Hydrogen Sulphide in the Black Sea (2)
  Profile of hydrogen sulphide in the Black Sea.
  But the zones of hypoxia have definitely expanded in recent years. From 1973 to 1990 the bottom area affected by hypoxia increased from 3,500 km2 to 40,000 km2 (Zaitsev, 1993). This undoubtedly led to increases in hydrogen sulphide in the bottom waters, although measurements of hydrogen sulphide are much less abundant than measurements of oxygen levels. Since the north-western shelf is only 64,000 km2 (limited by the 100 m isobath), the hypoxia has now extended to a significant proportion of the shelf area.
  What will happen to the hydrogen sulphide levels in the Black Sea in the future? On the north-western shelf hydrogen sulphide concentrations may decline as measures are implemented to reduce the nutrient loading. But the improvement in the ecosystem will not be immediate. The sequestering of nutrients in bottom sediments on the north-west shelf will continue to provide a source of nutrients through benthic fluxes. No reliable data is currently available on nutrient levels in the sediments or the rates of nitrification and denitrication. Additional research on nutrient sequestering on the shelf and on benthic regeneration, is required in order to make an accurate assessment of future levels of eutrophication on the shelf once nutrient input from the rivers has been reduced to acceptable levels (Aubrey et al., 1996 b).
  But what of the deep hydrogen sulphide in the Black Sea? Will the hydrogen sulphide boundary rise? Fortunately, existing data indicate that this boundary is relatively constant. There is no evidence that the average hydrogen sulphide boundary is shoaling over the basin or that it will do so at a later date.
  It is clear that the aggressive reduction of nutrient inputs from river and atmospheric sources will have a positive effect on the ecosystem of the north-western shelf and reduce the spread of the hydrogen sulphide across the shelf. But the sequestering of nutrients in bottom sediments means that it is still not possible to predict the response time of the system. In the deep Black Sea the hydrogen sulphide layer appears relatively stable, but large changes in fresh water inflow or physical mixing processes may produce changes in the hydrogen sulphide boundary, which may have a negative impact on the ecosystem as a whole (Aubrey et al., 1996 b).
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