The Sea of Marmara ecosystem has undergone great changes in the last 40 years due to human pressures. Primary pressure sources can be given as, domestic, industrial and agricultural wastes, over fishing, port and maritime activities, coastal destructions and deforestation. As a result of these human-induced pressures, the Marmara ecosystem has deteriorated and significant reductions in biological diversity and water quality have been observed.
Excessive organic matter and nutrients (Nitrogen-Phosphorus) discharges from point or diffuse sources to the Sea of Marmara have caused eutrophication. Light transperency, food web, biodiversity and dissolved oxygen concentrations have been altered due to eutrophication. The increase in the biomass of primary producers and the amount of organic matter production with the increasing nutrient load has led to a significant decrease in oxygen concentrations with the oxygen consumption of heterotrophs while decomposing organic matter, especially in the halocline and sub-halocline layers of the Marmara Sea, which has a two-layered water column system. In addition to the long-term oxygen depletion problem, mucilage, which is a phenomenon that has occurred in very few regions of the world, first occurred in the Marmara Sea between 2007-2008 according to official records and disappeared in 2010. Mucilage is a mucus structure containing carbohydrates and polysaccharides, formed by the extracellular release of phytoplankton due to many environmental effects, and can be found on the sea surface, column and floor by attracting the living and non-living particle structures around it.
The mucilage, which started to appear on the surface in the Marmara Sea in March 2021, was detected in the entire upper layer of the Marmara in June-July with the one-month oceanographic studies carried out with the R/V Bilim 2. Studies continued in the Sea of Marmara in September 2021 and March 2022 as well. Current measurements showed that almost all sub-halocline layer of the Marmara Sea was below the hypoxia limit (<2 mg/L), and in some regions anoxic (oxygen-free) conditions have been observed. Due to carbohydrate and polysaccharide content of the mucilage, it is speculated that oxygen will be consumed by bacterial decomposition, thus, oxygen concentrations in the water column will decrease. Oxygen consumption rates were found to be twice as high in the mucilage period compared to the non mucilage period on the surface and above the interlayer. In the studies performed from the beginning of June to the beginning of July, we showed that oxygen decreased in the interlayer and this decrease was caused by mucilage. In this conference proceeding, the current oceanographic features of the Sea of Marmara, water column oxygen concentrations and the possible effects of mucilage on oxygen concentrations are discussed.