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We have studied temporal variation in the sensitivity of phytoplankton photosynthesis to inhibition by ultraviolet radiation (UV; 280–400 nm) using biological weighting functions (BWFs) that quantify the biological effect of different wavelengths of UV. Variations in irradiance‐dependent BWFs were evaluated for natural phytoplankton assemblages from the Rhode River, a shallow subestuary of the Chesapeake Bay, Maryland, from October 1994 to July 1996. Phytoplankton assemblages were sensitive to UV throughout the year. Rhode River assemblages are inhibited more strongly in the UV‐B (280–320 nm), particularly below 300 nm, but there is a significant influence well into the UV‐A (320–400 nm). There was no inhibition of phytoplankton photosynthesis by photosynthetically available radiation (400–700 nm), but there was significant seasonal variation in the saturated rate of photosynthesis (P_s^B) and in the …