Toxic tides: another reason to worry about hurricanes.

When Hurricanes Charley, Frances, Ivan, and Jeanne struck Florida in the summer of 2004, they killed 116 people, left thousands homeless, and caused billions of dollars in damage. Now, scientists suggest that the storms may also have triggered an intense, widespread Gulf of Mexico algae bloom that afflicted the state's western coast throughout 2005.

Commonly called red tides regardless of their color, toxic algal blooms frequently occur in the shallow waters off Florida's west-central coast. The organism primarily responsible for red tides there undergoes population explosions that scientists have been working to explain for more than a century, says Chuanmin Hu, an oceanographer at the University of South Florida in St. Petersburg. Toxins produced by the algae accumulate in shellfish, kill sea creatures, and irritate the eyes and respiratory systems of boaters and beachgoers.

At its peak, the 2005 red tide covered more than 67,000 square kilometers of shallow coastal waters--an area larger than the state of West Virginia. Hu and his colleagues now suggest that blame for the bloom falls on algae-boosting nutrients that reached the Gulf via the discharge of groundwater beneath the Gulf's surface. That seepage was driven by precipitation that had collected on land during and after the 2004 hurricanes.

One theory holds that red tides are fueled by phosphorus- and nitrogen-bearing plant nutrients brought to the region by local rivers or carried there from the Mississippi River by ocean currents. However, Hu calculated, those sources probably provided only one-fifth the amount of nitrogen that was needed to sustain the algal bloom.

Another model suggests that iron-rich dust wafting across the ocean from the Sahara triggers red tides. However, satellite data show that no more African dust came to Florida's Gulf coast in 2005 than in any of the previous 8 years, Hu found.

Hu points out that the four big hurricanes of 2004 dropped more than 38 centimeters of rain on central Florida in a single month, a rate of precipitation that hadn't been seen since 1970. After several months, enough water had accumulated in local aquifers to increase the rate of discharge below the Gulf's surface. That water carried algae-boosting nutrients, Hu's team asserts in the June 16 Geophysical Research Letters.

Groundwater seeping from land often carries at least 10 times the concentration of nutrients that seawater does (SN: 10/15/0.5, p. 248). Therefore, underwater springs can have a substantial effect on coastal ecosystems, says Ivan Valiela, a marine biologist in a Boston University laboratory in Woods Hole, Mass. "Even a small flow can have a large effect," he notes.

Under normal conditions, underwater springs in Tampa Bay carry more than one-fourth of the nitrogen-bearing nutrients that reach coastal waters, says Kevin D. Kroeger, a research chemist at the U.S. Geological Survey in St. Petersburg. The time needed for rain to reach the coast via aquifers hasn't been measured in Florida, but it's reasonable to estimate that an increased flow rate would show up in a few weeks or months, he notes. "It's a theory worth studying," says Kroeger.