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Researchers for the first time have found definitive evidence that changing ocean chemistry from increased carbon-dioxide emissions are at least partially responsible for massive oyster die-offs in the Pacific Northwest.

The research published Wednesday by scientists from Seattle and Oregon State University is the first anywhere to show that increasingly corrosive seas already are killing marine organisms in North America.

“This is the smoking gun for oyster larvae,” said Richard Feely, an oceanographer and leading marine-chemistry researcher with the National Oceanic and Atmospheric Administration in Seattle and one of the paper’s authors. “This is the clearest experimental evidence yet that lower pH is making oysters die.”

Said Alan Barton, another of the paper’s authors, “It’s now an incontrovertible fact that ocean chemistry is affecting our larvae.”

Since 2005, wild oysters along the Washington coast and oysters at a commercial shellfish hatchery in Oregon have been dying by the billions. Leading scientists long have suspected that one of the causes is the increasing corrosiveness of ocean waters that frequently rise up from the deep during high winds to lap against the shore.

Scientists long have predicted that as carbon dioxide from greenhouse gas emissions get taken up by the seas, the chemistry of ocean waters would slide toward the acidic side of the pH scale. Feely and other researchers in 2007 and 2008 were among the first to show that ocean pH along the West Coast already had dropped more than had been expected for decades.

But it was always possible that the waters hitting the oyster-rich shores of Willapa Bay or being drawn into seaside hatcheries was either too warm or carrying bacteria or other pollutants that were killing oysters before they had a chance to form their shells.

But in their new paper published in the journal Limnology and Oceanography, researchers were able to control for temperature and bacteria and other factors.

When ocean water lower in pH got sucked directly into the Whiskey Creek Shellfish Hatchery in Netarts Bay, Ore., the oysters died. When the pH was higher, oysters survived just fine.

“This is not just some lab experiment,” Barton said. “This is real ocean water ­– from today, not from some predicted future — impacting shell formation. It’s a pretty important finding.”