Anoxic waters

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 groundwater as those with dissolved oxygen concentration of less than 0.5 milligrams per litre. 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, 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.

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 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.

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