Blooms of dinoflagellate Karlodinium veneficum are widely distributed in estuarine and coastal waters and have been found to cause fish kills worldwide. K. veneficum has a mixed nutritional mode and relies on both photosynthesis and phagotrophy for growth; it is a mixotroph. Here, a model of mixotrophic growth of K. veneficum (MIXO) was developed, calibrated with previously-reported laboratory physiological data, and subsequently embedded in a 3D-coupled hydrodynamic (ROMS)-biogeochemical (RCA) model of eutrophic Chesapeake Bay, USA. The resulting ROMS-RCA-MIXO model was applied in hindcast mode to investigate seasonal and spatial distributions. Simulations showed that K. veneficum blooms occurred during June-August and were confined to the upper and middle Bay, consistent with long-term field observations. Autotrophic growth dominated in spring but heterotrophic growth dominated during the summer. The number of prey ingested by K. veneficum varied from 0.1 to 0.6 day^-1 and the food vacuole content reached up to 50% of the core mixotroph biomass. The ingestion rate increased with prey density and also when P:N ratio fell below ∼0.03 (N:P ∼ 33), indicating that K. veneficum only switched to mixotrophic feeding in P-deficient waters when sufficient prey were available; this occurred during the summer months. The digestion rate increased with both the food vacuole content and temperature. The modeling analysis affirms K. veneficum as a phagotrophic 'alga' which is primarily photosynthetic but switches to mixotrophic feeding under nutrient deficient conditions.