Andros Island, Sept. 6 — Caribbean waters should be blue: turquoise, royal blue, azure, cerulean. Around Andros, the water is green; light green in some places, dark green in others.

That gorgeous blue we think of when we imagine tropical waters occurs when there's not much micro-algae in the water and the sunlight is reflected back from the bottom. The blue also is a sign that there aren't many chemicals in the water — nutrients that nourish algae and turn the waters green.

"Coral reefs have evolved to live in very low-nutrient conditions," says Brian Lapointe, an algal expert and director of the Marine Nutrient Dynamics Program at Harbor Branch Oceanographic Institution in Florida. "Recycling of nutrients is concentrated on the reef. Reefs have evolved to rely on that recycling, so when they get a big influx of nutrients, it throws the whole reef out of balance."

Video: The world's third largest coral reef off the island of Andros is teeming with colorful creatures.

Algae seem to be overabundant on the Andros reef, agree Brian and Peter Barile, a doctoral candidate at Florida Tech. They've seen this imbalance many times before: in the Bahamas, Florida Keys and, more recently, Negril, Jamaica. They theorize that nitrogen runoff from agriculture and settlements on Andros is responsible for the algae that coats the barrier reef.

In some places on the patch reef, behind the reef crest, blue-green algae that looks like mucus drips from the tops of caves eaten into dead coral. To me, this part of the reef is downright ugly. Different species of algae coat most things. The occasional healthy coral head stands out in the barrenness.

"It's like a yard," Brian says. "Look at fertilizer and what are the ingredients? Nitrogen and phosphorus. Add that to ocean plants and their growth will explode, too."

Peter stayed at Forfar Field Station 12 years ago and took a close look at the reef then. Thriving coral was all over the place, he remembers. Now, he says, there's much less coral and much more algae.

"Twelve years is fast for a shift to a different balance on a reef that has been growing for thousands of years," Brian says.

Corals, which have been evolving for 600 million years, are highly adapted to a difficult environment. They grow very slowly. Some species take a decade just to reach sexual maturity. Algae, on the other hand, are fast growers, and opportunistic. In a race to fill a bare patch on a reef, algae will almost always win out over corals.

The balance of reef ecosystems has changed before. Tens of thousands of years ago there was a relatively slow, natural shift in sea currents that caused a surge in algal growth on reefs, according to Brian. But this time, it appears as if humans are responsible.

"Coral reefs have never had to cope with the excess nitrogen they are now being subjected to," says Brian.

He and Peter have analyzed nitrogen levels in waters around reefs near resort areas in Negril and found the chemical signature of sewage. In other areas, usually near farms, nitrogen has the signature of fertilizer runoff.

Now, Peter is looking at the atmosphere as a source of nitrogen in the sea. Because the Caribbean is downwind from the biggest consumer of fossil fuels in the world — the United States — Peter suspects that nitrogen from fossil fuels is responsible for algal growth in remote locations.

"There's a lot of dead coral out there," says Sean Grace, an anthozoan expert from the University of Rhode Island who is looking closely at the condition of the reef.

Sean agrees that an explosive growth in algae here seems to be responsible for much of the coral death. Traditionally, he says, algae is kept in check by predators, primarily fish and sea urchins. But over-fishing and a 1980s die-off of certain sea urchins also contributed to the problem, he says.

"It's been noted that on a few carefully protected test reefs in Jamaica, when the fish come back, algae cover decreases," he says.

What does all this mean for the Andros reef?

Over-fishing and an excess nitrogen in the water must be addressed immediately to help reefs recover, Sean says, adding that sea urchins also need to be protected.

The future of reefs is not very bright, Brian says: "There's a lot of algae out there."

< Previous dispatch

Next dispatch >