Mesozooplankton communities in the Aegean and Black Seas: a comparative study
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An attempt was made for a comparative study of the mesozooplankton communities between the Aegean and Black Seas. These areas are in connected through the Sea of Marmara, the Bosporus and the Dardanelles straits. The comparison was mainly based on data collected both in coastal and offshore waters, by the Hellenic Centre for Marine Research (Greece), the Shirshov Institute of Oceanology (Russia), the Institute of Biology of Southern Seas (Ukraine) and the Institute of Oceanology (Bulgaria). Our data revealed important differentiation in terms of species composition and interannual fluctuations of zooplankton quantity. Among the common copepod species, those presenting an eurytherm and/or euryhaline character (e.g. Acartia clausi, Paracalanus parvus, Penilia avirostris, Podon polyphemoides) were found dominant in the Black Sea and common in the Aegean Sea. Decreased dissimilarity of species composition seems to exist between the North Aegean Sea and the Black Sea, as well as between the coastal areas of the Black Sea and those affected by pollution in the Aegean Sea. A common feature of both seas is the existence of spatial differentiation in terms of the quantity of zooplankters due to anthropogenic and/or natural influences. The pattern of this differentiation seems to vary interannually in the Black Sea, depending either on the abundance of mesozooplankton predators (indigenous or invasive species) or on global climatic changes. Although the Black Sea is generally richer in zooplankton than the Aegean Sea, the significant decrease in the quantity of zooplankton from the early 1980s to the mid-1990s, due to the above factors, resulted in quasi-similar values in both areas. Black Sea mesozooplankton has revealed strong interannual variability both quantitatively and qualitatively, whereas no significant variability was detected in the Aegean Sea. This difference could be attributed both to variant anthropogenic forcing on these ecosystems and to the sensitive character of the semi-enclosed Black Sea.