Anoxic waters are areas of sea water, fresh water or groundwater that are depleted of dissolved oxygen. Anoxic conditions are in a general a more severe condition of hypoxia. The US Geological Survey deigns anoxic waters as those with dissolved oxygen concentration of less than .5 milligrams per liter. 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) as well as by a pronounced density stratification, in which, for instance, heavier hypersaline 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. In fact, some postulate that the Permian–Triassic extinction event, a mass extinction of species from world's oceans, resulted from widespread anoxic conditions. At present anoxic basins exist, 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. Examples of which are fjords where shallow sills at there entrance prevent circulation and deep ocean western boundaries where circulation is especially low while production at upper levels is exceptionally high. 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. This results in the byproduct of hydrogen sulfide (H2S), a chemical toxic to most biota and responsible for the characteristic "rotten egg" smell and dark black sediment color.
PO4-2 + H+1 → H2S +H2O + chemical energy
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
Anoxia is quite common in muddy ocean bottoms where there are both high amounts of organic matter and the low levels inflow of oxygenated water through the sediment. Below a few centimeters from the surface the interstitial water (water between sediment) is oxygen free.
Anoxia is further influenced by biochemical oxygen demand (BOD), which is the amount oxygen used by marine organisms in the process of breaking down organic matter. BOD is influenced by the type of organisms present, the pH of the water, temperature, and the type of organic matter present in the area. BOD is directly related to the amount of dissolved oxygen available, especially in smaller bodies of water such as rivers and streams. As BOD increases, available oxygen decreases. This causes stress on larger organisms. BOD comes from nautral and anthropogenic sources, including: dead organisms, manure, wastewater, and urban runoff.
Human Caused Anoxic Conditions
Eutrophication, an influx of nutrients (phosphate/nitrate), often a byproduct of agricultural run-off and sewage discharge, can result in large but short-lived algae blooms. Upon a bloom’s conclusion, the dead algae sink to the bottom and are broken down until all oxygen is expended. Such a case is the Gulf of Mexico where a seasonal dead zone occurs, which can be disturbed by weather patterns such as hurricanes and tropical convection. Sewage discharge, specifically that of nutrient concentrated “sludge”, can be especially damaging to ecosystem diversity. Species sensitive to anoxic conditions are replaced by fewer heartier species, reducing the overall variability of the effected area. 
Daily and Seasonal Cycles
The temperature of a body of water directly affects the amount of dissolved oxygen it can old. Following Henry’s Law, as water becomes warmer it becomes less soluble to oxygen. This property leads to daily anoxic cycles on small geographic scales and seasonal cycles of anoxia on the larger scale. Thus, bodies of water are more vulnerable to anoxic conditions during warmest period of the day and during the summer months. This problem can be further exasperated in the vicinity of industrial discharge where warm water used to cool machinery is less able to hold oxygen than the basin to which it is released.
Daily cycles are also influenced by the activity of photosynthetic organisms. The lack of photosynthesis during nighttime hours in the absence of light can result in anoxic conditions intensifying throughout the night with a maximum shortly after sunrise.
Organisms have adapted a variety of mechanisms to live within anoxic sediment. While some are able to pump oxygenated from higher water levels down into the sediment, other adaptations include specific hemoglobins for low oxygen environments, slow movement to reduce rate of metabolism, and symbiotic relationships with anaerobic bacteria. In all cases, the prevalence of toxic H2S results in low levels of biologic activity and a lower level of species diversity if the area is not normally anoxic. 
- 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;
- . US Geological Survey http://water.usgs.gov/nawqa/vocs/national_assessment/report/glossary.html. Retrieved 3 December 2013. Missing or empty
- 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
- Castro and Huber,2005
- "5.2 Dissolved Oxygen and Biochemical Oxygen Demand". Water: Monitoring & Assessment. US Environmental Protection Agency. Retrieved 3 December 2013.
- Castro and Huber,2005
- "Dissolved Oxygen Depletion in Lake Erie". Great Lakes Monitoring. US Environmental Protection Agency. Retrieved 3 December 2013.
- Castro and Huber,2005
- Castro, Peter and Huber, Michael E. (2005) Marine Biology 5th ed. McGraw Hill. ISBN 0-07-250934-1
- Gerlach, S. (1994). "Oxygen conditions improve when the salinity in the Baltic Sea decreases". Marine Pollution Bulletin 28 (7): 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, G. J.; Huebner, A.; Varnavas, S. P.; Thomson, J.; Tamburini, C.; Marty, D.; McGenity, T. J.; Timmis, K. N.; Biodeep Scientific, P. (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. .