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 con...