Mechanisms impeding natural Mediterranization process of Black Sea fauna

The inherent low species biodiversity of the Black Sea (roughly one-third of the Eastern Mediterranean Sea) is a sign of ineffective bio-invasion process of Mediterranean species. This phenomenon is explained on the basis of a hypothesis that strong physical gradients and some accompanying physiological limitations along the colonization route of Turkish Straits System (TSS) make Mediterranean pelagic and benthic plankton species and fish larvae vulnerable to long-distance natural expansions. The physical barriers include strong inter-basin temperature and salinity contrasts, two-layer counterflow and stratification along the TSS, very strong local vertical mixing and hydraulic adjustment processes along the Canakkale and Istanbul Straits. The physiological barriers are related to stresses induced by strong and abrupt temperature, salinity, and oxygen changes that affect the growth, mortality and reproduction characteristics within the two layer water column structure of the TSS. Long-term behavioral, ecological, and evolutionary acclimatization processes in the Marmara Sea environment to overcome these barriers constitute an essential first stage of species expansions to the Black Sea. Among the new physiologically adapted „source‟ populations, only those which manage to arrive at the southwestern Black Sea shelf region may have a chance to acclimatize Black Sea conditions and ultimately establish self-sustaining populations. These physical and physiological stresses are so strong and robust that inter-annual and decadal climateand eutrophication-induced perturbations can hardly alter this persistent structure. Starting by the second half of the previous century, the TSS has also been serving as an ideal invasion corridor for intentional or non-intentional alien species introductions via intensifying ship traffic.