ABSTRACTThe effects of light on growth, RuBPCase activity, and chemical composition of Ulva curvata (Kütz.) De Toni and U.Lactuca L. were examined at a range of temperatures and N‐supply levels. Groeth of Ulva speices becomes more light‐dependent with increasing temperature and N. The effect of light on RuBPcase is N‐dependent, with a positive correlation under N‐sufficient and a negative correlation under N‐limited conditions. Light effects on pigment levels and ratios may be independent of effects on growth rate. These interactions uncouple growth rate from RuBPCase and pigments, and thus from tissue%N. The limits of variability of the growth‐%N relationship can be described by a parabola. Under relative light or temperature‐limitation, %N is negatively, growth increase with increasing %N. Tight coupling of seaweed wrowth and chemical composition may therefor be relatively rare in natural waters where growth can be simultaneously limited by light, temperature, and N.