Khomych V. Ecological features of formation of hydrobiocenoses on the rivers of the Forest-steppe of Ukraine.

The thesis deals with the peculiarities of the transformation of ichthyo- cenosis in the conditions of anthropogenic changes of the forest-steppe river systems as a result of the building of the mini hydropower plants (MHPP). It was found that the hydroelectrical conditions in the water reservoirs have significantly changed after the building of the hydroelectrical plants, which was done on a large scale in the second half of XX century. These changes led to the negative dynamics of the species richness of hydrobionts in general and fish in particular. In addition, such conditions made it impossible to inhabit fish populations with low ecological valence, but instead favored populations of adventitious species. The building of the reservoirs has a significant impact on river systems. There is a radical change of the hydrological regime. That's why the water flows slow down and disappear, the channels are muddy, and harmful substances of domestic and industrial waste gradually accumulate. The conversion of rivers into reservoirs of lake type is also accompanied by changes in species composition and structure of phytoplankton, zooplankton and benthos. Eutrophy are typical. All this leads to the significant changes of ichthyo-fauna. It is established that the degree of positive or negative impact on the fauna depends significantly on the size of the river.