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, and
have occurred during the geological history of the Baltic Sea.
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
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
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
HS- + 2 O2 →
- Bannock Basin, eastern Mediterranean Sea;
- Black Sea Basin, off
eastern Europe, below 50 metres (150 feet);
- Caspian Sea
Basin, below 100 metres (300 feet);
Basin, off north central Venezuela;
Deep, in the Baltic off Sweden;
Fjord, off Denmark;
- Orca Basin,
northeast Gulf of
Inlet, off Vancouver Island, Canada;
- Gerlach, S.A. (1994) “Oxygen conditions improve when the
salinity in the Baltic Sea decreases”. Marine Pollution
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företeelse?” Vatten, 28: 404-408.
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Evolution in Anoxic Worlds (Oxford Series in Ecology and
Evolution) Oxford University Press. ISBN 0-19-854838-9
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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)
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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.