nutrient inputs cause a shift to less desirable phytoplankton or bloom-forming benthic macroalgae. Even in tropical waters, where seagrasses may be more limited by nutrient (phosphorus) availability (Short et al. 1990), fast-growing phytoplankton and macroalgae have more rapid nutrient uptake potential and can replace seagrasses as the dominant primary producers in enriched systems (Duarte 1995; Hein et al. 1995). These fast growing, “nuisance” macroalgae are typically filamentous or sheet-like forms (e.g., Ulva, Cladophora, Chaetomorpha) that can accumulate in extensive thick mats over the seagrass or sediment surface. Massive and persistent macroalgal blooms ultimately displace seagrasses and perennial macroalgae through shading effects (Valiela et al. 1997). In addition to causing the loss of important habitat, these nuisance macroalgae are usually unsightly (Chapter 4).

Harmful Algal Blooms

Harmful algal blooms (HAB) include, but are not restricted to, those events referred to as red or brown tides, and are characterized by the proliferation and occasional dominance of particular species of toxic or harmful algae. As with most phytoplankton blooms, this proliferation results from a combination of physical, chemical, and biological mechanisms and interactions that are, for the most part, poorly understood.

Among the thousands of species of microscopic algae at the base of the marine food web are a few dozen that produce potent toxins or that cause harm to humans and marine mammals, fisheries resources, or coastal ecosystems. These species make their presence known in a variety of ways, ranging from massive blooms of cells that discolor the water (giving rise to the term red or brown tide) to dilute, inconspicuous concentrations of cells noticed only because of the harm caused by their potent toxins. The impacts of these phenomena include mass mortalities of wild and farmed fish and shellfish; human illness 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. HABs and related phenomena such as Pfiesteria outbreaks have attracted intense public and political attention (Box 1-5).

One major category of HAB impact occurs when toxic phytoplankton are filtered from the water as food by shellfish, which then accumulate the algal toxins to levels harmful or lethal to humans or other consumers (Shumway 1990). These poisoning syndromes have been given the names paralytic, diarrhetic, neurotoxic, and amnesic shellfish poisoning (PSP, DSP, NSP, and ASP). Whales, porpoises, seabirds, and other animals can be victims as well, receiving toxins through the food web via contami-



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