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Anoxic waters are areas of sea water, fresh water or groundwater that are depleted of dissolved oxygen and are a more severe condition of hypoxia. The US Geological Survey defines anoxic waters as those with dissolved oxygen concentration of less than .5 milligrams per litre.〔(【引用サイトリンク】publisher=US Geological Survey )〕 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 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,〔Richards, 1965; Sarmiento 1988-B〕 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,〔Jerbo, 1972;Hallberg, 1974〕 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〔(【引用サイトリンク】accessdate=2011-02-09 )〕 and Hood Canal in Washington State.〔http://wsg.washington.edu/mas/pdfs/hypoxia101.pdf〕 ==Causes and effects== Anoxic conditions result from several factors; for example, stagnation conditions, density stratification,〔Gerlach, 1994〕 inputs of organic material, and strong thermoclines. Examples of which are fjords (where shallow sills at their entrance prevent circulation) and deep ocean western boundaries where circulation is especially low while production at upper levels is exceptionally high. 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.〔Castro and Huber,2005〕 SO4−2 + 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 natural and anthropogenic sources, including: dead organisms, manure, wastewater, and urban runoff. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Anoxic waters」の詳細全文を読む スポンサード リンク
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