A mysterious mass of black water that appeared off the southwestern tip of Florida at the end of November may have been caused by a nontoxic algal bloom, scientists say. Details of the vanishing patch remain murky.
Crew aboard fishing vessels noticed the black water in January, but reports of it didn't reach the scientific community until March. When remote sensing scientist Chuanmin Hu dug through recent satellite images, he found traces of the black blob in Florida Bay as early as November. Hu, a researcher at the University of South Florida (USF) in St. Petersburg, says that the mass reached its peak size in early February--3400 square kilometers, or roughly two times the size of Florida's Lake Okeechobee. It has been dissipating ever since. "We're basically looking at ashes from two months ago and trying to figure out what burned," says USF chemical oceanographer Cindy Heil.
Analysis of the first samples, taken 19 March, are consistent with an algal bloom. The water's black color appears to come from high concentrations of organic matter and chlorophyll, says chemical oceanographer Paula Coble also of USF. Heil reports that the low nutrient levels in the water are about normal for the aftermath of algal bloom. Algae or plankton have not yet been identified.
What prompted the bloom also remains a mystery. Researchers at a meeting on 28 March in St. Petersburg leaned away from the idea that the bloom resulted from nutrient-rich runoff caused by heavy rains in the last few months, says Hu, as had been suggested by some scientists in earlier media reports. But there is not enough evidence to completely rule out a river source, Hu says. Contrary to what other media reports suggest, the black water is probably not a "dead zone," says Jan Lansberg, an aquatic health specialist at the Florida Marine Research Institute in St. Petersburg. There is no evidence of fish kills, and anecdotes from recreational and commercial divers about the death of bottom-dwelling creatures may be hard to link to the black water, now that the water has moved on. Answers could come from benthic ecologist and diver Jim Culter of Mote Marine Laboratory in Sarasota, who is collecting bottom samples and will return on Tuesday.