| Summary: | International audience Shipboard iron-addition incubation experiments were carried out in the Indian sector of the Subantarctic Southern Ocean duringthe Antares-IV campaign in late January–February 1999. The aim of these experiments was to estimate the dissolved iron requirements of the native phytoplankton community in this oceanic region, in order to improve the parameterisation of iron as a limitingnutrient in a coupled 1D physical–biogeochemical ocean model. The experiments were conducted with plankton collected from the upper water column (B20m depth) at three sites in the Crozet Basin between 43–461S and 61–651E: (1) the Polar Front Zone (PFZ, dissolved Fe=0.33 nM), (2) the confluence of the Subantarctic and Subtropical Fronts (SAF/STF, dissolved Fe=0.29 nM), and (3) the southern Subtropical Zone (STZ, dissolved Fe=0.09 nM). Experimental results from each site indicate that algal community growth rates varied as a function of added iron concentration. Monod saturation functions fitted to the experimental data yield estimates for the community half-saturation constant for growth with respect to iron (Km) of 0.41–0.45nM Fe (PFZ), 0.055–0.086nM Fe (SAF/STF) and 0.092–0.093nM Fe (STZ, with macronutrients added), each of which has an estimated uncertainty of 720%. The Km estimate for the SAF/STF site reflects the mixed algal assemblage (diatoms+nanoplankton+dinoflagellates) that grew in the experimental incubations, whereas the Km estimates for the PFZ and STZ sites probably reflect the Fe requirements of the small pennate diatoms such as Pseudo-nitzschia spp., which dominated the algal biomass produced in these experiments. The fact that there are significant differences between the Km estimates for the PFZ and STZ sites suggests that similar diatom assemblages may have quite different iron requirements, perhaps due to differences in environmental conditions (e.g., light, macronutrient levels). We also examine the sensitivity of a onedimensional coupled physical–biogeochemical model to the choice of Km for ...
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