Published online 19 November 2010 | Nature | doi:10.1038/news.2010.621

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Raised ocean temperatures result in severe damage to reefs in the Caribbean.

Fire coral from Flower Garden Banks National Marine Sanctuary, bleached in 2010. NOAA, FGBNMS

The year 2005 was devastating for coral, with unusually warm waters in the tropical Atlantic Ocean and Caribbean Sea causing one of the worst bleaching events on record. Researchers who monitored the event have now catalogued the full extent of the disaster — and they warn that 2010 is shaping up to be even worse.

Mark Eakin, coordinator of the US National Oceanic and Atmospheric Administration's Coral Reef Watch programme based in Silver Springs, Maryland, and his colleagues performed an extensive coral survey to record the effects of unseasonally high temperatures on reefs in 2005. The project involved more than 250 collaborators from 22 countries, and compared satellite data with field surveys to determine how heat stress affected the coral in different places.

In a paper published this week in PLoS ONE1, the researchers report the most severe coral bleaching ever recorded in the Caribbean: more than 80% of the corals surveyed were bleached, and at many sites more than 40% died. "Severe, widespread bleaching spells trouble for tropical marine ecosystems in general," says Eakin.

The researchers also saw bleaching in places where it hadn't been recorded before, for example at the Flower Garden Banks National Marine Sanctuary in the Gulf of Mexico. They also saw the first mass bleaching of elkhorn coral (Acropora palmata) in the Virgin Islands National Park. This coral was previously dominant in that reef ecosystem, but has been on the US Endangered Species Act list since 2006.

Bleaching occurs when stress causes corals to expel symbiotic algae. Several factors can cause bleaching, but, according to Eakin, the only thing that could cause mass bleaching on this geographical scale is high temperatures maintained over a period of months. "There is some evidence that local factors and other climate-scale factors, such as ocean acidification, sea-level rise and changes in storm intensity, may influence bleaching sensitivity," he says. "But temperature is the big driver."

Under stress

In 2005, regionally averaged temperatures were the warmest in more than 150 years, and researchers calculate that corals were subjected to some of the highest thermal stress in 20 years. Thermal stress is a measure of how much warmer temperatures are than normal and how long the anomalous temperatures persist.

Corals can recover from bleaching, but in many places, says Eakin, bleaching is happening faster than the reefs can recover. "When reefs are repeatedly hit with stress there is a continuing decline," he says. "The expectation is that it is only going to get worse in the future."

NOAA model released in September 2010, showing the coral bleaching risk for September-December 2010. Click for a larger image. NOAA

Eakin is concerned that 2010 has been particularly devastating for coral. Bleaching has been observed in every ocean and major sea in which coral occurs, from the Persian Gulf to southeast Asia, the Central Pacific to the Caribbean — only the second time this has happened (the first time was in 1998). "We're looking at an event of the same magnitude, with temperatures on a par with what we saw in 2005," he says. "As far as corals are concerned, 2010 is, in places, as bad as or worse than 2005."

Beyond the limit

Eakin attributes the unusually high temperatures of 2010 to weather patterns, specifically the knock-on effects of an El Niño event in 2009-2010 followed by a La Niña event in 2010-2011, combined with overall warming due to climate change. With base temperatures higher, says Eakin, any further temperature rise owing to weather patterns or other causes can push corals beyond their limit.

Paul Sammarco at the Louisiana Universities Marine Consortium in Chauvin, who researches environmental impacts on coral reefs but was not involved in the Coral Reef Watch programme, agrees that 2010 could be worse for coral than 2005, but he says it is still too early to say for certain. Either way, he says, temperature data and coral observations suggest that there will be more bleaching in the future. "What is much more predictable and more likely is that we will have a year much worse than 2005 between now and, say, 2015 or 2020."

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Researchers are continuing to monitor bleaching events to try to understand how they are affected by local factors, such as the presence of excess nutrients from surface runoff. "We need a better understanding of the relationship between temperature and nutrients, how those processes work and how we can actually reduce impacts," says Eakin. "But the major challenge is figuring out how to deal with atmospheric and ocean CO 2 levels."

If sea temperatures cannot be brought under control by altering atmospheric CO 2 concentrations or temperatures, researchers may be forced to take drastic measures to try to save small spots of particularly important coral. Two possible methods are shading corals with cloth, and pumping cool water from the depths over shallow reefs for short periods when temperatures are predicted to be particularly high, says Sammarco. "This would be dicey and difficult to control, but it may be worth investigating."