Abstract
Deep‐water nitrate is a major reservoir of oceanic combined nitrogen and has long been considered to be the major source of new nitrogen supporting primary production in the oligotrophic ocean. 15 N: 14 N ratios in plankton provide an integrative record of the nitrogen cycle processes at work in the ocean, and near‐surface organic matter in oligotrophic waters like the Sargasso Sea is characterized by an unusually low 15 N content relative to average deep‐water nitrate. Herein we show that the low dΔ 15 N of suspended particles and zooplankton from the tropical North Atlantic cannot arise through isotopic fractionation associated with nutrient uptake and food web processes but are instead consistent with a significant input of new nitrogen to the upper water column by N 2 fixation. These results provide direct, integrative evidence that N 2 fixation makes a major contribution to the nitrogen budget of the oligotrophic North Atlantic Ocean.