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Acid Oceans Threatening Marine Food Chain, Experts Warn

Scott Norris in San Francisco, California
for National Geographic News
February 17, 2007
 
The world's oceans are turning acidic due to the buildup of carbon
dioxide (CO2) in the atmosphere, and scientists say the effects on
marine life will be catastrophic.

In the next 50 to 100 years corrosive seawater will dissolve the shells of tiny marine snails and reduce coral reefs to rubble, the researchers say (coral photos, facts, more).

Four leading marine experts delivered this grim prognosis yesterday at the annual meeting of the American Association for the Advancement of Science in San Francisco, California.

The scientists stressed that increased ocean acidity is one of the gravest dangers posed by the buildup of atmospheric CO2.

"Ocean chemistry is changing to a state that has not occurred for hundreds of thousands of years," said Richard Feely of Seattle's Pacific Marine Environmental Laboratory.

"Shell-building by marine organisms will slow down or stop. Reef-building will decrease or reverse."

Already, Feely said, ocean acidity has increased about 30 percent since industrialization began spurring harmful carbon emissions centuries ago. Unless emissions are reduced from current levels, an increase of 150 percent is predicted by 2100.

Such an increase would make the oceans more acidic than they've been at any time in the last 20 million years, he added.

Sea Creatures' Uncertain Fate

The organisms most directly affected are those that build hard shells or other mineral structures of calcium carbonate. These include numerous species of corals, marine snails, and crust-building algae.

As oceans absorb CO2 from the air, the gas reacts with water to produce carbonic acid. The acid in turn consumes the carbonate that sea creatures need to build their shells.

"This is a problem that no living corals have encountered in their past evolutionary history," said Charlie Veron, of the Australian Institute of Marine Science.

Over time, coral reefs have been able to adjust to changes in ocean temperature and sea level, Veron said. But acidification appears to be a problem that the ancestors of today's corals were unable to solve.

Mass extinctions of marine life in the distant past, he said, were probably caused by chemical changes similar to those happening today.

"It took coral reefs about four to ten million years to recover each time," he added.

In shallow waters where most corals presently grow, carbonate is not in short supply. But at greater depths carbonate concentration decreases until it reaches a point beyond which shell- and reef-production is no longer possible.

That critical threshold is rising closer to the surface as oceans grow increasingly acidic, limiting the depths at which corals and other organisms can live.

Feely said that this limit has already risen several hundred meters, particularly in the Indian and Pacific oceans. Recent measurements in the Gulf of Alaska found that carbonate was in short supply less than 325 feet (100 meters) below the surface.

James Orr, of the International Atomic Energy Agency's Marine Environmental Laboratory in Monaco, said that by the end of this century, shell- and reef-building sea creatures will be unable to live at any depth across a huge area of the world's oceans.

"Two-thirds of cold-water corals will be exposed to corrosive waters by 2100," Orr said.

Far-Reaching Effects

Initial concerns about ocean acidification have focused on corals, which are already experiencing die-offs and "bleaching" due to warmer water temperatures.

(Read related story: "Global Warming Has Devastating Effect on Coral Reefs, Study Shows".)

Unfortunately, Veron said, "the very corals that will escape mass bleaching are those most prone to the effects of ocean acidification."

Scientists now recognize that the danger extends to other organisms as well. Robert Buddemeier, of the Kansas Geological Survey, said mineral-producing "coralline" algae are especially vulnerable.

The algaes' cementlike secretions are the "glue" that helps hold coral reefs together, Buddemeier said, and also help stabilize coastlines in nonreef areas.

Perhaps even more alarming is the threat to marine snails called pteropods.

Populations of these tiny creatures can reach up to ten thousand individuals per cubic meter (35 cubic feet) in the Southern Ocean. Their loss, Orr said, would have far-reaching effects.

"They're an integral component of marine food webs, a huge food source for many marine predators," he said.

The snails' calcium carbonate shells are so thin they are virtually transparent, Orr added, which makes them particularly vulnerable.

"Pteropod shells can start to dissolve in 24 hours," he said, "under [the seawater] conditions we expect for 2100."

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