Anoxic waters

Anoxic waters are areas of sea water, fresh water, or groundwater that are depleted of dissolved oxygen. The US Geological Survey defines anoxic groundwater as those with dissolved oxygen concentration of less than 0.5 milligrams per litre.[1] Anoxic waters can be contrasted with hypoxic waters, which are low (but not lacking) in dissolved oxygen. 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,[2] 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,[3] and have occurred throughout geological history. The Permian–Triassic extinction event, a mass extinction of species from the world's oceans, may have resulted from widespread anoxic conditions combined with ocean acidification driven by a massive release of carbon dioxide into Earth's atmosphere.[4] Many lakes have a permanent or temporary anoxic layer created by respiration depleting oxygen at depth and thermal stratification preventing its resupply.[5]

Anoxic basins exist in the Baltic Sea,[6] the Black Sea, the Cariaco Trench, various fjord valleys, and elsewhere.[7] Eutrophication has likely increased the extent of anoxic zones in areas including the Baltic Sea, the Gulf of Mexico,[8] and Hood Canal in Washington State.[9]

  1. ^ "Volatile Organic Compounds in the Nation's Ground Water and Drinking-Water Supply Wells: Supporting Information: Glossary". US Geological Survey. Retrieved 3 December 2013.
  2. ^ Bjork, Mats; Short, Fred; McLeod, Elizabeth; 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.
  3. ^ Richards, 1965; Sarmiento 1988-B
  4. ^ McElwain, Jennifer C.; Wade-Murphy, Jessica; Hesselbo, Stephen P. (2005). "Changes in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals". Nature. 435 (7041): 479–482. Bibcode:2005Natur.435..479M. doi:10.1038/nature03618. ISSN 0028-0836. PMID 15917805. S2CID 4339259.
  5. ^ Wetzel, Robert G. (2001). Limnology : lake and river ecosystems (3rd ed.). San Diego: Academic Press. ISBN 0-12-744760-1. OCLC 46393244.
  6. ^ Jerbo, 1972;Hallberg, 1974
  7. ^ Skei, J. M. (1983). "Permanently Anoxic Marine Basins: Exchange of Substances across Boundaries". Ecological Bulletins (35): 419–429. ISSN 0346-6868. JSTOR 20112877.
  8. ^ "Streamflow and Nutrient Delivery to the Gulf of Mexico for October 2009 to May 2010 (Preliminary)". Archived from the original on 2012-11-29. Retrieved 2011-02-09.
  9. ^ "Archived copy" (PDF). Archived from the original (PDF) on 2011-09-27. Retrieved 2013-03-05.{{cite web}}: CS1 maint: archived copy as title (link)