Authors: AYŞEN YILMAZ
Abstract: The lower trophic level ecosystems and the biogeochemical cycles of the seas surrounding Turkey were investigated in the last decade and the results are presented in this paper. The Black Sea has a multi-layer system and below the oxygenated surface layer, the concentration of dissolved oxygen decreases rapidly to <20 \mu M in the sub-oxic layer between the 15.5 and 16.2 density surfaces.The upper boundary of the hydrogen sulfide layer is observed at different depths but always at a density of 16.2. The nitrate/phosphate molar ratio is greater than 20 in the coastal Black Sea waters where the primary production is limited by phosphorus. This ratio is low (<8) in the open surface and sub-oxic waters. The reason for this is the denitrification reaction taking place in the sub-oxic layer and nitrogen is a limiting nutrient in such areas. The ecosystem of the Marmara Sea is influenced by the Black Sea inflow and by the input from the İstanbul metropolitan area. Mediterranean origin waters entering the Marmara deep basin through the Dardanelles change in character on the way to the Black Sea; for example the oxygen content of the Mediterranean waters decreases by 70-80% due to lack of contact with the atmosphere when it enters the basin. The nitrate/phosphate ratio, in the deeper layers of the Marmara waters, which varies between 8 and 10, is in agreement with the chemical composition of particulate matter sinking from the surface layer. The eastern Mediterranean is physically dynamic and therefore the water column is well oxygenated down to the bottom. This area is oligotrophic due to the limited amount of nutrient input. The nutricline coincides with the lower boundary of the euphotic zone and it is located at shallower depths (50-100m) in the Rhodes cyclonic region where the primary production is relatively high. The nutricline is far (200-700m) below the euphotic zone in the anticyclonic areas of the Mediterranean.
Keywords: The Black Sea, Mediterranean, The Sea of Marmara,Biogeochemical Cycles, Ecosystem
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