Influence of mesoscale eddies on new production in the Sargasso Sea

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 techniques2,3,4,5,6. 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.