Extracted from:

Donald F. Boesch, Donald A. Anderson, Rita A. Horner, Sandra E. Shumway,Patricia A. Tester and Terry E. Whitledge. 1997. Harmful Algal Blooms in Coastal Waters: Options for Prevention, Control and Mitigation. NOAA Coastal Ocean Program, Decision Analysis Series No. 10, Special Joint Report with the National Fish and Wildlife Foundation, February 1997.

Site developer's note: genus and species names of algae are indicated in red, rather than standard underlining (not a webpage option unless a link...) or italics, in order to make the names easier to find and read.

Neurotoxic Shellfish Poisoning

Massive fish kills off the west coast of Florida have been known since 1844 and according to the writings of sixteenth-century Spanish explorers, the Tampa Bay Indians long noted the seasonality of fish kills now associated with red tide blooms. Shellfish toxicity was documented in 1880 and aerosol-related respiratory symptoms in human inhabitants were described in 1917. Between 1844 and 1996, red tides (discoloration, fish kills, respiratory irritation, or shellfish poisoning) have occurred in 58 years. Since 1946, when the causative toxic dinoflagellate, Gymnodinium breve, was first discovered, red tide has been observed in 42 of the 50 intervening years.

Throughout the Gulf of Mexico and the U.S. South Atlantic Bight, G. breve is found in low background concentrations (1-1,000 cell/l) except in areas off the west Florida and Texas coasts where local circulation may play a role in concentrating cells. While G. breve blooms have occurred in many different areas within the Gulf of Mexico, from Yucatan in the south, along the Tamaulipas and Texas coasts, and recently to Alabama, Mississippi and Louisiana waters, they are most frequent along the west coast of Florida. Blooms there are especially frequent from Clearwater to Sanibel Island, occurring in 21 of the last 22 years. These blooms on the southwest Florida shelf served as a source for cells inoculating the U.S. South Atlantic Bight (Florida east coast and North Carolina) in 1987-88 (Tester et al. 1991).

Florida red tides affect humans, wildlife, fishery resources and the regional tourist-related economy. As G. breve cells die and break up, they release a suite of powerful neurotoxins, known collectively ad brevetoxins. Shellfish management regulations include a biotoxin control plan that is implemented during red tides to reduce the risk to humans from consumption of toxic molluscs. While some illness related to shellfish consumption occasionally has occurred, in general the highly cautionary regulations have been quite effective in preventing neurotoxic shellfish poisoning. While it is known that aerosols from red tides produce respiratory ailments in many humans exposed, the long-term consequences are poorly known, in part because statistics on individuals treated for respiratory and other associated maladies are not maintained. Also, it is difficult to assess long-term effects of acute exposure among tourists who leave the area. Fish kills, bird kills and occasional invertebrate kills are common sights during red tides. In 1996 more than 150 manatees, an endangered species, died due to brevetoxin exposure during a prolonged red tide along the southwest Florida coast (Steidinger et al. 1996).

The economic impacts of the Florida red tides are not well-documented but estimates of $15-20 million were published in the early 1970s when the blooms lasted for at least three months and impacted several counties. The 4-6 month red tide in North Carolina during 1987-88 was estimated to have cost the coastal community there $25 million (Tester and Fowler 1990). The 1995-96 west Florida red tide, an unusually prolonged one, had severe consequences for shellfish growers, beach resorts and tourist-dependent businesses and may have adversely affected real estate values. Although these impacts have not been comprehensively quantified, hoteliers and restauranteurs in e region testified to unprecedented reductions in business volume.

G. breve blooms are initiated on the continental shelf or at the shelf edge, rather than in nearshore waters where they produce the most deleterious effects. Low concentrations (<1,000 cells/l) of the organism occur in offshore waters throughout the year. Bloom initiation is characteristically associated with intrusion of deeper, offshore waters onto the shelf. This phenomenon is best known for the west Florida shelf where blooms may occur any time of the year, but typically occur in late summer and fall when more than 70% of the observed outbreaks have been initiated. Bloom concentrations first appear offshore (18-74 km) and are associated with fronts. Much as weather fronts mark the convergence of air masses, ocean fronts separate waters of different temperature and salinity characteristics. These fronts are caused by the onshore-offshore meanders of the Loop Current, part of the Gulf of Mexico current system through which water flows along the edge of the shelf and then through Florida Strait, eventually to become the Gulf Stream.

G. breve may grow rapidly in the dynamic nutrient regimes and light conditions along frontal gradients, dividing up to once a day, but usually once every 2 to 5 days (Steidinger et al. in press), gradually building high densities. G breve is well-adapted to such environments and can grow throughout the water column where there is sufficient light. It has a high photosynthetic capacity and can assimilate nutrients (both organic and inorganic) at low light levels. Once growth occurs, it takes 2 to 8 weeks to develop into a bloom of fish-killing proportions (1-2.5x105 cells/l) depending on physical, chemical and biological conditions.

Because of rapid growth and ability to out-compete or otherwise exclude other phytoplankton species, G. breve can develop almost monospecific surface blooms covering a surface area of 10,000 km2 or more. Although biomass concentration is patchy, chlorophyll a values (a good surrogate for biomass) from 10 > 100 mg/m3 make the resultant discolored surface water detectable by satellite color sensors. The Coastal Zone Color Scanner which provided data between 1978 and 1986 was able to detect chlorophyll a from G. breve cells at densities as much as three orders of magnitude less than are present when discolored water is detectable by the human eye, about 106 cells/l (Tester and Steidinger in press). New satellite-borne ocean color sensors, which have been recently launched or should be operational in the near future, thus offer prospects for routine bloom detection. Of course, the ability to detect subsurface patches and distinguish G. breve blooms from those of other algae will limit the use of technology.

The fate of a G. breve bloom, whether it grow denser or larger and, very importantly, whether it will be transported onshore and impact beaches and bays, is determined largely by the currents on the continental shelf, which are driven by winds, impingement of the Loop Current on the shelf, and the fact that the ocean level typically slopes ever-so-slightly down from north to south. On the west Florida shelf, currents are complex and include gyres, larger eddies, and filament-like incursions of offshore water across the shelf. These physical processes are capable of moving blooms onshore, to the north along the Florida panhandle (via a large clockwise gyre on the northern part of the shelf), or south toward the Florida Strait. During some periods there may be little net flow, allowing blooms to be maintained within the midshelf zone and occasionally inoculate or reinoculate the nearshore region. The annual cycle of wind stress, northward during the summer and southward in the fall, is responsible for the persistent upwelling (summer) and downwelling (fall) found over the west Florida shelf, which can concentrate or disperse blooms and transport them toward or away from shore depending on the site and timing of the bloom. Once in nearshore waters, transport of the blooms is affected by longshore flows and tidal exchanges with bays.

G. breve is essentially a continental margin species which can utilize low levels of nutrients very effectively. Dense blooms do not persist at salinities below 24 parts per thousand, conditions that occur in estuaries and coastal waters receiving fresh water from rivers. The offshore initiation of red tide blooms cannot be readily prevented. An important question, then, is the degree to which a bloom being moved into nearshore waters can be prevented from persisting or intensifying by reducing nutrient inputs from the land, including those of human origin. G breve can utilize land-based nutrients and grow rapidly in the coastal waters provided the salinity does not fall below 24 parts per thousand. Evidence suggests that dense blooms inshore cannot be sustained without inputs of "new" nutrients (Steidinger et al. in press). If so, human inputs of nutrient could be responsible for extending the duration and impacts of red tides once blooms enter the nearshore zone, including bays and canals.

Dissipation or termination of red tide blooms occurs when blooms are transported out of the area or when the integrity of the water is weakened by mixing and dilution. Both declining water temperature and increasing wind stress contributed to the dissipation of the 1987-1988 G. breve bloom off North and South Carolina (Tester et al. 1991). Unfortunately, the roles of density-dependent growth factors, nutrient limitation, and grazing pressure in the decline of red tide blooms are not well known.

UNIVERSITY OF MARYLAND College of Ag & Natural Resources Department of Veterinary Medicine Aquatic Pathobiology Center