Oxygen-starved ocean dead zones may be more widespread than thought.

Spanish researchers found that many species die off at oxygen levels well above what is now considered uninhabitable. The new study suggests that the extent of dead zones in coastal areas that support fishing industries is greater than previously known.

Since the mid-20th century, more than 400 dead zones have formed along continental coastlines, where fertilizer pollution causes algal blooms whose decomposition feeds oxygen-gobbling bacteria.

These so-called hypoxic regions now cover an area roughly equivalent in size to Oregon. Compared to Earth's total ocean area, that's relatively small, but they're grouped in places critical to commercial fishing. They're also spreading, in both size and frequency: Since the 1960s, the number of hypoxic areas has doubled every 10 years.

And as significant as the problem is, it's based on what may be outdated, overly permissive standards. The new study is a review of nearly 900 studies of 206 ocean floor-dwelling species, and suggests that the level of oxygen considered hypoxic needs to be raised.

"These results imply that the number and area of coastal ecosystems affected by hypoxia and the future extent of hypoxia impacts on marine life have been generally underestimated," write Carlos Duarte and Raquel Vaquer-Sunyer of the Mediterranean Institute for Advanced Studies. The paper was published yesterday in the Proceedings of the National Academy of Sciences.

The benchmark for hypoxia was set in 1983, when scientists found that fish and shrimp abandoned water containing less than two milligrams of dissolved oxygen per liter. To preserve the biodiversity of bottom-dwellers — and thus the functional structure of marine ecosystems that currently support coastal commercial fishing industries — Duarte and Vaquer-Sunyer would set an oxygen baseline of 4.6 milligrams per liter, if not higher.

"If we went by this definition, we'd probably double the number of hypoxic zones, or at least increase it by 100," said Robert Diaz, a Virginia Institute of Marine Science biologist who has tracked the rise of ocean dead zones since 1995, and was not involved in the current study.

Diaz noted that the figure produced by Duarte and Vaquer-Sunyer was a rule of thumb: In tropical regions, lower oxygen levels may be acceptable, while more oxygen may be needed elsewhere. More important than the figure itself, he said, was what it signified.

"The dead zones are occurring in areas that are very productive fishing grounds," said Diaz. "Everything is pointing towards a more desperate situation in all aquatic systems, freshwater and marine. That's pretty clear. People should be worried, all over the world."

Thresholds of hypoxia for marine biodiversity[PNAS] [open access]

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*Images: Above, the Gulf of Mexico dead zone, from NASA; at right, organism hypoxia thresholds and the chronological increase of hypoxic areas, from PNAS. To see dead zones in Google Earth or Google Maps, visit Robert Diaz' website.

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