Oxygen depletion may be caused by the oxygen demand of decaying organic material, such as algal blooms or effluents. Restricted vertical circulation may exacerbate the problem. When seawater absorbs CO2, its acidity increases. As temperatures rise, circulation in the oceans stalls, preventing oxygen from reaching the depths. Waters with dissolved oxygen levels lower than 2-3 milligrams per litre are termed hypoxic; oxygen concentrations become too low to sustain animal life.
In 1976, large scale oxygen depletion in the New York Bight caused mass mortality of benthic fauna. The German Bight and the Central North Sea are also showing indications of oxygen depletion, with resulting damage to fauna.
Botanists have described a "dead zone" in the northern Gulf of Mexico as the largest in the world. The "dead zone" is a result of pollution from the Mississippi river and is characterised by a lack of oxygen and life. The zone has been larger than 10,000 square kilometers since 1993, increasing in midsummer 1999 to 20,000 square kilometers.