Cold-Water Corals at Risk From Fishing Nets, Acidic Oceans

Sean Markey
for National Geographic News
May 1, 2006
Shortly after the last ice age, pioneering organisms near the Norwegian
coast founded a colony that would become, 10,000 years later, the
largest of its kind on Earth.

Its members built a stony fortress that still stands today, 115 feet (35 meters) tall and 8.7 miles (14 kilometers) long, despite an environment of ceaseless cold and dark.

These hardy settlers did not spring from Stone Age Viking ancestors. They are cold-water corals whose home, the Sula Ridge Complex, is the largest deepwater reef in the world.

Relatives of sea anemones and jellyfish, cold-water corals thrive in 39° to 54°F (4° to 12°C) waters at depths ranging from 160 feet (50 meters) to 3 miles (5 kilometers).

Only in recent decades have scientists begun to regularly probe these little-known marine ecosystems, which are as rich in species as rain forests.

But recent research suggests the organisms' historic success may not continue.

In an article in the current issue of the journal Science, J. Murray Roberts of the Scottish Association for Marine Science in Oban issues a warning.

"Threats to these fragile, long-lived, and rich ecosystems are mounting." Roberts and his colleagues write.

Chief among these threats, they say, are deepwater fishing practices and the increasing acidification of the world's oceans.

"The impacts of deep-water trawling are already widespread," they write, "and effects of ocean acidification are potentially devastating."

The effects on other marine species could be equally significant, the researchers add.

Cold-water coral reefs are complex structures that provide shelter, habitat, and food to myriad species. (See and download wallpaper photos of coral-reef life.)

One reef in the northeast Atlantic was found to host 1,300 species.

New Threats

As ocean fish stocks grow increasingly scarce, fishing trawlers have pushed into deeper waters.

Trolling heavy nets across the seafloor, the ships scar or break apart cold-water coral reefs with grim efficiency, experts say.

In one instance, Australian authorities banned bottom fishing in some areas after it was found that only 4 to 5 percent of the coral cover remained there.

More troublesome to marine habitats, biologists say, may be the potential impact of rising ocean acidity from fossil fuel use.

Acting as a carbon sink, the ocean absorbs atmospheric carbon dioxide. One 2005 study estimated that half of Earth's airborne CO2 was ultimately trapped by the ocean.

Through a series of chemical reactions, this dissolved CO2 turns ocean water more acidic.

Corals build their skeletons from waterborne calcium carbonate, the same mineral that makes limestone.

Rising ocean acidity retards the speed at which calcification can occur and in some cases may cause corals to build weaker skeletons.

The organisms, in effect, suffer a sort of osteoperosis.

Writing in Science, Murray and his colleagues note that some models suggest rising atmospheric CO2 "could cause the greatest increase in ocean acidification over the last 300 million years."

The researchers add that if atmospheric CO2 were to double, tropical coral-reef calcification rates could drop by half.

Cold-water corals "may be even more vulnerable," the scientists write.

In Deep Water

"The changes [in ocean acidity] aren't huge," said John Guinotte, a marine biogeographer at the Marine Conservation Biology Institute in Bellevue, Washington.

"But they're substantial enough to impact the biology of a lot of marine organisms."

Guinotte notes that waters rich in a form of calcium carbonate called aragonite, which corals need to survive, will not run deep enough in the future to reach some cold-water corals.

In another recent study, Guinotte and his colleagues used research data and computer models to predict that only 30 percent of cold-water corals will remain in aragonite-rich waters by 2100.

And unlike other, more mobile organisms, corals can't relocate when their habitat changes, he added.

"These deep-sea corals aren't going anywhere," Guinotte said.

"They can't pick up and move when their environment becomes more and more inhospitable."

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