A vast swath of oxygen-depleted water has reappeared off the central Oregon coast for an unprecedented sixth straight year, worrying ocean scientists that the recurring pattern has become the new norm. The return of this "dead zone" comes just as there were initial signs of life reemerging on the ocean floor after being devastated by record low oxygen conditions last year. Salmon and other fast swimming fish are sometimes able to escape the effects, but crabs, sea stars, and other slow movers suffocate when oxygen levels plummet.
As in previous years, the low oxygen conditions arise when northerly winds push the top layer of ocean water out to sea. That pulls up nutrient-rich, but oxygen-poor water from the deep ocean to take its place. The nutrients trigger a plankton bloom and subsequent die off, and the decomposing plankton are eaten by bacteria that draw even more oxygen from the water, leaving little for organisms left behind.
Oceanographers at Oregon State University (OSU) in Corvallis began monitoring ocean conditions this year beginning in April, deploying robotic underwater gliders and taking survey cruises. By the end of June, oxygen levels on reefs devastated by last year's dead zone had again dropped to 0.5 milliliters per liter--far below the 1.4 ml/L that is considered hypoxic for most marine life. Wind changes in mid July brought a brief respite. But the northerly winds returned last week bringing hypoxic waters back closer to shore. "We are definitely experiencing hypoxia once again," says Francis Chan, an OSU marine ecologist.
Over the last 6 years, it's become clear that changes in wind patterns and water currents are the main factors driving hypoxic events in Oregon, Chan says. These changes appear consistent with predictions from computer models of climate change from global warming. For now the evidence isn't definitive. But one concern, Chan adds, is that increasing temperature variations over land and sea surfaces brought on by climate change could whip up stronger winds that might trigger broader dead zones off the Oregon coast and beyond. In 2002, Chan says researchers thought Oregon's dead zone was an anomaly brought on by a perfect storm of changes in wind and water circulation patterns. However, he adds, "now we're seeing that the system is very sensitive, and that a little more mild [winds] might put us in this state year after year." If that occurs, it could turn a rich and productive ocean ecosystem into an underwater desert.