Scientists studying Caribbean reefs say that 2010 may be the worst year ever for coral death there. Abnormally warm water since June appears to have dealt a blow to shallow and deep-sea corals that is likely to top the devastation of 2005, when 80% of corals were bleached and as many as 40% died in areas on the eastern side of the Caribbean.
Bleaching occurs when crucial microorganisms leave coral reefs during stress. Corals, which shelter a quarter or more of all marine species, get bleached, and may die, after prolonged heating. A few weeks of water temperatures a few degrees above normal can be fatal. During the 2005 die-off, for example, water temperatures off the Virgin Islands rose just 3°C above the average in August—but stayed that way until November. “There has been little recovery in the Caribbean since,” says reef specialist C. Mark Eakin of the U.S. National Oceanic and Atmospheric Administration (NOAA) in Silver Spring, Maryland.
In July of this year, NOAA warned that bleaching was again likely. Now, temperature maps indicate that the water has remained hot for longer than in the 2005 episode and the abnormal warmth spreads over a much broader area (see images). The episode 5 years ago hit the Lesser Antilles on the eastern side of the Caribbean as far south as Guadeloupe. (Here’s a map of the islands.) This year, the bleaching and high temperatures have devastated reefs in the Dutch Antilles and affected coral along the western and southern sides of the sea. Those include reefs off Panama and the island of Curaçao near Venezuela. That island’s maximum monthly average surface water temperature is 28.5°C; it was 30.2°C for the month of September.
“I’ve never seen bleaching like [it] in Panama,” said Nancy Knowlton, a coral biologist at the Smithsonian Tropical Research Institute in Panama who has been studying the local flora for 25 years. She and colleague Hector Guzman have seen massive reefs die in recent weeks in the enclosed lagoon of Bocas del Toro in Panama after becoming coated with giant sheets of slime, the remains of dead microorganisms. “This is NOT a normal condition on reefs, even bleached reefs. Where last year there were healthy corals, this year there was only gray ooze,” she wrote in an e-mail.
A lack of wind prevents the mixing that would add cool water and alleviate the die-off. In some areas, says Guzman, the water “is like a Jacuzzi” and the sea life is dead, extra salty, and low in needed dissolved oxygen. “I’m trying to be positive,” says Guzman, who notes that some wind has remixed and cooled the waters. “I have seen reefs get wiped out and come back the next year.” But the devastation is “more dramatic” than he has seen before, affecting hard corals, sponges, and sea anemones.
A number of factors besides water temperature can cause coral bleaching and die-offs, including pollution and storms. But temperature is the number-one culprit in such a massive die-off, says Eakin. The warmest 12-month period in the NASA temperature record ended this summer; June through August was the fourth-warmest such period in the record. The extent of the devastation across the Caribbean will become clear in the coming months as biologists measure the deaths.