They still predict oxygen levels will move toward normal in the next few weeks as fall weather sets in. Winds from the south that become more prominent as winter approaches should help clear out the low-oxygen water that settled on the bottom like a carpet.

"There could be such a big pool that built up over the summer, it could take some time to flush it out," Chan said.

Oxygen levels sank in August to near zero, the lowest point recorded in about 50 years of regular measurements off the Oregon coast.

Researchers scanning the bottom with a remote-controlled submarine carrying a video camera were startled by what they found. Rocky stretches off Cape Perpetua that usually teem with rockfish and other marine life had turned lifeless, littered by dead crabs and decaying starfish.

Later they found the spread of bacteria that thrive in low-oxygen conditions and exhale toxic gases.

But their glimpse into the depths offered only a narrow window into what was happening offshore. They still do not know how big a toll the low-oxygen waters took on sea life.

Fish might have avoided problems by moving to more hospitable conditions. There are reports of Oregon bays crawling with crabs that may have fled the dead zone offshore.

"We're all in new territory here, so I don't know what the biological effects were," said Hal Weeks, a marine ecologist with the Oregon Department of Fish and Wildlife. "It's a very interesting jigsaw puzzle."

The return of life to the sea floor may resemble the regrowth of a charred forest after a wildfire, Chan said. Researchers do not know whether it will take longer because of the duration and severity of the dead zone episode or whether fish and other sea life will retake the ground quickly.

Low-oxygen water, an essential ingredient in Oregon's rich marine environment, wells up from the deep each summer, bringing essential nutrients with it.

That has always left some areas short of oxygen. The difference this year is that the oxygen-starved waters have pushed closer to shore, scientists say, blanketing the bottom for a longer period.

At the same time, tiny marine organisms fed by the nutrients die. Their decay draws even more oxygen out of the water.

This is the fifth summer in a row that a layer of low-oxygen water has blanketed the ocean floor along the Oregon coast, and it rapidly turned into the most severe episode so far. The layer this year is thicker, lower in oxygen and at least four times larger than in previous years.

"This is just lasting much longer than we anticipated," Chan said.

Low-oxygen waters also extended into waters off Washington, although less monitoring takes place there.

Researchers do not know precisely what is behind the long-lasting conditions but suspect it's related to climate shifts driven by global warming. Winds usually help cycle oxygen into the depths but they have been unreliable this year, OSU scientists said.

Michael Milstein: 503-294-7689; michaelmilstein@news.oregonian.com