To evaluate what drives phytoplankton photosynthesis rates in the Amundsen Sea Polynya (ASP), Antarctica, during the spring bloom, we studied phytoplankton biomass, photosynthesis rates, and water column productivity during a bloom of Phaeocystis antarctica (Haptophyceae) and tested effects of iron (Fe) and light availability on these parameters in bioassay experiments in deck incubators. Phytoplankton biomass and productivity were highest (20 µg chlorophyll a L⁻¹ and 6.5 g C m⁻² d⁻¹) in the central ASP where sea ice melt water and surface warming enhanced stratification, reducing mixed layer depth and increasing light availability. In contrast, maximum photosynthesis rate (P^*_max), initial light-limited slope of the photosynthesis–irradiance curve (α^*), and maximum photochemical efficiency of photosystem II (F_v/F_m) were highest in the southern ASP near the potential Fe sources of the Dotson and Getz ice shelves. In the central ASP, P^*_max, α^*, and F_v/F_m were all lower. Fe addition increased phytoplankton growth rates in three of twelve incubations, and at a significant level when all experiments were analyzed together, indicating Fe availability may be rate-limiting for phytoplankton growth in several regions of the ASP early in the season during build-up of the spring bloom. Moreover, Fe addition increased P^*_max, α^*, and F_v/F_m in almost all experiments when compared to unamended controls. Incubation under high light also increased P^*_max, but decreased F_v/F_m and α^* when compared to low light incubation. These results indicate that the lower values for P^*_max, α^*, and F_v/F_m in the central ASP, compared to regions close to the ice shelves, are constrained by lower Fe availability rather than light availability. Our study suggests that higher Fe availability (e.g., from higher melt rates of ice shelves) would increase photosynthesis rates in the central ASP and potentially increase water column productivity 1.7-fold, making the ASP even more productive than it is today.