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

During the first Southern Ocean Iron RElease Experiment (SOIREE), a suite o f biogeochemical measurements (water column Th-234 and delta C-13(org) inve ntories, particle fluxes from sediment traps, phytoplankton sinking rates) were undertaken to test the hypothesis that the vertical export of particul ate organic carbon (POC) is enhanced due to iron-induced increases in phyto plankton production. During the 13-days that the SOIREE bloom was monitored , export fluxes within the iron-fertilised patch were not substantially dif ferent to those in waters outside the bloom. On days 11-13, iron enrichment may have caused particle transformations that could lead to elevated futur e export via particle aggregation and/or diatom chain formation. The unknow n time-lag between increased production and export, the longevity of the SO IREE 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.