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It could be a bloom of microscopic plant life. Or a patch of discolored runoff from the Everglades. Or organic matter left over from a Red Tide bloom.

The patch of so-called black water _ about the size of Lake Okeechobee _ has captured the attention of a cadre of scientists from several research organizations. Their mission? To explain the "dead zone" drifting in the gulf about 25 miles northwest of Key West.

"My best guess is a very poor one at this point just because we haven't seen anything like it before," said Erich Mueller, director of Mote Marine Laboratory's Center for Tropical Research.

Scientists at the Florida Marine Research Institute (FMRI) in St. Petersburg, in conjunction with Mote scientists and a group from the University of South Florida in St. Petersburg, are studying water samples from the patch, looking at satellite images, checking the nutrient content of the water and analyzing the pigment.

"There are a lot of agencies doing investigations based on what we're good at," said Beverly Roberts, the FMRI research scientist coordinating the group.

Roberts said scientists ought to be able to compare notes and offer an educated guess about the patch of black water by late next week.

On Friday, there was consensus on a couple of points: The black patch seems to be getting smaller and less concentrated. Frank Muller-Karger, a USF professor of oceanography, said the patch has moved very slowly over the past three months from the Charlotte Harbor area to the Florida Bay.

"It has diluted quite a bit from what it was a few weeks ago," he said.

The scientists also concurred about the toxicity of the patch. It doesn't appear, at this point, to be killing fish or other marine life.

"All the reports from fishermen are that fish are scarce," said Mueller, of Mote. "Whatever it is doesn't seem to be terribly toxic. It's more interesting from a scientific standpoint. It's not as bad as Red Tide."

Among many theories, scientists are investigating whether the black water is being caused by either of two different bits of microscopic plant life found in black water samples.

They are known as Dactyliosolen fragilissimus and rhizosolenia. Just finding the organisms doesn't mean, however, that either is the cause of the black water since many organisms frequently are present in sea water.

Rhizosolenia is a suspect because it was found in medium concentrations in two different patches of water _ one of them described as "nasty" by a volunteer _ near Key West on March 16.

At a 1998 conference, rhizosolenia blooms were identified as the cause of brown surface discoloration along the western edge of Florida Bay, near the current black water patch.

Rhizosolenia is a type of phytoplankton, also called "grass of the sea." It is among the plant life that are building blocks for the marine food web.

"They're small and they're eaten by a lot of things," said Roberts, of FMRI.

Although it is not known to be toxic to marine life like its cousin Red Tide, rhizosolenia has the potential to kill fish, Roberts said. Under a microscope, the organism looks like thousands of toothpicks. If a bloom is thick enough, it has been known to clog the gills of fish. And it could deplete oxygen in the water, also causing distress to marine life.

Rhizosolenia, however, is not known to pose difficulties for humans.

On March 8, a volunteer collected a water sample about 20 miles northeast of Key West that appeared to be from a discolored patch. It had high concentrations of Dactyliosolen fragilissimus, another phytoplankton. It also is one of the "grass of the sea" organisms and is an important part of the marine food web in the same way as rhizosolenia. It, too, could clog gills in high enough concentrations and deplete oxygen.

Scientists also are looking into whether decaying plant matter could be causing the discoloration, or whether it is merely churned up water, or whether it is a patch of freshwater runoff, colored by dirt or dead plants.

In the early 1990s, researchers identified a large "dead zone" off the coast of Louisiana, thought to be caused by Mississippi River water that carried high concentrations of fertilizer and waste material from animals and humans. The huge area, last year measured at 8,000 square miles, lacks dissolved oxygen and cannot support sea life.

In the Florida black water episode, the relatively high salinity of the water, said Muller-Karger, would seem to rule out the possibility that the patch was caused by a burst of freshwater runoff.

Another possibility, said Muller-Karger, is that the patch of discolored water, which ranges from deep green to brown to black, is some leftover matter from Red Tide outbreaks. This fall, a large and reoccuring outbreak of Red Tide seemed to germinate in the same area in which the black water patch began.

While the possibilities are intriguing, the scientists involved were cautious about coming to any conclusions before they had a chance to examine the tests conducted by their peers.

"As scientists, we like to keep an open mind," Roberts said.