Hypoxia (environmental)
Encyclopedia : H : HY : HYP : Hypoxia (environmental)
- For other uses of the term "hypoxia", see hypoxia (disambiguation).
Causes of hypoxia
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 a highly detrimental outcome of a process known as eutrophication. Where 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 bloom reduces DO saturation during the night. When phytoplankton cells die, they sink towards the bottom and are decomposed by bacteria, a process that further reduces DO in the water column. If oxygen depletion progresses to hypoxia, fish kills can occur and invertebrates like worms and clams on the bottom may be killed as well.
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" can be created.
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.
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 seas at Kiel Fjord, Germany). New approaches of longterm monitoring of oxygen regime in the ocean observe online the behavior of fish and zooplankton, which changes drastically under reduced oxygen saturations (ecoSCOPE) and already at very low levels of water pollution.
References
- 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
See also
- Anoxic event
- Dead zone (ecology)
- Oxygen saturation
- Wastewater quality indicators discusses both BOD and COD as measures of water quality.
- Winkler test for dissolved oxygen — for instructions on how to determine the amount of oxygen dissolved in fresh water.
External link
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