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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Martin, J. H. Glacial-interglacial CO2 change: The iron hypothesis. Paleoceanography 5, 1–13 (1990).
Falkowski, P. G., Barber, R. T. & Smetacek, V. Biogeochemical controls and feedbacks on ocean primary production. Science 281, 200– 206 (1998).
Coale, K. H. et al. A massive phytoplankton bloom induced by ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean. Nature 383, 495–501 ( 1996).
Behrenfeld, M. J., Bale, A. J., Kolber, Z. S., Aiken, J. & Falkowski, P. G. Confirmation of iron limitation of phytoplankton photosynthesis in the equatorial Pacific Ocean. Nature 383, 508–510 ( 1996).
Cooper, D. J., Watson, A. J. & Nightingale, P. D. Large decrease in ocean-surface CO2 fugacity in response to in situ iron fertilization. Nature 383, 511–513 (1996).
Sarmiento, J. L. & Orr, J. C. Three-dimensional simulations of the impact of Southern Ocean nutrient depletion on atmospheric CO2 and ocean chemistry. Limnol. Oceanogr. 36, 1928–1950 (1991).
Sarmiento, J. L., Hughes, T. M. C., Stouffer, R. J. & Manabe, S. Simulated response of the ocean carbon cycle to anthropogenic climate warming. Nature 393, 245–249 (1998).
Broecker, W. S. & Henderson, G. M. The sequence of events surrounding termination II and their implications for the causes of glacial interglacial CO2 changes. Paleoceanography 13, 352–364 ( 1998).
Kumar, N. et al. Increased biological productivity and export production in the glacial Southern Ocean. Nature 378, 675– 680 (1995).
de Baar, H. J. W. & Boyd, P. W. in The Dynamic Ocean Carbon Cycle: A Midterm Synthesis of the Joint Global Ocean Flux Study Ch. 4 (eds Hanson, R. B., Ducklow, H. W. & Field, J. G.) 61–140 (International Geosphere Biosphere Programme Book Series, Cambridge Univ. Press, Cambridge, 1999).
de Baar, H. J. W. et al. Importance of iron for plankton blooms and carbon dioxide drawdown in the Southern Ocean. Nature 373, 412–415 (1995).
Mitchell, B. G., Brody, E. A., Holm-Hansen, O., McClain, C. & Bishop, J. Light limitation of phytoplankton biomass and macronutrient utilization in the Southern Ocean. Limnol. Oceanogr. 36, 1662–1677 (1991).
Nelson, D. M. & Smith, W. O. Jr Sverdrup revisited: Critical depths, maximum chlorophyll levels, and the control of Southern Ocean productivity by the irradiance-mixing regime. Limnol. Oceanogr. 36, 1650–1661 (1991).
Boyd, P. W., LaRoche, J., Gall, M., Frew, R. & McKay, R. M. L. The role of iron, light and silicate in controlling algal biomass in sub-Antarctic waters SE of New Zealand. J. Geophys. Res. 104, 13395–13408 ( 1999).
Banse, K. Iron availability, nitrate uptake and exportable new production in the subarctic Pacific. J. Geophys. Res. 96, 741– 748 (1991).
Orsi, A. H., Whitworth, T. W. & Nowlin, W. D. Jr On the meridional extent and fronts of the Antarctic Circumpolar Current. Deep-Sea Res. I 42, 641–673 (1995).
Nowlin, W. D. Jr & Klinck, J. M. The physics of the Antarctic Circumpolar Current. Rev. Geophys. Space Phys. 24, 469–491 ( 1986).
Zentara, S. J. & Kamykowski, D. Geographic variation in the relationship between silicic acid and nitrate in the South Pacific Ocean. Deep-Sea Res. A 28, 455– 465 (1981).
Rintoul, S. R. & Bullister, J. L. A late winter hydrographic section from Tasmania to Antarctica. Deep-Sea Res. I 46, 1417–1454 ( 1999).
Rintoul, S. R., Donguy, J. R. & Roemmich, D. H. Seasonal evolution of upper ocean thermal structure between Tasmania and Antarctica. Deep-Sea Res. I 44 , 1185–1202 (1997).
LaRoche, J., Boyd, P. W., McKay, R. M. L. & Geider, R. J. Flavodoxin as an in situ marker for iron stress in phytoplankton. Nature 382, 802–805 ( 1996).
Watson, A. J., Liss, P. S. & Duce, R. A. Design of a small-scale iron fertilisation experiment. Limnol. Oceanogr. 36, 1960– 1965 (1991).
Martin, J. H. et al. Testing the iron hypothesis in ecosystems of the equatorial Pacific Ocean. Nature 371, 123– 129 (1994).
Law, C. S., Watson, A. J., Liddicoat, M. I. & Stanton, T. Sulphur hexafluoride as a tracer of biogeochemical and physical processes in an open-ocean iron fertilisation experiment. Deep-Sea Res. II 45, 977–994 ( 1998).
Millero, F. J., Sotolongo, S. & Izaguirre, M. The oxidation kinetics of Fe(II) in seawater. Geochim. Cosmochim. Acta 51, 793–801 (1987).
Geider, R. J. Biological oceanography: complex lessons of iron uptake. Nature 400, 815–816 ( 1999).
Muggli, D. L., Lecourt, M. & Harrison, P. J. Effects of iron and nitrogen source on the sinking rate, physiology and metal composition of an oceanic diatom from the subarctic Pacific. Mar. Ecol. Prog. Ser. 132, 215– 227 (1996).
Greene, R. M., Geider, R. J., Kolber, Z. & Falkowski, P. G. Iron-induced changes in light-harvesting and photochemical energy conversion processes in eukaryotic marine algae. Plant Physiol. 100, 565–575 (1992).
Behrenfeld, M. J., Prasil, O., Kolber, Z. S., Babin, M. & Falkowski, P. G. Compensatory changes in photosystem II electron turnover rates protect photosynthesis from photoinhibition. Photosynth. Res. 58, 259–268 (1999).
Neale, P. J., Davis, R. F. & Cullen, J. J. Interactive effects of ozone depletion and vertical mixing on photosynthesis of Antarctic phytoplankton. Nature 392, 585–589 (1998).
Sunda, W. G. & Huntsman, S. A. Interrelated influence of iron, light and cell size on marine phytoplankton growth. Nature 390, 389–392 (1997).
Laubscher, R. K., Perissinotto, R. & McQuaid, C. D. Phytoplankton production and biomass at frontal zones in the Atlantic Sector of the Southern Ocean. Polar Biol. 13, 471–481 (1993).
Watson, A. J., Bakker, D. C. E., Ridgwell, A. J., Boyd, P. W. & Law, C. S. Effect of iron supply on Southern Ocean CO2 uptake and implications for glacial atmospheric CO 2. Nature 407, 730– 733 (2000).
Hutchins, D. A. & Bruland, K. W. Iron limited diatom growth and Si:N uptake ratios in a coastal upwelling regime. Nature 393, 561–564 ( 1998).
Verity, P. & Smetacek, V. Organism life cycles, predation, and the structure of marine pelagic ecosystems. Mar. Ecol. Prog. Ser. 130, 277–293 ( 1996).
Wolfe, G. V., Steinke, M. & Kirst, G. O. Grazing-activated chemical defence in a unicellular marine alga. Nature 387, 894– 897 (1997).
Buesseler, K. O. Do upper-ocean sediment traps provide an accurate record of particle flux? Nature 353, 420–423 (1991).
McKay, R. M. L., Geider, R. J. & LaRoche, J. Physiological and biochemical response of the photosynthetic apparatus of two marine diatoms to Fe stress. Plant Physiol. 114, 615–622 (1997).
Maldonado, M. T. & Price, N. M. Utilization of iron bound to strong organic ligands by phytoplankton communities in the subarctic Pacific Ocean. Deep-Sea Res. II 46, 2447 –2474 (1999).
Abraham, E. R. et al. Importance of stirring in the development of an iron-fertilized phytoplankton bloom. Nature 407, 727– 730 (2000).
Morel, F. M. M., Reuter, J. G. & Price, N. M. Iron nutrition of phytoplankton and its possible importance in the ecology of ocean regions with high nutrient and low biomass. Oceanography 4, 56–61 (1991).
Boyd, P. W. & Newton, P. P. Does planktonic community structure determine downward particulate organic carbon flux in different oceanic provinces. Deep-Sea Res. I 46, 63– 91 (1999).
Obata, A., J. Ishizaka & Endoh, M. Global verification of critical depth theory for phytoplankton blooms with climatological in situ temperature and satellite ocean color data. J. Geophys. Res. 101, 20657– 20667 (1998).
Peng, T.-H. & Broecker, W. S. Factors limiting the reduction of atmospheric CO2 by iron fertilisation. Limnol. Oceanogr. 36, 1919–1927 ( 1991).
Bishop, J. K. B. & Rossow, W. R. Spatial and temporal variability of global surface solar irradiance. J. Geophys. Res. 96, 16839–16858 ( 1991).
Trenberth, K. E., Large, W. G. & Olson, J. G. The mean annual cycle in global ocean wind stress. J. Phys. Oceanogr. 20, 1742– 1760 (1990).
Moore, J. K. et al. SeaWiFS satellite ocean color data from the Southern Ocean. Geophys. Res. Lett. 26, 1465– 1468 (1999).
Jackson, G. A. A model of the formation of marine algal flocs by physical coagulation processes. Deep-Sea Res. 37, 1197– 1211 (1990).
Law, C. S., Watson, A. J. & Liddicoat, M. I. Automated vacuum analysis of sulphur hexafluoride in seawater; derivation of the atmospheric trend (1970-1993) and potential as a transient tracer. Mar. Chem. 48, 57 –69 (1994).
Bradford-Grieve, J. M. et al. Pelagic ecosystem structure and functioning in the Subtropical Front region east of New Zealand in austral winter and spring 1993. J. Plank. Res. 21, 405–428 (1999).
Fitzwater, S. E., Knauer, G. A. & Martin, J. H. Metal contamination and its effect on primary production methods. Limnol. Oceanogr. 27, 544– 551 (1982).
Maldonado, M., Boyd, P. W., Price N. M. & Harrison, P. J. Co-limitation of phytoplankton by light and Fe during winter in the NE subarctic Pacific Ocean. Deep-Sea Res. II 46, 2475– 2486 (1999).
Montagnes, D. J. S., Berges, J. A., Harrison, P. J. & Taylor, F. J. R. Estimating carbon, nitrogen, protein and chlorophyll a from volume in marine phytoplankton. Limnol. Oceanogr. 39, 1044–1060 (1994).
Popp, B. N., Parekh, P., Tilbrook, B., Bidigare, R. R. & Laws, E. A. Organic carbon δ13C variations in sedimentary rocks as chemostratigraphic and paleoenvironmental tools. Palaeoclimatol. Palaeoecol. 132, 119– 132 (1997).
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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