1. Marine Biogeochemistry, IFM-GEOMAR Leibniz-Institut für Meereswissenschaften, Düsternbrooker Weg 20, D-24105 Kiel, Germany
2. Department of Biological Sciences, University of Essex, Colchester CO4 3SQ, UK
3. Marine Biological Association of the United Kingdom, Plymouth PL1 2PB, UK

Correspondence to: Richard J. Geider² Email: geider@essex.ac.uk

The role of iron in enhancing phytoplankton productivity in high nutrient, low chlorophyll oceanic regions was demonstrated first through iron-addition bioassay experiments¹ and subsequently confirmed by large-scale iron fertilization experiments². Iron supply has been hypothesized to limit nitrogen fixation and hence oceanic primary productivity on geological timescales³, providing an alternative to phosphorus as the ultimate limiting nutrient⁴. Oceanographic observations have been interpreted both to confirm and refute this hypothesis^{5, 6}, but direct experimental evidence is lacking⁷. We conducted experiments to test this hypothesis during the Meteor 55 cruise to the tropical North Atlantic. This region is rich in diazotrophs⁸ and strongly impacted by Saharan dust input⁹. Here we show that community primary productivity was nitrogen-limited, and that nitrogen fixation was co-limited by iron and phosphorus. Saharan dust addition stimulated nitrogen fixation, presumably by supplying both iron and phosphorus^{10, 11}. Our results support the hypothesis that aeolian mineral dust deposition promotes nitrogen fixation in the eastern tropical North Atlantic.

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