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While light limitation can inhibit bloom formation in dinoflagellates, the potential for high‐intensity photosynthetically active radiation (PAR) to inhibit blooms by causing stress or damage has not been well‐studied. We measured the effects of high‐intensityPARon the bloom‐forming dinoflagellatesAlexandrium fundyenseandHeterocapsa rotundata. Various physiological parameters (photosynthetic efficiencyF_v/F_m, cell permeability, dimethylsulfoniopropionate [DMSP], cell volume, and chlorophyll‐acontent) were measured before and after exposure to high‐intensity natural sunlight in short‐term light stress experiments. In addition, photosynthesis‐irradiance (P‐E) responses were compared for cells grown at different light levels to assess the capacity for photophysiological acclimation in each species. Experiments revealed distinct species‐specific responses to highPAR. While high light decreasedF_v/F_min both species,A. fundyenseshowed little additional evidence of light stress in short‐term experiments, although increased membrane permeability and intracellularDMSPindicated a response to handling. P‐E responses further indicated a high light‐adapted species with Chl‐ainversely proportional to growth irradiance and no evidence of photoinhibition; reduced maximum per‐cell photosynthesis rates suggest a trade‐off between photoprotection and C fixation in high light‐acclimated cells.Heterocapsa rotundatacells, in contrast, swelled in response to high light and sometimes lysed in short‐term experiments, releasingDMSP. P‐E responses confirmed a low light‐adapted species with high photosynthetic efficiencies associated with trade‐offs in the form of substantial photoinhibition and a lack of plasticity in Chl‐acontent. These contrasting responses illustrate that high light constrains dinoflagellate community composition through species‐specific stress effects, with consequences for bloom formation and ecological interactions within the plankton.