The determination of the tolerable and optimal limits of trace metals in the environment is very important. These limits are usually affected by the regional differences in ecosystems. Taking into account the above-mentioned statement, the experimental studies were carried out. Firstly, the effect of trace elements on production and destruction of organic matter was studied, taking into account that these processes are the most important stages in the matter cycle and energy flux and are essential for ecosystem functioning. Data analyses demonstrated that in all freshwater ecosystems, the same patterns concerning the effect of trace elements on phytoplancton production and destruction of organic matter function, most of these being described by polynoms of the 2-4 category (Zubcov E.,2000). Secondly, the effect of trace element content on highly valued industrial species of fish in early ontogenesis was studied. These species are considered to be the most vulnerable elements of the food chains.Experiments with eggs, larvae and fish fry of Cyprinus carpio, Aristichthys nobilis, Hypophtalmichthys molitrix, and Ctenopharyngodon idella were carried out in aquariums, reproduction installations and fish tanks placed in the fish ponds. Samples of eggs were collected immediately after fertilization, at the stage of the formation of the blastodisc, morula, blastula, gastrula, at the beginning and end of organogenesis and immediately after egg hatchling. Concentration of chromium, cadmium and arsenic in fertilized eggs increased insignificantly; some effect was observed only at the gastrulation stage, but a steady decrease up to analytical zero occurred at the hatching stage (Table 1). It was revealed that such metals as cadmium, chromium, and arsenic are toxic for egg development even at low concentrations, such as 2-5 g/l, these concentrations of metals were also critical for production and destruction processes in water ecosystems. Table 1. Percentage of survived eggs of Ctenopharyngodon idella in experiments after the addition of metal supplements in water Morula Blastula Gastrula Organo- genesis Embriorotation Beginning of hatching Finalization of hatching Cd, 2 g/l 50 48 40 24 15 14 0 Cd, 10 g/l 20 20 10 0 0 0 0 Cr, 5 g/l 40 40 5 0 0 0 0 Cr, 10 g/l 10 10 2 0 0 0 0 As, 2 g/l 40 30 3 1 0 0 0 As,10 g/l 25 15 2 1 0 0 0 Experiments with one month old fish fry demonstrated their higher resistance to investigated metals than fish eggs and larvae. In aquariums, where Cd, Pb, Ni, and Cu were added in concentrations 5-25 g/l, the fish fry (with a body weight under 40 g) fed poorly in the first 5-10 days and even some tendencies of weight losing were observed; after 12-24 days their growth was significantly lower than those of control specimens.