Scientists had suspected runoff from the enormous amount of water dumped on Florida by five tropical storms in 2004 contributed to red tides the following year, but a recent study indicates some of the nutrients actually came from groundwater leaching up through the ocean floor.
Red tides, which may not always be biologically harmful, are caused by rapid growth of algae in saltwater estuaries and coastal waters. With tens of thousands, even millions of cells per gallon of water, the blooms can make the water appear brown or red.
Along much of the Florida Gulf coastline, red tides are caused by a species called Karenia brevis, which can produce toxins that can kill marine mammals like whales, seals and otters and can lead to irritation of the eyes and respiratory systems of humans and other animals.
Researchers at the University of South Florida and the U.S. Geological Survey in St. Petersburg, Fla., sought to identify how the rainfall from the hurricanes of August and September 2004 were linked to the extreme red tide event that started early in 2005.
They used data from NASA environmental satellites as well as observations from ships, research buoys operated by the National Oceanic and Atmospheric Administration and geochemical tracers to study how the red tide evolved. They reported their findings last month in the journal Geophysical Research Letters.
Red tides occur in the same area off the west-central coast of Florida almost every year, mostly between August and March, from north of Tampa to south of Naples.
But last year's red tide was unusual because it lasted an entire year and killed fish, turtles and birds as well as marine mammals.
During the previous hurricane season, Tropical Storm Bonnie and Hurricanes Charley, Frances, Ivan and Jeanne crossed Florida, dumping as much as 27 inches of rain on portions of central Florida in less than two months.
Satellite images showed that by the end of December the red tide bloom extended over 425 square miles; by late September 2005, it covered more than 26,000 square miles.
"Runoff alone provided insufficient nitrogen to support this bloom," said USF researcher Chuanmin Hu. "Submarine groundwater discharge injects water coming from the water table underneath Florida into the ocean floor, below the ocean's surface and off the coast."
The researchers used dye tracers dumped into some of Florida's famous submarine springs to confirm that inland supplies of nutrients could migrate out into the ocean beneath the ground and sea floor.
Other studies already have shown that submarine groundwater is a larger source of nutrients into Tampa Bay than local rivers even when rainfall is average.
"We believe that submarine groundwater discharge provides the missing nutrients, and may trigger and maintain red tides off west-central Florida," he added.
Groundwater discharges may explain not only why red tides seem to happen in the same area most years, but also why they happen frequently in other coastal regions of the Gulf, the researchers said.
The study also put to rest another misconception about hurricanes and red tides. It had been thought that big tropical storms tend to break up the algae blooms.
But the researchers found that as Hurricane Dennis spun past the area on the way to the Florida panhandle in July and Hurricane Katrina brushed the Keys on the way to Louisiana and Mississippi in August, they caused significant water mixing and storm surges that washed dead fish ashore.
Yet after sediment settled on the ocean floor, the blooms appeared to grow even more extensive.