Friday 26 August 2005

PBA1
0850-1030 hours

456
Particle sedimentation from an iron-stimulated phytoplankton bloom in the
Southern Ocean
Strass, Volker H¹, Assmy, Philipp¹, Bathmann, Ulrich¹, Bellerby, Richard², Cisewski, Boris¹, Croot, Peter³, Gonzales, Santiago⁴, Hoffmann, Linn³, Klaas, Christine¹, Kraegefsky, Soeren¹, Leach, Harry⁵, Peeken, Ilka³, Savoye, Nicolas⁶, Smetacek, Victor¹
1 Alfred-Wegener-Institut fuer Polar- und Meeresforschung, Bremerhaven, Germany
2 Bjerknes Centre for Climate Research, Bergen, Norway
3 Leibniz-Institut fuer Meereswissenschaften, Kiel, Germany
4 Netherlands Institute for Sea Research, Den Burg, Texel, The Netherlands
5 Department of Earth and Ocean Sciences, University of Liverpool, UK
6 Department of Analytical and Environmental Chemistry, Vrije Universiteit Brussel, Belgium
Author email: vhstrass@awi-bremerhaven.de
The supply of iron to the phytoplankton living in the surface layer of the open ocean is thought to be of global significance for the uptake of carbon dioxide from the atmosphere. Of outstanding importance is the Southern Ocean, where the upwelling of deep water masses in the Antarctic divergence supplies a surplus of macronutrients, which, unused by phytoplankton primary production, however subduct again at the Antarctic Polar Front (APF) and Subantarctic Front. We report results from the European Iron Fertilization Experiment (EIFEX), which was performed to test the iron hypothesis directly at the APF. As the APF is associated with swift and meandering frontal jets, tracking a fertilized patch here poses a serious problem. This problem was solved by selecting a mesoscale eddy as the experimental site. Using this strategy it was possible to monitor the fertilized patch for over 5 weeks during the period January to March, 2004. The EIFEX data reveal an iron-stimulated increase of the mixed layer chlorophyll concentration by a factor of more than three, an increase of diatom biomass by a factor of almost three, and a drawdown of total carbon dioxide in the mixed layer of 20 mol/kg. EIFEX is the first iron-fertilization experiment to show a sedimentation of biogenic particles to the deep ocean and bottom nepheloid layer, and hence a hindering of the rapid return of part of the biologically fixed carbon to the atmosphere.

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