he possibility of using composite indices is being considered for: 1 - assessing the impact on a water body based on the results of simulation; 2 - checking the adequacy of results of system modeling according to field observations and simulation modeling; 3 - revealing ability of a system to maintain the class of state after impact on aquatic ecosystem based on statistical criteria for assessing the proximity of series. On two author’s simulation models a series of 8 experiments were performed to assess the permissible impact and seasonal effects of its manifestation based on changes: 1 - ability to produce organic matter; 2 - change quality and toxic pollution after exposure. In experiments with exposure, features of average annual and seasonal changes in the values of sub-indices of emergent properties of the model system are considered. Based on statistical criteria of the model before and after the impact by the values of the subindices, it is concluded the system can retain class in which it was before impact or transition of the system after impact to another class in a certain time interval and invariance of some components of the aquatic ecosystem for impact. As a result of analysis of obtained values of traditional statistical indices was obtained, some components can be characterized as more sensitive or vice versa - invariant. In all cases, it depends on the magnitude and specificity of the impact on the ecosystem. Different systemic transformations manifest themselves taking different times to form a response to impact.