| Summary: | International audience To evaluate the sufficiency of ambient iron and nutrient levels for algal growth in surface waters of the Crozet Basin, in the Indian-Subantarctic region of the Southern Ocean, shipboard iron- and nutrient-addition bottle-incubation experiments were performed during the Antares-IV campaign in January-February 1999. The experiments were conducted with plankton collected from near-surface (~20 m depth) waters at three locations between 43-46°S and 61-65°E, each characterised by relatively low algal biomass (0.2-0.7 µg L-1 chlorophyll a) and dissolved iron concentrations (~0.1-0.3 nM): (1) the Polar Frontal Zone (~19 µM nitrate and 1.2 µM silicate), (2) the confluence of the Subantarctic and Subtropical Fronts (~5.4 µM nitrate and 0.5 µM silicate), and (3) the Subtropical Zone north of the Agulhas Front (< 0.1 µM nitrate and ~1.4 µM silicate). Our experimental results demonstrate three different regimes of chemical limitation on phytoplankton production and community structure within this small oceanic area: in the Polar Frontal Zone, primary limitation of community production by iron deficiency, and secondary limitation of diatom production by silicate deficiency; within the Subantarctic-Subtropical Frontal Zone, primary and secondary limitation of community production by deficiencies in iron and silicate, respectively; and within the southern Subtropical Zone, primary and secondary limitation of community production by deficiencies in nitrate/phosphate and iron, respectively.
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