In the Arctic Ocean the peak of the phytoplankton bloom occurs around the period of sea ice break-up.  Climate change is likely to impact the bloom phenology and its crucial contribution to the production  dynamics of Arctic marine ecosystems. Here we explore and quantify controls on the timing of the spring  bloom using a one-dimensional biogeochemical/ecosystem model configured for coastal western Baffin  Bay. The model reproduces the observations made on the phenology and the assemblage of the  phytoplankton community from an ice camp in the region. Using sensitivity experiments, we found that  two essential controls on the timing of the spring bloom were the biomass of phytoplankton before bloom  initiation and the light under sea ice before sea ice break-up. The level of nitrate before bloom initiation  was less important. The bloom peak was delayed up to 20 days if the overwintering phytoplankton  biomass was too low. This result highlights the importance of phytoplankton survival mechanisms during  polar winter to the pelagic ecosystem of the Arctic Ocean and the spring bloom dynamics.