Geographic variations in the nitrogen isotope composition of surface particulate nitrogen and new production across the North Atlantic Ocean

Natural 15N/14N (δ15N) was determined in suspended particulate matter (SPM) and plankton tow samples (20- and 233-μm mesh nets) during extensive surveys along E–W transects from Halifax, Nova Scotia, to the Canary Islands in the fall of 1992 and the spring of 1993. This is the first study reporting δ15N measurements along with NO−3 and NH+4 uptake rates in the North Atlantic. During both cruises, the δ15N of SPM and net-plankton was lower in oligotrophic waters compared with values in eutrophic areas. The lack of correlation between the euphotic zone integrated δ15NSPM and NH+4 concentration and NH+4 uptake rate at both times suggested that the contrast between regions could not be explained by regenerated production, even though NH+4 was the preferred N substrate in most regions. In fact, based on a correlation analysis, this study suggested that isotope fractionation during nitrate uptake and the residence time of N in SPM (τ) were the two major factors controlling the δ15NSPM in 1992. During the fall of 1992, the δ15NSPM of the oligotrophic region (τ of 100–170 d) was consistent with the accumulated product equation in closed systems and a δ15N of new nitrate of 4–5‰. In contrast, in eutrophic regions (τ of 7–40 d), the δ15NSPM was consistent with an open system equation. Isotope fractionation (ε) for growth on nitrate was estimated to be 2.4‰ in the upwelling off Morocco. In 1993, no correlations were observed between δ15NSPM and nitrate dynamics, probably because of horizontal advection. Based on the low τ of 5–30 d, an open system equation was used with nitrate as the reactant, and it suggested variations of 2–3‰ in either the δ15NNO3 or ε.