AN ENIGMATIC mass of black water more than twice the size of London has appeared this year in waters off Florida, alarming fishermen and confounding scientists.
The first reports of the black water came from fishermen in the region in January. Archived satellite images revealed that the black patch emerged off south-west Florida late last November and eventually spread to cover about 3400 square kilometres of water, including a large portion of the Florida Keys National Marine Sanctuary.
鈥淐ertainly in recent history, as far as I know, this is a quite unusual phenomenon,鈥� says Brian Keller, science coordinator for the sanctuary. There are reports of a similar event occurring around 1900, but little is known about it.
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Initially, researchers thought an unusually dark 鈥渞ed tide鈥濃�攁 bloom of a toxic alga鈥攎ight be to blame. However, they didn鈥檛 find high concentrations of the alga, and there were no reports of the numerous dead fish usually associated with red tides.
But researchers who met in the region last week agreed that there鈥檚 a more likely explanation: a bloom of another type of algae called diatoms could be darkening the water. Heavy rains at the beginning of the year could have flushed high levels of silicates and nutrients from land, feeding the diatoms out at sea. And some samples of the water do indeed contain higher than normal levels of diatoms.
Ed Little, a biologist with the National Marine Fisheries Service based in Key West, says some fish catches from the region have been abnormally low this year, but other factors could easily explain this. A few observers have found dead marine creatures such as fish and sponges in the region, and there are anecdotal reports that fish are avoiding it. But there鈥檚 no proof that any of this is linked to the black water, and diatom blooms are not toxic, so these stories would not fit in with the favoured explanation.
Another hypothesis implicates an algal bloom as an indirect cause of the black water. Blooms can lower oxygen levels in water, causing plants on the sea floor to die. These conditions would promote the growth of bacterial mats that could have darkened the water as they broke up.
Jim Culter, an ecologist who specialises in seabed organisms at the Mote Marine Laboratory in Sarasota, Florida, says he saw this process happen on a small scale in the area last autumn. He did not see any darkened water but says the bacterial material was blackish, and it could blacken the water if it broke loose. 鈥淏ut it would take a pretty darn big growth,鈥� says Culter.
The massive patch of black water now appears to be dissipating. And although researchers think diatoms are almost certainly the cause of the strange phenomenon, they are keen to confirm that hypothesis, and still need to explain the dead marine life. 鈥淭here are still a lot of unanswered questions,鈥� says John Hunt of the Florida Fish and Wildlife Conservation Commission, based in the Florida Keys.