n order to unify the methods of monitoring aquatic ecosystems, in the course of an international expedition of the project MIS ETC 1676 «Cross-border interdisciplinary cooperation for the prevention of natural disasters and mitigation of environmental pollution in Lower Danube Euroregion» (Romania, Ukraine, Moldova) 13 - 15 June 2014 in the Danube Delta, hydrochemical and hydrobiological samplings were carried out at the stations Izmail, Chilia Veche, Vylkove, Isaccea. Samples were taken according to standard methods, fixation was made on board, and the processing of samples was done in the coastal laboratories.Hardness of the water of the studied area was in a small range: 3.35 - 3.50 mg eq./l. The minimum value of mineral nitrogen was registered in the area of vylkove (1234 μg/l), the maximum in the region of Izmail (1412 μg/l). A minimum of organic nitrogen was in the area of vylkove (75.0 μg/l), the maximum in the vicinity of Chilia veche (700 μg/l), in the region of Isaccea 536 μg/l /l, and 342 μg/l /l in the vicinity of Izmail. Maximum mineral phosphorus was registered in the area of Chilia veche (45.7 μg/l /l), minimum in the area of Isaccea (38.9 μg/l /l). Maximum organic phosphorus was in an area of Izmail (15.8 μg/l /l) and Isaccea (15.1 μg/l /l). By biological oxygen demand (BOD 5), the minimum was in areas of vylkove (0.62 μg/l) and Isaccea (0.63 μg/l), the maximum in the region of Izmail (1.04 μg/l). On the content of nutrients all the investigated areas were characterized as high eutrophic.In the algocenosis of the Lower Danube 43 species and varieties of algae were noted belonging to the 5 phyla of phytoplankton: Bacillariophyta (57%), Chlorophyta (30%), Cyanophyta (9%), Chrysophyta (2%) and Euglenophyta (2%). The greatest number of species was registered for diatoms (24 species) and green algae (13 species); small number of species was observed for cyanobacteria (4), golden algae (1) and euglenophyta (1). The greatest number of species was in an area of Chilia veche (31 species), here the maximum value of Shannon index (H = 1.061) was recorded. The lowest number of species was in an area of vylkove (11 species), which resulted in the lowest value of the index (n = 0,718). The maximum abundance of 641,0*103 and biomass 343.5 mg/m3 was registered near Chilia veche. By abundance and biomass at all stations the brackish diatom skeletonema subsalsum (A.Cl.) Bethge dominated, with the maximum abundance in the area of Chilia Veche (254,7 *103 cells/l). Subdominants were representatives of the brackish-water diatoms: thalassiosira nordenskioeldii Cl. (maximum abundance of 50,1*103 cells/l - Chilia veche) and stephanodiscus hantzschii Grun (the maximum abundance of 62,4 *103cells/l - Isaccea). Thus, in the summer time diatomic-green microalgae complex is formed, with domination of diatoms of brackish-water origin For quantitative assessment of zooplankton community we considered three groups of aquatic organisms (meroplankton was not taken into account): Rotatoria, Copepoda, Cladocera. In collected samples 11 taxa were identified, of which three taxa (27,3%) belonged to Rotatoria, 6 taxa (54,5%) to Copepoda and 2 taxa (18,2%) to Cladocera. The greatest number of species was registered in the area of Isaccea (7 taxa), the lowest in the area of Chilia veche (2 taxa). Constant component of river zooplankton in the studied sites were members of the order Copepoda: juvenile stages of Copepoda g. sp. stad. nauplius(euconstant, F> 75%), taxon Cyclopoida gen. sp. and species microcyclops gracilis (constant, F> 50%). Category of additional taxa (accessories, 25% <F <50%) were 2 species of rotifers brachionus budapestinensis and Keratella quadrata, as well as species acanthocyclops vernalis (Copepoda). The most numerous was the category of random taxa (accidental species, F <25%), consisting of the representatives of all the investigated groups of zooplankton: rotifer Cephalodella eva, copepods mesocyclops leuckarti and metadiaptomus asiaticus, cladocerans bosmina longirostris and Chydorus sp. The greatest importance in the formation of zooplankton taxa belonged to 2 eudominantas of copepods - microcyclops gracilis + Cyclopoida gen. sp. and 2 dominant taxa from the same group - acanthocyclops vernalis + Copepoda g. sp. stad. nauplius. The total share of these species in the taxonomic structure of the investigated biocenoses didn’t exceed 50%, while the share of recedent taxa accounted for 55%. Thus zooplankton complex of studied gidrocenoses can be attributed to polymix type.In meiobenthos there were registered 6 major taxonomic units: Nematoda, Harpacticoida, Turbellaria L., Oligochaeta L., Polychaeta L., Gastropoda L. Among the representatives of eumeiobenthos nematoda were found at all stations of the study and most contributed to the total abundance of meiobenthos. nematodes were the main component of the meiobenthos of bottom soils, especially in eutrophic areas. Organisms of this group are able to withstand the low values of dissolved oxygen. Of pseudomeiobenthos representatives ubiquitous were Turbellaria L., Oligochaeta L., Polychaeta l. Maximum abundance of meiobenthos was in an area of Izmail (212000 ind/m2), minimum in the area of Isaccea (64,000 ind/m2). Biomass of the total meiobenthos ranged from 0.8 to 4.5 g/m2. To assess the state of environment we used the ratio of the total abundance of meiobenthos to biomass (ntot/Btot). Meiobenthos reaction as a response to unfavorable conditions of existence may be expressed in a sharp increase in the abundance of meiobenthos organisms with minimum body size and short development cycles. with their massive development and a sharp increase in population density, the total biomass will decline. The more difficult is the environmental situation, the greater is the rate ntot/Btot, because in the stressful circumstances for each mg of total biomass will be more organisms. The maximum rate of ntot/Btot was registered in the area of Vylkove (80) and Isaccea (78), a less difficult situation in the benthic communities were in the area of Chilia veche (54.8) and Izmail (46.4).Thus, the studied areas of the Lower Danube river ecosystems are characterized by different conditions of habitats. Maximum nutrients were in an area of Chilia veche, which resulted in active vegetation of phytoplankton in the area, the maximum value of qualitative and quantitative indicators. The greatest number of zooplankton species was recorded in the area Isaccea (7 taxa), the lowest - in the area of Chilia Veche (2 taxa). The most difficult situation in the benthic communities by ntot/Btot index of meiobenthos was found in the areas of vylkove and Isaccea. acknowledgement: The study was performed in the frame of EU Joint Operational Program Romania - Ukraine - Republic of Moldova