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Low-Oxygen Levels in Ocean Threaten Sea Life

May 2, 2008

Researchers warn that as the oceans are heated by global warming, low-oxygen zones are threatening sea life, making it difficult for some to survive.

Over the last 50 years, Oxygen-depleted zones in the central and eastern equatorial Atlantic and equatorial Pacific oceans appear to have expanded, according to a report published in the journal Science.

Concerns about the threat to sea life in low-oxygen zones in the Gulf of Mexico have been studied in recent years.

Lothar Stramma of the University of Kiel in Germany, said if these zones continue to expand it could have dramatic consequences for both sea life and coastal economies.

“The finding was not surprising, because computer climate models had predicted a decline in dissolved oxygen in the oceans under warmer conditions,” Stramma said.

“Warmer water simply cannot absorb as much oxygen as colder water,” said co-author Gregory C. Johnson of the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Seattle.

There are complex biological and chemical interactions in these low-oxygen regions, Stramma said, adding that this needs to be more closely studied.

“As oceans lose oxygen, this will reduce habitat for many organisms,” said Frank A. Whitney of Canada’s Institute of Ocean Sciences.

Whitney was not a part of the study, but said “many species will lose their deep habitat, meaning competition will become stronger in the remaining favorable habitat, and increased vulnerability to predation will likely occur.”

He said the most rapid oxygen declines he has seen have occurred in the subarctic Pacific Ocean, and fish and crab kills have been reported in the last few years off Oregon.

“The findings are compelling but not surprising,” said Steven J. Bograd, a research oceanographer at the National Oceanic and Atmospheric Administration’s Environmental Research Division in Pacific Grove, Calif.

Bograd has studied trends in dissolved oxygen in the ocean off California, finding an expansion of the area of the continental shelf there that is exposed to low-oxygen conditions. He also was not a part of Stramma’s team.

He said most marine species have minimum oxygen thresholds needed for survival. “As oxygen decreases, these animals will suffer and/or be compelled to move to other areas. Over time, the optimal area for various species will be compressed,” he explained.

Bograd’s findings are to be published in Geophysical Research Letters.

“We are not able to say definitively what has caused the oxygen declines off California. But we do know that waters from the eastern tropical Pacific” – a reduced-oxygen area studied by Stramma – flow into this region, Bograd said.

He said both studies results are consistent and that there could also be other processes at work off California.

Johnson said the general pattern is for colder ocean waters in the north and south to absorb oxygen, cool and sink below the surface to then flow toward the equator. Along the way, organic matter drifts down into the deeper water and its decay uses up some of this oxygen.

He added: The oxygen balance depends on this movement and the amount of oxygen reaching the warmer waters and this can be reduced if less is absorbed and moved in the deep currents.

“That means that eventually, at the end of the line, there will be less oxygen,” he said.

Johnson said in cold surface water, oxygen levels can reach as high as 300 to 400 micromols per kilogram. A mol of a gas such as oxygen occupies a volume of just under six gallons and a micromol is one-thousandth of that. A kilogram of water is the amount that would weigh 2.2 pounds.

Dissolved oxygen varies widely in the oceans, and sea life becomes stressed when it reaches between 60 and 120 micromols per kilogram.

According to researchers, concentrations as low as 10 were found in parts of the eastern Pacific and the northern Indian Ocean and larger areas in the Atlantic and Pacific were below 150.

Declines in affected areas ranging from 0.09 to 0.34 per year over the last half century were noted in Stramma’s findings.

The German Research Council, U.S. National Oceanic and Atmospheric Administration and U.S. National Science Foundation founded Stramma’s study.

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