IRON GATE I RESERVOIR ? ECOLOGICAL EVOLUTION, ROMANIA

The construction of the dam on the Danube in the area known as the Iron Gates led to the formation of the biggest reservoir of the river the length of which is of about 100 km. The depth of the reservoir near the dam is of 30 m. Due to the construction of the reservoir more than 10,000 ha of Romania's territory were flooded. Under these circumstances, there took place major transformations in the structures of the Danube's biocoenosis. In this new environmental conditions certain species disappeared (those characteristic to the springs and to the terrestrial liable to floods areas), some restricted their spreading areas (it is about fish especially), while the others, which occupied a limited space within the structural spectre of the biocoenosis before the construction of the reservoir, rapidly increased in number and became dominant species in the ecological configuration of the ecosystem (as it is the case of the mollusc Dreissena polymorpha). Thus, there appeared biocoenosis characteristic to the lacustrine ecosystems. The research emphasized three major stages in the ecological evolution of the reservoir: 1971-1972, disappearance of the reophile biological processes, as well as the intensification of the eutrophication process characterized by the increase of phytoplankton and zooplankton production; 1972-1973, stabilization of plankton production; 1973-1974, diminution of plankton production; 1974 and further on, setting up of a dynamic balance of the fluvial-limnic ecosystem. At present, the structure of the zoobenthos, as well as of the phytoplankton and zooplankton is characteristic to the limnic-reophilic ecosystem.