In the spring of 2020, in most of the water area of the northern Caspian, the prevalence of saprotrophic floras did not exceed 50000 cells/ml, which microbiota level in summer. The number of oligotrophs in most of the water area was inferior to saprotrophs, which indicated an imbalance in the heterotrophic microbial community. In summer, in the northern part of the Caspian Sea, a decrease in the average heterotrophic group of bacterioplankton and bacteriobenthos was recorded, which indicated the process of self-purification of the marine environment from allochthonous elements. Viewing in the summer period in the waters of the Northern Caspian Sea, the zone of the zone with water saprobicity corresponding to the “polluted” category, the process of self-cleaning of the reservoir after the spring flood was not completed. In the autumn of 2020, the distribution of heterotrophic bacterioplankton and bacteriobenthos reached its lowest levels for all periods of research. In water, the number of saprotrophs was inferior to oligotrophs, which indicated the presence of a heterotrophic microbiota in the water, but they remained in the zone of increased saprobicity. An analysis of the taxonomic structure microbiota showed that heterotrophic communities of bacterioplankton and bacteriobenthos form representatives of the 6th family of gram-negative bacteria (Alsaligenaceae, Enterobacteriaceae, Flavobacteriaceae, Moraxellaceae, Pseudomonadaceae and Vibrionaceae), as well as gram-positive microorganisms. Gram-positive bacteria are most often found in enrichment cultures, mainly of the listed families. Some of the isolated bacteria belong to the group of opportunistic pathogens, which led to the identification of a potential danger to aquatic organisms. However, most of the identified representatives are true saprophytes, covering the inhabitants of water and bottom sediments. The predominance of indigenous microbiocenosis tends to reduce the bacterial pressure of the ecosystem in the water area and improve the state of the habitat of aquatic biological resources in terms of biological micro-characteristics.

Commercial development of aquatic biological resources (ABR) of the Caspian Sea necessitates constant monitoring of the ABR habitat. Microbiota studies are of particular relevance, since bacterial communities largely determine the quality of the FBR habitat. The purpose of this work is to identify opportunistic and sanitary-indicative bacteria that have pathogenicity factors and antibiotic resistance and pose a danger for aquatic biological resources. The paper presents data on the taxonomic composition of saprotrophic bacterioplankton and bacteriobenthos in the waters of the Northern Caspian Sea in the period from 2013 to 2018. Water samples were taken annually in summer and autumn. The work used generally accepted methods for identifying bacteria by morphological and physiological-biochemical characteristics, as well as methods for determining antibiotic resistance and pathogenic activity. During the research period (2013–2018), more than 2000 isolates of saprotrophic bacteria belonging to 28 taxa were isolated from water samples and bottom sediments of the northern part of the Caspian Sea. The taxonomic composition of saprotrophic bacterioplankton and bacteriobenthos did not differ significantly in the number of taxa identified, but differed somewhat in the proportion of species, especially at the level of larger taxonomic groups. The study of bacterial virulence showed that the occurrence of various pathogenic properties in bacteria did not have a common direction in a number of years. Microorganisms isolated from water and soil of the northern part of the Caspian Sea had multiple antibiotic resistance to various groups of drugs. During the study period, high resistance of isolated bacteria (62–92% of isolated isolates) to benzylpenicillin and ampicillin was noted. In general, the presence of bacteria in the water and bottom sediments of the Northern Caspian Sea that have pathogenicity factors and multiple antibiotic resistance indirectly indicated the stress of the bacteriocenosis in the research area.

Changing habitat conditions is an important driving force affecting the distribution of fish in the range. A quantitative assessment was made of the influence of environmental factors on the distribution of bream Abramis brama Linnaeus, 1758 and carp Cyprinus carpio Linnaeus, 1758 in the waters of the eastern part of the Northern Caspian. The number was estimated based on the results of research net catches. The preferred temperatures and habitat depths of bream and carp in the shallow zone of the Northern Caspian are shown. The collection of field material was carried out during complex expedition observations in 2014 and 2020. The installation of nets with a mesh of 30–200 mm was carried out in the shallow-water zone of the eastern part of the Northern Caspian. Bream and carp in the Northern Caspian Sea form concentrations in areas with depths up to 6–8 m. In the time interval from the end of April to October, bream was observed in the temperature range from 7.0 to 28.5 °C, carp — from 5.5 to 28.0 °C. A mathematical assessment of the influence of depth, temperature and water transparency on the number of these fish species was carried out. The obtained regression models took into account from 44.9 to 80.1% of the factors determining the number of fish in the studied water area. Carp are more sensitive to the effects of temperature, depth and transparency compared to bream. The obtained results provide a theoretical basis for predicting changes in the habitat of fish due to abiotic environmental factors, which is relevant in connection with pronounced climatic changes.

Both the habitat of aquatic biological resources and almost all ecological and trophic groups of biocenoses are subject to the negative impact of anthropogenic activities in the coastal zone and in the Caspian Sea. Anthropogenic eutrophication caused by domestic and industrial wastewater, fertilizers, disrupts the balance in the structure of biocenoses. Toxic technogenic emissions lead to changes in the state of natural aquatic ecosystems. In order to maintain balance in the face of anthropogenic changes, it is necessary to assess the state of these ecosystems. Оne of the indicators in assessing the quality of the habitat of aquatic biological resources is the sanitary condition of the reservoirs of the Volga-Caspian region. The sanitary and bacteriological state of the northern part of the Caspian Sea was studied as a fishery reservoir and habitat for aquatic biological resources. Based on the results obtained, the degree of pollution of the reservoir and the anthropogenic impact on the studied reservoir. In 2021 seasonal changes in the number of saprotrophic and oligotrophic bacteria in sea water and soil were noted in the waters of the Northern Caspian. In summer, water quality was classified as “clean” water (category 1), while in autumn it deteriorated and was assessed as “polluted” water (category 2). High values of the coli index and the presence of lactose-positive E. coli testified to the presence of household, including fecal, pollution in the coastal zone of the Northern Caspian Sea. At the same time, conditionally pathogenic bacteria (aeromonads, pseudomonads) were not isolated in the studied water samples. The obtained microbiological data indicate a slight anthropogenic impact on the study area.

In the Caspian Sea, as in any natural body of water, the distribution and dynamics of the number of fish, including mullet, is due to the combined influence of environmental factors. In the present period of regression of the sea and changes in the level of runoff of the Volga River, salinity plays an important role in the life cycle of all representatives of the ichthyofauna. The article is devoted to the topical issue of the impact of the salt regime on the spatial and quantitative characteristics of the mullet population. The distribution and its relative abundance in 2016 and 2019 observations are presented. The distribution areas are mapped with a detailed description of the features of the formation of concentrations during two contrasting periods: desalination and salinization of the sea. Analyzing the data of trawl catches, it was found that in 2016, when the powerful Volga runoff led to the expansion of desalination zones, the average catch of mullet was 7 copies/an hour of trawling. It is noteworthy that this indicator has grown to 11 copies/an hour of trawling, during the salinization of the Northern Caspian in low-water 2019 Graphical data confirming the presence of a direct correlation between the relative abundance of mullet and increasing salinity gradients are presented. It is shown that for two years the value of the correlation coefficient was 0.74–0.76. Mullet was caught in areas with salinity from 2 to 17%. It was found that the majority of concentrations (68–74%) were formed in marine waters from 8%. In conclusion, it is concluded that the increase in salinity indicators of the northern part of the Caspian Sea has a positive effect on the expansion of the range of mullet.