Dead zones off Oregon's coast that have threatened sea life for the past six summers are unprecedented in the historical record, say scientists...
Dead zones off Oregon’s coast that have threatened sea life for the past six summers are unprecedented in the historical record, say scientists at Oregon State University (OSU).
The researchers combed through more than 50 years of ocean data to see if the extremely low oxygen levels measured recently also occurred in the past.
“The answer was no,” said OSU marine ecologist Francis Chan, lead author of a report published in today’s issue of the journal Science. “It’s not normal to have oxygen levels this low and this close to shore.”
In 2006, the dead zone spread over 1,000 square miles. Oxygen levels plummeted to zero in some places. Using a remotely-operated submersible, Chan and his colleagues took pictures and video that showed a wasteland littered with dead crabs.
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When he revisited the areas the following year, some species had returned, but the diversity of invertebrates remained low.
“We used to see seven species of starfish on the reefs,” Chan said. “Now we saw one or two.”
While it’s impossible to link any one event to global climate change, the researchers say the dead zones are caused by wind shifts of the type expected as the planet’s temperature rises.
“In this part of the marine environment, we may have crossed a tipping point,” said OSU marine biologist Jane Lubchenco.
The dead zones form in the summer when coastal winds blow from north to south. The winds cause upwelling, which pulls nutrient-rich but oxygen-poor water from deep below the surface. Microscopic plants and animals feast on the influx of food, but when they die, their decomposition depletes the water of what little oxygen it has.
In 2006, northerly winds were twice as strong as usual, Chan said. “That was the smoking gun for why we were seeing those scenes of dead marine life,” he said.
It also shows how sensitive the marine ecosystem is to climate change.
“It’s remarkable to me that over 150 feet below the surface, what we’re seeing on the seafloor is really connected to what’s happening in the atmosphere,” Chan said.
The historical data for the Oregon coast included tens of thousands of separate measurements taken by researchers who lowered bottles to different depths then analyzed the oxygen contents in the lab. Oceanographers today use electronic instruments that can make eight measurements a second — but the old-fashioned method remains the gold standard, Chan said.
“We have high confidence in the historic data,” he said.
The 50-year period included several El Niño and La Niña cycles, as well as shifts in a decadelong climate pattern that influences Northwest weather and sea temperatures. None seemed to affect the formation of low-oxygen zones.
Off the coast of Washington, the old numbers tell a different story, said University of Washington oceanographer Barbara Hickey. Zones of low-oxygen water have long been common in the summers.
“Definitely in the ’60s and ’70s we saw numerous examples of extremely low oxygen,” she said. For the past three years, Hickey and her colleagues have been collecting more detailed data from a research buoy off Kalaloch, on Washington’s coast. But it’s too early to say whether the dead zones are intensifying or spreading, Hickey said.
Hickey also cautioned against drawing conclusions about the role of global warming in Oregon’s dead zones. “It’s dangerous to project out too soon.”
Sandi Doughton: 206-464-2491 or firstname.lastname@example.org