Large dead zone leaves Oregon coast

A large and persistent dead zone of low-oxygen water off Oregon's central coast that killed crab and other sea life has finally faded away thanks to shifting ocean winds, according to university researchers.

The low-oxygen conditions were of record intensity and encompassed a roughly 70-mile-long swath of water larger than Rhode Island just off the coast. These conditions lasted for about four months, far longer than any of the previous dead zones tracked off Oregon during the past five years, according to Oregon State University researchers who have monitored oxygen levels through the summer and fall.

In August, scientists documented record-low oxygen readings, which appeared to wipe out most bottom-dwelling sea life in one reef that the scientists explored with the aid of a remote-operated video camera. In a second August visit to the reef, the video camera relayed images of a white mat of bacteria that thrive in the absence of oxygen, a condition never previously documented off the Oregon coast.

"It's a big relief that it's finally over," said Jane Lubchenco, an Oregon State University professor of marine biology. "We are not going to know for some time what the real impact was. There were some pretty powerful images of dead stuff on the bottom."

This dead zone appears to have been caused by consistent winds from the north, which spread low-oxygen water through the near-shore zone. Southerly winds, which typically help flush the low-oxygen water, were absent for most of the summer and finally appeared in recent weeks to push the low-oxygen water out to sea, Lubchenco said.

A similar swath of low-oxygen water occurred this summer in the Pacific Ocean off Washington's Olympic Peninsula. During one two-week period in July, oxygen concentrations were 100 times lower than the cutoff considered dangerous for sea life, said Mary Sue Brancato, resource-protection coordinator for the Olympic Coast National Marine Sanctuary. The measurements corresponded with sightings of dead and dying fish on the beach and reports from fishermen who pulled up dead crab.

This was the first year researchers were able to collect continuous data off the Washington coast, thanks to eight specialized buoys provided by the National Oceanic and Atmospheric Administration. The sanctuary hopes to use the buoys again next summer if it receives the $75,000 it will take to repair the instruments and deploy them, Brancato said.

Without more data, it's impossible to say whether global warming is to blame for the changes in winds and currents that triggered the low-oxygen levels off Washington's coast, she said.

Seattle Times staff reporter Sandi Doughton contributed to this report.

Hal Bernton: 206-464-2581 or hbernton@seattletimes.com