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.

Microbiological monitoring of the Northern Caspian proved that for the research period in 2013-2018 the average annual value of the total number of bacteria (TBN) was 1.35 million cells/ml. The TBN maximum value (1.46 million cells/ml) was recorded in 2013, the minimum (1.19 million cells/ml) - in 2014. Concentration of saprotrophic bacteria in the waters of the Northern Caspian during the research period varied from 50.94 thousand CFU/ml in 2013 to 1.66 thousand CFU/ml in 2014. The concentration of saprotrophic bacteria remained practically unchanged within 2014-2018. The values of the ratio of the total abundance of microorganisms and saprotrophic bacteria show the eutrophication of the waters of the North Caspian in 2013 and in 2016, in other periods the water quality corresponded to the readings of an oligotrophic reservoir. The maximum number of oil-oxidizing bacteria (OOB) (8.28 thousand CFU/ml) in the waters of the Northern Caspian was recorded in 2013, the minimum (0.21 thousand CFU/ml) - in 2014. Starting from 2015 there was recorded an increasing number of OOBs and its stabilization until the end of the research period. The ratio of NOB and saprotrophs in water varied from 16.47 to 52.47%. Analysis of microbiological and hydrological-hydrochemical indicators revealed positive correlations of TBN and annual runoff (r = + 0.77), TBN and the content of mineral forms of nitrogen (r = + 0.60), the number of saprotrophic bacteria and nitrogen (r = + 0.83), the amount of NOB and mineral nitrogen and silicon (r = + 0.81). In the long-term aspect, an improvement in the microbiological situation was recorded against the background of a growing total number of bacterioplankton under a simultaneous decrease in the number of saprotrophic and oil-oxidizing bacteria.