Scientists stir 'black water' mystery

Marine biologists: No easy answers, little apparent danger

This true-color image of the affected water off southwestern Florida was taken on March 21 by NASA's SeaWiFS instrument aboard the SeaStar spacecraft.

MIAMI, Florida (CNN) -- Researchers are comparing notes on a giant, mysterious area of "black water" in Florida Bay that they say seems to be an algae bloom -- a natural and common event, if one of the biggest in many years.

At this point, scientists say they can see no reason for panic. They say that although the algae may chase away fish, it doesn't appear to kill them -- and it doesn't appear dangerous to humans.

Fishermen and environmentalists say this could be a major wake-up call: They've reported die-offs of sponges and other sea-bottom life in areas touched by the dark-water bloom.

Near Key West, the dark water still can be seen but it's a greenish-brown, not black.

And although marine biologists in Florida are accustomed to seeing the "red tide" bloom, they concede they're stymied in this case.

CNN NewsPass VIDEO U.S. marine scientists are trying to determine what is causing the mysterious 'black water' found off the coast of the Florida. CNN's Mark Potter reports (April 2) Play video

"This is truly a detective story," says John Hunt of the Florida Marine Research Institute. "We had events occur. We're trying to collect information today and project back to see what has caused this event."

At its peak in February, the darkened water covered an estimated 700 square miles north of the Florida Keys and west of the tip of the mainland.

And scientists are warning as their tests go on that they may never fully solve the mystery of the bloom they're seeing.

CNN's Mark Potter has more on this curious phenomenon.