Hydrogen Sulphide in the Black Sea (1)
  As the result of past geological events, its morphometry and specific water balance, nearly 87 percent of the Black Sea water volume is anoxic and contains high levels of hydrogen sulphide . The 13 percent of the volume that contains oxygen consists of the shallow surface water and the waters from the shelves. The recent eutrophication of the sea has placed even this 13 percent under severe stress. The introduction of excess nutrient loads has been accompanied by massive phytoplankton blooms (especially flagellates), whose death in turn depletes even the shallow shelf waters of oxygen as the oxidation of organic material consumes valuable oxygen resources. Up to 40,000 km2 of the north-west shelf of the Black Sea is now subject to hypoxia and the occasional formation of hydrogen sulphide rich bottom waters. The high levels of hydrogen sulphide, both naturally occurring and exacerbated by anthropogenic factors, have considerable socio-economic as well as ecological implications.
  Most hydrogen sulphide production is due to redox processes that occur in the water column. There is little evidence of hydrogen sulphide production by geothermal or other crustal processes; although Zaitsev reported that a single source of hydrogen sulphide was observed during bottom sampling on the north-western shelf, analogous to a “black smoker”. But the volumes produced by such geothermal sources are negligible compared with the main redox process.
  Despite the relatively stable hydrogen sulphide distribution over the last 7,500 years, the level of the interface separating the oxygenated water from the oxygen-deprived lower waters has fluctuated according to the physical oceanography of the region. The hydrogen sulphide layer lies some 100 to 200 m below the surface. There are also seasonal and annual fluctuations in the level at which hydrogen sulphide is first encountered. Seasonal atmospheric variations produce considerable variations in circulation (Oguz et. al., 1995). The hydrogen sulphide boundary is usually deepest in summer and shallowest in spring.
  Human use of the Black Sea drainage basin has also had a profound impact on the ecology and oceanography of the Black Sea (Aubrey et. al., 1996a).
  Eutrophication has risen as the nutrient load has increased, leading to hypoxia and occasional anoxia, particularly on the north-west shelf. This anoxia also leads to the formation of hydrogen sulphide in the shelf zones. Garkavaya (unpublished data) recently recorded hydrogen sulphide concentrations of 1.5 to 2.25 ml/l in the lower water column on the north-western shelf at depths of 10-30 m. This hydrogen sulphide only became apparent in the 1970’s as a consequence of increased levels of eutrophication. Yet hydrogen sulphide on the shelf is still transitory, occurring primarily in summer and autumn, as intense water column mixing during winter and spring reoxygenates the bottom waters.
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