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Mississippi River Nutrients May Feed Red Tide Blooms In Florida

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By MIKE SALINERO, The Tampa Tribune

Published: November 8, 2007

Federal scientists think they can link pollution flowing down the Mississippi River to red tide blooms that in recent years have devastated tourism, fishing and other water-related businesses along Florida's Gulf Coast.

Several aspects of the hypothesis were challenged, however, by the state's leading red tide scientist, who says a direct connection hasn't been proved.

Research by the National Oceanic and Atmospheric Administration suggests early summer winds blow nitrogen-laden water east from the mouth of the Mississippi to waters off Florida's shore. The nutrients then feed thinly dispersed red tide algae cells, called Karenia brevis, until they multiply in sufficient quantities to form blooms.

According to the NOAA hypothesis, a combination of wind and currents carries blooms in from as far as 100 miles offshore.

If true, the theory would answer a riddle that has puzzled researchers for years: Where do the red tide cells get the necessary food for growth in the relatively low-nutrient waters of the Gulf?

"What this means is we can expect the presence of the red tide bloom in a low-nutrient area, and that's why it happens every year," said Richard Stumpf, a NOAA scientist involved in the study.

But Cindy Heil, senior research scientist at the Florida Fish and Wildlife Research Institute in St. Petersburg, challenged several aspects of the NOAA research. For instance, NOAA scientists say red tide blooms start 100 miles or more offshore. That's where the Karenia cells come in contact with the plume of nutrients from the Mississippi River, according to NOAA scientists.

Heil disputed that, citing data collected by her predecessor at the institute, Karen Steidinger, for whom Karenia brevis was named. Steidinger's research showed red tide blooms form 11 to 40 miles offshore.

"There's a disconnect between the red tide initiation zone and the location of the plume once it's off Southwest Florida," Heil said.

Heil said she is skeptical a plume of nutrients pushed to Florida from the Mississippi River would have enough nitrogen left to fuel a red tide bloom. Huge quantities of the nutrients are consumed by algae where the Mississippi River flows into the Gulf. These algae blooms are responsible for a 7,000-square-mile dead zone around the mouth of the river.

"All the microorganisms are in competition for these nutrients," Heil said. "So you wouldn't expect a lot of the nutrients to be left in this plume by the time it gets to South Florida."

Richard Pierce, senior scientist at Mote Marine Laboratory in Sarasota, said he thinks Heil's points are valid but don't necessarily make the NOAA hypothesis wrong. He said red tide blooms may be forming farther out as Stumpf has suggested, but nobody is monitoring there to prove it.

"I think they're both right," Pierce said. "Cindy Heil is saying we need to be out there and doing more testing to see where these blooms actually start."

Pierce agreed with Heil that there are other nutrient sources that have been identified as fueling red tide. For instance, tricodesmium, a type of bacteria, uses iron dust blown here from the Sahara Desert to excrete nitrogen gas into the water. That gas is used as food by red tide algae.

"The bottom line is this is an excellent piece of research, it's a very good hypothesis, but it's not 'the answer' to red tide," Pierce said. "It's part of the puzzle, but there are other contributing factors to red tide."

Mote will be one of the agencies testing the NOAA hypothesis, Pierce said. Scientists at the lab have developed sensors they call "BreveBusters" to detect red tide cells on a "real time" basis. The sensors will cruise Gulf waters on remote-controlled vehicles, documenting the chemical and biological environment in which Karenia brevis grows.

Reporter Mike Salinero can be reached at msalinero@tampatrib.com or (813) 259-8303.

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