As a result of eutrophication caused by increased nutrient input via major northwestern rivers during the last few decades, the Black Sea ecosystem has been subject to extreme changes in recent years. Abnormal changes due to altered nutrient balance were reflected in the qualitative and quantitative composition of phytoplankton and zooplankton. The increase observed in the quantity of plankton was probably responsible for the rise of Turkish anchovy catches observed over the last few decades. However since 1988, the Black Sea has been invaded by a voracious zooplankton predator, the ctenophore Mnemiopsis leidyi which was accidentally introduced into this sea from the northwest Atlantic. This mass occurrence of Mnemiopsis appears to be one of the most important reasons for the sharp decrease of anchovy and other pelagic fish stocks in the Black Sea. By October 1997, new ctenophore Beroe ovata has appeared in shallow waters of the Black. Species of genus Beroe almost exclusively feed on other ctenophores and feeding interaction within ctenophores form an ecological feed-back system which also affects other compartments of the planktonic community.