In the multi-coloured glory of a healthy coral reef, white can be a sign of death or decay. Patterson is searching for clues to a global malady called coral bleaching, which some researchers consider the greatest threat to the world's reefs.

In a complex experiment, the chambers are attached by cables snaking through the sand to the nearby Aquarius undersea laboratory, which is touted as the world's only active live-aboard underwater lab. Patterson sends water flowing over the corals to measure how currents might affect bleaching.

Patterson, an associate professor at the College of William & Mary, and his team of scientists lived on Aquarius for 10 days recently and spent hours each day in the water, studying bleaching.

"It's probably the No. 1 problem facing reefs on a global scale," said Steven Miller, director of the National Undersea Research Center at the University of North Carolina at Wilmington. Aquarius is operated by the university and owned by the US National Oceanic and Atmospheric Administration.


Coral reefs, often referred to as undersea gardens, are life generators, offering food, shelter and safety to hundreds of marine species.

"It has the potential to destroy coral reefs as we know them. This is the No.1 issue."

Although vast areas of the world's oceans have not been examined, some scientists believe 20-30% of coral reefs have been lost in recent years, a dangerous signal on the health of the seas. 
 
Corals get stressed

Coral reefs, often referred to as undersea gardens, are life generators, offering food, shelter and safety to hundreds of marine species.

The coral polyps that build reefs on the sea floor are actually tiny animals that fashion whitish rock-like structures of calcium carbonate.

But the reefs in tourist photos are riotously coloured in browns, tans, blues and greens due to symbiotic algae, zooxanthellae, that live in the coral.

Healthy coral has a happy relationship of food-and-oxygen give-and-take with its algae. Bleaching, which leaves patches of the multi-hued reef white, happens when coral expels algae.

Scientists say the corals reject algae when they are stressed, primarily by unusually high sea temperatures caused by global warming or abnormal local heating. Other factors such as currents or pollution may also play a role.

When the algae leave, the corals can sicken. Some may survive. Others may die.

Bleaching, once rare, has become routine. In 1998, reefs from the Caribbean to Australia were hit by the worst coral bleaching episode in recorded history. On the Great Barrier Reef, 700-year-old corals died as a result, Australian scientists said.

So common is bleaching that the National Oceanic and Atmospheric Administration runs a warning Web page called "Tropical Ocean Coral Bleaching Indices," giving sea surface temperatures from the Molasses Reef in the Florida Keys to Fiji and the Easter Islands in the Pacific to Ningaloo, Australia.

The site is designed to indicate the "accumulated thermal stress" that coral reefs experience.

Global warming


Many scientists believe global warming is a greater threat than development, pollution, coral diseases or any other risk factor facing the reefs.

In his experiment, Patterson set up six clear chambers, measuring about 0.7m long and 18cm wide, in the sand around the Aquarius lab. Using electric heating elements powered from the lab, he heated the water in the chambers a couple of degrees, from about 29C (84F) to nearly 31C (88F) "enough to give the corals some stress."

Small boat bilge pumps were used to move water through the bleaching chambers and over the corals.

Patterson was testing a theory. "I think the greater the flow, the faster it will turn white," he said in an interview aboard Aquarius.

Scientists believe the more they know about coral bleaching, the better the chance of protecting reefs. For example, if they know coral in certain currents will survive better than others, they can guard that coral from other threats.

Patterson and his team, taking advantage of the Aquarius laboratory to examine the test subjects every day, took instrument readings and tissue samples from the corals frequently. The experiments were meant to determine if their "photosynthetic machinery is happy or unhappy," he said.

With global temperatures expected to rise another 2 to 2.5C in the 21st century, many scientists believe global warming is a greater threat than development, pollution, coral diseases or any other risk factor facing the reefs.

Miller has said he is not optimistic.

"The extra degree or two we're going to see in the next 50 years will push us over the edge," he said.