ST. CROIX - National Oceanic and Atmospheric Administration scientists on Tuesday confirmed that coral bleaching in the Caribbean basin last summer was the most extensive and severe they have seen since they began collecting satellite data more than 20 years ago.

Discussing the bleaching on the second day of a coral monitoring workshop at the Nature Conservancy in Estate Little Princess, Mark Eakin, coordinator of NOAA's Coral Reef Watch program, said his organization was in the process of collecting data from more than 70 researchers measuring 1,000 sites throughout the Caribbean. The early returns are grim.

"This was an extremely severe event," Eakin said. "It was unlike anything we've ever seen."

Eakin said it was still unclear how many of the corals had rebounded from the bleaching and how many had died. During a bleaching event, corals expel the essential algae that live within their tissue. The algae supply corals with the majority of their food through photosynthesis. Stresses including high ocean temperatures and abundant sunlight can provoke the bleaching and are believed to be the culprits behind this summer's extensive bleaching.

Corals cannot survive long periods without the all-important algae and will die eventually if waters do not cool and allow the algae to recolonize. The exact length of time a coral can live without its algae varies by species, location and number of other stresses - such as poor water quality and pollution.

Corals in the Virgin Islands bleached heavily this summer, with some areas around St. Croix experiencing 95 percent bleaching on the reefs. Monitoring sites on St. John also reported extensive bleaching, and initial surveys of the corals in recent months have shown that some species - specifically elkhorn coral - have had some significant die off.

NOAA scientists are in the territory this week to meet with representatives of several environmental groups to discuss bleachings and the devices that NOAA has available to monitor the events. NOAA scientist Al Strong said Tuesday that the three-day workshop serves to both educate the local managers as to what tools are available and help NOAA to gather input on what tools might be most useful for the managers in the future.

"We're driven by what's available and what the managers want, they are our customers," he said. "We want to develop the best products possible."

Eakin said the weather and climate conditions in the Caribbean and across the Atlantic that led to an active and devastating hurricane season last year were the same ones that led to the coral bleaching. With strong winds to the north and south of the tropics, warm water became pinned in a band and continued to heat during the summer. Winds blowing over that hot band were weak, he said, preventing cooler, deeper water from drifting toward the surface. When strong winds blow across the surface of the ocean, the top layer is pushed aside, and cooler waters from deeper layers rise to fill the gaps. When winds are weak, the surface layer acts like an insulating blanket, trapping heat in the upper layers and overheating the sensitive corals. After bathing in this water for weeks, most corals bleach.

Eakin said NOAA is working to develop a seasonal forecast similar to the ones generated for the hurricane season. While it is nearly impossible, at this point, to predict exactly where and when bleaching might occur, Eakin said that with enough data NOAA could issue a seasonal report that could categorize the general risk of bleaching. NOAA also is developing the ability to determine which areas are most vulnerable to bleaching, using data from tidal currents and underwater geography studies.

Eakin said the hurricane forecast and the bleaching forecast often would go hand in hand.

"In years when you're experiencing an active hurricane season, you are also more likely to see bleaching," he said, explaining that the hot and stagnant water that provides near-perfect fuel for hurricanes is the same water that overheats corals.

With forecasters already predicting a powerful Atlantic storm season in 2006, local environmental managers need to be on the lookout for bleaching this coming summer.

"Warm waters, plus doldrums and high light penetration are what bleach corals," Eakin said. "These are the same conditions that make for an active hurricane season."

While managers cannot cool the waters to prevent bleaching, they can work to limit other stresses such as construction work, pollution or runoff. Eakin said NOAA's satellite monitoring tools can give these managers a few weeks of lead time to prepare.

"You can't really stop the bleaching," he said. "But you can address the other forms of stress. You can help the coral survive the bleaching event."

- Contact Patrick Joy at 774-8772 ext. 458 or e-mail pjoy@dailynews.vi.