Endeavors magazine: In Hot Water

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In Hot Water

A one-of-a-kind study exposes the sharp decline of coral reefs around the world.

by Mark Derewicz

[filed under: marine sciences; article date: january 2008]

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Half of the world’s reef-building corals have already died,” says John Bruno. Even where humans try their best to tread lightly.

Bruno, a marine ecologist at Carolina, and graduate student Elizabeth Selig came to this conclusion after two years of compiling and analyzing 6,001 scientific surveys of 2,667 coral reefs in the Pacific Ocean from Indonesia to Hawaii. They found that nearly six hundred square miles of these ecologically diverse underwater forests have disappeared every year since the mid-1980s. That’s about 1 to 2 percent of the world’s coral reefs dying each year, nearly twice the rate of tropical rainforest loss.

The Indo-Pacific is home to 75 percent of the world’s coral reefs, which are composed of tiny marine animals. The reefs support thousands of fish species and other aquatic life, and coastal communities depend on healthy coral reefs for fisheries, tourism, and protection from storm surges. “These reefs are incredible buffers,” Bruno says. “You can literally have fifty-foot waves break on a reef during a cyclone, and the waves on the beach are one or two feet high.”

When corals die, they leave skeletons that erode quickly. Fish go away; fisheries suffer. And sea walls disappear. “And the next time there’s a hurricane,” Bruno says, “those fifty-foot waves will slam on the shore.”

Bruno and Selig tracked the decline of coral cover — the measure of how much live coral covers the ocean floor in a given area. Coral cover is a key indicator of reef health in the same way that canopy cover is indicative of tropical rainforest health. In places where coral grows, it has historically tended to cover about 50 percent of the ocean floor. But Bruno and Selig found that coral cover throughout the Indo-Pacific declined from 40 percent in the early 1980s to 20 percent by 2003. Today, only 2 percent of Indo-Pacific reefs come close to the 50 percent baseline.

This same rate of decline exists in Hawaii, Indonesia, Australia, and all points in between. And Bruno and Selig say that such a region-wide decline is the most surprising result of their research because not all coral reefs face the same dangers. Some coral live in tight quarters, which means that viruses can more easily infect and kill nearby coral. Some reefs are over-fished and face more pollution and sediment runoff from coastal development and agriculture. Other reefs are isolated and pristine.

The Great Barrier Reef off Australia’s northwest coast, for instance, is a protected area. Scientists had assumed that coral cover would be better there. They were wrong.

“Coral cover on the Great Barrier Reef was not significantly greater than reefs in the Philippines, where reefs are often thought to be highly threatened and poorly managed,” Bruno says. Coral cover in Hawaii is just as good — or bad — as coral cover in Australia, despite the particular protection policies of the two areas and the fact that the Great Barrier Reef is much more ecologically diverse.

“The Great Barrier Reef is in the coral triangle,” Bruno says. “So it’s the center of biodiversity for the whole ocean. It’s where most of the fish species are, and where most of the different types of invertebrates such as crabs and lobsters live.”

There are hundreds of different species of coral off Australia’s coast. There are about fifty in Hawaii.

“Ecologists had thought that the Indo-Pacific reefs were dying at a slower rate than say, Caribbean Sea reefs, which don’t support as many species and face at least as many threats,” he says. “We assumed that the Indo-Pacific was better off. But it’s just not, at least in terms of coral cover.”

All this leads to a troubling conclusion: coral decline is likely due to large-scale stressors.

In a separate study, Bruno and Selig demonstrated that warmer ocean water can cause coral bleaching and exacerbate coral disease outbreaks.

Other researchers had studied single reefs over the course of a year and found that warmer ocean temperatures created a breeding ground for coral disease. “But we looked at forty-eight reefs over six years,” Selig says. “That hadn’t been done before.”

And they documented just what warmer water can do to a coral.

Bruno says, “If you raise the temperature of the ocean just one degree Celsius in the summer, coral can bleach, turn stark white. And if the water remains too warm for too long, the coral will die; you’ll have mass coral die-off.”

This is what scientists found in 2005 when the Caribbean Sea stayed warmer than usual all summer. There was also massive coral bleaching in 1998 when there was a strong El Niño weather pattern. And all over the world, as Bruno and Selig are proving with every piece of evidence they keep in their databases, coral are bleaching and dying faster than previously thought.

Pollution from fossil fuel is not only warming the atmosphere and the oceans but increasing the acidity of the oceans, Bruno says, making it harder for corals to secrete their calcium carbonate skeletons, which they must constantly do to keep pace with erosion.

“So the implication is that local reef management doesn’t have that much to do with coral cover,” Bruno says. “Local management does affect other things — if you stop fishing, you get lots of big fish on your reef. But it doesn’t seem to affect the measure of the health of the reef. And the reason is probably that managers just can’t manage the big impacts, such as climate change. We need to deal with that at a national and global scale.”

John Bruno is an associate professor of marine sciences in the College of Arts and Sciences. Elizabeth Selig is a fifth-year doctoral candidate in Bruno’s lab.