The return of oxygen-depleted water off the Oregon coast is a sign of a warming climate, which could have ill effect on populations of sea creatures, scientists said Monday.

It's the sixth year the water, known as a dead zone, has formed.

''It does, indeed, appear to be the new normal,'' said Jane Lubchenco, professor of marine biology at Oregon State University. ''The fact that we are seeing six in a row now tells us that something pretty fundamental has changed about conditions off of our coast.''

Unlike the dead zone in the Gulf of Mexico, which is caused by fertilizer washing down the Mississippi River, the Oregon dead zone is triggered by northerly winds, which create an ocean-mixing condition called upwelling.

This brings low-oxygen waters from deep in the ocean close to shore, and spreads nitrogen and other nutrients through the water column, kicking off a population boom of plankton, the tiny plants and animals at the foundation of the ocean food web.

Normally, this is good for salmon, giving them lots of food to eat. But when huge amounts of plankton die, they fall to the bottom of the ocean, where they decompose, depleting the water of oxygen.

Oregon State researchers the found conditions returning during a survey of the 25 miles of continental shelf between Newport and Cape Perpetua last Friday.

Instruments towed back and forth from one mile offshore to 12 miles offshore found oxygen levels as low as one-sixth of normal, said Francis Chan, a research professor of marine ecology.

That is not as bad as last year, when scientists plotted a dead zone that stretched from southern Oregon to the tip of Olympic Peninsula in Washington, a distance of nearly 300 miles.

Video from a remotely operated submersible showed a crab graveyard on the Perpetua Reef south of Newport, and fishermen reported unusually large numbers of rockfish -- apparently able to swim away from the dead zone -- in unexpected areas on its edges, Lubchenco said.

New video from May showed that some rockfish and sea stars had returned, but that less mobile creatures such as sea anemones and sea cucumbers had not.

''The current low oxygen conditions may knock the system back to the starting line, delivering another setback to an already stressed system,'' Lubchenco said. ''This marine ecosystem may take as long to recover as the terrestrial ecosystem did from the eruption of Mount St. Helens.''