What Fuels Florida Red Tide?

September/October 2011
aerial view of red tide bloom
Aerial view of a Karenia brevis bloom located offshore of
Pinellas County, 2005.

Florida's red tide blooms predate its European settlers. Researchers have traced the appearance of the toxic algae back nearly 500 years. Florida red tide blooms can kill massive quantities of fish and other coastal organisms. Humans can become ill after eating tainted shellfish and can experience respiratory irritation after breathing sea spray during a red tide. Biologists with the FWC's Fish and Wildlife Research Institute (FWRI) are studying how offshore and inshore nutrients influence red tide blooms and whether coastal nutrient sources fuel them.

Red tide nutrient studies date back to 1948, when scientists first described the Florida red tide organism that has since become known as Karenia brevis. In an ongoing study since 2006, FWRI's Harmful Algal Blooms researchers and their collaborators at six institutions have been using a multifaceted approach to answer their questions. They are analyzing data from a severe 2001 bloom (the first in which they collected extensive nutrient data), examining the organism's responses to different nutrient forms and sources in laboratory studies, and collecting samples and taking measurements in the field. Researchers have examined three different red tides in the same general area of southwestern Florida: a bloom in the initial stage of forming, a nearshore bloom, and an older bloom. Because blooms occur in the region almost yearly, the lack of a 2010 red tide provided a chance to compare and contrast conditions with and without blooms.

The research has shown that many different nutrient sources contribute to red tides. They range from natural and man-made nearshore sources to nutrients from recycled sources and the ocean floor. No single nutrient source will support large Florida red tide blooms. Karenia brevis is extremely flexible in the nutrient forms it can use, which include simple nutrients such as phosphate, nitrate, and ammonia and more complex organic compounds. This flexibility, along with an abundance of nutrient sources and local wind and current conditions, facilitates Karenia brevis blooms in southwestern Florida. Nutrients in fertilizer runoff were not observed to trigger red tide blooms, although they may contribute to the persistence of blooms in certain areas. Researchers have found that urea-a nutrient associated with runoff-supports blooms of nontoxic algae rather than red tide.

In addition to their nutrient analyses, project scientists documented the wind patterns and currents favoring the transport of these blooms from the Gulf of Mexico to the Atlantic Ocean. With this information, federal and state managers can better predict when conditions favor the movement of these blooms and thus alert coastal communities to possible risks. Upon completion of the study, researchers will distribute their findings to environmental managers and the public.

The National Oceanic and Atmospheric Administration provides funding for this study, in which FWRI's partners are the University of South Florida, Mote Marine Laboratory, the University of Miami, Old Dominion University, the University of Maryland, and Virginia Institute of Marine Science.

FWC Facts:
Four species of horseshoe crabs exist today. Only one species, Limulus polyphemus, is found in North America, along the Atlantic and Gulf coasts from Maine to Mexico.

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