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14th MEDCOAST Congress on Coastal and Marine Sciences, Engineering, Management and Conservation, MEDCOAST 2019

2019

Appearance and spread of severe oxygen deficiency, “dead zones”, have become usual for coastal areas of the World Ocean. Consequences of these phenomena are always extremely negative. Redox conditions in bottom sediments and water greatly depend on oxygen, which is the key factor for benthic communities. This work is aimed to study biogeochemical processes, oxygen dynamic and its deficiency in coastal Black Sea areas of Crimea. Data has been obtained from sites with various hydrodynamic regimes and anthropogenic pressures. Sediments texture and organic carbon inventory are primary factors to limit the oxygen flux, while active processes of water exchange support its flux and the level of water saturation. Organic carbon enriched pelitic sediments are typical for oxygen deprived conditions in areas under severe anthropogenic pressures. Restricted water exchange results in oxygen deficiency in the most anthropogenically stressed areas of Sevastopol Bay. Oxygen deficiency is a permanent characteristic of the upper bottom sediments of the bay. It gradually becomes anaerobic and shifts anaerobic/reduced processes closer to the bottom surface and further to bottom waters. Biogeochemical processes in anaerobic sediments of Sevastopol Bay are driven by sulfate reduction and iron recycling. Water exchange in Balaklava Bay is also restricted, but the wind regime and the geomorphology support the downward flux of oxygen to sediments. Oxygen deficiency is a seasonal characteristic there, when biogeochemical processes are driven by Fe (II, III) transformations. In areas of the open coast of Crimea, hydrodynamic processes support actively remove nutrients and organic carbon loaded from the coast and provide oxygen into bottom waters and upper sediments. The oxygen saturation is typically from 50 – 99 % in the bottom waters and depends on the site depth. In the bottom sediments, the oxygen saturation is 8–70 % and oxygen penetrates down to 13 mm depth. Biogeochemical processes in the bottom sediments are governed by reactions with oxygen and sulphate reduction does not usually occur. The main features of investigated open coastal areas are: (a) the deficiency of oxygen near the western coast of Crimea is a combination of the water column depth (80 m), fine structure of sediments and additional load of organic carbon from the coast; (b) the oxygen deficiency (46 % sat.) and even hypoxia (8–30 % sat.) in bottom sediments near the eastern coast of Crimea (for example, in Feodosia Bay) is a result of severe anthropogenic pressures and specific near bottom currents from the Kerch Strait providing additional organic matter from the Sea of Azov.

BIOGEOCHEMISTRY, COASTAL ZONES, HYDRODYNAMICS, ORGANIC CARBON, SULFUR COMPOUNDS, TEXTURES

2019-12-12 15:49:00