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A Rocky Course

Since the reef first found footing, ice ages have come and gone, tectonic plates have crept forward, and ocean and atmospheric conditions have fluctuated wildly. The reef has seen many iterations—expanding and eroding, being defaced and reinhabited at nature's whim.

"A history of the Great Barrier Reef," Veron says, "is a catalog of disasters" caused by planetary chaos. But they are disasters from which the reef has always recovered.

Today new disasters endanger the reef, and the prospect for recovery is uncertain. The relatively quick shift in the world's climate, scientists say, appears to be devastating for reefs. In corals, warming temperatures and increased exposure to the sun's ultraviolet rays lead to a stress response called bleaching—when the colorful algae in coral cells become toxic and are expelled, turning the host animals skeletal white. Fleshy seaweeds may then choke out the remains.

Major bleaching in the Great Barrier Reef and elsewhere in 1997-98 was linked to a severe El Niño year and record-high sea-surface temperatures—in some spots 3°F higher than normal. Another round began in 2001 and again in 2005. By 2030, some reef experts say, these destructive episodes will occur every year.

Heat is also implicated in a 60-year decline in ocean phytoplankton—the microscopic organisms that not only gobble greenhouse gases but also feed, directly or indirectly, almost every other living thing in the sea. Reef fish, too, respond to warmer waters—sometimes with bolder, more aggressive behavior toward both predators and prey. Changes in sea level, either up or down, have a dire impact as well, exposing shallow corals to too much sun or drowning them in deeper water, where they're hidden from the light.

A more immediate concern is massive flooding in Australia that earlier this year sent huge plumes of sediment and toxin-laden waters onto the reef off Queensland. The full harm to marine life won't be clear for years, but long stretches of the Great Barrier Reef could experience disastrous die-offs.

And then there's the acid test.

Reef ecosystems worldwide took a pounding during each of Earth's five mass extinctions, the first about 440 million years ago. Greenhouse gases spiked naturally over the millennia, and Aussie biologist Veron says massive spewing of carbon dioxide during periods of heavy volcanic activity was likely a big player in coral decimation, notably the most recent mass extinction some 65 million years ago. At that time, oceans absorbed more and more of those greenhouse gases from the atmosphere, causing ocean acidity to rise. The lower pH—a sign of high levels of acidity—ultimately thwarted the ability of marine creatures to build their limestone shells and skeletons.

In some oceans this acidification is once again happening. The most vulnerable to acid's corrosive bite are the fast-growing branching corals and vital calcium-excreting algae that help bind the reef. The more brittle the reef's bones, the more wave action, storms, diseases, pollutants, and other stresses can break them.

In ancient times many corals adapted to changing ocean acidity, says Veron, who paints a particularly bleak picture of the Barrier Reef's future. "The difference is there were long stretches in between; corals had millions of years to work it out." He fears that with unprecedented CO2, sulfur, and nitrogen emissions by human industry, added to the increasing escape of methane as a result of Earth's melting ice, much of the reef will be nearly bereft of life within 50 years. What will be left? "Coral skeletons bathed in algal slime," he says.

Edging Forward

Of course, to the two million tourists who visit the reef each year, the promise of an underwater paradise teeming with life is still fulfilled. But the blemishes are there if you know where to look. The reef bears a two-mile-long scar from a collision with a Chinese coal carrier in April of last year. Other ship groundings and occasional oil spills have marred the habitat. Sediment plumes from flooding and nutrients from agriculture and development also do very real damage to the ecosystem. But Aussies aren't inclined to let the reef fall apart without a national outcry. The captain of the boat who took me diving put it this way: "Without the reef, there's nothing out here but a whole lot of salty water." To many locals, he adds, "the reef is a loved one whose loss is too sad to contemplate." And it is also crucial economically: The visitors he motors to the reef's edges provide more than one billion dollars annually for Australia's books.

The challenge scientists face is to keep the reef healthy despite rapid change. "To fix a car engine, you need to know how it works," says marine biologist Terry Hughes of James Cook University. "The same is true for reefs." He and others have been investigating how these ecosystems function so that efforts to prevent damage can be doubly effective.

High on the to-do list: Determine the full impact of overfishing. Traditionally, commercial fishermen could work along the reef, even after 133,000 square miles of ocean habitat was designated a marine park in 1975. But with rising concern about the big take, the Australian government in 2004 made a third of that area, in strategically placed zones, off-limits to all fishing—including for sport. The biological recovery has been bigger and faster than expected; within two years after the ban, for example, numbers of coral trout doubled on once heavily fished reef. Some scientists speculate that protective zones may also lead to declines in outbreaks of a devastating coral-eating sea star.

Scientists also want to know what makes specific corals extra tenacious during times of change. "We know some reefs experience much more stressful conditions than others," says reef ecologist Peter Mumby of the University of Queensland. "Looking at decades of sea temperature data, we can now map where corals are most acclimated to warmth and target conservation actions there." He says understanding how corals recover from bleaching—and figuring out where new polyps are likely to grow—can help in designing reserves. Even the outspoken Veron acknowledges that coral survival is possible long-term if the onslaughts against reefs are halted—soon.

Nature has some safeguards of her own, including a genetic script for corals that may have helped them ride out past environmental disruptions. Many reef builders evolve through hybridization—when different species mix genes. As Veron puts it, "everything is always on its way to becoming something else." On the reef, about a third of the corals reproduce in annual mass spawning. During such events, as many as 35 species on a single patch of reef release their egg and sperm bundles simultaneously, which means millions of gametes from genetically different parents mingle in a slick at the ocean surface. "This provides outstanding opportunities to produce hybrids," explains marine biologist Bette Willis of James Cook University. Especially with climate and ocean chemistry in such flux, she says, hybridization can offer a speedy path to adaptation and hardiness against disease.

Indeed, one lesson is that despite today's weighty threats, the Great Barrier Reef won't easily crumble. It has, after all, toughed it out through catastrophic change before. And all kinds of marine life are around to help keep the reef whole. In studies conducted in 2007, scientists found that where grazing fish thrive, so do corals, especially in waters polluted with excess nutrients. "If you take away herbivores, say through over­fishing, seaweed replaces corals," says Hughes. If voracious vegetarians are protected, corals can prevail.

A human visitor to the reef can see the fish doing their vital job. In dappled afternoon light toward the reef's northern tip, palatial walls of coral tower over a rare species of batfish, long finned and masked in black, that nibbles back strands of sargassum. And a school of parrotfish—fused teeth like wire cutters—chip away noisily at the rocks, where algae in mats of green and red have quietly taken hold.

David Doubilet divided his time between the Great Barrier Reef’s remote north and its touristed south and central areas. Jennifer S. Holland is a staff writer.
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