary of the epilimnion. At the same time, whitefish (Coregonus lavaretus), being main planktivorous species in the lake, preferred the hypolimnion, but was leaving this layer if dissolved oxygen concentrations decreased to 1–5 mg/L. The study was conducted in Bolshoy Sevan (the southern part of Lake Sevan) on July 29–30, 2019. The vertical profiles of water temperature and dissolved oxygen concentration, as well as distribution of major taxonomic groups of aquatic organisms were assessed four times a day: in the dark (night time), in the light (day time), and in twilight (morning and evening). Fish population density was assessed using a “Simrad EK80” echosounder. Vertical distribution of zooplankton and fish observed within the study period did not correspond to that described earlier. Maximal zooplankton biomass during the study was recorded in the hypolimnion, and minimal biomass, in the epilimnion. Most of fish (89–100%) located in the metalimnion. At night, about 10% of the fish population descended to the hypolimnion, despite the low dissolved oxygen concentrations found there (3.0–3.1 mg/L). At dawn, about 7% of the fish ascended to the epilimnion, reaching a 7-m depth, where the water temperature was 20.8 °C. The water temperature range preferred by most fish individuals varied during the observation period and did not correspond to previously reported ranges.

A morphological analysis of the populations of rotifer Kellicottia bostoniensis (Rousselet, 1908) from 23 waterbodies in European Russia has been made. It is shown that the length of the posterior spine is the most variable character; the length of the lorica is the least variable one. There is a positive correlation between the length of anterior and posterior spines. The length of spines increases with an increase in lorica length. The sizes of rotifers differ in waterbodies with different trophic statuses, depths, and pH values. The body size of K. bostoniensis is larger in mesotrophic, acid, and deep waterbodies.