National Geographic News  [Printer-friendly version]
November 24, 2008

OCEANS TEN TIMES MORE ACIDIC THAN THOUGHT

[Rachel's introduction: "The increase in [ocean] 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.]

By Helen Scales

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.

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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