Although nutrient-limited growth of phytoplankton in the laboratory produces well-defined responses in the absorption coefficient, pigmentation, and quantum yield of photosynthesis, natural populations show only weak or inconclusive effects. Further analysis of the data from Biowatt-II, a seasonal study in the NW Atlantic in 1987, suggests two things. First, the phytoplankton absorption coefficient changes as a function of chlorophyll (alpha) , although photoprotectant accessory pigments are important in summer. Second, with the exception of one cruise, the data suggest that an estimated maximum quantum yield varies by less than 20%. Resolution of the issue of a nutrient effect may come with detailed measurements in a tropical region, in an ocean province without seasonally varying irradiance but having a strong seasonal signal in nutrient concentrations.