Structural and functional indicators of zooplankton can be used in the system of environmental and toxicological monitoring, which still awaits due attention from environmental toxicology, designed to conduct research in natural ecosystems exposed to toxicological pollutants. The entry into the water body of toxic substances almost always has a negative, stressful effect on the ecosystem, leading to a deterioration of its condition or a transition from optimal to extreme ecological state. As many studies have noted (Pashkova, 2012; Vandysh, 2006; Braginskii et al. 1987), the reaction of zooplankton communities to toxic substances is fundamentally different in comparison with the action of an excess of nutrients or organic matter leading to eutrophication, during which, along with structural changes, intensification of the functioning of communities, increasing the biological productivity of the reservoir as a whole is observed. As a result of the action of toxic pollutants, when structural parameters change, profound disturbances in the functioning of communities are observed, including their gradual degradation, loss of ecosystem functions, and community death under extreme toxic conditions. With the introduction of toxic substances in water, the total number of zooplankton species and its quantitative development decrease, as a result of which the quantitative structure of the community and the decrease in the species diversity index are simplified (Shurganova, 2014; Albanese et al, 2013). The decrease in species richness occurs mainly due to a decrease in the specialized stenobiont endemic and relict species, as well as species with large individuals and a long life cycle. At the same time, an increase in the number of euribiotic, cosmopolitan species and of small species and those with short life cycle is observed (Xiong et al. 2017, Yang et al., 2018). As a result of toxicity, certain changes also occur in structural quantity of the communities: from the main taxonomic groups of zooplankton, the dominance passes from cladocerans to rotifers and copepods, which are more resistant to the presence of toxic substances. The resistance of rotifers to the toxicity of the aquatic environment is due to the high reproduction rate and the ability to lay dormant eggs protected by the thick capsule from adverse factors. From the copepod group, cyclopids have a special resistance to the action of toxic substances due to the chitinuous layer of the body, pradotorism and sexual reproduction. Cladocerans and Calanoids are vulnerable to toxic substances due to their type of nutrition by filtering, thus quickly absorbing and accumulating dissolved and toxic organic substances. As a result, omnivorous and predatory consumers (Andronikova, 1996) begin to dominate among trophic groups instead of peaceful, non-predatory ones. In the process of toxicification, the rate of reproduction of zooplankton is significantly reduced due to a decrease in fertility and the speed of production processes - the daily P /B coefficients decrease on average from 0.22 to 0.14 (Pashkova, 2012). In addition to the composition and quality of pollutants entering water bodies, it is important to consider the degree of intensity and duration of exposure to pollutants. One should bear in mind that during a prolonged supply of pollutants, some harmful substances accumulate in the bottom sediments, which over time pass in the the water. Therefore, the effect on planktonic organisms can occur without visible sources of pollution (Litvinchuk, 2019). Representatives of zooplankton filtrators play a leading role in the transformation of matter and energy transfer along the trophic chain from phytoplankton to fish, which affects the functioning of the aquatic ecosystem. Vorozhun (2009) studied the rate of water filtration and the feeding of Daphnia with green algae, where it was found that the consumption of green algae by the species Daphnia magna decreased when exposed to an anionic surfactant. The suppression of the filtration activity of zooplankton by some water polluting substances is a certain danger, since as a result, the regulatory function of zooplankton in the aquatic ecosystems may decrease by a range of parameters. Assessment of the indicative capabilities of zooplankton in the diagnosis of the ecological and toxicological state of aquatic ecosystems is one of the promising areas of hydrobiological research.

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