Dead zone off coast of Oregon explained

THE ASSOCIATED PRESS
The Olympian

10/28/02

EUGENE, Ore. -- Scientists have an answer to a puzzling "dead zone" found this summer in the ocean off the central Oregon Coast.
Karina Nielson, an Oregon State University zoology researcher, said the area became so low in oxygen that fish and crab couldn't survive.

"The 'why' part, we're still working on," she said.

Scientists had found ocean dead zones before but had never documented one on the West Coast, Nielson said. An investigation was begun in July after crab fishers began pulling up pots containing dead crab. Then crab and fish began washing up on the beaches in unusual numbers. Video from a remotely operated undersea vehicle showed only dead fish remaining in the area.

Nielson, who studies the annual summer upwelling of deep, cold ocean water off much of the West Coast, reported her findings recently at a meeting of the Oregon Shores Conservation Coalition in Newport.

She said the formation of the dead zone from north of Florence to north of Newport resulted from a convergence of unusual circumstances: The upwelling was colder than usual this year and also contained more nutrients and less oxygen than normal.

The upwelling is an annual occurrence caused by the spinning of the Earth and by surface water being pushed offshore by prevailing north summer wind.

The nutrient-rich water from the deep encouraged a bloom of phytoplankton, the algae that are the foundation of the ocean food chain.

When the phytoplankton died and sank to the ocean floor, the decomposition process took still more oxygen from the already oxygen-deficient cold water.

Ocean water in the dead zone became more stratified than usual because of greater-than-normal temperature differences between the top layer and the bottom layer -- the result of summer weather that was warmer than usual, and the extra-cold upwelling. That made the water at the bottom even more starved of oxygen.

By the end of August the summer upwelling began to subside and conditions were returning to normal. Fish that had left the area were coming back, Nielson said, and winter storms that disturbed the sea surface helped mix oxygen back into the water.

The Oregon dead zone could be a symptom of a yet-undiscovered oceanographic phenomenon, or a result of global warming, Nielson said, but it's most likely related to changes in natural ocean conditions that seem to be making Northwest waters more productive for marine life.