A mysterious patch of black water off the Florida coast, the extent of which was detailed by satellite images, is most likely the result of a bloom of algae, officials said today after examining the results of water tests.

The bloom, which cropped up early this year and peaked around Feb. 4, is a nontoxic cousin of the red tide, which has been blooming off southwest Florida since last August. Gil McRae, a scientist with the Florida Marine Research Institute, said the black water bloom does not appear to be hazardous to humans, adding that the event is not likely the result of polluted river runoff or any other human or mysterious cause.

"Nothing indicates that this is anything other than a natural event," McRae said in a telephone interview. Importantly, he added, "there's no toxicity associated with it like there is with a red tide."

"Some of these things were probably quite common in the past, but we didn't know about them because we didn't have the imagery," he said. "It's invaluable to us."

Researchers from several institutions have been studying samples of the black water in recent weeks to try and solve what one scientist said was a type of discoloration that had never been seen before in the region.

McRae said the bloom was likely caused by organisms called diatoms. The discoloration is due to extra plant mass in the water. He said there remains concern that as this plant matter settles to the sea floor, bottom-dwelling plants and animals could suffer.

Why the diatoms generated a bloom remains unknown. McRae said it could be a transition of some sort from the red tide that has been present in the region and is caused by another organism.

Through the weekend and into early next week, divers will collect more water samples. Scientists will analyze the samples for nutrients and algae species in order to learn how other sea life fares. They will also conduct a chemical analysis that "may give us a handle on the source," McRae said.

In a joint statement, scientists said the discolored water "does not appear to pose a direct threat to marine life" other than the bottom-dwellers.

The latest satellite imagery indicates that the discolored water seems to be dissipating, the researchers said. The initial mass had broken into patches that now are spread along more than a hundred miles of southwest Florida, north of the Keys. The area is west of Miami and within the Gulf of Mexico.

The investigation included researchers from the Mote Marine Laboratory, the University of South Florida, the Florida Keys National Marine Sanctuary, and the Florida Department of Health.