ABSTRACT: Ciguatera fish poisoning in humans is a serious and widespread syndrome associated with the consumption of reef fishes that have accumulated lipid-soluble toxins known as ciguatoxins. These toxins are piscine metabolites of ciguatoxin precursors produced by benthic dinoflagellates in the genus Gambierdiscus. This investigation employed a novel experimental approach to identify and characterize the environmental factors and their interactions that influence the dynamic balance between cellular growth and loss of Gambierdiscus populations in situ. Field studies were conducted in St. Thomas (US Virgin Islands) at 3 sites and 2 depths (10 and 20 m). At each site and depth, Gambierdiscus was subjected to treatments designed to reduce grazing pressure (disturbance and removal) and elevate nutrient availability to elicit a population abundance response attributable to one of these treatments. We hypothesized that Gambierdiscus abundance would respond positively to increased nutrient availability, increasing depth (reduced water motion), and decreased grazing pressures. We found communities of Gambierdiscus were significantly higher by, on average, 138% when the effects of grazing were limited (p = 0.0002). Among sites, the effects of depth and nutrients on Gambierdiscus populations were not significant. The significant effect of grazing and disturbance observed in this study suggests that changes in reef herbivore and detritivore feeding selectivity and grazing rates may have large impacts on the areal density of Gambierdiscus in natural systems. Whether or not reduced grazing rates or disturbances translate into higher cell (toxin) ingestion rates for consumers and ultimately cause changes in toxicity for humans is unknown and in need of further investigation.