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About Red Tides in Florida

Aerial view of lake

Red tides, also called harmful algal blooms (HABs), occur when microscopic algae multiply to higher-than-normal concentrations, often discoloring the water. Although more than 50 HAB species occur in the Gulf of Mexico, one of the most well-known species is Karenia brevis, the red tide organism. K. brevis is found year-round at background concentrations of 1,000 cells per liter or less. Each cell is typically 20 to 45 micrometers long and 10 to 15 micrometers deep. It has two whip-like appendages called flagella that propel and direct it through the water at a speed of one meter per hour. In Florida waters, K. brevis thrives in high-salinity (salt content) areas but can tolerate a wide range of salinities and temperatures. The species forms nearly monospecific (single species) blooms by outcompeting or excluding other phytoplankton species.

dead fish in water

K. brevis produces brevetoxins capable of killing fish, birds and other marine animals. Brevetoxins may also cause health problems in humans, including respiratory irritation when wave action breaks open cells and the toxins become airborne. People who consume shellfish contaminated with brevetoxins can suffer Neurotoxic Shellfish Poisoning.

Blooms of K. brevis also affect Florida’s economy. Coastal communities that rely on tourism lose millions of dollars when dead fish wash up on beaches or beachgoers experience eye and respiratory irritation, and shellfish-harvesting businesses lose income when shellfish beds are closed. A study of three red tide blooms that occurred in the 1970s and 1980s estimated losses from each to be between $15 million and $25 million.

K. brevis blooms occur in the Gulf of Mexico almost every year, generally in late summer or early fall. They are most common off the central and southwestern coasts of Florida between Clearwater and Sanibel Island but may occur anywhere in the Gulf. Blooms are less common but do occur along the southeastern Atlantic coast as far north as North Carolina. Most blooms last three to five months and affect hundreds of square miles, but they can continue sporadically for as long as 18 months, affecting thousands of square miles.

Prior to the early 1970s, red tides in Florida were believed to originate inshore because blooms and respiratory irritation were most often observed first around passes and barrier islands. We now know that Florida's red tides begin in nutrient-poor water 18 to 74 kilometers (11 to 46 miles) offshore.

Blooms develop in four stages. The initiation stage occurs when a K. brevis population first accumulates and moves into an area. During the second stage, growth, the population steadily increases. Within a few weeks, K. brevis concentrations may be high enough to kill fish. The third stage is maintenance, during which wind and currents control the bloom’s movement. If the bloom moves inshore, nutrient runoff from land may promote bloom expansion. A bloom can linger in coastal areas for days, weeks or even months. The fourth stage is dissipation or termination. During this stage, mechanisms such as winds and currents disperse the cells, introduce new water masses that reduce the concentration of K. brevis cells or move the bloom to a different area.

Biologists have documented the occurrence and abundance of K. brevis for more than 50 years, during which detection and monitoring technologies have changed dramatically. To monitor and track blooms of K. brevis, the HAB group combines water sampling and enumeration (cell counts), molecular tools, toxin analysis, detection via satellite imagery and predictive models of bloom movement. Data generated through traditional environmental sampling, in combination with data generated through newer approaches such as remote sensing and modeling, may give scientists the ability to forecast red tides and potentially mitigate their effects.