The Pacific Ocean off the West Coast is acidifying at twice the rate of the rest of the world’s oceans, according to researchers, with potentially catastrophic effects on shellfisheries.

The findings, published Monday in the journal Nature Geoscience, were the product of a seven-year study led by Emily Osborne, a researcher with the National Oceanic and Atmospheric Administration’s ocean acidification program.

Osborne began collecting samples of sediment from the Santa Barbara Basin off the coast of Southern California in 2013. Because the basin has little oxygen and a dearth of plants and animals, the ocean floor has remained relatively undisturbed. The sediment cores she pulled from the basin contained the shells of foraminifera, a single-celled plankton, dating back to 1895.

After weighing and photographing each specimen, Osborne was able to use the thickness of the shells to establish a 100-year record of pH levels in the waters of the California Current, which runs along the West Coast.

She found that, since 1895, shell thickness had decreased by 20 percent due to increased ocean acidification.

“I started looking at the lowest, oldest parts of core and worked my way to top,” she said. “It was crazy to see visually how the shell changed throughout the core.”

5. The graph shows the decline from 1900 to 2000 in the concentration of carbonate ions in the waters off California. Carbonate ions are the building blocks used by foraminifera and other shelled marine species to build their shells. As carbon dioxide concentrations have risen in these Pacific Ocean waters due to absorbing excess carbon dioxide emissions and upwelling of carbon dioxide-rich waters from the deep, the carbonate ion concentration has declined and acidity has increased, making it more difficult for marine species to build shells. The inset photos show (a) a typical foraminifera shell; (b) cross section of a shell from 1900 showing thickness of the shell; and (c) a shell from 2000 showing a thinner shell wall. Courtesy/Emily Osborne/NOAA

The ocean absorbs carbon dioxide from the atmosphere, which reduces pH levels and concentrations of calcium carbonate, a mineral used by shellfish to calcify their shells. As more carbon is released into the atmosphere, concentrations of calcium carbonate decrease and the shells of organisms like foraminifera get thinner, a trend Osborne saw clearly in the sediment cores she examined.

“The shell thickness record instantly showed a long-term declining trend,” she said. “It was really obvious across entire record.”

That trend could spell trouble for shellfish farmers on the West Coast, who depend on calcium carbonate-rich waters for their oysters, clams and mussels to grow. In 2007, the Whiskey Creek Shellfish Hatchery in Netarts Bay on the Oregon coast saw a catastrophic larval failure because of acidic conditions.

Looking at the chemical composition of the shells, Osborne found that the type of carbon in the shells was indicative of carbon dioxide absorbed from the atmosphere, the same greenhouse gas that is driving changes to the climate across Oregon and around the globe.

A 2015 survey of shellfish farmers, conducted by Oregon State University, found that 75% of those asked reported some level of concern over ocean acidification. Of those respondents who said they have been affected by ocean acidification, 97% reported financial damage, while 68% cited emotional stress.

The record also showed evidence of natural variability, she said. The pH levels rose and fell with natural phenomenon like El Niño and the Pacific Decadal Oscillation, a longer-term pattern of warming and cooling ocean waters. Those natural changes will offset effects of ocean acidification in some years, but amplify it in others.

“We now know that the Pacific Decadal Oscillation can intensify or alleviate (ocean acidification) in this region,” Osborne said. “For those large swings to also have a human imprint, it’s going to make the extremes more extreme.”

-- Kale Williams

kwilliams@oregonian.com

503-294-4048

@sfkale

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