The causes and physiological-biochemical mechanisms of migration of marine species of hydrobionts to fresh waters of the Dnieper River and the cascade of its reservoirs have been investigated. The possible adaptation of marine fish species and invertebrates is related to the economic human activity: regulation of river runoff, creation of a cascade of reservoirs with large water-surface area, anthropogenic pollution of natural water, and also changes of climate conditions on the Earth (global warming). It is presumed that these processes result in significant change of physicochemical indicators of aquatic medium. One can observe the rise of mineralization, increase of the number of biogenic compounds, variation of their ratio in fresh water that, in turn, allows the marine species of hydrobionts to adapt to new conditions and advance upstream.