Abstract
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.
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