Red Tide



Numerous types of algae produce harmful toxins that are responsible for poisonings usually involving seafood. However, the primary source of toxic algae in Florida is the Red Tide, which is produced by a dinoflagellate, Karenia brevis. It produces neurotoxic shellfish poisoning (NSP) that can cause gastrointestinal problems following ingestion of contaminated seafood. Symptoms may also include respiratory difficulties that are a consequence of inhaling the spray from contaminated seawater.

Another dinoflagellate commonly found in Florida waters is Pfiesteria piscacida, and poisonings were attributed to this organism in North Carolina and Maryland. Symptoms include neurological complications involving memory loss and impaired judgment. These symptoms are usually temporary, and the relationship of a toxin to human disease is controversial.




Algal blooms have become common to coastal waters of Florida for reasons that are not clearly understood. Encroachment of human impacts may favor their growth as the levels of essential nutrients, such as phosphates and nitrogen, are altered. One of the longest lasting red tide events is still ongoing on the west coast around Sarasota. Periodic outbreaks have closed oyster and clam harvesting all along the Florida Gulf coast. These tides are not only are a human health risk but are thought to be responsible for significant mortality in dolphins and manatees.


Florida seafood is monitored for possible "Red Tide" contamination. At the first possible indication that the dinoflagellates are present in shellfish-harvesting waters, harvesting is halted and can be resumed only after the shellfish are to be free of any toxin.

Prepared by Anita C. Wright, Ph.D.
Associate Professor of Food Microbiology
University of Florida Food Science and Human Nutrition Department

Food Safety