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From Wikipedia, the free encyclopedia

Anoxic waters are areas of sea water or fresh water that are depleted of dissolved oxygen. This condition is generally found in areas that have restricted water exchange.

In most cases, oxygen is prevented from reaching the deeper levels by a physical barrier (sill) as well as by a pronounced density stratification. Anoxic conditions will occur if the rate of oxidation of organic matter by bacteria is greater than the supply of dissolved oxygen.

Anoxic waters are a natural phenomenon,[1][2] and have occurred during the geological history of the Baltic Sea.[3][4] Recently, there have been some indications that eutrophication has increased the extent of the anoxic areas in areas including the Baltic Sea, and the Gulf of Mexico.

Anoxic conditions result from several factors; for example, stagnation conditions, density stratification,[5] inputs of organic material, and strong thermoclines. The bacterial production of sulphide starts in the sediments, where the bacteria find suitable substrates, and then expands into the water column.

When oxygen is depleted in a basin, bacteria first turn to the second-best electron acceptor, which in sea water is nitrate. Denitrification occurs, and the nitrate will be consumed rather rapidly. After reducing some other minor elements, the bacteria will turn to reducing sulphate. If anoxic sea water becomes reoxygenized, sulphides will be oxidized to sulphate according to:

HS- + 2 O2 → HSO4-


Anoxic basins

See also


  1. ^ Richards, 1965
  2. ^ Sarmiento 1988-B
  3. ^ Jerbo, 1972
  4. ^ Hallberg, 1974
  5. ^ Gerlach, 1994


  • Gerlach, S.A. (1994) “Oxygen conditions improve when the salinity in the Baltic Sea decreases”. Marine Pollution Bulletin, 28: 413-416.
  • Hallberg, R.O. (1974) “Paleoredox conditions in the Eastern Gotland Basin during the recent centuries”. Merentutkimuslait. Julk./Havsforskningsinstitutets Skrift, 238: 3-16.
  • Jerbo, A. (1972) “Är Östersjöbottnens syreunderskott en modern företeelse?” Vatten, 28: 404-408.
  • Fenchel, Tom & Finlay, Bland J. (1995) Ecology and Evolution in Anoxic Worlds (Oxford Series in Ecology and Evolution) Oxford University Press. ISBN 0-19-854838-9
  • Richards, F.A. (1965) “Anoxic basins and fjords”, in Riley, J.P., and Skirrow, G. (eds) Chemical Oceanography, London, Academic Press, 611-643.
  • Sarmiento, J.L., Herbert, T.D., and Toggweiler, J.R. (1988-A) “Causes of anoxia in the world ocean”. Global Biochemical Cycles, 2(2): 115-128.
  • Sarmiento, J.A. et al. (1988-B) “Ocean Carbon-Cycle Dynamics and Atmospheric pCO2”. Philosophical Transactions of the Royal Society of London, Series A, Mathematical and Physical Sciences, Vol. 325, No. 1583, Tracers in the Ocean (May 25, 1988), pp. 3-21.
  • Zilli M., Guarino C., Daffonchio D., Borin S., Converti A. (2005) “The enigma of prokaryotic life in deep hypersaline anoxic basins”. Science, 307:121-123.


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