Sea ice acts as an iron (Fe) reservoir in the Southern Ocean (SO) where primary productivity is largely Felimited. The mechanisms leading to Fe enrichment in sea ice result from the combination of poorlyunderstood and largely unexplored physical and biological processes. We analyze the biogeochemicalimpacts of three plausible idealized formulations of dissolved Fe (DFe) incorporation into sea icecorresponding to (i) constant Fe concentration in sea ice, (ii) constant ocean‐ice Feflux, and (iii) ocean‐iceFeflux linearly varying with seawater Fe concentration in a global ocean‐sea‐ice‐biogeochemical model,focusing on the SO. The three formulations simulate different geographical distributions of DFeconcentrations in sea ice. Iron in sea ice remains largely uncertain due to the limited number of spatial andseasonal observations, poorly constrained Fe sources and sinks, and significant uncertainties in simulatedsea ice and hydrography. Despite these differences, the fertilization effect by sea ice on phytoplanktonphotosynthesis is qualitatively similar regardless of the formulation considered. Iron incorporation duringsea‐ice formation, transport, and melt release, common to all formulations, dominates over differences insea‐ice Fe concentrations. Formulating the Fe incorporation rate as proportional to seawater Feconcentrations gives the closest agreement tofield observations. With this formulation, sediments work insynergy with Fe transport to fertilize the waters north of the continental shelf. Southern Ocean primaryproduction and export production increase by 5–10% and 9–19%, respectively, when Fe incorporation intosea ice is considered, suggesting a moderate effect of Fe‐bearing sea ice on marine productivit