Underwater fireworks

Underwater fireworks

On most moonless nights strings of kayaks float along the surface of Bahía Mosquito. The people have come here, to this otherwise unremarkable bay on the island of Vieques, to witness one of nature's most eye-popping events. As a paddle skims the top of the water, thousands of points of light appear just below the surface. Behind each kayak is a blue-green glow that slowly fades. Those who trail their fingers in the water can see the outline of every digit.

This glow is produced by dinoflagellates, single-celled organisms that are found everywhere. But the species found in the Caribbean, called Pyrodinium bahamense, happens to glow when the surrounding water is disturbed in any way. The concentration of dinoflagellates in Bahía Mosquito is an astounding 720,000 per gallon -- more than anywhere else on earth. This accounts for the nightly display of underwater fireworks.

Bahía Mosquito -- which means, of course, Mosquito Bay -- is not the only bioluminescent bay in Puerto Rico. There are a handful of others, including Fajardo's Laguna Grande and La Parguera's Bahía de Fosforescente. But they can't compare to the glittering waters of Bahía Mosquito. That's why travelers flock here from all around Puerto Rico.

The conditions at Bahía Mosquito are perfect for growing dinoflagellates. A mangrove forest surrounding the bay constantly drops leaves and branches into the water. As bacteria eat the decaying matter, they produce prodigious amounts of vitamin B12 -- an essential part of the dinoflagellate diet. The lagoon's mouth, resembling the neck of a bottle, keeps the nutrients from being swept out to sea. The narrow mouth also keeps the brackish water in and the salty water out. These tiny creatures, like the rest of us, are healthier when they don't have so much salt in their diet.

Other bioluminescent bays around the world have lost their luster, mostly because of pollution. Light pollution is one of the main causes, as the dinoflagellates can't compete with the glare of headlights or the glow of streetlamps. (Even the moon makes the glow difficult to discern.) On Vieques, conservation-minded citizens constantly remind their neighbors to make sure their outdoor lights are shaded so they point downward. They have also fought development near the bay, including a new road and a sports complex that would have destroyed the bay almost overnight.

Water pollution is another problem, including industrial waste, sewage drainage, and agricultural runoff. Ironically, spillage from the gas- and diesel-powered boats that take tourists to see the spectacle is often the culprit. The smoke-belching boats that depart for La Parguera's Bahía de Fosforescente every night are slowly putting themselves out of business. The display in La Parguera, once as dazzling as the one in Vieques, is dismal.

On Vieques these boats are banned from Bahía Mosquito. Instead, there are kayaks and electric-powered pontoon boats that barely cause a ripple on the surface. But there are plenty of waves when the passengers dive into the inky black water, leaving behind them a trail of light that reminds many people of the tail of a comet.