Sarasota County Water Atlas

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Red Tide

What does this mean?

Red tide is a descriptive term for discolored water that is applied by people around the world to some very dissimilar organisms. In Florida, red tide describes a higher-than-normal abundance of Karenia brevis, a naturally occurring dinoflagellate alga. This is an example of a Harmful Algal Bloom (HAB), a condition known to plague oceans of the world. The occurrence of various HABs are increasing globally, but the spotty evidence showing local increases in the frequency, intensity, duration, or distribution of Karenia blooms is the source of much spirited debate among scientists and the public. Unlike many algal blooms, it has been difficult to establish a relationship between man-made pollution and Kareniablooms. It is known that Karenia blooms can begin in low-nutrient waters of the offshore Gulf. Experts agree that reducing coastal wastewater and stormwater pollution is a worthwhile endeavor that should not be postponed until a scientific correlation is made to red tide.

The Gulf of Mexico between Tampa and Fort Myers is the epicenter of Karenia blooms that usually occur in late summer or fall almost every year. Karenia is usually present in the Gulf at background concentrations of about 1000 cells per liter. In August of 2005, a rare mass mortality event killed Gulf aquatic life over a 2,100 square mile area about 10 miles offshore from the Tampa Bay area. The low oxygen concentrations that devastated marine life were correlated to red tide and a thermocline. A similar event may have happened in 1971.

A brief but comprehensive overview of Red Tide is available online at:

Effects of Red Tide Concentrations

Description Karenia brevis (cells/liter) Possible Effects (K. brevis only)
PRESENT background levels of 1,000 cells or < None
VERY LOW a >1,000 to <5,000 Possible respiratory irritation
VERY LOW b 5,000 to 10,000 Possible respiratory irritation and shellfish harvesting closures
LOW a >10,000 to <50,000 Respiratory irritation, but chlorophyll levels too low to be detected by satellites
LOW b Respiratory irritation, maybe fish kills, & bloom chlorophyll probably detected by satellites
MEDIUM Respiratory irritation & probable fish kills
HIGH >1,000,000 As above plus discoloration

How are the data collected? (Methods)

Microscopic analysis of water samples is a common way to assess the abundance of red tide and is the source of data shown on the Sarasota Water Atlas. To do this, Karenia brevis cells are laboriously counted in a known water volume, a concentration is calculated, and reported as number of cells per liter. Cell counts are a widely used and mature technique that is approved by the Food and Drug Administration (FDA) for closure of commercial shellfishing operations.

There are several other ways to measure red tide and some new methods under development. Using bioassays of shellfish tissue is the FDA approved method for reopening shellfish beds closed because of red tide. Other techniques involve satellites, genetic probes, and various optical methods. EPA is currently (2007) reviewing nine new methods and comparing them to cell count data.

A sampling network is coordinated by the Fish and Wildlife Research Institute (FWRI) that involves volunteers and professionals and includes the Florida Fish and Wildlife Conservation Commission, Mote Marine Laboratory, Florida Department of Agriculture and Consumer Services, Rookery Bay National Estuarine Research Reserve, and the health departments of Sarasota, Lee and Collier County. More information about this effort is available online at


Using a microscope, Karenia brevis cells are counted in a known water volume, a concentration is calculated, and results are reported as a number of cells per liter.

Caveats and Limitations

Karenia concentrations often vary widely depending on depth, salinity and currents. A discrete sample from a specific location is unable to characterize the concentration throughout a large water body that is not uniformly mixed. This concern is no different than with many other environmental sampling efforts and is no reason to discount the data. Having an abundance of samples is a suitable offset for spatial variation in situations like this.

Additional Information

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