The longevity (>50d) of the phytoplankton bloom following mesoscale iron-enrichment of Southern Ocean waters (SOIREE) exceeded that for naturally occurring polar blooms (15--20d). During SOIREE, SF6-labelled waters increased 20-fold, and the greatest algal loss term was lateral advection. To test whether such advective losses could delay the onset of mass sedimentation, an algal aggregation model was employed. It successfully simulated temporal trends in mass sedimentation during the tropical IronEx II bloom, yet suggested no such event during SOIREE. However, when an iron-enrichment of 100 km length-scale (i.e. tenfold greater than for SOIREE) was mimicked, a marked increase in algal aggregate size occurred after 15d, indicative of increased export. Thus, careful interpretation of experimental results-especially for the fate of algal carbon-is essential, particularly for polar studies where the ratio of net algal growth to advective losses is low, if they are to be extrapolated to open-ocean waters.