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Waiting to Inhale: Deep-Ocean Low-Oxygen Zones Spreading to Shallower Coastal Waters

Oxygen-deprived areas in the world's oceans usually found in deeper water are moving up to offshore areas and threatening coastal marine ecosystems by spurring the die-off of some species and overpopulation of others



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A plague of oxygen-deprived waters from the deep ocean is creeping up over the continental shelves off the Pacific Northwest and forcing marine species there to relocate or die. Since 2002 tongues of hypoxic, or low-oxygen, waters from deeper areas offshore have slipped into shallower near-shore environments off the Oregon coast, although not close enough to be oxygenated by the waves. The problem stems from oxygen reduction in deep water, a phenomenon that some scientists are observing in oceans worldwide, and that may be related to climate change.

The hypoxic seawater is distinct from the well-known "dead zones" that form at the mouths of the Mississippi and other rivers around the world. Those areas result from agricultural runoff, which lead to algae blooms that consume oxygen. Rather, the Pacific Northwest problem is broader and more mysterious.

Shelf waters off the Pacific Northwest extend anywhere from 30 to 80 kilometers offshore and lie beneath the California Current, one of the richest marine ecosystems in the world. Francis Chan, a senior research professor at Oregon State University, has been monitoring the area's low-oxygen events, which normally peak in the late summer months. "Oxygen is just about the most crucial necessity for anything biological," he says.*

Chan is one of a number of scientists alarmed at the dramatically reduced oxygen levels showing up in these waters. In fact, the Oregon Department of Fish and Wildlife  put submersible vehicles off Oregon's coast during a hypoxic event that went anoxic (oxygenless) in 2006, he says, monitoring conditions and recording numerous carcasses of sea stars, sea cucumbers, marine worms and fish.**

Lothar Stramma, a physical oceanographer at the Christian Albrechts University of Kiel in Germany and his associates describe the hypoxic problem as global in a paper accepted for publication in Deep-Sea Research , stating that tropical low-oxygen zones have expanded horizontally and vertically around the world, and that subsurface oxygen has decreased adjacent to most continental shelves. Low-oxygen zones where large ocean species cannot live have increased by close to 5.2 million square kilometers since the 1960s, the team found. Where this expansion intersects with the coastal shelf, oxygen-deprived waters are slipping up and over shelf floors, killing off creatures such as crabs, mussels and scallops. Such bottom-dwellers normally have a lot to eat in such rich ecosystems, but these species are sensitive to oxygen loss. Similarly, the anoxic ocean at the end of the Permian period (around 250 million years ago) was associated with elevated carbon dioxide and massive terrestrial and oceanic extinctions.

Lisa Levin of the Scripps Institution of Oceanography in La Jolla, Calif., says that as oxygen-starved layers move upward, large animals such as marlin, tuna and sailfish will be forced into ever-shallower waters. "That may be good for fishermen, but it also makes it a lot easier for fishermen to fish these species out of the ocean," says Levin, who worked with Stramma on Deep-Sea Research .

Biodiversity will be the big loser as these low-oxygen zones knock out some species and promote others. Among the big winners is the Humboldt squid, which can tolerate low oxygen; it has expanded its range in the northeastern Pacific in the past 10 years, from the Gulf of California all the way to southeastern Alaska. Biologists worry about the hunting pressure the squid will put on other species.

Increases in jellyfish blooms also are likely to be part of the process. Levin encountered such blooms recently in low-oxygen environments off India's coast, where "the jellyfish were as thick as soup," she says. Larval fish are especially susceptible to low-oxygen ocean zones. "Larvae are really a ball of cells with a mouth and a gut. There is only so much they can do. They're not as mobile as fish," she says. Reproducing female crustaceans and fish may be adversely affected, as well.

Levin says that the Pacific's deeper currents keep its waters less oxygenated than those of the Atlantic. "It's what we call 'old water,' since deeper Pacific waters haven't been at the surface in a long time," Levin says. Stramma thinks that some of the Pacific's oxygen problems could also result from El Niño. But climate models predict reductions in dissolved oxygen in all oceans as average global air and sea temperatures rise, and this may be the main driver of what is happening there, she says.

Chan says that lighter warm water creates a cap over the colder depths, making it less likely that deeper waters—where everything from "plankton to whale poop" sucks up oxygen—will rise to mix with the oxygenated surface. Plus, warmer water simply holds less oxygen. According to Chan, most hypoxia-intolerant species engulfed in low-oxygen waters quickly move away. "But for those whose stress response is to hunker down and wait," he adds, "they will die."

*Erratum (2/24/10): This sentence was changed after publication. It originally stated that Francis Chan is a professor at the University of Oregon.

**Erratum (2/24/10): This sentence was changed after publication. It originally stated that NOAA put submersibles off the Oregon coast; it was completely rewritten both to correct the error and for clarity.

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