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Red Tides

Florida Red Tide Bloom of Karenia brevis. (Photo courtesy Woods Hole Oceanographic Institution Harmful Algal Bloom Page) Florida Red Tide Bloom of Karenia Brevis. (Photo courtesy Woods Hole Oceanographic Institution Harmful Algal Bloom Page)
Immunofluorescently-labeled Alexandrium cell.  (Photo courtesy Kristin Gribble, WHOI) Immunofluorescently-labeled Alexandrium cell. (Photo courtesy Kristin Gribble, WHIO)


Red tides can often have a devastating effect on fisheries is some coastal areas. Red tides are caused by several species of microscopic plants called phytoplankton. These species produce strong chemical toxins poisons called toxins.

In Florida red tide typically starts in the Gulf of Mexico 40-80 miles off shore and slowly moves southeasterly with the prevailing ocean currents toward the Tampa Bay area. As the bloom increases the density of red tide organisms increase to several million cells in each liter of seawater. The result is a deadly mass of toxin-containing water moving toward southwest Florida, leaving a wake of dead and dying fish. These toxins cause extensive fish kills, contaminate shellfish and create respiratory irritation for people who live along the shore. The Scandinavian and Japanese fisheries, Caribbean and South Pacific reef fish, and shellfishing along US. coasts are sometimes affected by red tides.

The cause of red tides is not well known. Some scientists think coastal pollution helps produce red tides. In Florida, red tides are caused by a natural process. Because of the severe economic and public health effects of red tide, many oceanographers are using satellites such as SeaWiFS to map the location of red tides, and to understand what causes red tides.

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