Unknown Future for Coral Reefs

When Nerilie Abram first arrived at the Mentawai Islands southwest of Sumatra in 2000, she was expecting to find a thriving coral reef and its ecosystem. Instead, she and her co-workers found dead coral with no fish. Local people told Abram that the coral began to die in 1997, an El Niño year, when an algal bloom had smothered the reef.

But the upwelling warm water from El Niño alone was not strong enough to create the bloom, Abram says. And evidence from coral cores showed the reef had survived even stronger upwellings. “So we looked for a stronger nutrient source,” she says, and found it in the wildfires that raged across Indonesia that year. The iron from heavy smoke in the region fertilized the red tide, enhancing the effects of warmer water conditions.

In 2002, researchers observed bleached corals for the first time in the Northwestern Hawaiian Islands. In Hawaii and elsewhere, communities rely on reefs for fish and tourism dollars. They may also serve as colonizers for reefs in other regions. Photo courtesy of NOAA.

That conjunction of events worked to asphyxiate the reef, which has not yet recovered. And as climate change and human impacts shift forest conditions, “wildfires are only going to be worse,” Abram says. “On a broader scale, the combined threats to reefs are all increasing.”

Abram and her co-workers published their detective work this summer; their report was one of several published in Science on Aug. 15, in a series of papers delivering a broader picture on coral reefs. One group reported the loss of 80 percent of Caribbean coral cover over three decades, much of it from disease. Coral reefs are in danger, scientists wrote, and their recovery, when compared to historic coral reefs, is not assured. Though action is necessary to preserve reefs today, researchers do not know enough about how reefs function to guarantee that conservation and remediation will work.

“People have been talking about the decline in coral reefs for 10 years, but nobody really showed the long-term decline,” says John Pandolfi, lead author on one of the Science papers, and a reef paleo-ecologist at the Smithsonian Institution’s National Museum of Natural History. But the global decline started centuries ago, wrote Pandolfi and his co-authors, and followed the same general trend for each major reef system they reviewed.

The team used archeological and fossil records to show that in coral reef ecosystems around the world, first the large herbivores and predators disappear, followed by declines in smaller creatures. Then the coral and resident suspension feeders begin their own descents. The team pegged the declines to human cultural periods, from the tropical western Atlantic to the Red Sea and northern Australia: Hunting and gathering marks the beginning of the end. The additional stresses of human overfishing and agricultural runoff — both polluting and over-fertilizing ocean waters — have sent coral reef ecosystems further down their spiral.

This global deterioration may have left corals more susceptible to warming sea-surface temperatures, enhanced by global climate change, the researchers argue. Temperature fluctuations have immediate effects on the symbiotic relationship between corals and their resident microbes, known as zooxanthellae. The microbes, which were only recently reclassified from one species to four genetically different groups, each with different tolerances for different temperature ranges, remain somewhat of a mystery to biologists. These single-celled algae provide their coral hosts with their products from photosynthesis, and the coral give back excretion products.

Yet the microbe-coral coexistence is not always friendly. Corals sometimes expel their microbes, particularly in warmer than usual water, taking on a “bleached” appearance. Fossil evidence suggests that this bleaching occurs for shallow coral reefs in rising waters, as they become too deep for their microbes to obtain enough sun. Or too much sun leads to overexposure of the zooxanthellae to UV radiation, with the same result. Only sometimes do reefs recover; why and how is unknown.

Bleaching and coral disease are symptoms of prior debilitation, Pandolfi says, like a hospital patient getting a secondary exploitative disease. “We need to keep it in context: Coral reefs are very sick right now and have been for a long time,” he says. “Bleaching and disease have not caused this major decline in coral reefs,” but fishing and environmental pollution, among other conditions, have.

Pandolfi and others have looked to fossil coral reefs for answers as to how modern reefs might respond to inclement conditions. Some modern species can be traced across the Quaternary, says Brian Rosen, a coral reef researcher at the Natural History Museum in London (and co-author of another paper in Science with Pandolfi and others). “You can start matching the history of reefs in any one area to global temperature plots and sea-level changes,” he says, even if the fossil record does not preserve individual bleaching events or other shorter time-scale disasters.

In the past, the corals themselves changed, stretching or retracting their geographic outer limits as temperatures warmed. “We see, in a sense, recovery or survival on big time scales,” he says. “But if you are managing the Great Barrier Reef, it doesn’t matter if someday reefs will colonize the coasts of France.”

That ability to adapt, preserved in the fossil record, means that reefs will somehow survive on Earth, says Roger Griffis, the coordinator for the Coral Reef Conservation Program under the National Oceanic and Atmospheric Administration. “These systems will change and adapt,” he says, but “may be really different looking than they are now.” However, while the amount of change and temperature variation has been as high in the past as projected today, Rosen says, researchers do not yet have evidence as to whether corals will be able to survive the current fast rate of global climate change.

The disease epidemics that swept across the Caribbean in the 1980s led to an 80 to 90 percent loss of Acropora species, once the most common shallow species of coral. “That’s like losing your most common tree in a forest,” Griffis says.

Management practices need to focus quickly on what factors most harm coral reefs and which ecosystems seem most resilient to change, Griffis says. “Corals have a better chance of surviving things like temperature changes if they are less stressed from other things like overfishing,” he says.

Griffis and other representatives from the United States and its territories met last October for the 10th Coral Reef Task Force meeting, hosted by Guam and Saipan (in the Northern Mariana Islands), two island economies that rely heavily on coral reef tourism. The task force hammered out several resolutions and the next steps for reef management. Management plans will have to limit the impacts of fishing, along with other stresses like pollution and runoff.

In the meantime, fish are beginning to return to the Mentawai Islands reef, Abram says, and in 2001, small corals began growing. But, she says, “in the best-case scenario, if there are no other stresses on the reef, it will take 50 to 100 years” to recover its several-meters thickness. And the stresses — from climate change, El Niño sea-surface temperature changes, fishing, agricultural runoff and even fires — are unlikely to dissipate soon.

Naomi Lubick

NOAA's Coral Reef online
"The future of coral reefs," a review by N. Knowlton, PNAS, May 8, 2001

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