THE FLORIDA KEYS REEF TRACT—Earlier this month, outfitted in scuba gear and bobbing above lumpy brown coral 6 kilometers off Key Largo, Lauren Toth, a coral geologist at the U.S. Geological Survey (USGS) in St. Petersburg, Florida, set out to learn just how much time Florida's coral reef has left.
Around the world, warming oceans are killing coral. In Florida, Toth and others have found, heat-induced bleaching is just the latest in a millennialong series of insults, which have brought the reef 's growth to a standstill and left it vulnerable to erosion and rising seas. As a result, the barrier reef—the third longest in the world—is not simply dying. It appears to be vanishing.
At stake is a 320-kilometer-long bulwark that protects the Keys from waves while providing habitat for fish and a lure for tourists. Recent measurements by Toth and her colleagues have confirmed that the coral is eroding, in some places by several millimeters per year. Now, she and others are surveying the entire reef to learn how fast, and where, it is being lost.
The recent toll of warming, disease, and pollution on Florida's reef has been even heavier than on some other iconic reefs, such as Australia's Great Barrier Reef. "We've lost 90% of our coral cover in recent decades," says Erinn Muller, a coral biologist at the Mote Marine Laboratory in Sarasota, Florida. Some species, such as the fast-growing elkhorn coral with its distinctive wide branches, have nearly vanished. A host of others have died as a disease called stony coral tissue loss has marched down the Keys. "The last corals alive are getting hammered," says Derek Manzello, an oceanographer at a Miami, Florida, lab of the National Oceanic and Atmospheric Administration.
Yet Toth and her peers have shown that Florida's corals haven't been healthy for millennia. Core samples from the reef record that it stopped growing 3000 years ago. Florida's reef lies near the northern temperature limit for corals, and Toth and her colleagues reported in a study last year that a cooling trend around that time likely made the waters more prone to cold snaps that would periodically kill off corals, leaving the reefs at a delicate tipping point.
Now, although cold snaps still occur, global warming is bringing hotter summers, causing bleaching and mass die-offs. It is also raising sea level. Healthy corals readily cope with sea level rise, growing with the rising ocean. But Florida's ailing reefs probably can't keep pace. Anecdotes abound of patches of reef eroded flat to sand.
Data, however, are rare, with one exception: In 1998, USGS scientists drilled rods into 12 dead coral colonies at one site here, hoping to gauge erosion. Recently, a USGS team led by Ilsa Kuffner and Toth revisited the rods and used the cement that holds the rods in place as a reference point. In work close to publication, they found that the dead corals are eroding by 5.5 millimeters a year—almost double the rate of global sea level rise—as they are chewed apart by parrotfish and other species.
Toth is now trying to get a broader view by taking advantage of rods that Florida state scientists embedded at 46 sites up and down the Keys as reference points for an annual photographic survey. At many of these sites, the epoxy that cements each rod in place, once flush with the coral, is now sticking out because of erosion. To get more precise measurements, Toth and her co-workers developed a portable tool that can sit on top of the stakes.
On this April morning, she and a colleague were testing the device, floating above boulder and mountainous star coral and massive starlet coral, with a fleeting sighting of one staghorn. Silvery chubs, red grouper, and snappers seemed to watch the scientists work. Each rod presented its own challenge. Some were covered with small fire corals, which the divers dislodged with taps of a hammer. Others had been coopted by sponges, or in one case had fused with the spine of a fanlike soft coral. When a coral might be damaged or a sponge squished, the scientists skipped measuring it.
"The prototype works," Toth said as she climbed back into the boat. In this case, the rods recorded little erosion—perhaps because the reef was already squat. "Once it's flat, it probably doesn't get targeted [by fish] anymore," she said. This summer, the divers who conduct Florida's photographic survey will use the USGS tool to measure about half the rods at each site, giving a more complete picture of the reef's decline.
For coral biologist Alina Szmant at the University of North Carolina in Wilmington, Toth's work confirms the reef is doomed, as the assaults overwhelm a fragile ecosystem. "I don't want to give up, but you have to be honest with yourself," she says.
A patchwork of restoration efforts, largely from nonprofit groups, continues on the reef, often aiming to replace dead coral with heat-resistant transplants. Backers should temper their expectations, Toth says: "If these reefs haven't been growing for 3000 years, it's going to be really tough to get reefs like exist elsewhere in the Caribbean."
Toth still hopes some living coral can be saved. But she says biologists need to introduce not just corals that can resist heat or disease, but also species that can build structure, like staghorn or elkhorn. It's also time, she adds, to think about saving the reef structure and the services it provides, even if its coral dies. "How do we keep from losing what was built over the last 8000 years?" she says. "Because we don't have another 8000 years to rebuild it."