London, May 18 : Due to the gradual heating of the oceans under global warming, the "marine dead zones" - areas of water with low levels of dissolved oxygen - have spread across the world's tropical oceans, a new study has warned.
Researchers warn that the lack of oxygen is 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," Times Online quoted Stramma, as saying.
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.
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.