In most cases, oxygen is prevented from reaching the deeper levels by a physical barrier (e.g. mud) 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, and have occurred throughout geological history. Anoxic basins exist at present, for example, in the Baltic Sea, 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 and Hood Canal in Washington State. 
Causes and effects
Anoxic conditions result from several factors; for example, stagnation conditions, density stratification, 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
- 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;
- L'Atalante basin, eastern Mediterranean Sea
- Mariager Fjord, off Denmark;
- Orca Basin, northeast Gulf of Mexico;
- Saanich Inlet, off Vancouver Island, Canada;
- Bjork, Mats ; Short, Fred; McLeod, Elizabeth and Beer, Sven (2008). Managing Sea-grasses for Resilience to Climate Change. Volume 3 of IUCN Resilience Science Group Working Papers. Gland, Switzerland: International Union for Conservation of Nature (IUCN). p. 24. ISBN 978-2-8317-1089-1.
- Richards, 1965; Sarmiento 1988-B
- Jerbo, 1972;Hallberg, 1974
- "Streamflow and Nutrient Delivery to the Gulf of Mexico for October 2009 to May 2010 (Preliminary)". Retrieved 2011-02-09.
- Gerlach, 1994
- Gerlach, S. (1994). "Oxygen conditions improve when the salinity in the Baltic Sea decreases". Marine Pollution Bulletin 28: 413–384. doi:10.1016/0025-326X(94)90126-0.
- 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.; Toggweiler, J. R. (1988). "Causes of anoxia in the world ocean". Global Biogeochemical Cycles 2 (2): 115. Bibcode:1988GBioC...2..115S. doi:10.1029/GB002i002p00115.
- 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.
- Van Der Wielen, P. W. J. J.; Bolhuis, H; Borin, S; Daffonchio, D; Corselli, C; Giuliano, L; D'auria, G; De Lange, GJ et al. (2005). "The Enigma of Prokaryotic Life in Deep Hypersaline Anoxic Basins". Science 307 (5706): 121–123. Bibcode:2005Sci...307..121V. doi:10.1126/science.1103569. PMID 15637281. .