This past summer, fishermen off the Oregon coast hauled up their crab pots only to find them filled with dead crabs. Scientists say that extremely low levels of oxygen in the area’s shallow ocean waters were to blame for the crustacean carnage, and are now trying to understand exactly what drives the phenomenon.

The so-called dead zone, which also killed fish, sea stars and other marine organisms, began around the middle of July and covered an area the size of Rhode Island. It was marked by the lowest dissolved oxygen levels ever observed in shallow waters on the West Coast, according to a team of researchers at Oregon State University (OSU) in Corvallis.

Oxygen levels below about 1.4 milliliters of oxygen per liter of water are harmful to most marine life. Normally, summertime oxygen levels along the Oregon coast can reach lows ranging from 1.2 to 1.5 milliliters per liter. But this summer, the researchers measured oxygen levels between 0.05 and 0.5 milliliters per liter — about three to 30 times lower than normal — making the dead zone especially potent and “the worst we’ve seen on record,” says Francis Chan, a marine ecologist at OSU.

Chan and his colleagues have been collecting data about Oregon’s coastal waters — including measurements of oxygen, salinity, temperature, chlorophyll and ocean currents — since the summer of 2002, the first year they ever observed a dead zone there. Since then, the phenomenon has occurred every summer. Based on pre-2002 water-quality data from the National Oceanic and Atmospheric Administration and anecdotal reports from longtime fishermen, the team says that isolated dead zones also may have occurred in the area between the 1950s and 1970s, but not with the regularity or severity of the last few years’ events.

“We haven’t seen a disruption of the near-shore ecosystem like this before,” says Jack Barth, an oceanographer at OSU. “We honestly don’t know where that will lead.”

Dead zones occur in coastal estuaries and lakes around the world and generally result when a proliferation of plankton die, sink and are decomposed by bacteria, which consume oxygen. In places like the Gulf Coast or the Chesapeake Bay, nutrients from agricultural runoff fuel the dead-zone-inducing plankton blooms.

Oregon’s dead zone, however, is triggered naturally, when winds drive coastal surface water offshore and deep-ocean water moves up to replace it. The deep water is rich in nutrients that feed plankton, which eventually die, and is already low in oxygen — a “one-two punch” in terms of promoting a dead zone, Chan says.

The five-year string of dead zones along Oregon’s coast is largely due to winds that have been much more persistent than in previous years, and therefore have sustained the upwelling of deep-ocean water for a longer period. “We’re getting a glimpse into what happens when coastal winds change,” Chan says.

Some scientists predict that stronger, more persistent coastal winds could be one of the effects of climate change — a result of an increased temperature gradient between the air above increasingly warmer land masses and the air above the cooler oceans. Barth says, however, that it is too early to tell if the dead zones are related to changes in the climate or simply some other cycle not yet identified. “I’m not prepared to say we have the data to pin it” on climate change, he says.

Parker MacCready, a physical oceanographer at the University of Washington in Seattle, says it’s possible that dead zones have always happened to some extent in the area, but have just been worse in the last few years. “It’s not yet clear to me that we know this is out of the ordinary,” he says, but adds that more research into the mechanisms that promote low oxygen levels in the region will help shed light on the phenomenon.

Indeed, Barth says, the OSU team is aiming to build a more sophisticated monitoring system. They recently acquired “gliders,” battery-powered underwater vehicles that are “literally thrown overboard” and left to move through the water by themselves, following a programmed route. The gliders will collect weekly data on a cross section of the ocean that will help the researchers to better monitor and better understand the dead zones.

Jennifer Yauck

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