Sans O2

By Sumana Narayanan
Published: Saturday 15 November 2008

Oxygen threshold for dead zones needs revision

ABOUT 246,000 square km of oceans--bigger than the size of the uk--turn into dead zones: patches of waters with too little oxygen for life. The Gulf of Mexico, the Baltic, the East China Sea and the Black Sea are some of the worst affected. About 400 such zones are known across the world. The Indian Ocean, too, has patches depleted of oxygen, (see box: Indian Ocean Blues).

Dead zones result from excessive nutrients: nitrogen and phosphorus, primarily, from agricultural, industrial and urban effluents. Nutrient-rich waters sustain algal blooms. After the algae die, they sink to the ocean floor, where bacteria decompose them, using up all the oxygen in the water and leading to what scientist called hypoxia.

Scientists demarcate dead zones when the dissolved oxygen drops below 2 milligramme per litre of water. Three scientists from Spain said recently this threshold must be redefined, because life is damaged even when there is more oxygen than the limit. Because different organisms require different levels of oxygen, it is not easy to settle upon a threshold.

The Spanish team reviewed 872 research papers located through a search on Google Scholar and Web of Science. They found why the threshold of 2 mg/litre was arrived upon: by fisheries managers to indicate collapse of fisheries. This does not account for the fact that various organisms die at varying oxygen levels. Even the intensity of the effect of low oxygen varies.

Crustaceans like prawns are more sensitive, begin dying in large numbers when oxygen drops below 5.72 mg/ litre; fish are hardier, and are unaffected till the level drops to 2 mg/l. Then there are animal groups that adapt to low oxygen. Mobile animals migrate to oxygen-rich waters, and burrowing species like mussels prefer to not go too deep into the soil. Others such as cnidarians, like jellyfish and corals, lower their metabolic rate to deal with less oxygen.

Oxygen levels should not be studied in isolation, the scientists caution; environmental conditions like temperature and toxcity determine how an animal or a plant deals with oxygen stress. This is not understood well because most studies rely on data collected in the laboratory, not the open seas. In their paper published in the Proceedings of the National Academy of Sciences (Vol 105, No 40), they point out that oxygen levels are held steady in the lab, unlike in natural conditions, where environmental factors play a varying and critical role.

Revising the threshold would make conservation efforts more effective, setting management targets to avoid biodiversity loss due to hypoxia, the authors said, adding that a revision would include more sea waters in dead zones.

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