The biochemical composition of newly produced phytoplankton biomass in Lake Ontario varied systematically with respect to experimentally manipulated incident light intensity and environmentally imposed water temperature and daylength, but was insensitive to light quality. Total uptake of 14C-labeled bicarbonate was light dependent (Popt:dark = 60–200), while 35SO42− uptake was light stimulated (Popt:dark < 5). Subcellular allocation of 14C for relative protein, carbohydrate, and lipid polymer synthesis responded sensitively to subsaturating light. Pathways of 35S assimilation were unaffected by light intensity. Night protein synthesis and attendant respiration of polymeric carbohydrates was a function of prior light history: with daytime illumination at Popt, day and night rates of 35SO4-S incorporation into protein were often indistinguishable. Using April–November data from Popt only, allocation of carbon to carbohydrate polymer storage for night growth was strictly proportional to nightlength. The proportion of carbon contained in protein was strongly correlated with in situ water temperature. The lack of cross-correlation suggests that temperature and daylength exert independent constraints on the biochemical composition of lake microplankton.
