PFIESTERIA PISCICIDA AND OTHER PFIESTERIA-LIKE DINOFLAGELLATES - BEHAVIOR, IMPACTS, AND ENVIRONMENTAL CONTROLS

Toxic Pfiesteria-like dinoflagellates have been implicated as causativ e agents of major fish kills (affecting 10(3)-10(9) fish) in estuaries and coastal waters of the mid-Atlantic and southeastern U.S. Transfor mations among an array of flagellated, amoeboid, and encysted stages i n the complex life cycle of the representative species, Pfiesteria pis cicida, are controlled by the availability of fresh secretions, blood, or other tissues of fish prey. P. piscicida also is a voracious preda tor on other estuarine microorganisms. Pfiesteria-like dinoflagellates require an unidentified substance(s) commonly found in fresh fish exc reta-secreta to initiate toxin production. P. piscicida is lethal to f ish at low cell densities (>250-300 cells ml(-1)), and at sublethal le vels (similar to 100-250 cells ml(-1)) it has been shown to cause ulce rative fish diseases. P. piscicida also has been linked to serious hum an health impacts. This dinoflagellate is eurythermal and euryhaline, with optima for toxic activity by the most lethal stage (toxic zoospor es, TZs) at greater than or equal to 26 degrees C and 15 psu, respecti vely. Thus far it has shown no light optimum and is capable of killing fish at any time during a 24-h cycle. In warmer waters (greater than or equal to 15 degrees C) flagellated stages predominate while fish ar e dying, whereas amoebae predominate in colder conditions and when fis h are dead. Nutritional stimuli influencing P. piscicida are complex; inorganic phosphate apparently can directly stimulate TZs, whereas ino rganic phosphate and nitrate indirectly promote increased production o f nontoxic zoospores (NTZs, maintained in the absence of live fish, as potential precursors to lethal TZs) by stimulating their algal prey. Organic phosphate (P-o) and nitrogen are taken up by P. piscicida osmo trophically, and P-o is stimulatory to both TZs and NTZs. The availabl e data point to a critical need to characterize the chronic and acute impacts of toxic Pfiesteria-like dinoflagellates on fish and other tar geted prey in estuarine and coastal waters that are adversely affected by cultural eutrophication.