In Hot Water: Global Warming Takes a Toll on Coral Reefs

By Schmidt, Charles W. | Environmental Health Perspectives, July 2008 | Go to article overview

In Hot Water: Global Warming Takes a Toll on Coral Reefs


Schmidt, Charles W., Environmental Health Perspectives


In the summer of 2005, while Atlantic hurricanes battered coastlines from Cuba to Mexico, the Eastern Caribbean baked under a relentless sun with barely a breeze to cool the air. Tourists and locals alike wilted in the heat, and below the sea, marine life and corals in particular suffered as well. The windless calm settled in just as a buildup of unusually warm water began accumulating in the region. Ordinarily, easterly trade winds would have churned the sea, helping it to cool. But thanks to an unprecedented heat wave beginning in May--the result of a confluence of factors related to climate change, scientists say--water temperatures in the Eastern Caribbean climbed and stayed high for months, reaching levels that by September would be warmer than any recorded in 150 years.

The heat disturbed a symbiotic partnership that coral animals normally maintain with a type of algae called zooxanthellae. Zooxanthellae supply corals with essential nutrients produced by photosynthesis, particularly carbon, in return for the shelter and access to sunlight provided by the reefs. The algae also impart color to the corals, which themselves are colorless. But as sea temperatures rose, the zooxanthellae disappeared, leaving their carbon-deprived hosts behind to starve. The reefs turned snow white, the color of the underlying stonelike structures they had built up over centuries, in a phenomenon known as coral bleaching.

As the heat wave progressed, it left a trail of bleached reefs the likes of which had never been seen in the Caribbean. By year's end, coral cover ranging from 90% in the Virgin Islands to 52% in the French West Indies was affected.

Coral bleaching isn't always fatal--if water temperatures cool in time, the zooxanthellae might return, allowing corals to recover. But in parts of the Eastern Caribbean, the reefs never got a chance. Almost as soon as their recovery started, they were attacked by diseases affecting a range of coral species down to 60 feet. By 2007, roughly 60% of the coral cover in the Virgin Islands and 53% in Puerto Rico's La Parguera Natural Reserve was dead--an unprecedented tragedy.

The Eastern Caribbean disease outbreak came on the heels of what's been a rough several decades for coral reefs worldwide. Long suffering from land-based pollution, habitat destruction, and overfishing, coral reefs now must also contend with climate change, which has accelerated their global decline. This puts a wealth of biodiversity at risk. Reefs support up to 800 types of coral, 4,000 fish species, and countless invertebrates. Reef-dwelling species numbering in the hundreds of thousands may not even be catalogued yet, some scientists speculate.

The implications of these declines could be as disastrous for human health as they are for marine life. Globally, reefs provide a quarter of the annual fish catch and food for about 1 billion people, according to the United Nations Environment Programme. Reefs protect shorelines from storm surges, which could become more powerful as sea levels rise with climate change. Tourism--a mainstay of coastal economies in the tropics, worth billions in annual revenue--could suffer if reefs lose their appeal.

Reefs are also a long-standing source of medicines to treat human disease. Being attached to reefs, corals and other immobilized marine animals can't escape predators, so they deploy a range of chemical compounds to deter hunters, fight disease, and thwart competing organisms. Two antiviral drugs (vidarabine and azi-dothymidine) and the anticancer agent cytarabine were developed using compounds extracted from Caribbean reef sponges. Another product called dolastatin 10, isolated from the sea hare (Dolabella auricularia) of the Indian Ocean, has been investigated as a treatment for breast and liver cancers and leukemia. Many more lifesaving medicines and useful chemical products could one day be derived from reef dwellers, experts say. …

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