Ocean dead zone linked to low oxygen.

Byline: LARRY BACON The Register-Guard

Scientists have found the answer to a puzzling summer phenomenon that created an ocean "dead zone" 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 fishermen 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 just north of Florence to north of Newport resulted from a convergence of unusual circumstances:

The upwelling - an annual occurrence caused by the spinning of the Earth and by surface water being pushed offshore by prevailing north summer wind - was colder than usual this year and also contained more nutrients and less oxygen than normal.

The nutrient-rich water from the deep encouraged a bloom of phytoplankton, the minute 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.

The increased stratification and the fact that winds were lighter than normal for much of the summer made it more difficult for oxygenated water in the top layer to mix with the low-oxygen water in the bottom layer.

The result was a condition at the bottom of the ocean that scientists call "hypoxic," in which the oxygen concentration in the water falls to less than two milligrams per liter.

The concentration normally ranges between 3.5 milligrams and 8 milligrams per liter, depending on temperature and depth, Nielson said. In water below 2 milligrams per liter, many organisms will die, she said, noting that in the central coast dead zone it got as low as about half a milligram per liter.

Scientists documented the hypoxic condition in a layer of water 10 meters to 20 meters from the bottom (33 feet and 66 feet) in an area just beyond the surf line to water as deep as 280 feet.

Tests and water sampling were done as far as eight miles offshore, Nielson said. The hypoxic zone likely extended beyond the testing area, she said, but it's unknown how far.

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.

Some of the first scientists on the scene were state Department of Fish and Wildlife researchers using a remotely operated undersea vehicle, known as a ROV, which transmits live video to a boat on the surface.

The work was part of a multi-year study to document possible changes in rockfish populations.

One of the researchers, Dave Fox, said the team expected to see much the same assortment of fish and other sea creatures they had seen in a similar excursion the previous summer.

But they were astounded to find that the rockfish had vanished, the bottom was littered with dead crabs and worms, and the water was cloudy with brown phytoplankton, he said. The only fish they saw were small sculpins, and they were dead.

"Most fish are mobile enough that when conditions get bad they move," Fox said.

"But sculpins - they're behavior is that when they get enough stress they hide rather than swim away."

Reports from sport divers on some reefs on the edge of the dead zone indicated an amazing number of fish, so concentrated that they appeared to be "stacked like cord wood," Nielson said. Those fish were probably refugees from the dead zone, she said.

Fox and Nielson agreed that the loss of marine life in the dead zone was not catastrophic because many creatures left when the oxygen level dropped. Fox said that subsequent excursions with the ROV indicated that fish began returning even before the oxygen levels returned to normal.

Future research will try to find out why the hypoxic event happened when it did and where it did, the scientists said.

Nielson said researchers believe that what happened in Oregon was a natural occurrence rather than the result of pollution. Probably the best-known dead zone is in the Gulf of Mexico, she said, caused by nutrients coming down the Mississippi River that trigger the growth of phytoplankton that die and rob the water of oxygen.

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 in recent years that seems to be making Pacific Northwest waters more productive for marine life.

She said much of the data being collected by scientists may not answer the question of why the dead zone occurred when and where it did, but it will provide baseline information that will help scientists spot significant changes in the future.

She said it's possible other dead zones may have occurred in the area in the past and not been noticed.

Similar zones have been recorded in other parts of the world, including the East Coast of the United States, she said.

Fox said if it happens off the Oregon Coast again, it most likely will happen as it did this summer - a small-scale event in a near-shore area.

"If we see things like this starting to happen on a big scale," he said, "then we'll be concerned."

CAPTION(S):

Oregon Department of Fish and Wildlife A yelloweye rockfish (left) and a quillback rockfish swim off of Cape Perpetua in the summer of 2001.