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 that plagues oceans around the world. The occurrence of various HABs is increasing globally, and the evidence showing increases in the frequency, intensity, duration, or distribution of Karenia blooms and their source has caused 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 Karenia blooms. It is known that Karenia blooms can begin in low-nutrient waters of the offshore Gulf, and that nutrient pollution in coastal waters feed algae, including Karenia. Experts agree that reducing coastal wastewater and stormwater nutrient pollution is a worthwhile endeavor that should not be postponed until a definitive scientific correlation is made to red tide.

The Gulf of Mexico is an epicenter of Karenia blooms, which historically have occurred in late summer or fall. 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. In recent years, Karenia blooms have persisted nearly year-round in concentrations that have had devastating consequences for aquatic wildlife, fisheries, tourism, and human health.

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

Effects of Red Tide Concentrations

Description Karenia brevis
Possible Effects (K. brevis only)
PRESENT background levels of
1,000 cells or less
VERY LOW a 1,000 to 5,000 Possible respiratory irritation
VERY LOW b 5,000 to 10,000 Possible respiratory irritation; shellfish harvesting closures
LOW a 10,000 to 50,000 Respiratory irritation; chlorophyll levels too low to be detected by satellites
LOW b 50,000 to 100,000 Respiratory irritation, fish kills probable; chlorophyll detectable by satellites
MEDIUM 100,000 to 1,000,000 Respiratory irritation, fish kills, other wildlife impacts probable
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 coastal Water Atlas websites. During this process, 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.

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 Counties. 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 vary widely depending on depth, salinity, water temperature, 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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