Nitrogen fixation and nitrogen isotope abundances in zooplankton of the oligotrophic

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. 15N: 14N 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 15N content relative to average deep-water nitrate. Herein we show that the low d15N 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 N2 fixation. These results provide direct, integrative evidence that N2 fixation makes a major contribution to the nitrogen budget of the oligotrophic North Atlantic Ocean. Recent biological and geochemical studies have produced greatly increased estimates of the importance of biological N2 fixation in supporting new production in oligotrophic ar-eas of the ocean (Carpenter and Romans 1991; Lipschult