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AUSTRALIA'S CORAL PERILS
It's the superstar of the coral world. Australia's Great Barrier Reef—the largest system of its kind on Earth—is everything a reef should be: a fantasia of hard and soft coral teeming with sea turtles, giant clams, sea anemones, spiny lobsters, shrimp, squid, and fish that look as if they've rolled across a Peter Max palette.
Australian Environment Minister Robert Hill recently reported that the reef generally is in good shape—protected in part by its sheer gargantuan size. But conservationists on the island continent and elsewhere are sounding alarms over the future of this 1,250-mile (2,000-km) underwater wonderland.
Ominous developments over recent years include damage by overzealous fishermen, repeated invasions of coral-eating starfish, and that deadly affliction of reefs around the globe: coral bleaching. All of this—including the starfish plague—is more or less directly related to human activities in the opinion of some leading experts.
The executive director of the Australian Conservation Foundation, Don Henry, has labeled the various threats to the reef, a United Nations-designated World Heritage Area , as "extremely serious," and cautioned that it is at risk of being put on the UN's endangered list.
Last year, four respected scientists warned the UN World Heritage Bureau that the reef will "steadily degrade" if the current management regime continues. They said the corals face "slow death" from "a thousand cuts," and called for an independent assessment of the threats.
An even direr warning came last October from the Geneva-based World Wildlife Fund for Nature, which predicted that rising sea temperatures attributed to global warming could destroy the reef entirely by 2010.
UNDERWATER OLYMPIC FLAME
Along with the famed landmark Uluru (previously called Ayers Rock), the Great Barrier Reef is the pride of Australia.
In June, the Olympic flame made a short lap through its sapphire blue waters on its way to Sydney for the 2000 Olympic Games. The swim was an historic first for the Olympic flame, which was carried by a scuba diver in a specially designed torch.
The largest system of coral and associated plants and animals in the world, the reef actually is a formation of 2,900 separate reefs off of Australia's northeast coast. Its waters are home to 1,500 species of fish, 400 kinds of corals, and 4,000 species of mollusks.
Some of these waters off the state of Queensland have become increasingly murky during the past decade or so. One study found that runoff from cattle grazing lands and sugar-cane fields flowing into Queensland's rivers washes 16.5 million tons of sediment into the Coral Sea each year. The silt smothers the polyps—the tiny animals that build the coral—on the inner reefs. The runoff in this study also contained nearly 85,000 tons of nitrogen and 11,000 tons of phosphorous, much of it from fertilizers used on farms.
Under certain wind and tidal conditions, this mud soup can carry harmful chemicals 125 miles (201 kilometers) offshore, where nitrogen and phosphorous enhance the growth of phytoplankton, a type of algae. The green goo blocks sunlight needed to nurture coral and other reef creatures, and damages the polyps' hard exterior skeleton—making it weaker and more prone to damage from storms.
The issue of agricultural runoff is contentious, especially in the farming areas. A long-term reef monitoring project begun in 1992 by the Australian Institute of Marine Science (AIMS) thus far has not established a statistical correlation between human activities and the nutrient content of sediments. However, the institute noted that most nutrients probably don't wind up in the layers of silt.
Increased nutrients in the water, along with higher salinity and warmer temperatures, are suspects in the most recent invasion of crown-of-thorn starfish, which feed on polyps. The latest incursion is the third in recent decades, prompting a debate as to whether their arrivals are entirely natural, or are at least partly brought on by human activities. The nutrient-rich runoff that stimulates algae growth also contains microscopic diatoms and dinoflagellates that provide nourishment for the starfish.
Conservationists argue that overfishing on the reefs is upsetting the natural balance. Anchors and chains dropped by commercial trawlers and weekend sailors take a further toll—as do the coral chunks that are inevitably broken off or otherwise damaged by the more careless sight-seeing snorklers and scuba divers.
Perhaps most ominous—and possibly the least controllable of the threats—is coral bleaching. Recent episodes, including a massive 1998 outbreak, are thought by some to have been brought on by global warming.
The corals' rich colors are provided by tiny algae called zooxanthellae, which live in symbiotic relationships with coral polyps. When the polyps come under various kinds of stress, including rising sea temperatures, they expel their neighbors and turn white—the rough equivalent of running a fever. Bleaching isn't necessarily fatal unless the stress continues, in which case the polyps may starve to death. An especially severe episode of bleaching in 1983 killed 95 percent of the corals off in another part of the world: the Galapagos Island.
GREAT BARRIER LAWN?
AIMS believes that another major bleaching episode may have been prevented last year by a lucky event in the region: a heavy summer downpour that lowered the sea temperature and provided two other bleaching preventatives at just the right time: cloud cover and strong winds. Up until then, according to Great Barrier Reef Marine Park Authority scientist Ray Berkelmans, the summer weather had been "a perfect recipe for a bleaching disturbance."
Last year, AIMS long-term monitoring project leader Hugh Sweatman reported that despite the 1998 bleaching episode and the abundance of starfish in certain locales, reefs in other areas have shown net increases in hard coral over the past two years. But he said the future is uncertain.
"It depends on how fast changes in climate occur," he said, "and whether marine organisms, particularly corals and their symbiotic algae, can adapt to warmer conditions rapidly enough."
In one nightmare scenario, naturally occurring coral enemies such as cyclones and flood plumes, along with human interference such as overfishing and higher nutrient concentrations, could kill off vast areas of corals—opening those areas up to invasions of seaweed. In sufficient quantities, fish, shrimp, and urchins that normally feed on these hardy plants could be overwhelmed.
In case of a massive coral die-off, "All going well, corals can become dominant again over about a decade," said Terry Done, project leader of AIMS' Sustaining Coral Reefs project. "However, we are concerned that the balance can be tilted in favor of the seaweeds long-term."
The Great Barrier Reef could become the Great Barrier Lawn.
Eye in the Sky is a weekly series that brings you the story behind the headlines using satellite imagery, remote sensing, aerial photography, and maps. This feature is developed by National Geographic News with the sponsorship of the National Imagery and Mapping Agency (NIMA) and Earth-Info. Check out maps and imagery at http://www.earth-info.org.