Mention a hot tub to most people, and images of relaxation, perhaps a little romance, probably come to mind. But when marine biologist Thomas J. Bright, who works at Glover's Reef Marine Research Station off the coast of Belize, uses the term, he is anything but content. As head of the station, which is run by the Wildlife Conservation Society, Bright spends a fair bit of time in the water monitoring the health of Belize's coral reefs. Some of his most recent observations, particularly of deepwater reefs, have been surprising--and troubling.

It all started last September, Bright recalls, as he was sailing back to Belize from a research trip to Honduras, when he stopped to examine one of the Caribbean's abundant coral reefs. "The water felt like a hot tub," Bright says, and he immediately knew something was amiss. Just as he suspected, the coral reef where he was snorkeling had suffered heavy damage as a result of a process called coral bleaching. Bleaching occurs when coral--often in response to an increase in water temperature--expel algae that typically live on the reef. The algae, known as zooxanthellae, exist in a symbiotic relationship with the coral, providing nutrition to the reef. The algae also give the reef its color. Without the zooxanthellae, the coral's calcium carbonate skeleton is exposed, and the reef appears pure white and will eventually die.

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