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Blooms of the Karenia brevis dinoflagellate can create devastating effects on marine communities off the west coast of Florida when they release a suite of neurotoxins known as brevetoxins (PbTx). These blooms, informally referred to as red tides, can cause massive fish kills, necessitate closures of shellfish fisheries, and can even leave lingering toxins that impact shelf communities long after the bloom has dissipated. Much effort has been put into studying K. brevis bloom initiation and dynamics, however, predicting the impact of these blooms on Florida's fish and fisheries is problematic because K. brevis cell counts and fish mortality is not well correlated. Ecopath with Ecosim (EwE) is an ecosystem modelling platform that is used in this study to simulate K. brevis bloom effects on the West Florida Shelf ecosystem. The model produced for this study, which has been named the West Florida Shelf K. brevis model, is an application of the EwE modelling platform that employs a recently developed time series to index K. brevis bloom severity. This index was used to drive bloom-related mortality as a fishing fleet in the model in a historical reconstruction scenario from 1980 to 2009. The study suggests that K. brevis blooms can impose top-down food web effects causing trophic cascades. Piscivores, planktivores and detritivores experienced biomass increases when bloom-related mortality was applied, while invertebrate eaters and omnivores experienced biomass decreases. Changes in community structure and biodiversity are evidenced by effects on species evenness and richness. The model indicated an overall improvement in fits to observational time series of biomass and catch when using the K. brevis index. These results provide evidence that K. brevis mortality should be considered in population models, particularly for species with low predation mortality such as gag.