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Climate change is the likely cause of unprecedented mass of oxygen-poor water off the Sonoma Coast, a phenomenon that could harm the region's prized Dungeness crab and other marine life.

Scientists at the Bodega Marine Laboratory, who were the first to detect the hypoxic (low-oxygen) waters, aren't calling it a "dead zone" yet, despite the similarity to a lethal condition along the Oregon coast for the past 12 years and forecasts that it will occur worldwide with global warming.

"There's nothing dead," said John Largier, an oceanographer at the UC Davis research facility on Bodega Head. But equipment on a bright yellow buoy anchored about a mile offshore has recorded dissolved oxygen levels low enough to cause "significant distress" for some marine organisms, he said.

Oxygen-poor water is common in deep water of the open ocean, but until this year had never been documented over the continental shelf close to the Sonoma coast, he said.

In Oregon, near-shore hypoxia was discovered in 2002 and has occurred every year since, punctuated by sporadic die-offs of crab and other bottom-dwelling species, including anemones, starfish and sea cucumbers.

Masses of dead crabs washed onto a rocky beach near the town of Yachats on the central Oregon coast in 2004, and a 2006 event in the same area — in which dissolved oxygen approached zero — devastated life on the seafloor.

Low-oxygen episodes in Oregon are "here to stay," said Francis Chan, a marine ecologist at Oregon State University, where scientists are tracking hypoxia with small undersea craft called gliders that operate on their own for weeks at a time.

The range and intensity of Oregon's dead zones varies from year to year, but they are concentrated in a 50-mile stretch of the central Coast from Florence to Newport, Chan said.

A comparable stretch of the Sonoma Coast would run from Bodega Bay to Gualala. Low-oxygen water has been measured on the seabottom about 100 feet below the marine lab buoy and within 10 feet of the surface, Largier said.

He doesn't know how widely it extends, but suspects it reaches all along the Sonoma and Mendocino coasts.

Dissolved oxygen readings from the buoy go as low as 2 milligrams per liter, the definition of hypoxia and one-fourth of a normal oxygen level.

Chan and his colleagues thought it was "only a matter of time" before dead zone conditions were detected in Northern California.

Oregon's hypoxia was first reported in 2002 by crab fishermen, who found up to 75 percent of their catch in crab pots was coming up dead. Checking oceanographic records, Oregon scientists found that oxygen levels in the open ocean had been trending downward for five decades, a likely precursor to the advent of nearshore hypoxia.

Dead zones like Oregon's also have been reported in Washington state, as well as Peru and Chile in South America and Namibia and South Africa.

No one has documented the number of crabs killed during Oregon's hypoxic events, which typically begin in mid- to late summer and last for one to six weeks, Chan said.

Oregon's Dungeness crab catch, worth an average of $30 million a year over the past decade, fluctuates dramatically from year to year.

About six of Oregon's best crab seasons have occurred since the dead zone was discovered in 2002, but it's possible they would have been better with no hypoxia, said Troy Buell of the Oregon Department of Fish and Wildlife.

The crab season is winding down by the time low-oxygen conditions appear, Chan said, adding that crab die-offs are infrequent and localized.

Crab loss so far "isn't big enough to have an imprint on the population," he said.

Still, scientists are concerned about the future impacts of west coast hypoxia, generally regarded as a symptom of climate change.

California's commercial Dungeness crab catch also fluctuates, with a record $95 million haul — the price paid for 31.7 million pounds of crabs in 2011-12 — far exceeding the 10-year average of $40 million.

"Deoxygeneration of the ocean will have potentially serious consequences," said a study last year by four Oregon State scientists titled "Declining Oxygen in the Northeast Pacific."

Chan, Largier and Tessa Hill, another Bodega Marine Lab faculty member, are members of a panel of 22 scientists convened last month at Stanford University with the goal of interpreting research on hypoxia and ocean acidification for public policymakers.

Largier said he is "fairly confident" that west coast hypoxia is a consequence of climate change, but the presumed connection is complex.

It's a two-fold process in which warming ocean surface water absorbs less oxygen and also curtails a natural mixing action that drives dissolved oxygen into deeper waters.

At the same time, the coastal upwelling brings nutrient-rich water toward the surface, where it fosters an abundance of marine life but is also low on oxygen.

The net result is a plummeting level of dissolved oxygen that stresses marine organisms similar to the way a human struggles to breathe enough oxygen at high altitudes.

The vast majority of more than 400 dead zones documented around the world in a 2008 report are of a entirely different nature, created by fertilizer-laden runoff from rivers and burning of fossil fuels.

The largest dead zone in the nation is in the Gulf Of Mexico, where the Mississippi River discharges nutrients from the nation's agricultural heartland, creating a hypoxic zone that swells as large as the state of New Jersey.

Largier discounted that factor on the North Coast, saying the neither the Russian River nor San Francisco Bay contribute to nearshore hypoxia.

But if climate change truly is the trigger, dead zones are likely to occur wherever there is an ocean upwelling, experts say.

In retrospect, Largier said, the arrival of giant Humboldt squid in local waters — where party boat anglers began hauling them in by the droves in 2005, according to news reports — should have been a warning.

The large squid, which inhabited deep, oxygen-poor waters off the South American coast, may have moved north as the oxygen levels declined, Largier said.

An article in Nature magazine in 2010 noted that the arrival of Humboldt squid is considered a sign "of a system in trouble."