The results of long-term complex hydrobiological studies of the cooling pond of the South Ukrainian Nuclear Power Plant (SU NPP) are presented. The abundance of zoo-and phytoplankton increased until 1984-1985, and then in the summer of 1986, against a background of extremely high temperatures in the reservoir, there was a significant (32-fold) drop in the mean biomass of phytoplankton, coinciding with the minimum abundance of zooplankton. In subsequent years, the abundance of these groups recovered, but has not reached previous levels. At present, the zooplankton contains a considerable amount of thermophilic species. Research in recent years indicates that the zooperiphyton is dominated by the invasive gastropods Melanoides tuberculata (Müller, 1774) and Terebia granifera (Lamarck, 1822). In the epiliton, the number of LDT (lowest determined taxa) and groups of invertebrate was more than double that in the epiphyton (17 and 7, respectively). At the first stages of the development of the pond ecosystem, the periphyton communities were dominated by zebra mussel Dreissena polymorpha Pall. These communities were eliminated as temperature increased, and after the commissioning of the NPP, the second and third power units were not completely restored in the environment of a constantly high thermal load. Over years, with the formation of bottom biotopes, the abundance of zoobenthos increased, and with an increase in the technogenic load, it decreased. At the present stage, the zoobenthos is impoverished (9 taxa) and is dominated by mainly juvenile tubificids.

well as abundance and biomass dynamics of phytoplankton, zooplankton, zoobenthos, zooperiphyton, and higher aquatic plants in the cooling pond and channels were found to be related to both technogenic and biogenic factors. The invasion of alien species constantly occurred in the teсhnoeсosуstem. Thirteen invader species were noted during the period of research. Invasion of Zebra mussel (Dreissena polymorpha) and submerged plant Najas marina prevented regular operation of the system; sponge Eunapius carteri reached mass development. Some invaders appeared for a short period; others naturalized, but did not reach the mass development. The phenomenon of contourization associated with the invasion and the mass development of the powerful filter feeder-Zebra mussel was observed in the ecosystem during the period of research. Signs of contourization (for example, a significant increase of water transparency) were observed already in years 2-3 after the invasion of the clam; significant changes in the pelagic subsystem were observed only in years 5-6. At the time of the invasion of the second dreissenid species (D. bugensis), population of D. polymorpha was in the stage of stabilization at low level of abundance. The assumption of the possible outbreak of D. bugensis abundance during the first years after its invasion was not confirmed. Thus, the successional processes in the teсhnoeсosуstem are determined not only by technogenic and natural abiotic, but also biotic factors.