The Florida red tide is a naturallyoccurring phenomenon that con-tinues to challenge researchers

seeking clues to its origin and cause.It has been documented alongFlorida’s Gulf Coast since the 1840sand probably occurred much earlier. Fish kills aroundTampa Bay were mentioned in the logs of Spanish ex-plorers. The source of these red tides—a group of tiny,plant-like organisms called dinoflagellates—was notdiscovered until the massive red tide of 1946–47 insouthwest Florida.

DescriptionRed tides with various characteristics have been docu-mented worldwide for thousands of years in cold tem-perate to tropical waters. Dinoflagellates, the organismsthat cause most red tides, are microscopic, single-celledorganisms characterized by two whiplike structures,each called a flagellum. One flagellum spins the cellaround and the other propels it through the water atabout three feet per hour. Dinoflagellates and othertypes of microscopic algae, collectively called “phyto-plankton,” are commonly referred to as the “grass of thesea” because they are so plentiful and have plant-likenutritional characteristics. They use the sun’s energy toproduce their own food and, in turn, are eaten by manyother kinds of marine life. In this way, they serve as a

foundation for the marine food web.Dinoflagellates can produce

some of the most powerful poisonsin nature. When certain dinoflagel-lates are present in higher-than-normal concentrations, a “bloom” is

created that releases poison, or toxin, into the water.This toxin can cause various effects; for example, it mayparalyze fish, causing them to stop breathing. Some-times, a bloom discolors the surrounding water. Thecolor may be red, but a bloom may also be yellow,orange, brown, or reddish-brown.That’s why scientistsprefer the term Harmful Algal Bloom (HAB).

In Florida, the most common cause of red tides isa toxic marine dinoflagellate named Karenia brevis (fre-quently abbreviated to K. brevis), which is a yellow-greendinoflagellate measuring only about 1⁄1000 of an inch long.A stingray-shaped single cell, it contains one flagellumencircling a groove around the middle of the cell anda second flagellum trailing behind like a ship’s rudder.The cell’s forward motion resembles a gently falling leaf,turning over and over in the water as it swims, but K.

Scientific name Karenia brevis (pronounced Kah-REN-ee-uh BREV-is, often abbreviated to K. brevis). Formerly known as Gymnodinium breve and Ptychodiscus brevis.

Size About 1⁄1000 of an inch longRange Documented throughout the Gulf of Mexico and along the Atlantic coastline to North

CarolinaEffects Red tides can kill fish and other marine animals and contaminate shellfish such

as clams and oysters. People can become ill by eating shellfish tainted with redtide toxins; additionally, toxic particles in sea spray at the shore can cause respi-ratory discomfort.

RED TIDEFlorida’s Unwelcome Visitor

Florida Fish and Wildlife Conservation Commission Fish and Wildlife Research Institute

Scientists prefer to call red tides Harmful Algal Blooms, or HABs.

brevis is a weak swimmer and progresses mostly bydrifting along with currents.

Like other dinoflagellates,K. brevis reproduces by celldivision, with a single cellsplitting into two about every48 to 120 hours. In addition toa dividing cycle, K. brevis hasa sexual cycle that may include“resting” stages whereby itcould remain inactive duringnon-bloom periods.

Karenia brevis is probablyalways present in Florida marine waters at very lowlevels of less than or equal to 1,000 cells per liter (ap-proximately equal to one quart) of water. Periodically,due to a combination of environmental or biologicalconditions, K. brevis can accumulate in concentrationsof up to millions of cells per liter. Water samplescollected during a red tide that plagued southwestFlorida in 1995 and 1996 contained over 20 million cellsper liter. Counts exceeding 100 million cells per liter havebeen recorded.

Scientific research shows that the growth of K.brevis is influenced by a variety of factors, includingsunlight, temperature, salinity, and the amount andtypes of nutrients available in the water. Winds and cur-rents also play a role in determining when and whereblooms will occur. Studies indicate that K. brevisprobably blooms annually in offshore waters as part

of its normal growth cycle. It becomes a problem forpeople only when winds and currents drive the bloomsclose to shore, where they can be concentrated.

Because Florida red tides caused by K. brevis startoffshore, one theory is that pulses of warm water fromthe Caribbean moving into the deeper waters of the Gulfof Mexico may “awaken” K. brevis and spark a red tidebloom. Another theory is that another phytoplanktonorganism precedes K. brevis and conditions the waterfor red tide growth.

People frequently ask whether red tides are a resultof increasing pollution of coastal waters. Althoughexcess nutrients associated with human activities havebeen linked to red tides caused by other species inenclosed areas in Japan, Europe, and elsewhere, thereis no evidence to suggest a similar connection betweenpollution and Florida’s offshore K. brevis blooms. K.brevis red tides begin offshore and have occurred in theGulf of Mexico for hundreds of years, long before man-made pollution became prevalent. However, pollutioncan cause other types of algal blooms in Florida’scoastal waters and estuaries, and researchers areinvestigating the possibility that pollution or nutrientenrichment may influence K. brevis blooms after theblooms are transported and concentrated inshore.

Distribution Karenia brevis red tides have been observed at leastonce along almost the entire coastline of Florida. Theyhave also occurred at least once in the coastal watersof the other Gulf states (most frequently in Texas) andin Mexico. On the Atlantic coast, K. brevis has beentransported as far north as the Carolinas. Blooms occurmost frequently from August through February buthave been documented in every month of the year.Offshore surveys have shown that Florida red tidesgenerally begin 10 to 40 miles from the coast in the Gulfof Mexico on the mid-continental shelf. Winds andcurrents may push the patches of red tide onshore oralong the shore to other areas.

If conditions are right, a bloom may remain in anarea for several weeks or may move up and downalong the coast for months at a time. One red tide thatfirst appeared near Naples in November 1946 spreadas far north as Sanibel Island and Englewood by January1947. Red tide surfaced again in the spring of 1947 inouter Florida Bay and a few months later as far northas Tarpon Springs. It was during this event, charac-

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Karenia brevis, magnified1,160 times.

The life cycle of Karenia brevis. The dominant cell can reproducein two ways: by dividing into two cells (asexual division) and bymerging with another cell (sexual cycle). Stages 1 through 9 areknown, but stages 10 through 12 are still in question.

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terized as one of the worst red tide episodes on record,that scientists first identified K. brevis as the toxicorganism responsible for Florida red tides.

How Red Tides Affect Marine LifeKarenia brevis toxins, called “brevetoxins,” primarilyaffect the nervous system of fishes, causing death byparalyzing the nerves and effectively suffocating the fish.Karenia brevis can become lethal to fish at concentrationsgreater than 100,000 cells per liter. This organism hasbeen implicated in the mortality of marine mammals,birds, and invertebrates during red tides such as the onethat occurred in 1996.

Although K. brevis red tides can kill thousands oreven millions of fish, there is no evidence that they causepermanent damage to marine fish and invertebratepopulations. The impact of a red tide often appears tobe short-lived, and fishermen have reported bettercatches of some species, such as crabs, in the monthsfollowing an outbreak. This may occur because the redtide organism has killed specific predators, allowingcertain prey species to survive in greater numbers, orbecause red tides introduce more food into the system.Thus, although large numbers of fish may be killed bya bloom, other species may benefit. Indeed, theecosystem currently in the Gulf of Mexico is composedof populations that are the product of an environmentthat has included red tides, storms, and otherdisturbances for probably thousands of years.

Slow-moving fish, unable to flee from the path ofred tides, are usually the first to die, along with territorialor bottom-dwelling fish. Nearly all fish are susceptible,especially if the bloom is dense or prolonged. Inverte-brates are usually not killed by red tide toxins, althougha greater variety of animals, including snails and crabs,may be killed if the bloom is severe enough.

Bivalve shellfish such as clams and oysters, whichfeed by filtering plant matter from the water, may ingestK. brevis and, consequently, become toxic to consumers.Even when K. brevis concentrations are only slightlyabove normal, these filter-feeders may become toxic ifthey are exposed to low levels of toxin long enough.

In southwest Florida in 1996, an unprecedented

event of 149 manatee deaths was finally linked to a redtide bloom that had extended into winter. As a result,both the bloom and the manatees were present at thesame time in one of the manatee wintering areas. Redtide toxin was found in the organs and stomach contentsof manatee carcasses. Given the results of detailedexamination of the carcasses, scientists hypothesizedthat these animals died quickly after being exposed tolarge quantities of toxin. Additional manatees died inthe winter of 1982 and in recent years during red tideevents; these animals also showed signs of exposureto red tide toxin.

How Red Tides Affect PeopleThe greatest threat to humans posed by K. brevis redtides is through consumption of bivalve shellfish thathave been contaminated with the red tide toxin. Atpresent, no humans have died from eating taintedclams, mussels, oysters, or coquinas, but some peoplehave become seriously ill with an ailment calledNeurotoxic Shellfish Poisoning (NSP). Symptoms includenausea, diarrhea, tingling of fingers and toes, andsometimes a reversal of sensations—hot seems cold andcold seems hot. Illness occurs within a few minutes toseveral hours after consumption of the shellfish. NSPis often confused with a more dangerous and commonlyknown shellfish poisoning called Paralytic ShellfishPoisoning (PSP). PSP is caused by other dinoflagel-lates that produce an entirely different set of symptomsin humans.

As part of a routine shellfish management plan, theFlorida Department of Agriculture and Consumer Servicescloses harvesting areas when shellfish beds arethreatened by a bloom. The harvesting ban is lifted onlyafter meat from shellfish passes a laboratory test for thetoxin. Generally, most bivalves can purge the toxin fromtheir systems within two to six weeks after the red tidedissipates. The shellfish harvesting bans do not applyto shrimp, crabs, or lobsters because the edible parts ofthese and other crustacean shellfish do not becometoxic when the animals are exposed to Florida red tides.

Fish caught during K. brevis red tides are safe to eatif they are filleted. However, at any time, experts adviseagainst eating a fish that appears sick or lethargic.

People can also be affected by airborne toxins.Wave action breaks apart the red tide cells, and thetoxins, associated with particles in the sea spray, causesneezing, coughing, and general respiratory irritation.

The red tide bloom of 1946–47 is estimated to have killed 500 million fish.

In addition, red tide can cause aesthetic problems incoastal areas; it often dumps smelly, dead fish—some-times hundreds or thousands of them—on area beaches.Most local communities dispose of the rotting fishquickly, but these cleanups can be costly.

Should Humans Seek to Eliminate Red Tides?Although it has long been debated whether researchshould strive to find ways of eliminating or otherwisecontrolling red tide, many scientists believe that thereis no practical way to totally eradicate Florida redtides. Getting rid of red tide would be extremely diffi-cult and costly because red tide blooms often occur overhundreds to thousands of square miles of water, aredistributed throughout the water column, can be movedgreat distances along the coast, and fluctuate dailywith the tides. The use of chemical or biological controlagents to disperse the red tide blooms or neutralize thetoxins may adversely affect other forms of marine life.Yet, the possibility of controlling the bloom at a locallevel, by mitigating either its effects or its distribution,has recently gained popularity. Researchers are pursuingthe possibility of applying techniques that have beenused for limiting localized blooms of other specieselsewhere. Overall, scientists and managers agree thatwe must be careful about introducing control agents intoour coastal system. Indeed, there is speculation that thered tide phenomenon may serve an important, althoughcurrently unverified, role in making the marineecosystem off Florida’s coast more productive.

If red tides and their paths could be predicted,alerted communities might have time to mobilizecleanup crews and establish warning systems beforethe bloom arrives. With prediction as one of theirgoals, scientists from the Florida Fish and WildlifeConservation Commission (FWC), the University of

South Florida College of Marine Science, and MoteMarine Laboratory are collaborating on a federallyfunded project to develop and deploy new technologyto monitor Florida’s coastal waters for red tide.Additionally, FWC and Mote are collaborating on astate-funded program to identify specific nutrientsources that support red tides and to assess potentiallinks between coastal nutrient pollution and thenearshore stages of red tides. The use of satellites indetecting ocean currents and blooms also holds promisefor tracking the movement of red tide and possiblypredicting its occurrence.

The FWC and a number of other agencies andresearch entities are acquiring scientific knowledgeabout the Florida red tide organism in order to manageits effects on humans and natural resources. Becauseof FWC’s long-term experience with this organism andothers, FWC scientists have made valuable contributionsto investigations of harmful algal blooms.

This satellite image, showing a bloom (light gray) in the Gulf ofMexico off the southwest coast of Florida, is an example of howsatellites are used to detect and track red tides. The original imageuses colors to show different concentrations of red tide. These bluearrows point to areas of greatest concentration.

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K. brevis is one of only a relatively few red tideorganisms known or suspected to produce noxious, airborne toxic particles that can

irritate human respiratory systems.

June 2005

Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute100 8th Avenue SE, St. Petersburg, FL 33701 • (727) 896-8626 • http://research.MyFWC.com