A review of zooplankton investigations of the Black Sea over the last decade

Investigations performed in the last decade indicate that there have been important changes in the zooplankton composition and structure in the Black Sea. However, contrasting events taking place in different regions of the Black Sea indicate a non-uniform structure of its ecosystem. Several fodder zooplankton species have either disappeared from or substantially decreased in number at different sampling sites of the Black Sea over the last one or two decades. Some other species adapted to thrive in eutrophic conditions have either appeared or increased in quantity. Meanwhile the biomass of the fodder zooplankton has also fluctuated considerably through the years. However, there seems to be a reverse trend in the long-term variation of fodder zooplankton between the shallow western and deep eastern areas. Over the last few decades the abundance of fish larvae has decreased significantly when compared either to past records or with larval abundances of other seas. This was shown to be due mainly to malnutrition of larvae. One of the most striking changes in the ichthyoplankton has been the shift in the spawning areas of the main fish species, the anchovy Engraulis encrasicolus from the northwestern to the southeastern Black Sea. Even the invading ctenophore Mnemiopsis were found to be starving. The condition of other species ( Calanus euxinus and Pleurobrachia pileus) disclosed the fact that cyclonic regions where chlorophyll and nutrient concentrations are high, provide better nutrition than anticyclonic regions.