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 the University of Oregon in Eugene, 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 National Oceanic and Atmospheric Administration (NOAA) put submersible vehicles off Oregon's coast during a hypoxic event that went anoxic (oxygenless) in 2006, he says, monitoring conditions and recording the numerous fish carcasses strewn across the bottom of the continental shelf.
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.
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http://www.scientificamerican.com/article.cfm?id=low-oxygen-ocean-coastal
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