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.
To report problems or provide comments,
Andrew Kane (Aquatic Pathobiology Center) at:email@example.com
Dan Jacobs (Maryland Sea Grant)
UNIVERSITY OF MARYLAND
College of Ag & Natural Resources
Department of Veterinary Medicine
Aquatic Pathobiology Center