Learn More: Red Tide

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 2018 red tide outbreak off the coast of central and southwest Florida devastated marine life and caused an estimated $184 million in damage to the local economy. 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: https://myfwc.com/research/redtide/

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 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.

The process of counting the cells was once manual and laborius, but many scientists now use a NASA-funded cellphone microscope app, the ‘HABscope,’ that can, within minutes, analyze the concentration of cells of the red tide organism in a water sample. Water quality monitors place drops of water onto a microscope slide and take a video of the sample. Once they file the video of the sample, the app calculates the cells per liter, which in turn determines the level of red tide in the water. The app has an algorithm that can identify K. brevis based on its swimming pattern.

There are several other ways to measure red tide. 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 https://myfwc.com/research/redtide/statewide/.

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.

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.

• An Assessment of Florida Red Tide: Causes, Consequences and Management Strategies
• Aquatic Toxins Program
• Collaboration for Prediction of Red Tides (CPR)
• Comparative "Systems" Approach to HAB Research, The
• Counts of Red Tide Organisms 1954-57
• Effects of Florida Red Tide on Marine Animals
• Florida Gulf Coast Red Tide
• Frequently Asked Questions: Red Tide
• Harmful Algal Blooms and Coastal Business: Economic Consequences in Florida
• Harmful Algal Blooms in Coastal Waters of Lee County, FL: Bloom Dynamics and Identification...
• Harmful Algal Blooms in Stratified Environments
• Introduction to Red Tide
• Invasion of the Red Tide In North Carolina Coastal Waters
• Karenia brevis Fact Sheet
• Monitoring Approaches for Early Warning of Domoic Acid Events in Washington State
• Mote/FWC-FWRI cooperative red tide research program, 2008-09
• New Approaches and Technologies for Observing Harmful Algal Blooms
• Offshore Red Tide-Associated Mortalities and FWRI Event Response 08-17-05
• Primer on Harmful Algal Blooms in the Gulf of Mexico, A
• Red Tide and Other Harmful Algal Blooms (FWC)
• Red Tide Control and Mitigation Program: Report to Stakeholders
• Red Tide Current Status from FWC
• Red Tides In Florida
• Solutions to Avoid Red Tide (START)