Researchers studying black water off Floridaz

ST. PETERSBURG, Fla. -- Scientists who study the waters off Florida are trying to determine what's causing a huge patch of dark water off the southwest Florida coast.

Commercial fishermen who first spotted it in January have described it as black water; others said it's dark brown; still others characterized it as brownish-green.

It has covered hundreds of miles from Naples south to Key West and west to the Marquesas.

Researchers at the Florida Marine Research Institute are pooling results of various water sample tests. They're comparing charts of dark water sightings and studying satellite pictures of the mysterious patch near the Florida Keys as it moves across Florida Bay from the northwest carried by currents.

"It's dissipated and diluted, but still there," said Chuanmin Hu of the University of South Florida College of Marine Sciences.

Scientists have a lot of information, some evidence but no answers.

"We are being detectives looking back in time," said John Hunt, a marine biologist who runs the institute's laboratory in the Florida Keys. "We have a lot of uncertainties."

There is life in the water, so water samples may help resolve the mystery, researcher Gil McRae said. "The characterization of black water as a dead zone is not accurate," he said.

Scientists have not seen the fish kills that are characteristic of an outbreak of red tide, an oxygen-depleting algae that kills marine life, but they are concerned about the tiny animals of the coral reefs.

Researchers don't have enough information to determine whether the dark water is an outgrowth of red tide.

They don't believe the dark water poses any health risk to divers.

Diver sent in

Mote Marine Laboratory sent a diver into the darkest areas off the Keys to collect bottom samples, hoping that sheds some light on the oddity.

Sampling yielded a mixed bag. Surface water levels ranged from clear spots to areas showing medium levels of red tide and diatoms -- a specific type of algae bloom. Bottom sampling showed low concentrations of these algae, although testing is incomplete.

Hunt thinks the dark water is a natural phenomenon that's been there all along, but is 10 times larger this year. What factors influence size he doesn't know.

Water samples seem to support this theory.

As red tide moves in toward shore it dies out, he said. Decaying red tide mixes with diatoms, which feed on a nutrient found in freshwater flow. The bloom that develops grows in size and is pushed by the currents, Hunt said.

Plankton needs nutrients to grow and nutrients come from the land through river runoff. But researchers don't know what nutrients are involved in the origin of black water because they're all gone, said Paula Coble of the University of South Florida.

She said scientists need to discover a patch of black water when it's new to study it.

John Walsh of USF College of Marine Sciences said black water is not new. He cited a newspaper clipping from 1902 that reported black water off the Dry Tortugas.