Seasonal variations of biochemical and optical properties, physical dynamics and N stable isotopic composition in three northeastern Mediterranean basins (Aegean, Cretan and Ionian Seas)

Multidisciplinary surveys were conducted in October 2014, May 2015 and December 2015 in three major deep basins of the Eastern Mediterranean Sea (EMED) - the north Aegean Sea, the south Aegean (Cretan Sea) and its straits, and the south Ionian Sea - providing the most recent information on physical, biochemical and particle dynamics, along with nitrogen stable isotope composition. A north-south gradient in oligotrophy was observed, presumably related to more pronounced nutrient supply through the inflow of Black Sea Water and riverine inputs in the surface waters of the north Aegean Sea, coinciding with the relatively higher productivity in this area. Consumption rates of Dissolved Oxygen (DO) in the deep waters of the north Aegean Sea reveal the physical dynamics of the subsurface layers. According to this, it seems that no deep water formation has occurred in the Athos basin after 2008. In the Cretan Sea, the Transitional Mediterranean Water mass was identified at the layer 700-1200 m showing a slight decrease in its core depth, when compared with previous studies in the area. Seasonal variations of DO and salinity in the deep waters of the Ionian Sea indicate the intrusion of more oxygenated and less saline waters of Adriatic origin in the bathypelagic layer of the south Ionian Sea. Detailed determination of stable isotope ratios for nitrate (delta N-15-NO3) and suspended particulate nitrogen (delta N-15-PN) in various water column depth of the three basins showed a further decrease of the overall low EMED delta N-15 signal, when compared with previous studies in the area. This trend can be attributed to the onward atmospheric deposition of anthropogenic N to the EMED. The delta N-15-PN depth profiles exhibited clear Rayleigh-type isotope fractionation and associated with remineralisation throughout the water column, implying that the unique, isolated oceanic basins of the Eastern Mediterranean serve as important marine observatories of anthropogenic influences on marine nutrient budgets.