Unusually warm ocean temperatures in the summer and fall of 2005 caused a mass die-off of Caribbean corals that is the worst ever recorded there, according to new research published yesterday in the online journal PLoS ONE.

More than 80 percent of corals bleached and over 40 percent died at many sites in the Caribbean and Gulf of Mexico that year, the study says, arguing the 2005 event will have long-term consequences for the health of reefs.

Such events are also likely to become more common as global warming continues, concludes a team of 65 authors in 22 countries. They predict "a troubled future for tropical marine ecosystems under a warming climate."

Lead author Mark Eakin, who coordinates the National Oceanic and Atmospheric Administration's Coral Reef Watch, said the new paper presents the "first time there's been a full analysis of what happened across the Caribbean in 2005."

It also draws a link between the events of 2005 and an overall rise in the temperature of the world's oceans.

"We'd seen all these bleaching events in the Caribbean and we thought we knew what thermal stress would look like," Eakin said of the aftermath of the 2005 bleaching event. "But this was just hideous in its scope, it was so big."

The heat stress on corals during the 2005 event was greater than any recorded by scientists in two decades prior and the average temperatures in the region were the warmest in more than 150 years, the study says.

Warming temperatures 'starving' the corals
The analysis notes that, for the first time, scientists recorded bleaching at Saba, an island in the Netherlands Antilles, mass bleaching in the Flower Garden Banks National Marine Sanctuary in the Gulf of Mexico, and bleaching of elkhorn coral -- a threatened species -- in the Virgin Islands National Park.

Bleaching occurs when corals expel the microscopic algae that normally live inside their hard skeletons, providing them with food and their bright coloration. Changes in ocean salinity, nutrient runoff and other pollution can cause small-scale bleaching, but scientists say that large-scale bleaching like the 2005 Caribbean event is a symptom of unusual ocean warmth.

Such bleaching doesn't always spell death for corals, but those that survive are often weaker and more susceptible to disease.

"A bleached coral is still alive," Eakin said. "If the bleaching event is mild enough in the short term, they can recover their algae. But they're starving at the point they lose their algae."

An example of that sort of recovery occurred off the Florida Keys during the 2005 bleaching event. A string of hurricanes -- Katrina, Rita and Wilma among them -- that came through the area helped cool down water temperatures. Although the storms were relatively weak when they passed by the Keys, the wind, rain and clouds they brought were enough to halt bleaching there.

But long-term prospects for coral appear to be grim, the study notes, since reefs recover slowly from large-scale bleaching events, and those events are becoming more frequent. Major bleaching is now occurring in the Caribbean "every five years or less," the study says.

That includes a bleaching event that has rippled around the globe this year, affecting reefs in Kuwait, the Maldives, Southeast Asia, the Philippines, central Pacific islands and now the Caribbean (ClimateWire, Nov. 10).

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500