Particle transformations and export flux during an in situ iron-stimulated algal bloom in the Southern Ocean

During the first Southern Ocean Iron RElease Experiment (SOIREE), a suite of biogeochemical measurements (water column 234Th and δ13Corg inventories, particle fluxes from sediment traps, phytoplankton sinking rates) were undertaken to test the hypothesis that the vertical export of particulate organic carbon (POC) is enhanced due to iron-induced increases in phytoplankton production. During the 13-days that the SOIREE bloom was monitored, export fluxes within the iron-fertilised patch were not substantially different to those in waters outside the bloom. On days 11-13, iron enrichment may have caused particle transformations that could lead to elevated future export via particle aggregation and/or diatom chain formation. The unknown time-lag between increased production and export, the longevity of the SOIREE bloom, and the absence of nutrient limitation over days 1-13, however, prohibit prediction of any iron-induced export. This conclusion highlights the difficulties of fully testing the "Iron Hypothesis" and for evaluating the implications for global climate change.