The cause for the daily rhythm in the photosynthetic capacity of phytoplankton has been examined. Alternative hypotheses have been modeled with the Michaelis-Menten equation. In one model rates of photosynthesis oscillate in response to a forcing from the external concentration of limiting nutrient while photosynthetic potential (Umax) of the assemblage remains constant. In the alternative model photosynthetic potential oscillates in response to intrinsic organization of the cell. The alternatives may be deduced from the photosynthetic response to added nutrient.The photosynthetic response to added phosphate was tested with water from Lake George, New York. Rates of carbon assimilation in the unenriched controls described a daily oscillation with an amplitude of approximately two and a phase maximum in midmorning. The degree of stimulation to added phosphate was also rhythmic. The maximum and minimum corresponded with the daily maximum and minimum, respectively, in unenriched controls. The oscillating int...