By Jonathan Leake
    The Sunday Times UK

    Sunday 18 May 2008

    Marine dead zones, where fish and other sea life can suffocate from lack of oxygen, are spreading across the world's tropical oceans, a study has warned.

    Researchers found that the warming of sea water through climate change is reducing its ability to carry dissolved oxygen, potentially turning swathes of the world's oceans into marine graveyards.

    The study, by scientists from some of the world's most prestigious marine research institutes, warns that if global temperatures keep rising there could be "dramatic consequences" for marine life and for humans in communities that depend on the sea for a living.

    Organisms such as fish, crabs, lobsters and prawns will die in such zones, warned Lothar Stramma of the Leibniz Institute of Marine Sciences in Kiel, Germany, who co-wrote the research paper with Janet Sprintall, a physical oceanographer at Scripps Institution of Oceanography in California.

    In the study, published in the journal Science, they collated hundreds of oxygen concentration readings taken over the past 50 years in the Atlantic and Pacific over depths ranging from 985ft to 2,500ft.

    "In the central and eastern tropical Atlantic and equatorial Pacific the oxygen-minimum zones appear to have expanded and intensified during the past 50 years," Stramma said.

    The researchers found that such regions now extend deeper into the oceans and closer to the surface. Fish and other sea life cannot survive in such waters, said Sprintall.

    The researchers say the change is closely linked to rising sea water temperature. At 0C, one kilogram of sea water can hold about 10ml of dissolved oxygen but at 25C this falls to just 4ml.

    This impact is amplified by a host of other factors. One of the most important is that parts of the eastern Atlantic, eastern Pacific and the Indian Ocean are naturally low in oxygen - so a small additional decline has a disproportionately greater effect.

    Examples of partly dead zones include a stretch of the Pacific about 5,000 miles wide off the west coast of South America. Others are found off the west coasts of Africa and India.

    Additionally, as surface water heats up it becomes less dense and forms an insulating layer that stops oxygen percolating into the colder layers beneath.

    Climate change is also suspected of altering the direction and strength of ocean currents, causing dead zones such as the one that suddenly appeared off Oregon, in America's Pacific Northwest, six years ago and which appears to have become an annual event, killing marine life at every level from plankton to salmon, seals and sea birds.

    Lisa Levin, professor of biological oceanography at Scripps, and a world expert on the expansion of oxygen depletion in the oceans, predicted that similar zones would eventually appear off California.

    "Around the world there are already around 150 areas suffering from low or declining oxygen levels," she said.

    Many of these are close to coastlines where the main cause is not climate change but pollution, especially agricultural chemicals washed off the land. The nitrogen in such run-off effectively fertilises the sea, causing a sudden "bloom" of algae and other planktonic life.

    As such organisms die they are decomposed by bacteria that multiply so fast they suck all the oxygen from the water.

    A report by the United Nations Environment Programme found that such coastal dead zones have doubled in number since 1995, with some extending over 27,000 square miles, about the size of the Republic of Ireland.

    Among the worst affected are the Baltic Sea, the Black Sea, and parts of the Mediterranean. Perhaps the biggest of all is found in the Gulf of Mexico, where the Mississippi carries thousands of tons of agrochemicals into the sea every year.

    Recent research has revealed that about 250m years ago average oxygen levels in oceans fell almost to zero - a reduction associated with dramatic changes in climate that resulted in the extinction of 95% of the world's species.