The article highlights the data on the abundance of saprotrophic bacterioplankton and bacteriobenthos in the water area of the Northern Caspian in the period from 2013 to 2017. There has been described the influence of the Volga river runoff during the spring flood on bacterioplankton and bacteriobenthos in the northern part of the Caspian Sea. The number of saprotrophic bacterioplankton in the spring period depended on water content; the minimum of saprotrophs was recorded in 2015 (low water year), maximum - in 2016 (high water year). Studying the vertical distribution of saprotrophs number showed that near the bottom it was somewhat less than on the surface. The most favorable sanitary situation in the Northern Caspian was registered in 2015 (throughout the year, water quality was estimated as clean), unfavorable periods occurred in 2013 and 2016 (during nearly whole vegetation period water quality was estimated as contaminated). In the bottom sediments seasonal changes of saprophytes completely corresponded to bacterioplankton, the abundance of bacteriobenthos exceeding the parameters of bacterioplankton. There has been studied species composition of saprophyte bacteriocenoses of the North Caspian, determined the dynamics of yearly occurrence of different groups and families of microorganisms. In the period from 2013 to 2016 there was observed a decline in the number of species of microorganisms, in 2017, species diversity increased slightly. In summer months and at the beginning of autumn species maximum was observed, in spring and late autumn there was registered species minimum. Most of isolated bacteria belonged to a group of opportunistic pathogens, while true saprophytes were much less common. Among the isolated species there have been determined potentially pathogenic microorganisms, their pathogenic activity and antibiotic resistance have been studied. The presence of one or more pathogenic factors, together with multi-drug resistance of bacteria to antimicrobial agents proved the potential danger of these representatives of bacterioplankton and bacteriobenthos for hydrobionts and humans.

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 mbalance 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.

The assessment of the quality of sea water samples was carried out according to biological indicators - the total number of microorganisms, which react sharply to changes in their environment. To ensure control over the ecological state of the aquatic environment, it is necessary to conduct regular monitoring studies on such microbiological indicators as the total number of microorganisms. Conventional microbiological methods for determining the total number of microorganisms are laborious and require a well-equipped laboratory and qualified personnel, and such analysis usually takes 24-48 hours. For a comparative assessment of the quality of sea water samples, the number of microorganisms was recorded by traditional methods and by the express method of chemiluminescence. As traditional methods, the method of inoculation on nutrient media, counting on a fluorescent microscope and on colored membrane filters were chosen. A chemiluminescent method was chosen for the rapid assessment of water by the total microbial abundance, which makes it possible to measure the content of bacterial cells by their ATP content. It is shown that the chemiluminescent method is superior to traditional methods for determining the total bacterial contamination (TBC, one of the main indicators of the natural object quality). At the same time, the chemiluminescent reaction made it possible to significantly reduce the measurement time and speed up data processing (measurement time did not exceed 2.5 min), i.e., it became possible to quickly analyze the samples under study in the field and in mobile laboratories. Direct positive correlations between the data determined by the traditional and new methods are revealed. The results obtained in the framework of a study on the determination of the number of microorganisms by classical methods and the chemiluminescence method allow us to recommend their use for express assessment of the quality of various natural objects.

Data on the abundance of heterotrophic bacterioplankton and bacteriobenthos in the northern and middle parts of the Caspian during the period from 2013 to 2017 are presented. In the waters of the northern part of the Caspian, a peak in the number of saprotrophs and crude oil degraders was recorded in 2013. In the middle part of the Caspian and in the region of the Mangyshlak regression, high numbers of heterotrophic bacteria were recorded in 2013 and 2016. The biodiversity of bacterioplankton in the surveyed areas is almost the same; the species composition of saprotrophs and crude oil degraders is identical. In the bottom sediments of the northern and middle parts of the Caspian Sea, two peaks of saprotrophic bacteriobenthos development (in 2013 and 2016) were recorded; in the region of the Mangyshlak threshold, the concentration of saprotrophs remained stable. The number of crude oil degraders was inferior to those of saprotrophs, while the proportion of crude oil degraders to saprotrophs changed annually. The species composition of saprotrophs is relatively stable, the biodiversity of oil-oxidizing bacteriobenthos has changed in the direction of increasing the proportion of non-fermenting bacteria. Constant changes in the ratio of the number of saprotrophs and crude oil degraders, together with the transformations of the dominant groups of the species composition of bacteria, indicated the instability of bacteriobenthos, and, as a result, the vulnerability of the microecosystem of bottom sediments under changing environmental conditions.

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