Andros Island, Sept. 21 — As I sit on the reef floor in about 15 feet of water, gazing around at the seascape, I remember the writings of 18th-century naturalists, who described the sea turtles, rays and huge fish teeming in these waters.

I wonder what has happened.

Large marine animals are rare now, and the world's third-longest coral reef, which once shone with health, is mostly covered with slime.

Algae coats almost everything, from live coral to rubble on the reef crest. It seems to fill every crevice in the reef's limestone base. It even flies from blades of turtle grass like transparent pirate flags.

The Andros reef is classified by the United Nation’s World Resources Institute as facing "medium" threat from human activities. But in places, the reef seems to be suffocating beneath the blue-green alga Lyngbya, the red macro-alga Spiridia filmentosa, and the abundant Microdictyon marinum.

Most of the scientists in this month-long survey — from mollusk specialists to plant experts to fish afficianodos — have not found their creatures in the numbers they expected here.

While the scientists aren't sure why that is, crustacean expert Chris Boyko says that if algae increases dramatically, crabs (for instance) might be unable to find nooks and crannies in which to live.

"It's horrible out there," says coral expert Sean Grace. "There's dead coral everywhere. I tried to save some today — scraped off algae — but then realized what I was doing. I'd say there's only about 5 percent coral cover in most places I've looked."

Sean's other specialities — sea anemones and hydrozoans — seem to be doing fine here. "(But) corals are the reef builders," Sean notes. "Without that three-dimensional coral structure, other species in this area will drop dramatically."

The algae extends fairly deep, says Sean, who scuba dived to the edge of the Tongue of the Ocean, where the sea floor abruptly plunges into the depths of the Atlantic. "At the wall — 130 feet down — there was algae. The algae guys said they'd never seen algae so deep before."

Like pirates who have hijaked the reef, algae kill corals in several ways. Some algae coat entire areas of the reef with slick layers of slime. The interconnected polyps that make up living coral contain single-celled zooanthellae. These plant-like organisms are thought to help feed their hosts. If algae blocks the light necessary for photosynthesis, the zooanthellae die and the coral can starve.

Algae also can chafe the coral, joining the coral-munching parrotfish and hurricanes that wear bare spots on the reef. And in a race to fill this empty real estate in one of the hottest markets on Earth, algae will almost always win against slow-growing coral.

What's causing this algal growth? Sean thinks a decimated Caribbean sea urchin population and over-fishing of the reef have robbed the ecosystem of two major algae grazers.

In addition, algae experts Brian Lapointe and Peter Barile note that inadequate waste treatment and fertilizer runoff is saturating the water with extra nutrients that drive algae wild.

Sean wonders — as do other scientists — how far along the 104-mile-long Andros barrier reef the damage extends, and whether the reef might look healthier outside our 26-square-mile survey zone.

"I've done three fore-reef dives and one deep dive," Sean says. "I'm not convinced this is representative of the whole reef, here. I'd need to see more sites."

"Initially, going out there, it looks pessimistic, though you have to look at it with some kind of optimism," Sean says. "There are positive signs: for instance, there's lots of juvenile coral around."

"But," he adds, after a moment of reflection, "it can take 100 years for the coral to grow."

< Previous dispatch