Phytoplankton in the Southern Ocean support important ecosystems and play a key role in the earth’s carbon cycle, hence affecting climate. However, current global biogeochemical models struggle to reproduce the dynamics and co-existence of key phytoplankton functional types (PFTs) in this Ocean. Here we explore the traits important to allow three key PFTs (diatoms,coccolithophores and Phaeocystis) to have distributions, dominance and composition consistent with observations. In this study we use the Darwin biogeochemical/ecosystem model coupled to the Massachusetts Institute of Technology (MIT) general circulation model (Darwin-MITgcm). We evaluated our model against an extensive synthesis of observations, including in situ microscopy and high-performance liquid chromatography (HPLC), and satellite derived phytoplankton dominance, PFTchlorophyll-a (Chla), and phenology metrics. To capture the regional timing of diatom blooms obtained from satellite required including both a lightly silicified diatom type and a larger and heavy silicified type in the model. To obtain the anticipated distribution of coccolithophores, including the Great Calcite Belt, required accounting for a high affinity for nutrients and anability to escape grazing control of this PFT. The implementation of two life stages of Phaeocystis to simulate both solitary and colonial forms of this PFT (with switching between forms being driven by iron availability) improved the co-existence of coccolithophores and Phaeocystis north of the Polar Front. The dual life-stages of Phaeocystis allowed it to compete both with other phytoplankton of larger size and/or similar sizes. The evaluation of simulated PFTs showed significant agreement to a large set of matchups with in situ PFT Chl-a data derived from pigment concentrations. Satellite data provided important qualitative comparisons of PFT phenology and PFT dominance. With these newly added traits the model produced the observed >50% coccolithophore contribution to the biomass of biomineralizing PFTs in the Great Calcite Belt. The model together with the large synthesis of observations provides a clearer picture of the Southern Ocean phytoplankton community structure, and new appreciation of the traits that are likely important in setting this structure.