Mesoscale eddies are ubiquitous oceanic features that exert profound control on nitrogen (N) supply and cycling, yet simultaneous rate measurements of multiple N transformation processes within eddy remain sparse. Here, we present the first co-located rate measurements of N₂ fixation, nitrate (NO₃^-) and ammonium (NH₄⁺) uptake, ammonia and nitrite oxidation, and export production within a cyclonic eddy in the western North Pacific Subtropical Gyre. Compared with the edge, the eddy core exhibited enhanced NO₃^- upwelling and N₂ fixation. Unexpectedly, enhanced new N inputs did not proportionally increase export production, but instead intensified N recycling via NH₄⁺ uptake and nitrification. Depth-integrated N₂ fixation, NO₃^- uptake, and NH₄⁺ uptake rates were tightly correlated, indicating that new N inputs stimulated rapid internal N recycling within the euphotic zone. We propose that cyclonic eddies can act as transient N retention zones, temporarily delaying export and altering the fate of new N in oligotrophic oceans.
 

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