A mysterious patch of "black water" that brought widespread death to reefs in the Florida Keys contained massive concentrations of microscopic plants, a new study shows. Measuring about 100 kilometers wide at its peak, the black water was spawned by the one-two punch of a "red tide" followed by an algal bloom.
Population explosions of reddish-brown microscopic plants called dinoflagellates periodically cause "red tides," killing marine life and sometimes sickening humans. During fall 2001, satellite images showed an extensive red tide off Florida's central west coast. To investigate the much-publicized phenomenon, a team of researchers led by oceanographer Chuanmin Hu of the University of South Florida in St. Petersburg analyzed ocean color and sea-surface temperatures recorded by satellites. The red tide began migrating in late 2001, darkening as it moved southward, and then stagnating over the Florida Keys (ScienceNOW, 1 April, 2002).
Samples of the stagnant water contained the toxin-producing dinoflagellate Karenia brevis, which was abundant enough to kill fish. But even greater numbers of nontoxic diatoms (also microscopic plants) were present in the dark water, Hu and colleagues reported online 15 February in Geophysical Research Letters. The extraordinarily high concentrations of sunlight-absorbing plants, in addition to dissolved materials, caused the water to appear black in satellite images, Hu says. Although dark water associated with diatom blooms and turbid river discharges is not unusual in the region, the 2002 event was the biggest and longest lasting one since ocean-color measurements began in 1997, he says.
In the wake of the black water invasion, divers surveyed two sites 55 kilometers apart, where corals had been relatively healthy. They found 40% fewer coral species, 70% less coral cover, and other losses to seafloor life. The researchers believe that nutrient-rich freshwater, draining from southern Florida during winter storms, fostered the diatom bloom in the red tide waters, darkening them. This, combined with the dinoflagellate toxin, likely overwhelmed marine life. By May, the black water moved through the Keys and dispersed into the Atlantic Gulf Stream.
"The research provides a direct connection between harmful algal blooms and coral reef decline," says Raphael Kudela, an oceanographer at the University of California, Santa Cruz. The researchers conquered devilish problems in using remote sensing as a tool for coastal monitoring, he adds, and others are likely to follow suit.