The four hurricanes that crossed the Sunshine State in 2004 dumped as much as 27 inches of rain - nearly double the historical values - in central Florida, which increased ground water levels and rates of surface runoff. These two factors are thought to create conditions ripe for the bloom of a red tide.

Red tides, which may or may not be harmful, are primarily caused by the toxic phytoplankton, Karenia brevis, on the west Florida Shelf. This species of phytoplankton can produce toxins that can kill marine organisms and lead to irritation of the eye and respiratory systems of animals and people.

Scientists Chuanmin Hu and Frank Muller-Karger of the University of South Florida, St. Petersburg, Fla., and Peter Swarzenski of U.S. Geological Survey, St. Petersburg, Fla., note that several factors may have contributed to the 2005 red tide - the 2004 hurricanes and their heavy rain, runoff and submarine ground water discharge. The scientists used NASA satellites, as well as observations from ships, buoys from the National Oceanic and Atmospheric Administration (NOAA), and geochemical tracers to study red tide.

Red tides occur in the same area off west-central Florida almost every year, mostly between August and March, from north of Tampa to south of Naples, Fla. The 2005 red tide poisoned the ocean waters and killed fish, turtles, birds and marine mammals. This red tide was unusual because it lasted an entire year.

"Runoff alone provided insufficient nitrogen to support this bloom," says Hu. "Submarine ground water discharge injects water coming from the water table underneath Florida into the ocean through the ocean floor, below the ocean's surface and off the coast. We believe that submarine ground water discharge provides the missing nutrients, and may trigger and maintain red tides off west-central Florida." Hu and his colleagues propose that the unusual number of hurricanes in 2004 resulted in high runoff, and in higher than normal discharge of ground water that was deposited along the west Florida coast throughout 2005. The ground water discharge also may explain why the red tides happen in the same area almost every year, but also why they happen in other coastal regions of the Gulf of Mexico, Hu and Swarzenski explain.

In the past, hurricanes were thought to dissipate red tides. However, a series of hurricanes in summer 2005, including Hurricane Dennis and Hurricane Katrina, only caused significant water mixing, and storm surges where dead fish were washed ashore. After sediment settled on the ocean floor, the red tide appeared even more extensive.

NASA satellites were used to detect the red tide blooms in December 2004. By the end of that month, satellite images showed that the bloom covered 425 square miles immediately off Tampa Bay. By late September 2005, the bloom expanded to approximately 26,062 square miles.

Submarine ground water in Tampa Bay has been shown to be a larger nutrient source than local rivers. There also are numerous submarine springs off the Florida coast, and some are located near where red tides occur every year. Hu, Muller-Karger and Swarzenski suggest that frequent hurricanes result in elevated rainfall that leads to an increase in the surface runoff and possibly discharge of submarine ground water, which lead to a larger, persistent red tide event.