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Dead Sealife On Beach Signals Changes Offshore

It's Saturday and you're strolling the Oregon beach, glad the week is over. The summer sun has finally dislodged the stubborn winter chill of the North Pacific coast. The sky is blue, the surf is up, and the air smells of . . . dead crabs and fish.

Weekend visitors, fishermen, and even marine scientists were surprised in 2002 when they discovered beaches along the central coast of Oregon strewn with the dead bodies of crabs, rockfish, and other marine organisms. Scientists at Oregon State University (OSU) tracked the cause of death to low levels of oxygen in water much closer to shore than usual.

The reappearance of this so-called "dead zone” in the summer of 2004 suggests that the 2002 anomaly may not be so anomalous after all. Instead, it may be a harbinger of fundamental changes in the Northeast Pacific Ocean, with large-scale effects on winds, currents, and distribution of low-oxygen water. "The climate question is the 800-pound gorilla in the room,” says Francis Chan, a biogeochemist in the interdisciplinary OSU team monitoring salinity, temperature, oxygen levels, and other water variables from their 54-foot research vessel off Newport, Ore.

But whether climate change is responsible and what long-term effects we can expect from changes in wind and current patterns remain open questions.

Barbara Hickey, an oceanography professor at the University of Washington in Seattle doesn't consider the 2002 mass die-off so anomalous in the first place. She pulls a 1989 Coastal Oceanography volume off the shelf and opens it to a figure showing nearshore oxygen levels in long-term data sets from the 1970s and 1980s along the Washington coast only slightly higher than the lethal levels documented by the OSU researchers.

Respiration becomes a problem for inshore organisms when the dissolved oxygen content drops below a threshold level of 1.43 milliliters of oxygen per liter of water, about a fourth of levels typical for turbulent water. Normally, the shallowness of nearshore waters permits enough gas exchange to replenish the dissolved oxygen, but when tongues of oxygen-depleted water move shoreward from the outer shelf, they can suffocate marine organisms not adapted to such low levels, dumping their dead bodies on the beach.

Oxygen levels are diminished in oceans around the world when surface organisms die and drizzle down through the water column. Several hundred meters down, armies of bacteria feed on their dead bodies, consuming the oxygen in the water around them as they grow and divide.

Nutrient levels in the water determine how fast the surface food-chains grow and thus how heavy the drizzle is. High nutrient zones, either natural or caused by humans, favor robust surface food-chains, and greater oxygen depletion below.

Off Washington and Oregon, strong summer currents far offshore create a productive natural upwelling zone, delivering high levels of nutrients along the coast. This results in strongly oxygen-depleted water near the bottom about 25 kilometers offshore. Chan and his colleagues suggest that large-scale changes in wind patterns are pulling up more nutrients into the food chain and drawing the oxygen-depleted water closer inshore than usual, precipitating the die-offs.

Oxygen depletion has become a serious human-caused problem in marine areas around the world. The dead zone in the Gulf of Mexico off Louisiana, where fertilizer from Midwestern agriculture flowing out through the Mississippi River has spawned a dead zone about 7,500 square miles in extent, is among the largest. By contrast, the Pacific Northwest's natural upwelling zone is so productive that Columbia River water shows lower nutrient loads than the ocean, despite heavy upstream fertilizer use.

If climate change is indeed responsible for shifts in wind, then die-offs as in Oregon may spread up and down the coast since low-oxygen water is already regularly present farther out on the shelf, at least until the organisms at risk have succumbed or reestablished in areas unaffected by low-oxygen water.

Dan Froehlich is a graduate student in zoology at the University of Washington.


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