Among the thousands of species of microscopic algae at the base of the marine food chain are a few dozen which produce potent toxins. These species make their presence known in many ways, ranging from massive ``red tides'' or blooms of cells that discolor the water, to dilute, inconspicuous concentrations of cells noticed only because of the harm caused by their highly potent toxins. The impacts of these phenomena include mass mortalities of wild and farmed fish and shellfish, human intoxications or even death from contaminated shellfish or fish, alterations of marine trophic structure through adverse effects on larvae and other life history stages of commercial fisheries species, and death of marine mammals, seabirds, and other animals.
``Blooms'' of these algae are commonly called red tides, since, in some cases, the tiny plants increase in abundance until they dominate the planktonic community and change the color of the water with their pigments. The term is misleading, however, since non-toxic species can bloom and harmlessly discolor the water; conversely, adverse effects can occur when algal cell concentrations are low and the water is clear. Given the confusion surrounding the meaning of ``red tide,'' the scientific community now prefers the term ``harmful algal bloom,'' with HAB as the obligatory acronym. This new descriptor applies not only to microscopic algae but also to benthic or planktonic macroalgae which can proliferate in response to anthropogenic nutrient enrichment, leading to major ecological impacts such as the displacement of indigenous species, habitat alteration, or oxygen depletion. The causes and effects of macroalgal blooms are thus similar in many ways to those associated with harmful microscopic phytoplankton species.
HAB phenomena take a variety of forms. One major category of impact occurs when toxic phytoplankton are filtered from the water as food by shellfish such as clams, mussels, oysters, or scallops, which then accumulate the algal toxins to levels which can be lethal to humans or other consumers (reviewed in Shumway, [1990], Ahmed [1991]). Typically, the shellfish are only marginally affected, even though a single clam can sometimes contain sufficient toxin to kill a human. These poisoning syndromes have been given the names paralytic, diarrhetic, neurotoxic, and amnesic shellfish poisoning (PSP, DSP, NSP, and ASP). Except for ASP, all are caused by biotoxins synthesized by a class of marine algae called dinoflagellates. ASP is produced by diatoms that until recently were all thought to be free of toxins and generally harmless [ Bates et al., 1989]. A fifth human illness, ciguatera fish poisoning (CFP) is caused by biotoxins produced by epibenthic dinoflagellates attached to surfaces in many coral reef communities (reviewed in Anderson and Lobel [1987]). Ciguatera toxins are transferred through the food chain from herbivorous reef fishes to larger carnivorous, commercially valuable finfish. In a similar manner, the viscera of other commercially important fish such as herring or sardines can contain PSP toxins, endangering human health following consumption of whole fish. Whales, porpoises, seabirds, and other animals can be victims as well, receiving toxins through the food chain via contaminated zooplankton or fish [ Geraci et al., 1988; Anderson and White, 1992]. All of these poisoning syndromes except DSP are known problems within the U.S. and its territories.
Another type of HAB impact occurs when marine fauna are killed by algal species that release toxins and other compounds into the water, or that kill without toxins by physically damaging gills or by creating low oxygen conditions as bloom biomass decays. Farmed fish mortalities from HABs have increased considerably in recent years, and are now a major concern to fish farmers and their insurance companies. The list of finfish, shellfish and wildlife affected by algal toxins is long and diverse (Table 1) and accentuates the magnitude and complexity of the HAB phenomena. In some ways, however, this list does not adequately document the scale of HAB impacts. A general trophic routing and impact model proposed by Smayda [1992] is perhaps a better representation of the many ways in which HAB species affect other organisms within marine ecosystems, not just those that are commercially important or easily visible to humans (Figure 1). This model shows where adverse effects on viability, growth, fecundity, and recruitment can occur within different trophic levels, either through toxin transmitted directly from the algae to the affected organism or indirectly through food web transfer. In virtually all trophic compartments of the marine food web, there can be impacts from toxic or harmful blooms. Smayda points out that the model resembles energy flow diagrams because bloom events impact ecosystems via the movement of toxins in a manner that is analagous to the flow of carbon or energy. Examination of Figure 1 in light of our present knowledge of HAB impacts quickly reveals the many areas where information is lacking. Most HAB research activity has focused on shellfish, fish, and zooplankton, but many other organisms are being affected by toxins in ways that we can only guess at right now. This is a good example where expanded research on HAB phenomena can provide useful information to many other disciplines in marine biology and geochemistry.