1. Department of Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
2. Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
3. Institute for Computational Earth System Science and Department of Geography, Santa Barbara, California 93106, USA
4. Ocean Physics Laboratory, University of California, Santa Barbara, California 93106, USA
5. Monterey Bay Aquarium Research Insitute, Moss Landing, California 95039, USA
6. Bermuda Biological Station for Research, Ferry Reach, GE01, Bermuda
7. Wrigley Institute for Environmental Studies, University of Southern California, Avalon, California 90704, USA

Correspondence to: D. J. McGillicuddy, Jr¹ Correspondence and requests for materials should be addressed to D.J.McG. (e-mail Email: dmcgillicuddy@whoi.edu).

It is problematic that geochemical estimates of new production — that fraction of total primary production in surface waters fuelled by externally supplied nutrients — in oligotrophic waters of the open ocean surpass that which can be sustained by the traditionally accepted mechanisms of nutrient supply.¹,² In the case of the Sargasso Sea, for example, these mechanisms account for less than half of the annual nutrient requirement indicated by new production estimates based on three independent transient-tracer techniques^{2, 3, 4, 5, 6}. Specifically, approximately one-quarter to one-third of the annual nutrient requirement can be supplied by entrainment into the mixed layer during wintertime convection⁷, with minor contributions from mixing in the thermocline⁸,⁹ and wind-driven transport¹⁰ (the potentially important role of nitrogen fixation¹¹ — for which estimates vary by an order of magnitude in this region¹² — is excluded from this budget). Here we present four lines of evidence — eddy-resolving model simulations, high-resolution observations from moored instrumentation, shipboard surveys and satellite data — which suggest that the vertical flux of nutrients induced by the dynamics of mesoscale eddies is sufficient to balance the nutrient budget in the Sargasso Sea.