Acid Oceans Threatening Marine Food Chain, Experts Warn

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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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