In recent centuries, people with an increasing rate create more and more of new artificial compounds (chemicals, biocides, pharmacological preparations, etc.), most of which, entering the environment, and then into water bodies, become toxicants for hydrobionts, and ultimately – through food chains – and humans. Throughout the evolution of life on Earth, microorganisms are the main barrier to toxicants in aquatic habitats. In order for bacterioplankton to effectively perform the function of biodegradation of pollutants and toxicants, the temperature of water and oxygen regime must be favorable for the development of aquatic microflora. The main hydrochemical parameters that characterize the oxygen regime are "Dissolved oxygen" (DO) and "Biochemical oxygen demand" (BOD). The specificity of the BOD parameter is that its value is determined by the composition of aquatic microbial communities and the conditions of their habitat. In a normally functioning aquatic ecosystem, the correlation between these parameters is negative - with increasing oxygen concentration, the BOD value decreases. An empirical model of the mutual change of these parameters (DO-BOD interaction) in the flowing water bodies was proposed by Streeter H.W. and Phelps E.B. (1925) and then modified by other researchers (Schnoor J., 1996; Lung W., 2001; Gotovtsev A., 2010). The depth and speed of flow in this model are considered as the main factors influencing overcoming oxygen deficiency and reducing BOD. In this study, an attempt was made to detect a trend in the correlation of DO-BOD parameters for various types of aquatic ecosystems. The values of the oxygen regime parameters are the result of the mutual influence of many factors: time of day, hydrological season, presence or absence of current, climatic conditions, ecological state, etc. To evaluate this multifactor process, a statistical analysis of the long-term data of the Laboratory of Hydrobiology and Ecotoxicology of the Institute of Zoology was carried out. For the Lower Prut, the data from Cahul, Cislita-Prut and Giurgiulesti stations (2013 – 2019) were used. For the Danube Delta, they were taken from Galati, Reni, Isaccea, Ismail, Chilia and Vilcovo stations (2013 – 2014).