Anoxic Sea Water

Anoxic waters are areas of sea water, fresh water or groundwater 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 (e.g. mud)[1] as well as by a pronounced density stratification, in which, for instance, heavier hyper saline waters rest at the bottom of a basin. 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,[2] and have occurred throughout geological history. Anoxic basins exist at present, for example, in the Baltic Sea,[3] and elsewhere (see below). Recently, there have been some indications that eutrophication has increased the extent of the anoxic areas in areas including the Baltic Sea, the Gulf of Mexico[4] and Hood Canal in Washington State. [5]

Causes and effects

Anoxic conditions result from several factors; for example, stagnation conditions, density stratification,[6] inputs of organic material, and strong thermoclines. The bacterial production of sulfide starts in the sediments, where the bacteria find suitable substrates, and then expands into the water column. In wastewater treatment, the absence of oxygen alone is indicated anoxic while the term anaerobic is used to indicate the absence of any common electron acceptor such as nitrate, sulfate or oxygen.

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 sulfate. If anoxic sea water becomes reoxygenized, sulfides will be oxidized to sulfate according to the chemical equation:

HS + 2 O2 → HSO4

or, more precisely:

(CH2O)106(NH3)16H3PO4 + 53 SO42- → 53 CO2 + 53 HCO3- + 53 HS- +16 NH3 + 53 H2O + H3PO4

In the Baltic Sea, the slowed rate of decomposition under anoxic conditions has left remarkably preserved fossils retaining impressions of soft body parts, in Lagerstätten.

Anoxic basins

See also

Notes

References

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  • 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.
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  • 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.
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