>Global Warning / Coral Reefs

Color Blindness

The brilliant beauty of coral reefs has begun to disappear before our eyes, along with the incredible variety of marine life that surrounds them. It has taken only a slight increase in sea-surface temperatures to unravel one of the earth’s most ancient ecosystems.

By Susan McGrath

 

In still dazzling late-afternoon light, a smallish school of marine biologists snorkels out over the reef flat at Heron Island, a speck of white sand and guano anchoring the southern end of the Great Barrier Reef, 50 miles off Australia's northeast coast. The reef here is a fairy garden of stony shapes—staghorn, table, branch, lily pad, and sea fan—each upholstered in velvet, like a buck's spring antlers, and pocked and whorled with individual coral polyps. Tucked in and around the colonies of coral and in narrow niches between their branches, in little hollows between the clumps and in labyrinthine caves in the underlying rock, and down the face of the outer reef wall and coursing over the tops are fish, crustaceans, mollusks, worms, and other creatures, bright and bland, striped and spotted, large and small. Lying in the silky, aquamarine water, I watch fleets of silvery fish dart by, maintaining their distance from one another with naval precision. Down among the stony branches, parrot fish, pink and green, grind nubbins of coral with their beaky jaws. Cemented into a spacious hollow in the reef flat are three giant clams. They look like plush jewel boxes, their fleshy emerald lips propped open, revealing ermine innards. I long for a careless shrimp to stray in and for a watchful clam to snap shut. No luck. I drift into deeper water, and an elegant whitetip reef shark slants past, with a metallic stare. This is a world so original, so alien, so unlike its terrestrial counterparts, I could gawp here for hours.

Not so my companions, alas. While I loll, these researchers dive and surface and discuss and dive and surface and discuss again. For them, this outing is work, and work of a rather urgent nature.

A baffling malady called bleaching is draining the color—and the life—from the world's coral reefs. And now, right here at Heron and all across its 1,250-mile arc, the Great Barrier Reef itself is bleaching. To my untrained eye, the reef beneath us, alive with movement, looks more or less intact. But below the distractions of vibrant fish, I look again; the corals themselves have taken on the color of dirty socks. The tapestry of life here has begun to unravel.

In the two decades since mass bleaching was first identified, it has killed more corals than all other causes combined. More than 16 percent of the world's corals have succumbed to bleaching and died. Countless millions of marine animals have died as well—anemones, sponges, sea fans, mollusks, crabs, shrimp, and fish, not to mention the seabirds and turtles that depend on them.

For the natural world, the loss of corals is a calamity beyond calculation. Coral reefs are keystone ecosystems, the richest and most diverse to be found in any ocean—an aquatic equivalent to rainforests. In barren tropical waters, corals build their metropolises, providing shelter and food for literally billions of other aquatic creatures. A quarter of all sea animals depend on them during some part of their life cycles. And we humans rely on them, too—for food, as natural breakwaters to protect our coastlines from storms, and as generators of billions of dollars in tourist income. Some tropical-island economies are almost wholly dependent on their coral reefs. Tourism and fisheries linked with the Great Barrier Reef, the world's largest, contribute $3.3 billion a year to Australia's gross national product.

Coral ecologist Peter Glynn was the first to document the eerie phenomenon experts now call mass bleaching. It was March 18, 1983, a hot, still morning in western Panama. Glynn humped his oxygen tank onto his back, clamped his regulator into his mouth, and took a giant stride off the boat deck. Later he would remember feeling no apparent difference in the temperature of water or air.

When the bubbles cleared, Glynn turned down to survey the reef. To his horror, it was as white as a Vermont snowdrift—"as if a blizzard had blown across it during the night," he says. The transparent, jellylike flesh of the coral still cloaked its limestone frame, but its color had been sucked away. Thousands of yards of Technicolor coral cups now looked like bones. In the following weeks and months, the mysterious malady hopped around the tropics. Scientists reported mass bleaching on reefs in the Caribbean, in southern Japan, in Indonesia. Some of the bleached coral recovered its color. Much of it simply died.

What Glynn and the other scientists were seeing was a rupture in one of the oldest marriages in nature. For some 200 million years, reef-building corals have operated in an intimate symbiotic partnership with unicellular algae that live within their tissues. These microscopic algae, called zooxanthellae, or zoox (it rhymes with "folks"), photosynthesize food for themselves and for their coral polyp hosts; the corals, in turn, provide the zoox with a place to live and basic compounds such as ammonia and carbon dioxide from their waste materials. The polyps come out to hunt at night, extending tiny tentacles to snag passing zooplankton, which supply both host and tenant with essential nitrogen.

The brown-gold colors we see in most corals are those of symbiotic zooxanthellae inside their cells, packed 2 million to 20 million per square inch. Many corals also have their own pigments, called pocilloporins, proteins that are responsible for the blue, purple, bright green, yellow, and pink colors that also protect algae and polyps from excessive sunlight.

When Peter Glynn saw the snow-white coral reef, he knew he was looking through zooxanthellae-less coral flesh to the calcium-carbonate skeleton below. Corals had been observed to dump their zoox in response to stressors such as dramatically lowered salinity, but that had been seen only on a small scale. Now some mysterious, invisible factor was causing all the corals to jettison their zooxanthellae.

"We started ruling out the possibilities," says Glynn. There was no contamination by heavy metals, no massive pesticide or herbicide spill, no current-borne pathogen, no major influx of fresh water from inland flooding. "Then we thought of looking at temperature. And there it was. Very high—unusually high temperatures." It was an El Niño year. Sea-surface temperatures (SSTs) in the eastern Pacific were the highest ever recorded at that time. So Glynn put coral samples in the lab and slowly turned up the heat. "Sure enough," he says, "the samples bleached."

Here's what happens: Reef-building corals live in conditions that are very stable year-round in water that is clear, saline, and about as warm as they can tolerate. When SSTs rise just two degrees Fahrenheit above normal summer high temperatures for a period of just two weeks, and skies are clear and windless, things go terribly wrong. Polyps overheat and start expelling their microalgal partners. They whiten and begin to starve. If conditions are really extreme, entire reef systems bleach—not just the corals but other marine animals that contain zooxanthellae. Anemones, sponges, jellyfish, even some mollusks turn ghostly pale. If conditions improve quickly, corals can recover. If conditions remain bad, week after week, corals die. Without the living corals, the limestone structure gradually erodes. The phenomenon is so well documented now that scientists at the National Oceanic and Atmospheric Administration track warming water worldwide and predict where bleaching may be about to occur.

In the past 100 years the ocean's surface temperature has warmed an average of about 1.8 degrees Fahrenheit. The consensus among scientists is that the warming is caused primarily by a buildup of certain gases in the atmosphere—carbon dioxide is the most prominent among them—which act as a heat-trapping blanket. The gases are released principally by the burning of fossil fuels: coal, oil, and natural gas.

Less than two degrees doesn't sound like much. But when added to the cyclical warming caused by El Niño, it's too much for coral reefs. In the El Niño event of 1997–98, the greatest ever recorded, almost all of the world's reefs bleached in a concerted way for the first time. In that bleaching episode alone, 16 percent of the world's corals died, among them some Indo-Pacific reefs that were considered the most diverse and pristine anywhere.

At Heron Island and across the Great Barrier Reef, the water is unusually warm this summer. Though the effect is not the dramatic bleaching Glynn saw in 1983 that causes reef-wide mortality, it is particularly bad news for one reason: For the first time, mass bleaching is occurring out of sync with an El Niño event.

Gathered here at the University of Queensland's Heron Island Research Center for an international workshop on bleaching are some of the world's ranking marine biologists. It's a rarefied group of scientists who alternately look as though they have been shipwrecked, sporting bare feet and tattered T-shirts, or are on location for a science fiction movie, striding around in wet suits peeled to the waist and dangling black hoses and valves. They are a semiaquatic bunch. On a late-afternoon reconnaissance, they effortlessly porpoise through the water, while I struggle to keep my snorkel clear and my fins from clobbering the coral.

Ross Jones, a marine biologist from the University of Queensland, tells me what they're looking at. "A lot of the colonies are pale but not entirely white. You can dump up to half your zoox, after all, before you even start to look bleached." Snowy-white corals are fully bleached yet still alive. Live corals produce and shed mucus to protect their surfaces from foreign debris, burrowing sponges, and colonization by seaweeds. Once corals die, however, algae take over. "These pale green ones," Jones says, gesturing to branching colonies limned in lettuce-green, "have died within the last few days. This coral"—he points to ancient-looking, red-and-brown turf-covered stone branches—"died within the last two months. Once you know what to look for, you can see that none of the corals really look as they should."

Why corals react to increased water temperature by pitching their zooxanthellae is not yet fully understood by scientists. Many believe the corals somehow recognize that the algae inside their cells are dangerously sick, and that they are not photosynthesizing as efficiently as normal. Sadly, scientists are getting some idea of what happens to other reef dwellers when corals bleach. In a jumbo tank on the tarmac, a University of Queensland honors student named Mike Phillips points out coin-size crabs wedged between the branches of a coral colony no bigger than a basketball. "Normally, out on the reef, you'll find 30 or 40 animals living inside a healthy colony that size: small shrimp, several pairs of these trapeziid crabs, some large snapping shrimp." The corals produce excess food in the form of mucus. The tenants eat it. Trapeziid crabs, in particular, are fiercely territorial. When a crown-of-thorns starfish—a voracious animal, 20 to 30 inches across, that can strip a colony of flesh in an hour—attacks a colony of corals, the tiny crab hurls itself at it, nipping its tube feet.

Within two months of a bleaching event, the crabs and other little watchdogs have disappeared, along with most of the animals that prey on them. Dead branching corals are soon reduced to rubble by waves, and the rubble, in turn, batters the reef during storms. The community unravels, and life on the reef deteriorates. Bioeroders—sponges, burrowing bivalves, worms, and others—riddle the defenseless limestone superstructure, and it, too, gradually crumbles under the relentless assault of the waves.

This time most of the corals here at Heron, and throughout the bulk of the Great Barrier Reef, will survive, although their reproduction will suffer. But with each subsequent bleaching event, more permanent damage occurs.

Ove Hoegh-Guldberg, professor of marine studies and director of the University of Queensland's Centre for Marine Science, is the chair of the bleaching workshop here and its manic and cheerful master of ceremonies. In 1999 he ran three different global-climate-change models to predict what would happen to sea-surface temperatures over the next century. The results showed that, on top of monster events, within 30 to 50 years temperatures reached during the 1998 El Niño will be reached every summer. "I was stunned," Hoegh-Guldberg says.

Most corals are extremely widely dispersed, so species probably won't become extinct in the short term, he explains. "But coral reefs most likely will no longer be the immensely useful and productive ecosystems they are today." Marine organisms will be deprived of vital life support. And people who depend on coral reefs for food and for income will have to look elsewhere—though where that "elsewhere" could be is anybody's guess.

"Humans are a pretty optimistic bunch," Hoegh-Guldberg says. "We tend not to believe the worst until we have to. You know, 'Aw, come on, there are plenty of woolly mammoths left!' The key thing is to plug the last gaps in our information, roll up our sleeves, and get to work.

"There's no point in being gloomy," he says, then gives a great shout of laughter. "Though, of course, I am a bit gloomy!"

I try not to be, but I return home only to obsessively monitor the coral reef listserv, an open
e-mail exchange among scientists and reef watchers. Six weeks after I'd left Heron Island, an American marine biologist posts this message: "Water temperatures started to warm up very early this spring in Florida. I fear a bad bleaching year."

My thoughts drift back to Heron. It's a privileged little reef, isolated from the sewage and sedimentation, boat anchors, and intense coastal development that threaten more accessible inshore reefs. As part of a marine park in a wealthy nation, it's safe from cyanide fishing, blast fishing, overfishing, dredging, mining for limestone, and all the other insults that degrade reefs in many poorer tropical countries. But these advantages are no longer enough to guarantee a reef's health or even its survival. Today a reef like Heron requires stewardship on a global level. It requires a stable climate.

Susan McGrath volunteers at Climate Solutions, a nonprofit group working to make the Pacific Northwest a leader in practical, profitable solutions to global warming. Her story "Spawning Hope," on salmon conservation in Washington State, ran in the September issue of Audubon.