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Nitrogen (N) limitation significantly increased the sensitivity of photosynthesis to inhibition by ultraviolet radiation (UV) in two estuarine dinoflagellates, Akashiwo sanguinea (= Gymnodinium sanguineum) and Gymnodinium (= Gyrodinium) cf. instriatum. Biological weighting functions (BWFs) and the kinetics of photosynthetic response to UV indicated that the main mechanism for the increase in sensitivity was less efficient repair. A decrease in cell size and in the concentration of the photoprotective mycosporine‐like amino acids also elevated sensitivity. The BWFs predict that increased UV‐B due to ozone depletion would cause a more than 1.5‐fold greater additional inhibition of N‐limited compared to nutrient‐sufficient dinoflagellates. The BWFs of the N‐limited cultures are similar to those measured for natural assemblages of phytoplankton in the Chesapeake Bay under low N availability.