Hypoxia (environmental) 5769311 225599567 2008-07-14T14:20:00Z Tj9991 192259 Reverted edits by [[Special:Contributions/166.109.198.83|166.109.198.83]] to last version by Richerman (using [[WP:HG|Huggle]]) {{Pollution}} :''For other uses of the term "hypoxia", see [[hypoxia]].'' '''Hypoxia''' or '''oxygen depletion''' is a phenomenon that occurs in aquatic environments as [[oxygen|dissolved oxygen]] ('''DO'''; molecular oxygen dissolved in the water) becomes reduced in concentration to a point detrimental to aquatic organisms living in the system. Dissolved oxygen is typically expressed as a percentage of the oxygen that would dissolve in the water at the prevailing temperature and salinity (both of which affect the solubility of oxygen in water; see [[oxygen saturation]] and [[underwater]]). An aquatic system lacking dissolved oxygen (0% saturation) is termed anaerobic, [[Reducing environment|reducing]], or '''anoxic'''; a system with low DO concentration&mdash;in the range between 1 and 30% DO saturation&mdash;is called '''hypoxic'''. Most fish cannot live below 30% DO saturation. A "healthy" aquatic environment should seldom experience DO less than 80%. == Causes of hypoxia == [[Image:Oxygendepletion1.gif|thumb|250px|left|Decline of oxygen saturation to anoxia, measured during the night in [[Kiel]] Fjord, Germany. Depth = 5 m]] Oxygen depletion could be the result of a number of factors including natural ones, but is of most concern as a consequence of [[pollution]] and as an outcome of a process known as [[eutrophication]] in which [[Nutrients#Nutrients and the environment|plant nutrients]] enter a river, lake, or ocean, [[phytoplankton]] blooms are encouraged. While phytoplankton, through photosynthesis, will raise DO saturation during daylight hours, the dense population of a [[Algal bloom|bloom]] reduces DO saturation during the night. When phytoplankton cells die, they sink towards the bottom and are decomposed by [[bacterium|bacteria]], a process that further reduces DO in the water column. If oxygen depletion progresses to '''hypoxia''', fish kills can occur and invertebrates like [[worm]]s and [[clam]]s on the bottom may be killed as well. [[Image:Fishkillk.jpg|thumb|left|Still frame from an underwater video of the sea floor. The floor is covered with crabs, fish, and clams apparently dead or dying from oxygen depletion.]] Natural occurrences of hypoxia have been observed. Water flowing from a river into the sea is less dense than salt water. When this water does not mix with the underlying saline water, the oxygen concentration in the bottom layer may become low enough for hypoxia to occur. Hypoxia is particularly problematic in shallow waters of semi-enclosed bodies of water like the [[Waddenzee]] or the [[Gulf of Mexico]] where land runoff is substantial. In these areas, a so-called "[[dead zone (ecology)|dead zone]]" can be created. [[Image:Jubilee-Mobile-Bay-Alabama-crabs-flounders.jpg|thumb|200px|right| Jubilee photo from [[Mobile Bay]]]] Hypoxia may also be the explanation for periodic phenomena such as the [[Mobile Bay jubilee]], where aquatic life suddenly rushes to the shallows, perhaps trying to escape oxygen-depleted water. Recent widespread shellfish kills near the coasts of Oregon and Washington are also blamed on cyclic [[dead zone (ecology)|dead zone]] ecology. <ref>[http://oregonstate.edu/dept/ncs/newsarch/2006/Aug06/dieoff.html oregonstate.edu] &ndash; Dead Zone Causing a Wave of Death Off Oregon Coast (8/9/2006)</ref> == Solutions == To combat hypoxia, it is essential to reduce the amount of land-derived nutrients reaching rivers in runoff. Defensively this can be done by improving sewage treatment and by reducing the amount of fertilizers leaching into the rivers. Offensively this can be done by restoring natural environments along a river; marshes are particularly effective in reducing the amount of phosphorus and nitrogen (nutrients) in water. <p clear="left"> [[Image:Oxygen depletion.png|thumb|250px|Graphs of oxygen and salinity levels at Kiel Fjord in September 1998.]] In a very short time the [[oxygen saturation]] can drop to zero when offshore blowing winds drive surface water out and anoxic depthwater rises up. At the same time a decline in temperature and a rise in salinity is observed (from the [[longterm ecological observatory]] in the [[sea]]s at Kiel Fjord, Germany). New approaches of long-term monitoring of oxygen regime in the ocean observe online the [[behavior]] of [[fish]] and [[zooplankton]], which changes drastically under reduced [[oxygen saturation]]s ([[ecoSCOPE]]) and already at very low levels of [[water pollution]]. ==Bog chemistry== In certain northern European [[sphagnum]] acidic bogs, a condition of hypoxia arises that prevents tissue decay by impeding micro-organisms in the soil and groundwater. Remarkable preservation of human mummies has occurred in some cases such as the discovery of [[Haraldskær Woman]] and [[Tollund Man]] in [[Jutland]], [[Denmark]] and [[Lindow man]] in [[Cheshire]], [[England]]. == References == {{reflist}} * Kils, U., U. Waller, and P. Fischer. 1989. The Fish Kill of the Autumn 1988 in Kiel Bay. International Council for the Exploration of the sea C M 1989/L:14 * Fischer P. and U. Kils. 1990. ''In situ'' Investigations on Respiration and Behaviour of Stickleback ''Gasterosteus aculeatus'' and the Eelpout ''Zoaraes viviparus'' During Low Oxygen Stress. International Council for the Exploration of the Sea C M 1990/F:23 * Fischer P., K. Rademacher, and U. Kils. 1992. ''In situ'' investigations on the respiration and behaviour of the eelpout Zoarces viviparus under short term hypoxia. Mar Ecol Prog Ser 88: 181-184 <!-- POV - The underwater videos of dying fish were released for broadcasting in the [[primetime]] TV news [http://www.tagesschau.de TAGESSCHAU] and articles in key newspapers in [[Germany]] (against the order of the director of the [http://www.ifm.uni-kiel.de/index-ifm.htm Institut für Meereskunde], with the consequences that the tourism in the area declined drastically, and the research group with all students and [[EcoSCOPE|instrumentation]] were destroyed by political forces of the that-time [[:de:Barschel-Affäre|quite]] currupt capitol [[Kiel]]. --> ==See also== *[[Anoxic event]] *[[Dead zone (ecology)]] *[[Oxygen saturation]] * [[Wastewater quality indicators]] discusses both [[Biochemical oxygen demand|BOD]] and [[COD]] as measures of water quality. *[[Winkler test for dissolved oxygen]] &mdash; for instructions on how to determine the amount of oxygen dissolved in fresh water. == External links == *[http://www.nos.noaa.gov/products/pubs_hypox.html Hypoxia in the Gulf of Mexico] *[http://web.archive.org/web/20001002161825/www.ecoscope.com/oxygenny.htm Dead zone in front of Atlantic City] [[Category:Aquatic ecology]] [[Category:Chemical oceanography]] [[Category:Environmental science]] [[Category:Water quality indicators]] [[Category:Oxygen]] [[da:Iltsvind]] [[de:Hypoxie (Ökologie)]] [[nl:Hypoxie (water)]] [[ja:貧酸素水塊]] [[pl:Anoksja (biologia)]] [[fi:Anoksia]]