Oceans Becoming Acidic Ten Times Faster Than Thought

Helen Scales
for National Geographic News
November 24, 2008
Increasing levels of carbon dioxide in the atmosphere may make Earth's oceans more acidic faster than previously thought—unbalancing ecosystems in the process, a new study says.

Since 2000, scientists have measured the acidity of seawater around Tatoosh Island off the coast of Washington state. The acidity increased ten times quicker than climate models predicted.

The research also revealed the corrosive effect of acidic oceans could trigger a dramatic shift in coastal species and jeopardize shellfish stocks.

"The increase in acidity we saw during our study was about the same magnitude as we expect over the course of the next century," said study co-author Timothy Wootton, a marine biologist from the University of Chicago.

"This raises a warning flag that the oceans may be changing faster than people think," he said.

Increased carbon dioxide emissions from human activities have led to a 30 percent rise in ocean acidity in the past 200 years.

(Related: "Acid Oceans Threatening Marine Food Chain, Experts Warn" [February 17, 2007].)

When atmospheric carbon dioxide dissolves in the oceans it forms carbonic acid, which in turn has a negative impact on marine life.

Laboratory studies show that as seawater acidity increases, the calcium carbonate shells and skeletons of many marine species, such as hard corals, sea urchins, and stony seaweeds, begin to corrode.

A Shifting Balance

Wootton and colleagues built models of an ecosystem based on field data of how species interact along Tatoosh Island's rocky shores.

Surprisingly, in a scenario of increasing acidity, not all species with calcium carbonate shells faired badly.

Instead, a shift took place: Larger mussels and barnacles suffered, leaving smaller barnacles and some calcium-based seaweeds better off.

In nature, "species are competing for space, they are eating each other, it's an incredibly dynamic system," said James Forester, a Harvard University ecologist who co-authored the study in this week's journal Proceedings of the National Academy of Sciences.

"When you change the playing field—in this case by altering acidity—you can get unexpected responses," he said.

"Mussels usually dominate the ecosystem because they are good at overgrowing and crushing out other species that grow on the rocks," said co-author Wootton.

"But when the mussels decline, it means other species—no matter whether or not they have a shell—can do better," he said.

Nancy Knowlton is a marine biology professor at the Scripps Institution of Oceanography in La Jolla, California, who was not involved in the study.

She pointed out the importance of adopting this "enemy of my enemy is my friend" approach when trying to understand how various shelled species respond to ocean acidification.

An acidity-driven shift in coastal ecosystems could spell disaster for shellfish industries that rely on mussels and other similar species, Wootton warned.

A Wider Pattern?

While the field surveys did show an overall decline in mussels, the predictive models were needed to hunt for longer-term changes.

"There is inertia in the system because many of these species live for a long time," said co-author Wooton.

"The little changes we see in the dynamics of the ecosystem may magnify over time."

These are the first data on ocean acidity from temperate—rather than tropical—waters. No one knows whether similar rapid changes are taking place elsewhere.

"The rules might be quite different on Tatoosh Island," Wootton suggested.

"There could be mechanisms going on in the waters around our island that are unique.

"We really need to get more data from other sites away from the equator to see what patterns are there," he added.

Marine biologist Knowlton said, "This is typical of so many climate studies—almost without exception things are turning out to be worse than we originally thought."

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