ABSTRACT. The response of photosynthesis, and of the uptake of NO₃^-, NH₄⁺ and serine, to irradiance was measured in diatom-dominated sea ice microbial assemblages from bottom ice and surface ice of McMurdo Sound, Antarctica. Uptake responses for dissolved inorganic nitrogen (DIN; NO₃^- and NH₄⁺) could be fitted to a standard model used for photosynthesis after the addition of a dark uptake parameter; serine uptake showed no dependence on irradiance. The derived uptake models were used to predict the patterns of photosynthesis and DIN uptake over diel irradiance cycles. According to model predictions, uptake rates in the bottom ice assemblage were always limited by irradiance; neither light saturation nor photoinhibition regulated photosynthesis or DIN utilization in this assamblage. Conversely, photosynthesis in the surface ice assemblage was nearly always light-saturated, whereas DIN uptake was photo-inhibited near midday and saturated at the minimum irradiance. Integrated daily C:DIN uptake ratios (g:g) in the bottom ice and surface ice assemblages were 8.6 and 9.7, respectively, corresponding to particulate C:N ratios (g:g) of 8.1 and 5.8 for these respective diatom-dominated communities. Our results indicate that information on diel patterns of photosynthesis and N uptake is required to evaluate accurately the stoichiometric balance of essential elements in sea ice microalgae.