The Amundsen Sea in West Antarctica features rapidly thinning ice shelves and large, seasonally recurring polynyas. Within these polynyas, sizable spring phytoplankton blooms occur. Although considerable effort has gone into characterising heat fluxes between the Amundsen Sea, its associated ice shelves, and the overlying atmosphere, the effect of the phytoplankton blooms on the distribution of heat remains poorly understood. In this modelling study, we implement a feedback from biogeochemistry onto physics into MITgcm-BLING and use it to show, for the first time, that high levels of chlorophyll – concentrated in the Amundsen Sea Polynya and the Pine Island Polynya – accelerate springtime surface warming in polynyas through enhanced absorption of solar radiation. The warm midsummer anomaly (on average between +0.2°C and +0.3C°) at the surface is quickly dissipated to the atmosphere, by small increases in latent and longwave heat loss as well as a substantial (17.5%) increase in sensible heat loss from open water areas. The summertime warm anomaly also reduces the summertime sea ice volume, and stimulates enhanced seasonal melting near the fronts of ice shelves. However larger effects derive from the accompanying cold anomaly, caused by shading of deeper waters, which persists throughout the year and affects a decrease in the volume of Circumpolar Deep Water on the continental shelf. This cooling ultimately leads to an increase in wintertime sea ice volume, and reduces basal melting of Amundsen Sea ice shelves by approximately 7% relative to the model scenario with no phytoplankton bloom.