The importance of nitrogen fixation to the nitrogen budget of the North Atlantic Ocean

Samples of seawater and suspended particles in the mixed layer (top 100m) were collected on Seward Johnson cruises SJ9603, SJ9612, and SJ0005 to the tropical and subtropical North Atlantic Ocean. Deep-water nitrate has been proposed to provide most nitrogen to the upper water column of the North Atlantic Ocean. Recent evidence has shown that N2 fixation is plays a significant role in supplying nitrogen for new production. The ratio of 15N: 14N, referred to as δ15N, provides a useful tracer for identifying major sources to new nitrogen in the upper water column. Persistently low δ15N values coupled with high N* values imply a large contribution of N2 fixation, with 83 of 85 stations suggesting some contribution of N2 fixation. The highest levels of diazotrophic contribution were recorded in the southwestern tropical Atlantic Ocean, where a large bloom of the N2 fixing diatom/ cyanobacterial association Hemiaulus/ Richelia association in addition to the N2 fixing cyanobacterium Trichodesmium was previously recorded. The isotopic data, N* data, and diazotrophic contribution estimates show N2 fixation is making a significant contribution to the nitrogen budget of the nutrient-poor North Atlantic Ocean.