Last year, the waters off western Florida turned red and stayed that way for 12 months. The cause was a dense concentration of algae, known as red tides, which sucked oxygen from the water, suffocating marine animals and releasing allergy-inducing toxins into the air. Scientists have long linked red tides to dust clouds from Africa, but a new study indicates that the destructive U.S. hurricane season of 2004 may have been the primary culprit for last year's algal explosion.
Dust can drift tremendous distances. Clouds originating in the Saharan desert reach Florida waters and seed them with nutrients that a blue-green phytoplankton known as Trichodesmium needs to flourish. In turn, this organism produces nitrogen, which stimulates the proliferation of the red algae Karenia brevis. Scientists have often blamed the complex phenomenon for red tides, but Frank Muller-Karger, a biological oceanographer at the University of South Florida (USF) in Saint Petersburg, was skeptical. Red tides always seem to occur at around the same time of year and in approximately the same place, he says: "I have a hard time imagining that dust from Africa falls only on the central coast of Florida." Instead, Muller-Karger and colleagues wondered whether the intense rains brought by hurricanes were flushing nitrogen out to sea.
To test the theory, the team measured the nitrogen content in the aquifers below Florida's Tampa Bay area. As water seeps from these underground reservoirs to the Gulf of Mexico, it picks up minerals from the earth. The researchers found that the groundwater discharge from Tampa Bay alone contained 35% as much nitrogen as seen in all of the westward draining central Florida rivers combined. Torrential rains can cause rivers to overflow, but this wouldn't supply enough nitrogen to cause red tides, says Muller-Karger. However, such rains would increase the output of groundwater discharge, he says, and when every spring is taken into account, they could supply the nitrogen needed to fuel a red tide. Because groundwater takes time to reach the ocean, it could take months for these effects to be seen. The team reports its findings in the June issue of Geophysical Research Letters. Co-author Chuanmin Hu adds that red tides don't occur in the dry season, when little groundwater reaches the ocean.
Groundwater definitely warrants a close look, says Gabe Vargo, a biological oceanographer at USF. But he cautions that because the researchers only focused on the Tampa bay area, it is not known how much these springs contribute to the nitrogen runoff into Florida's coastal waters in general. The next step, says Muller-Karger, is to locate and evaluate more elusive submarine springs, and to measure the connection between rainfall on land and the strength of the groundwater flow.