The study of the microbiological parameters of bacterioplankton in different types of aquatic ecosystems is one of the mail directions in the applied researches. Water samples were collected seasonally during 2015 at 11 stations of the Dniester River and Dubossary reservoir: Naslavcea, Valcinet, Soroca, Camenca (medial sector of the river - 12 samples), Erjovo, Goiani, Cocieri (Dubossary reservoir - 9 samples), Vadul lui Voda, Varnita, Suclea, Palanca (lower sector of the Dniester River - 12 samples). The trophic category of monitored ecosystems was appreciated based on such microbiological parameters as bacterial production and destruction, total number of bacteria, number of saprophytes and some physiological groups of microorganisms involved in cycles of nitrogen, phosphorus and carbon. Sampling and analysis of water samples was carried out by methods accepted in hydrobiology (Gak, 1975; Ambrazene, 1984; Kopylov and Kosolapov, 2008; Water Quality Monitoring and assessment of ecological status of aquatic ecosystems, 2015 etc.). The following results were obtained: - A large range of values has been revealed for such microbiological parameters as total number of bacteria (Ntot – from 0.6 to 8.0 million cells / ml) and number of saprophytes (Nsapr – from 0.1 to 62.0 thousand cells / ml). Under the investigated period, the highest values of these parameters were recorded for two stations of the Dniester River (Fig. 1): Fig.1. Dynamics of season average for the total number of bacteria (Ntot) and number of saprophytes (Nsapr) in the Dniester River and Dubossary reservoir: Ns - Naslavcea, Vc - Valcinet, Sr - Soroca, Cm - Camenca, Ej - Erjovo, Gn - Goiani, Cr - Cocieri, VV Vadul lui Voda, Vn - Varnita, Sc - Suclea, Pl - Palanca (2015) - In the temporal aspect, a significant increase of microbial mass has been mentioned from spring to summer due to the weather factors such as rise in water temperature, rainwater surface runoff, summer floods etc. - A wide range of variability was recorded for the physiological characteristics (production and destruction) of bacterioplankton communities, with growth of these parameters downstream of urban and rural areas that are caused by anthropogenic factors. The range of parameter values is the follows: 0.08 - 5.47 cal. / L / 24 h (production) and 0 - 26.6 cal. / L / 24 h (destruction). - The numerical density of microorganisms, involved in the circulation of phosphorus, nitrogen and carbon, was analysed in relation to lotic and lentic ecosystems of the Dniester River that have been studied. The density of ammonifying microorganisms varied from 0.2 to 35.0 thousand cells / ml, amylolytic bacteria - from 0.03 to 3.2 thousand cells / ml, and bacteria involved in mineralization of phosphorus - from 0.21 to 25.0 thousand cells / ml. The seasonal upward trend of the quantitative values for these groups of microorganisms was also registered in summer, along with increase of bacterial mass downstream of urban and rural areas. - Based on the total number of bacteria (Ntot) in 60 % of cases, the Dniester River is a mesotrophic water body, in 32 % - eutrophic, in 8 % - polytrophic. According to an estimate based on the number of saprophytes (Nsapr), Dniester River is a mesotrophic water body in 47 % of cases, in 29 % - eutrophic, in 12 % - oligotrophic, in 8 % - polytrophic, in 4 % - hypertrophic. The percentage share for the Dubossary reservoir trophicity is shown in Fig. 2. Fig.2. Percentage share of the Dubossary reservoir trophicity for the vegetation period of 2015, based on microbiological parameters Acknowledgements: The research was carried out within the framework of the National project 15.817.02.27A using the equipment of the Laboratory of Hydrobiology and Ecotoxicology and the material base of the Institute of Zoology for the field and laboratory investigations.