Low-Oxygen Zones Spreading to Deep Ocean

The world's oceans have always had large zones of low-oxygen water. But new studies find that those zones appear to have been growing for the past 50 years. The cause is uncertain, but it could be related to human-induced climate change.

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Life in the sea requires oxygen. Recently, scientists are finding that oxygen in the sea appears to be more and more scarce.

For example, off the coast of Oregon they've repeatedly found or noticed a dead zone with little or no oxygen for the past six years. Scientists have gotten similar readings off the coast of South America and Africa.

And now as NPR's Richard Harris reports, studies show oxygen-poor waters are spreading.

RICHARD HARRIS: Oceanographer Gregory Johnson says when you look at the ocean, you'll find more oxygen in some places and less in others.

Mr. GREGORY JOHNSON (Oceanographer): Oxygen is exchanged between the ocean and the atmosphere at the ocean surface. And those waters sometimes sink and travel in some instances very long distances over a long time.

HARRIS: And as those waters travel, oxygen can be used up by fish or it can be consumed when organic material in the water decays. As a result, some parts of the ocean have always had very little oxygen.

Johnson now reports in Science magazine that some of those low-oxygen regions appear to have been expanding over the past 50 years.

Mr. STEVEN JOHNSON (Writer): We found in several of the places oxygen minimum zones had expanded vertically - that is, they had crept upwards and crept downwards.

HARRIS: Creating an ever growing pocket of oxygen poor water beneath the surface. Why this is happening is not exactly clear to Johnson and his co-authors. It's possible that the oxygen is being consumed in part by more decaying organic material. It's also possible that it's being driven by large scale changes in ocean circulation.

Some computer models suggest those changes in ocean circulation could be due to human induced global warming.

Dr. JOHNSON: Decreases that we're seeing as far as I can tell are probably a little bit larger than you'd expect from the climate model, so I'm not sure we can really unravel that mystery yet.

HARRIS: Johnson works at the National Oceanic and Atmospheric Administration lab in Seattle. He studies ocean chemistry, so he's reluctant to speculate about the possible biological effects of this trend. But biologist William Gilly at Stanford's Hopkins Marine Lab is troubled.

Professor WILLIAM GILLY (Stanford University): I think it's an amazingly serious thing. To have, you know, oceans running out of oxygen is really a bad thing.

HARRIS: Gilly has seen similar declines of ocean oxygen in the Gulf of California and off the coast of Southern California. But it's tough to figure out what effect this will have on marine life.

Prof. GILLY: Normally you'd think, oh, this is an ocean dead zone, it's really bad, but it's not in this case because this is a vital dynamic player of some of the world's most productive eco-systems.

HARRIS: The area in and right above these low oxygen zones is actually favored by some species such as the Humboldt Squid.

Prof. GILLY: We know that Humboldt squid utilizes this upper boundary of the oxygen minimum zone where they eat these lanternfishes and things like that. There are working in this layer 24 hours a day sometimes. In the meantime, yellowfin tuna and elephant seals and sperm whales and everything else are diving down repetitively to this deep layer trying to eat, so it's clearly a very rich layer

HARRIS: The flipside is some fish need a lot of oxygen to live. So as the low oxygen layer expands, they could get forced to go elsewhere; who knows where.

Lisa Levin at the Scripps Institution of Oceanography is also concerned about what happens when these masses of low oxygen water flow up to the continental shelves and start intruding into areas of rich and productive marine life.

Dr. LISA LEVIN (Scripps Institution of Oceanography): Certainly where this occurs on our continental shelf we will see major changes in our eco-systems, including in the distribution and abundance of fishery species.

HARRIS: That is, the fish we catch to eat. Levin says whatever is causing the lost of oxygen, there doesn't seem to be much we can do to make it better. But, she adds, there are some ideas out there designed to combat climate change that could make it worse. Dumping iron or other fertilizer in the ocean might help the seas take up carbon dioxide but it would reduce oxygen levels too.

Dr. LEVIN: If we're seeing the open ocean systems already exhibiting trends of reduced oxygen, then we want to careful to any additional changes we make in these systems.

HARRIS: She says we'd better figure out what's going on first and try not to make it worse.

Richard Harris, NPR News.

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