Florida red tides linked to Mississippi runoff

updated 4:30 p.m. ET Nov. 7, 2007

WASHINGTON - Harmful red tide blooms along Florida's west coast in the fall are spurred when seasonal changes in wind patterns move fertilizers and other runoff east from the Mississippi River, scientists reported Wednesday.

Harmful algal blooms occur from time to time in most coastal areas, with different algae affecting different areas. Florida's red tide blooms are caused by an organism called Karenia, and researchers have been seeking ways to better forecast when they will appear.

Researchers led by Richard Stumpf of the National Oceanic and Atmospheric Administration concluded that Mississippi River nutrients, especially nitrogen from fertilizers, were encouraging the blooms, which occur along fronts of changing water density in the ocean. Changes in temperature or salinity can result in differing water density.

Karenia can swim up and down in the water, allowing them to feed on deep nutrients and then can come to the surface for light, forming the toxic blooms, Stumpf explained at a news conference.

Now that they have found the algae congregating below the surface, scientists are experimenting with an underwater mechanism to detect the blooms before they come to the surface.

"The key goal is to do a better forecast," Stumpf said.

Nationwide, NOAA reports, harmful algal blooms have a direct economic impact estimated to average $75 million annually, including public health costs, commercial fishing closures, recreation and tourism losses and in management and monitoring costs.

Scientists had been puzzled about why the west Florida blooms formed in water that is normally low in nutrients that the algae live on.

"We found that the concentrations of nutrients needed to start the Florida red tides is much lower than previously suspected," said Stumpf. "The hypothesis means that offshore areas should be examined for both small increases in nutrients and modest concentrations of the algae at the start of the bloom season."

Normally, water from the Mississippi travels west, he explained, but seasonal wind changes in late summer and fall move it eastward toward Florida.

Findings of the research team are published in the journal Continental Shelf Research.

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