The influence of trace metals and biogenic elements on the production processes and biochemical oxygen consumption in situ (Dniester, Prut, Danube rivers and Dubasari, Costesti-Stinca and Cuciurgan reservoirs) and laboratory modelling was estimated. For the first time, it was determined the influence of trace elements (Cu, Zn, Mn, Pb, Ni, Cd, Bi, Hg Co, Se, Mo, V ) on primary phytoplankton production and destruction of organic matter processes as well as on biochemical oxygen consumption-CBO5 (inhibition and increases of these processes). For the first time, analysing was done on the impact of trace elements on primary phytoplankton production and destruction of organic matter processes, CBO5. This data allowed also to asses the impact of trace elements on growth and development of fish. Supplementary to this, the research allowed to calculate the buffer capacity of aquatic ecosystems and water quality in relation to the studied metals. Therefore, a new method of evaluation of the status of aquatic ecosystems was proposed and proved theoretically. According to this methodology, three classes of water quality was estimated, according to the oligo-elements concentrations: • optimal or favourable (clean ecosystems, oligotrophic) • admissible (moderately polluted ecosystems, mesotrophic) • extreme or dangerous (polluted, eutrophicated - politrophic) The buffer capacity of the Prut River was higher than that of the Dniester River, that that could be explained by the influence from the functioning of the Dniester Hydropower Complex. Acknowledgement: The study was performed within the projects:15.817.02.27A AQUASYS, 18.51.07.08A/PS,, BSB 165 HYDROECONEX, BSB 027 MONITOX,DANUBIUS RI, MIS ETC 1150, MIS ETC 1676.