Scientists have struggled to understand a huge "dead zone" that has formed off the coast of Oregon every summer for the last five years, killing marine life over a wide area.
Unlike the dead zones in areas like the northern Gulf of Mexico, which are caused primarily by agricultural runoff, this appears to be a somewhat natural phenomenon. However, global climate change resulting from human activities may be partly responsible.
For the first time, the Oregon dead zone appears to be spreading to the north, reaching as far as central Washington, according to researchers at Oregon State University and other institutions.
The scale of the problem is hard to measure, but the stories from fishermen suggest some grim results. Crab fishermen, for example, have reported pulling up their crab pots along the Washington coast and finding them filled with dead crabs.
"You can't sell a dead crab," said Francis Chan, a marine ecologist at Oregon State.
Along much of the coastline, hundreds of dead fish have washed up on the beaches. And scientists fear thousands more have drifted off or been eaten by other animals, leaving a very unclear picture.
It's a serious problem for fishermen, who are already plagued by declining stocks in some species of fish, particularly salmon, and rising fuel costs. They didn't really need a dead zone, but they've got it now.
"Many marine species live in fairly specialized ecological niches, and any time you change the fundamental physics, chemistry and nature of the system, it's a serious concern," said Jack Barth, an oceanographer at Oregon State and at the Partnership for Interdisciplinary Studies of Coastal Oceans, a marine research consortium of West Coast universities.
The dead zone surprised scientists when it was detected for the first time in 2002, and it has surprised them further by returning every summer since. This one may rival the 2002 event in its scale and impact, but even that is uncertain because no one knows how long it will last.
"It could all disappear if the winds blew from the south tomorrow," Chan said.
The dead zone is apparently the result of changing wind patterns and some unusual topography on the ocean floor.
The continental shelf is very broad off the Oregon coast, Chan said, and there is a large plateau under the surface that has a dynamic impact on ocean currents.
Farther out from the plateau is very deep water, rich in nutrients but low in oxygen because it has remained relatively static until recent years. That nutritious water feeds algae and phytoplankton, which in turn feed everything from small fish to whales.
But since 2002, summer winds out of the north have altered ocean circulation patterns, causing an "up-welling" of that oxygen-poor water, bringing more nutrients to the shallow coastal areas that are normally rich with marine life. That sounds good, but in this case, it isn't.
"You can have too much of a good thing," Chan said.
The up-welling forces the oxygen level down to as low as 1 milliliter per liter -- substantially below the level that seawater becomes hypoxic, or unable to support life. Fish, crabs and other marine organisms die, as well as the algae and phytoplankton, which sink to the bottom where micro-organisms devour them, further depleting the oxygen.
The result is a dead zone.