A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization

Abstract

Changes in iron supply to oceanic plankton are thought to have a significant effect on concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration, a theory known as the ‘iron hypothesis’. For this reason, it is important to understand the response of pelagic biota to increased iron supply. Here we report the results of a mesoscale iron fertilization experiment in the polar Southern Ocean, where the potential to sequester iron-elevated algal carbon is probably greatest. Increased iron supply led to elevated phytoplankton biomass and rates of photosynthesis in surface waters, causing a large drawdown of carbon dioxide and macronutrients, and elevated dimethyl sulphide levels after 13 days. This drawdown was mostly due to the proliferation of diatom stocks. But downward export of biogenic carbon was not increased. Moreover, satellite observations of this massive bloom 30 days later, suggest that a sufficient proportion of the added iron was retained in surface waters. Our findings demonstrate that iron supply controls phytoplankton growth and community composition during summer in these polar Southern Ocean waters, but the fate of algal carbon remains unknown and depends on the interplay between the processes controlling export, remineralisation and timescales of water mass subduction.

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Figure 1: Site selection, pre-release survey and water-column structure.
Figure 2: Temporal evolution of physical, chemical and biological properties during SOIREE.
Figure 3: Time-series measurements made during SOIREE.
Figure 4: A vertical section (from the southeast to the northwest) through the iron-enriched patch on the night of 22 February.
Figure 5: Changes in chlorophyll a levels with time during an on-deck iron/light perturbation experiment.

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Acknowledgements

We thank the officers and crew of the RV Tangaroa, and the management and staff of the NIWA Vessel Management Company. We also acknowledge the provision of physical oceanographic data by J. Church, M. Morris, V. Strass, RV Astrolab, and J. Barth and T. Cowles. We also thank M. Walkington (CTD calibration), J. Aiken (instrument loan), P. Nightingale (argos reception for tracking buoys), S. Groom (near-time high resolution Ocean Color satellite images), NASA (SeaWiFS images), and the companies CEFIC (UK) and BHP (Australia) for their support. We acknowledge two grants from the UK NERC to S.T. (Advanced Fellowship Award), and to A.J.W. and C.S.L. (SOIREE). K.O.B. and M.C. were funded by the US NSF. M.T.M. was funded by the NSERC Canada and the Center for Environmental Bioinorganic Chemistry, Princeton, USA. P.C., A.J.W. and D.C.E.B. received a grant within CARUSO from the European Union. New Zealand scientists were funded by the Government PGSF fund for Antarctic Research. J. Cullen, K. Hunter and C. Hurd provided comments and advice that improved this manuscript.

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Correspondence to Philip W. Boyd.

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