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Hypoxia occurs when the oxygen content in a waterbody becomes so low that animals cannot function and die.
Causes
Natural hypoxia
Water flowing from a river into the sea is lighter than salt water. When this water does not mingle enough with the underlying water, the oxygen levels may become low enough for hypoxia to occur.
Pollution
Pollution-caused hypoxia is usually related to eutrophication. When water is contaminated with high quantaties of nutrients such as phosphate or nitrogen fertilizers, large algae blooms occur. When these algae die, particular in summer, when water can contain less oxygen, the rotting processes eat up all available oxygen. In autumn when water circulation and oxygen content go up again, some recovery occurs, however it does take many years for the environment to restore itself.
Hypoxia is particularly problematic in undeep water and in enclosed bodies of water like the Waddenzee and the Gulf of Mexico. These waters are essential for maintaining a healthy marine environment as many fish use these areas to breed.
Measures
To combat hypoxia, it is essential to reduce the amount of nutrients coming into the rivers. 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 the environments along the river, particularly marshes are effective in reducing the amount of phosfor and nitrogen in water.
see also oxygen depletion
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