Anoxic waters

Anoxic waters are bodies or areas of sea water, fresh water or groundwater that are depleted of dissolved oxygen. Anoxic waters can be contrasted with hypoxic waters, which are low but not completely lacking in dissolved oxygen, often defined as having oxygen concentration less than 2 milligrams per litre. This condition is generally found in areas that have water stagnation and/or dead stratification. The United States Geological Survey defines anoxic groundwater as that with a dissolved oxygen concentration of less than 0.5 milligrams per litre.

In most cases, oxygen diffusion is prevented from the shallower photic zone to deeper levels by a physical barrier, as well as by pronounced stratifications due to temperature or salinity, in which, for instance, denser, colder or hypersaline waters rest at the bottom of a basin. Anoxic conditions will occur if the rate of oxidative decomposition (aerobic digestion) of organic detritus by aerobic microorganisms (mainly fungi and aerobic bacteria) is greater than fresh supply of dissolved oxygen.

Anoxic waters are a natural phenomenon and have occurred throughout Earth's geological history. The Permian–Triassic extinction event, a mass extinction that wiped out most species from the world's oceans, may have resulted from widespread anoxic conditions combined with ocean acidification driven by a massive release of volcanic carbon dioxide into Earth's atmosphere by the supereruption of Siberian Traps, which concurrently also devastated the terrestrial vegetations responsible for photosynthetically removing carbon from the atmosphere. Many lakes have a permanent or temporary anoxic layer created by biotic aerobic respiration depleting oxygen at depth and thermal stratification preventing its replenishment to the depth.

Anoxic basins exist in the Baltic Sea, the Black Sea, the Cariaco Trench, various fjord valleys, and elsewhere. Eutrophication has likely increased the extent of anoxic zones in areas including the Baltic Sea, the Gulf of Mexico, and Hood Canal in Washington State.