Florida red tides occur almost every year in the Gulf of Mexico. The Florida red tide organism, Karenia brevis, produces a toxin that can kill marine animals and affect humans. Scientists have studied this organism for more than 50 years.
Red tides are harmful algal blooms (HABs) that occur when toxic, microscopic algae in seawater proliferate to higher-than-normal concentrations (bloom), often discoloring the water red, brown, green, or yellow. More than 40 species of toxic microalgae live in the Gulf of Mexico. The most common is the dinoflagellate Karenia brevis, the Florida red tide organism (formerly named Gymnodinium breve; see Taxonomy section below).
The Florida red tide organism was identified in 1947, but anecdotal reports of the effects of red tide in the Gulf of Mexico date back to the 1530s. Florida red tides occur in the Gulf of Mexico almost every year, generally in the late summer or early fall. They are most common off the central and southwestern coasts of Florida between Clearwater and Sanibel Island, but they may occur anywhere in the gulf. They also occur, but are less common, along the southeastern Atlantic coast as far north as North Carolina. Most blooms last three to five months and may affect hundreds of square miles. Occasionally, however, blooms continue sporadically for as long as 18 months and may affect thousands of square miles. Red tides can kill fish, birds, and marine mammals; cause health problems for humans; and adversely affect local economies.
Karenia brevis is a common, unarmored, photosynthetic dinoflagellate found year-round throughout the Gulf of Mexico at background concentrations of 1,000 cells per liter or less. Each cell is typically 20 to 45 micrometers (µm) long, 10 to 15 µm deep, and slightly wider than long. It has two whip-like appendages, or flagella, that propel and direct it through the water at a speed of 1 meter per hour. The cell contains a nucleus, numerous chloroplasts, and other organelles. In Florida waters, K. brevis thrives in high-salinity areas, but it can tolerate a wide range of salinities. It survives most temperatures common to the Gulf of Mexico. The species is able to outcompete or exclude other phytoplankton and forms nearly monospecific blooms.
Taxonomy, the science of identification and classification, is a dynamic discipline in which conclusions change as advances in technology result in new information. Taxonomists revise species descriptions and classifications as improved microscopic and fixation techniques enable them to identify or clarify significant characteristics.
The Florida red tide organism has undergone several taxonomic investigations and revisions. Karenia brevis was originally named Gymnodinium brevis in 1948, but the name was later corrected to Gymnodinium breve in accordance with guidelines of the International Code of Botanical Nomenclature. In 1979, G. breve was transferred to the genus Ptychodiscus and named Ptychodiscus brevis because of new findings concerning the dinoflagellate's morphology, biochemistry, and characters of its ultrastructure. This transfer was reevaluated in 1989, and scientists agreed that the species should be referred to by its original name, G. breve, until additional studies were completed.
Subsequently, several species formerly assigned to the genus Gymnodinium have been reclassified. Gymnodinium breve has now been transferred to the new genus Karenia, which was established in November 2000 by Gert Hansen and Øjvind Moestrup at the University of Copenhagen. The current name for the Florida red tide organism is Karenia brevis (Davis) G. Hansen & Moestrup, 2000. Ptychodiscus brevis (Davis) Steidinger, 1979 and Gymnodinium breve Davis, 1948 are thus synonyms of K. brevis. Additions to the genus Karenia are likely as taxonomic investigations continue and other species are more accurately characterized.
Karenia brevis = Ptychodiscus brevis = Gymnodinium breve
Karenia brevis produces brevetoxins that are capable of killing fish, birds, and other marine animals. Bottom-dwellers such as groupers and grunts are usually the first fish to die in a Florida red tide, although most fish are probably susceptible. Mortality, in terms of numbers killed and species affected, can be severe and is dependent upon factors such as bloom density and the length of time animals are exposed to the toxins.
Brevetoxins may also cause health problems in humans. The toxins accumulate in shellfish that are filter-feeders, such as oysters, clams, and coquinas, and may reach levels capable of causing neurotoxic shellfish poisoning (NSP) when ingested. NSP is a temporary illness characterized by gastrointestinal and neurological distress. Symptoms include nausea and diarrhea; dizziness; muscular aches; and tingling and numbness in the tongue, lips, throat, and extremities. Symptoms of NSP usually appear within a few hours of eating contaminated shellfish and disappear within a few days. Brevetoxins can also irritate eyes and respiratory systems when the toxins become airborne in sea spray; the irritation disappears once a person is no longer exposed. Other public health effects caused by red tides include puncture wounds from spines when beaches are littered with dead fish and, rarely, contact dermatitis from exposure to brevetoxins in seawater.
During the 1980s, to reduce the public health risks associated with red tides, the State of Florida formalized a federally approved biotoxin control plan that regulates shellfish harvesting during Karenia brevis blooms. Under this plan, guidelines were established for monitoring cell concentrations and closing shellfish beds when K. brevis populations reach dangerous levels. Harvesting bivalve (filter-feeding) shellfish is prohibited in a defined bloom area when concentrations in the area reach 5,000 K. brevis cells per liter. When the bloom terminates and the K. brevis population drops below 5,000 cells per liter, shellfish usually purge the toxins from their systems in two to six weeks. The shellfish meats are tested for toxicity during that period, and the harvesting ban is lifted when test results verify that bivalves in the area are again safe for human consumption. Harvesting bans are not applied to crabs, shrimp, lobsters, or fish, which are safe to eat even during red tide blooms, because brevetoxins do not accumulate in the parts consumed by humans.
Florida red tides have economic impacts as well. Tourist communities lose millions of dollars when dead fish wash up on beaches or beachgoers experience eye and respiratory irritation. Shellfish-harvesting businesses lose income when shellfish beds must be closed because of Karenia brevis blooms. Even tourism, recreational activities, and other businesses not actually at the bloom site may be adversely affected. Although it is hard to calculate actual dollars lost, a study of three red tide blooms that occurred in the 1970s and 1980s estimated losses from each event at between $15 million and $25 million.
Initiation and Transport
Prior to the early 1970s, Florida red tides were believed to originate inshore because discolored water, fish kills, and respiratory irritation were most often observed first around passes and barrier islands. However, later review of the historical data compiled from research cruises showed that Florida red tides begin, instead, in nutrient-poor water 18 to 74 kilometers offshore. Resting populations of Karenia brevis are believed to exist in the water column or sediments in specific areas on the west Florida continental shelf.
Blooms develop in four stages. The initiation stage occurs when a Karenia brevis population is first introduced into an area. The second stage is growth, during which 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 the bloom may be maintained in a circulation feature offshore or moved inshore by wind and currents. If the bloom moves inshore, increased nutrient levels there allow the cells to multiply, and physical factors like currents may concentrate the bloom even further. A bloom may linger in coastal areas for days, weeks, or even months. The fourth stage is dissipation/termination. Mechanisms that contribute to this stage, such as winds and currents, may disperse the cells, introduce new water masses, or move the bloom to a different area.
For years, scientists have tracked Florida red tides whenever they could to better understand bloom dynamics. Project Hourglass was a long-term (28-month) sampling/tracking program conducted in the 1960s. Biological and hydrographical samples for red tide studies were collected from a series of stations in the Gulf of Mexico between Tampa Bay and Charlotte Harbor. Drift bottles were deployed monthly, and the data gathered from these provided information on transport direction and areas of landfall. The movement patterns of the drift bottles showed that K. brevis blooms along the southwestern coast of Florida may be transported along the Gulf of Mexico coastline or entrained by the Loop Current into the Gulf Stream.
Detection, Monitoring, and Research
The occurrence and abundance of the Karenia brevis organism has been documented by biologists for more than 50 years. Although there were a few targeted research programs, most sampling occurred after a bloom had already begun as evidenced by reports of dead fish, discolored water, or respiratory irritation. Data collected from such response-oriented monitoring is incomplete and limited, because it is by then too late to study the initiation and growth phases of the bloom and because it is logistically difficult to mobilize resources quickly enough to document the event adequately.
Bloom detection using satellite technology and color imagery began in the 1970s. In 1978, the validity of this approach was verified by interpretation of a color image of the southwestern coast of Florida and comparing that interpretation with data collected from the water column. In the satellite image, different concentrations of chlorophyll were seen as different color densities, and the color densities were correlated with cell densities of Karenia brevis. The use of satellite technology to monitor red tides is limited, however. Satellites can be used to track surface blooms as they move, but they cannot yet detect bloom development or subsurface blooms. Many researchers now collect field data using autonomous, continuously recording instruments such as moored buoys. This method of in situ sampling can measure specific variables at short time intervals for an extended period, and data can be transmitted to a home base via satellite or cell phone for analysis and modeling.
Several current research programs have been initiated specifically to study the possibility of mitigating the effects of red tides through prediction or advanced warning. One such project is the federally funded Ecology and Oceanography of Harmful Algal Blooms (ECOHAB) program. ECOHAB investigators in Florida collect data from research cruises and from moored buoys to study the biology of Karenia brevis and its movement in response to environmental variables (such as temperature, salinity, and currents) on the continental shelf. Physical, chemical, and biological data are used to model and predict bloom initiation, growth, maintenance, and dissipation or termination; to evaluate life-cycle processes; and to study the transport and eventual fate of the brevetoxins. A pilot project called the Harmful Algal BloomS Observing System (HABSOS) is another federally funded program in the Gulf of Mexico. The purpose of HABSOS is to collect from federal, state, and academic laboratories all available data on red tide events and to compile the information in a central, accessible, continually updated repository. Such a database will give investigators the ability to study events as they occur and perhaps forecast the movement and probable effects of a bloom.
Other efforts to understand and predict red tides include a task force to recommend research and a volunteer program for statewide sampling. The Florida Harmful Algal Bloom Task Force was established in 1997 to identify gaps in the data that have been collected and to recommend additional research and monitoring needed on Florida red tides, other HABs, and their associated effects. The Task Force consists of representatives from federal agencies, state and local governments, water management districts, universities, private laboratories, and a citizen organization. The volunteer program was established in 2000 to help monitor the extensive region over which Florida red tides may occur. Volunteers collect water samples from established offshore transects in a network extending from Pensacola to the Florida Keys and send them to the Florida Fish and Wildlife Research Institute (FWRI) for analysis.
A substantial amount of information has been accumulated during the past 50 years of red tide research. Early data were recorded and maintained manually on paper and subsequently transferred to digital files on computers, but the data were not centrally located or easily retrievable, and long-term analysis was difficult. In 1998, scientists from FWRI decided to address this problem and, with the cooperation of Mote Marine Laboratory and the National Oceanographic and Atmospheric Administration (NOAA), assumed responsibility for creating a customized red tide database and associated metadata. Data transferred to the historical red tide database include location coordinates, cell counts of Karenia brevis (the Florida red tide organism) and other HAB species, and oceanographic and water-quality measurements such as temperature, salinity, and dissolved oxygen. The database is continually updated and analyzed by database managers.
Red tides are a part of Florida's history and will most likely remain a part of its future. Scientists continually strive to learn more about factors affecting the growth and intensity of Karenia brevis blooms. Although the biology of the organism and the role that red tides play in the dynamics of the Gulf of Mexico ecosystem are still not fully understood, predictive two- and three-dimensional models are being developed and tested. Data generated through traditional environmental sampling and monitoring, in combination with data generated through newer approaches such as remote sensing and modeling, may give us the ability to forecast red tide events and mitigate, or even eliminate, their effects.