Distribution of mollusc assemblages in the surface sediments of the Black Sea, Marmara Sea and Aegean Sea

The Marmara Sea Gateway connects the hypersaline (<36 ‰) Aegean Sea and the low-salinity (∼17-22 ‰) Black Sea through the Straits of Bosphorus and Dardanelles and the landlocked Marmara Sea. This gateway forms a natural laboratory in which to study the effects of climate change, sea-level fluctuations and water-mass exchange between small basins.;In the Marmara Sea, CTD data revealed the presence of a low salinity surface water mass representing Black Sea outflow and a high salinity deeper water mass, separated by a sharp mixing zone. Across the Southwestern Black Sea shelf the CTD data showed the presence of 3 water masses: (1) a low salinity surface water mass extended down to 10 m; (2) a low salinity, but colder water mass occupies water depth below 40 m; (3) and a higher salinity Mediterranean water occurs between these two water masses. In the Aegean Sea CTD data revealed a relatively low salinity Black Sea outflow water mass at the surface, and high salinity water mass at the bottom separated by a mixing zone.;The mollusc shells from grab samples were identified using a number of taxonomic keys and analyzed qualitatively and quantitatively in order to investigate the relationship between the community structure and the environment. The mollusc absence and presence data in quadrats was used to delineate seven different mollusc assemblages, with each assemblage representing a distinct set of environmental conditions. Principal component analysis was used to constrain mollusc faunal assemblages and their relationship to environmental variables. Seven hypothetical faunal assemblages and three hypothetical environmental variables were extracted, explaining 73.9 % and 73.8 % of the variance in the faunal and environmental data, respectively. Cross-plots of scores for the major faunal and environmental components revealed empirical relationships between the three oceanographically different seas. The separation of the Black Sea from the Marmara Sea and the Aegean Sea assemblages was found in the cross-plot of Faunal Component 1 versus Environmental Component 1. The separation from the Marmara Sea to the Aegean Sea was found in the cross-plots of Faunal Component 1 versus Environmental Component 3 and Faunal Component 4 versus Environmental Component 3.;Mollusc shells were also identified and counted in a ∼8 m long piston core recovered from the SW Black Sea shelf. Visual inspection of the data revealed four distinct faunal assemblages in the core. To test the validity of using the principal components determined from the surface samples to deduce past environments in a core, the same mollusc species found both in the surface samples and core units/subunits were used to calculate faunal scores for each unit/subunit. Principal components scores for units/subunits provided little basis for making strong interpretations about the changing paleoenvironment. Because only five species that were used in the principal component analysis on surface samples were present in the core MAR 02--45P.;Surface sediment samples were collected from 137 stations across six transects in the Black Sea, the Marmara Sea and the Aegean Sea. At each station grab samples and CTD (conductivity, temperature, depth) measurements were also collected.