One of the purposes of biogeography is to identify areas within which a characteristic ecosystem is expected to occur. In the case of phytoplanktonic communities this knowledge is key to separate regions characterized by different biogeochemical processes, design efficient sampling strategies and recognize ecological hotspots. Meso- and submesoscale (1–100 km, few days to months) high variability interacts with typical bloom spatial and temporal scales and makes investigating phytoplankton biogeography challenging especially in remote regions like the Southern Ocean, where in-situ observations are extremely sparse. In this study we use a Lagrangian approach to interpret the mesoscale biogeography of diatom dominance of iron-enriched Kerguelen bloom. We define a “threshold” model relating diatom dominance with Lagrangian properties of water parcels. We find out that, in spite of the simplicity of this approach, the threshold model can reproduce the plume of diatom dominance that can be observed using the ocean color re-analysis PHYSAT.