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Importance of stirring in the development of an iron-fertilized phytoplankton bloom


The growth of populations is known to be influenced by dispersal, which has often been described as purely diffusive1,2. In the open ocean, however, the tendrils and filaments of phytoplankton populations provide evidence for dispersal by stirring3,4. Despite the apparent importance of horizontal stirring for plankton ecology, this process remains poorly characterized. Here we investigate the development of a discrete phytoplankton bloom, which was initiated by the iron fertilization of a patch of water (7 km in diameter) in the Southern Ocean5. Satellite images show a striking, 150-km-long bloom near the experimental site, six weeks after the initial fertilization. We argue that the ribbon-like bloom was produced from the fertilized patch through stirring, growth and diffusion, and we derive an estimate of the stirring rate. In this case, stirring acts as an important control on bloom development, mixing phytoplankton and iron out of the patch, but also entraining silicate. This may have prevented the onset of silicate limitation, and so allowed the bloom to continue for as long as there was sufficient iron. Stirring in the ocean is likely to be variable, so blooms that are initially similar may develop very differently.

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Figure 1: Images of the SOIREE bloom from the SeaWiFS ocean colour satellite.
Figure 2: Changes in the size and shape of the fertilized patch following the initial infusion.


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We thank all the SOIREE team, particularly the captain and crew of RV Tangaroa for their assistance throughout the experiment; S. Nodder, R. Murdoch, A. Watson and T. Trull for experiment planning and coordination; and G. Jameson, M. Liddicoat and R. Ling for their work on the tracer release and mapping. We also thank S. Groom for additional interpretation of the remote sensing data; P. Nightingale for ARGOS data transfer; S. Rintoul for providing the XBT data; and M. Morris, C. Stevens and A. Martin for comments on the original manuscript. The SeaWiFS data, provided by the NASA DAAC/GSF and copyright of Orbital Imaging Corps and the NASA SeaWiFS project, was processed at CCMS-PML. E.A. and P.B. acknowledge the financial assistance of the NZ Public Good Science Fund for Antarctic research; C.L. thanks the UK Natural Environment Research Council for support; M.M. was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Center for Environmental Bioinorganic Chemistry, Princeton (CEBIC); A.B. was supported by the University of Plymouth and Plymouth Marine Laboratory.

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Correspondence to Edward R. Abraham.

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Abraham, E., Law, C., Boyd, P. et al. Importance of stirring in the development of an iron-fertilized phytoplankton bloom. Nature 407, 727–730 (2000).

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