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Science 3 August 2001:
Vol. 293. no. 5531, pp. 847 - 849
DOI: 10.1126/science.1059251
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Reports

Soluble and Colloidal Iron in the Oligotrophic North Atlantic and North Pacific

Jingfeng Wu,1* Edward Boyle,1 William Sunda,2 Liang-Saw Wen3

In the oligotrophic North Atlantic and North Pacific, ultrafiltration studies show that concentrations of soluble iron and soluble iron-binding organic ligands are much lower than previously presumed "dissolved" concentrations, which were operationally defined as that passing through a 0.4-micrometer pore filter. Our studies indicate that substantial portions of the previously presumed "dissolved" iron (and probably also iron-binding ligands) are present in colloidal size range. The soluble iron and iron-binding organic ligands are depleted at the surface and enriched at depth, similar to distributions of major nutrients. By contrast, colloidal iron shows a maximum at the surface and a minimum in the upper nutricline. Our results suggest that "dissolved" iron may be less bioavailable to phytoplankton than previously thought and that iron removal through colloid aggregation and settling should be considered in models of the oceanic iron cycle.

1 Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
2 Beaufort Laboratory, National Oceanic and Atmospheric Administration, Beaufort, NC 28516, USA.
3 National Center for Ocean Research, National Research Council, Post Office Box 23-13, Taipei, Taiwan.
*   To whom correspondence should be addressed. E-mail: jingfeng{at}mit.edu

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