The Southern Ocean Seasonal Sea Ice Zone (SSIZ) is characterized by the development of spring phytoplankton blooms following retreating sea ice. Until recently, assessing SSIZ bloom carbon export has been limited by a lack of under ice observations. Here, we relate the timing of phytoplankton growth to the drawdown of surface nutrients and sea ice cover and estimate spring bloom net community production (bNCP) using biogeochemical profiling float observations. The onset of biological production follows initial sea ice breakup with 64% of bNCP under partial sea ice cover. Estimates of bNCP range from <1 to >4 mol C m^-2 bloom^-1, with earlier sea ice breakup associated with higher bNCP, and the highest bNCP where micronutrient supply is likely enhanced by topographic-driven mixing. These results indicate that satellite-derived export estimates will underestimate bNCP in the SSIZ and have implications for future carbon export in a changing Southern Ocean sea ice regime.