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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

• Bannock Basin, eastern Mediterranean Sea;
• Black Sea Basin, off eastern Europe, below 50 metres (150 feet);
• Caspian Sea Basin, below 100 metres (300 feet);
• Cariaco Basin, off north central Venezuela;
• Gotland Deep, in the Baltic off Sweden;
• Mariager Fjord, off Denmark;
• Orca Basin, northeast Gulf of Mexico;
• Saanich Inlet, off Vancouver Island, Canada;

See also

• Anoxic event
• Dead zone (ecology)
• Meromictic
• The Revenge of Gaia

Notes

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

References

• 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.