Growing Ocean Acidity May Erode Coastal Ecosystems

John Roach
for National Geographic News
May 22, 2008
Ocean waters along North America's west coast are becoming more acidic than expected in response to atmospheric carbon emissions, which will likely cause significant changes to economically vital marine ecosystems, a new study says.

At one spot in northern California, waters acidic enough to corrode seashells now rake the shore, researchers point out.

"The models suggested they wouldn't be corrosive at the surface until sometime during the second half of this century," Richard Feely, a chemical oceanographer with the Pacific Marine Environmental Laboratory in Seattle, Washington, said via email.

Scientists have long known that the oceans serve as a giant carbon sink, moderating the effects of global warming by absorbing about a third to a half of human-caused carbon dioxide (CO2) emissions.

But the added carbon dioxide is lowering the oceans' pH, changing their chemistry and biology, explained Feely, whose lab is run by the National Oceanic and Atmospheric Administration.

Acidic waters inhibit marine organisms from producing the calcium carbonate that makes up their exoskeletons and shells.

"Scientists have also seen a reduced ability of marine algae and free-floating plants and animals to produce protective carbonate shells," Feely said.

For example researchers have seen a decline in swimming mollusks called pteropods that are eaten by creatures ranging from shrimplike krill to whales. The mollusks are particularly vital to juvenile salmon and other commercial fish.

"The impact of ocean acidification on fisheries and coral reef ecosystems could reverberate through the U.S. and global economy," Feely said.

Corrosive Upwelling

Feely and colleagues measured the acidity of waters along the west coast of North America from central Canada to northern Mexico.

They found deep-ocean waters corrosive enough to eat away at seashells and coral reefs are upwelling each spring and summer onto the continental shelf.

"The deep waters are always more acidic than surface waters," Feely explained.

In addition to carbon dioxide absorbed from the atmosphere, deep waters contain carbon dioxide that is a by-product of the breathing of marine organisms and the decay of organic matter.

"What's more, there is a 50-year lag between the time when ocean surface waters [are] exposed to the atmosphere, its sinking, and ultimate upwelling on the continental shelf," noted co-author Burke Hales from Oregon State University.

"This means that even if we were to stop instantaneously the current rate of rise of CO2 in the atmosphere ... the corrosivity of these upwelled waters would increase for the next 50 years," he said in a telephone briefing with reporters.

These corrosive waters already reach the surface at one point in northern California. Elsewhere, they reach depths between 130 and 390 feet (40 and 120 meters).

Feely, Hales, and colleagues report their findings in tomorrow's issue of the journal Science.

"Amazing and Frightening"

Ken Caldeira is a geoscientist who studies ocean acidification at the Carnegie Institute of Washington's office in Stanford, California. He said the new finding is dramatic.

"The idea that [calcium carbonate] shells might already be dissolving in coastal waters is pretty amazing, and it's frightening," he said.

Global models, he noted, had predicted corrosion in coastal waters at the Poles around the middle of this century and the California coast sometime next century.

However, the impact of these acidic waters remains uncertain, Caldeira added.

Experiments have shown that ocean acidification makes coral reefs vulnerable to erosion, potentially impacting thousands of species that depend on reef habitat.

Shellfish such as crabs, mussels, and oysters also weaken with increasing carbon dioxide levels, and experiments suggest squid and sea urchins are susceptible to acidic oceans.

But scientists have not conducted any experiments on how entire coastal ecosystems respond to increasing ocean acidification, Caldeira said. Most research has been in the lab and focused on single organisms.

In addition, the chemistry of coastal waters is generally more variable than the open ocean, suggesting coastal environments may be more resilient to the changing conditions.

"It could be either something serious that could completely restructure the coastal ecosystem, or it could be something that within a few years these things basically adapt to," he said.

He added, however, that no experiments have shown an organism becoming more efficient at making shells in response to ocean acidification.

"It looks like the biological capacity for adaptation there in calcification is pretty low."

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