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Global Warming Has Devastating Effect on Coral Reefs, Study Shows

Sean Markey
for National Geographic News
May 16, 2006
 
Eight years after warming seas caused the worst coral die-off on
record, coral reefs in the Indian Ocean are still unable to recover,
biologists say.

Many reefs have been reduced to rubble, a collapse that has deprived fish of food and shelter.

As a result, fish diversity has tumbled by half in some areas, say authors of the first long-term study of the effects of warming-caused bleaching on coral reefs and fish.

The study focused on reefs near Africa's Seychelles islands, north of Madagascar (see Seychelles map), which sustained heavy losses from bleaching in 1998.

"The outlook for recovery is quite bleak for the Seychelles," said lead study author Nicholas Graham, a tropical marine biologist at England's University of Newcastle Upon Tyne.

The study, in today's Proceedings of the National Academy of Sciences, predicts that isolated reef ecosystems like that around the Seychelles will suffer the most from global warming-caused bleaching events.

Warming Oceans

Small but prolonged rises in sea temperature force coral colonies to expel their symbiotic, food-producing algae, a process known as bleaching.

While the dying reefs, which turn ghostly white, can recover from such events, many do not.

In 1998 an El Niño weather pattern sparked the worst coral-bleaching event ever observed.

"Over 16 percent of the world's reefs … were lost in that one year," said Graham, part of a team that recently received an unrelated research grant from the National Geographic Society Committee for Research and Exploration. (National Geographic News is a part of the National Geographic Society.)

"It was a huge event."

In the western Indian Ocean, regional currents compounded the heat effect, bleaching 90 percent of the coral reefs there.

With data from a 1994 survey in hand, researchers returned to the Seychelles in 2005 to study the bleaching event's long-term impact on coral reefs and fish communities.

Surveying 60,000 square yards (50,000 square meters) of coral reef across 21 sites, researchers found that fish diversity declined the most on reefs that had sustained physical and biological erosion.

The finding by U.K., Australian, and Seychelles researchers confirms what many scientists had long suspected.

The census also revealed that four fish species—butterfly ish, damselfish, and two wrasses—may now be locally extinct. Six other fish species have declined to critically low numbers.

Describing reefs in the inner Seychelles as in "various states of collapse," Graham says it appears unlikely they can recover.

He says the reefs are too isolated to recruit young coral from other reef systems.

"Coral cover at the moment is at about seven and a half percent [of previous levels] in the [inner] Seychelles," Graham said.

"However less than one percent of that is fast-growing [branching] and plating corals, which in other places in the world are often the ones that come back and start a recovery process."

Reefs Vanishing Worldwide

Experts say word of vanishing coral reefs has become all too familiar.

"By and large, reefs have collapsed catastrophically just in the three decades that I've been studying them," said Nancy Knowlton, a marine biology professor at the Scripps Institution of Oceanography in La Jolla, California.

Knowlton, who is also a member of National Geographic Society Committee for Research and Exploration, notes that corals live precariously close to their thermal limits.

As a result, even the most isolated reefs are vulnerable to the effects of global warming.

"These increasingly warm temperatures that we've been seeing in the last couple of decades have been tipping reefs over in terms of these fast bleaching events," she said.

Graham, the study author, says that while local and regional resource managers can mitigate some damage to coral reefs, broader action is required.

"Bleaching is a global issue, and it's driven by global warming," Graham said. "So the onus is on all of us, really."

"We need to reduce greenhouse gases and take these issues seriously."

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