Coral bleaching is a condition often associated with the summer doldrums, but extreme cold weather, like what the Florida Keys experienced earlier this month, also can cause coral to bleach and die.

This month's cold snap has the Florida Keys National Marine Sanctuary and other coral conservation groups conducting a survey to determine the extent of the damage. During the next two weeks, teams of scientific divers from federal and state agencies and nongovernmental and academic organizations will be surveying coral colonies from the Dry Tortugas through Martin County to assess coral reef health.

Temperatures in some Keys nearshore waters dropped to 52 degrees for several days -- well below average for this time of year -- with fatal results for some corals.

Mote Marine Laboratory BleachWatch Coordinator Cory Walter was surprised at the extent of the affected corals when she dove various patch reefs in the Keys last week. Most of the bleaching and death occurred in the mid-Hawk Channel and nearshore reefs, Walter said. The offshore reefs fared better. The cold seems to have affected all species equally, Walter said.

The coordinated and comprehensive assessment from the scientific community was sparked by initial reports from divers, Walter and The Nature Conservancy marine biologist Meaghen Johnson.

"If there is any good news, it's that reef managers and scientists are able to quickly respond to this event and are in a good position to learn more about how reefs will rebound following such a rare occurrence," said Chris Bergh, director of The Nature Conservancy's Coastal and Marine Resilience Program.

Keys divers are encouraged to report the location of observed coral bleaching to Mote Marine Laboratory's BleachWatch program at http://www.mote.org/bleachwatch. This early-warning network helps alert managers to major disturbances.

Divers also should be aware that bleached corals are extremely vulnerable to additional stress. Divers are encouraged to seek non-stressed areas to enjoy at this time and, as always, not to touch corals, conservationists said.

Coral bleaching occurs when corals are stressed and lose their symbiotic algae called zooxanthella. Prolonged stress can result in coral death. Coral bleaching is most frequently associated with elevated water temperatures, but stress also occurs when water temperatures dip below the preferred 60-degree threshold, scientists say.

A cold-water bleaching and die-off hasn't occurred in Florida since the late 1970s.

Former sanctuary superintendent and 39-year Keys resident Billy Causey vividly recalled the damage from the winters of 1977 and 1978, when he saw "light flurries" of snow in Big Pine Key. Nearly 10 acres of healthy and "robust" staghorn coral on the Western Sambos Reef and another 10 acres of "unblemished" staghorn off Fort Jefferson in the Dry Tortugas died, he said.

He fears the long-term effects of the recent cold snap, which lasted much longer than the previous event, when Keys corals were much healthier than today. Years of pollution, overfishing and global warming have taken their toll, he said.

"Since (1977), our corals have been struggling," Causey said. "Our corals were just starting to see recovery."

A healthy coral results in big money for the Keys and South Florida. Reef-related expenditures generate more than $4.4 billion annually in southeast Florida and reef recreation supports more than 70,000 jobs, according to a 2001 economic survey.

tohara@keysnews.com