Bulk vs. amino acid stable N isotope estimations of metabolic status and contributions of nitrogen fixation to size-fractionated zooplankton biomass in the subtropical N Atlantic

A comparative analysis of natural abundance of stable N isotopes (δ15N) in individual amino acids and bulk organic matter of size-fractionated plankton revealed the differential impact of nitrogen fixation through the food web in a transect across the subtropical North Atlantic. All δ15N measurements showed low values in the central region, followed by the western zone, while maximum δ15N values were found in the eastern zone. These results were consistent with the prevalence of nitrogen fixation in the central and western zones, and the influence of the west Africa upwelling in the eastern zone. Use of compound-specific amino acid isotope data (CSI-AA) revealed relatively low variability in the impact of diazotrophic nitrogen within the different plankton size fractions, while δ15N of bulk organic matter showed high variability with size. Explicit CSI-AA trophic position estimates showed a small increase with mean plankton size class and varied in a relatively narrow range 1.8–2.5), with the lowest values in the central zone. High correlations between bulk plankton δ15N and individual amino acids (in particular Phe and Thr), as well as reconstructed total protein δ15N values, suggest a set of new relationships that may be important to tracing direct plankton contributions to nitrogen recycling in the ocean, including detrital organic nitrogen pools. Overall, these new results represent the most detailed investigation of CSI-AA data in plankton size classes to date, and indicated a greater importance of diazotrophic N than suggested by concurrent measurements of bulk δ15N, abundance of large nitrogen fixing organisms or nitrogen fixation rates. •The most extensive data on δ15N amino acid patterns in oceanic plankton is reported.•Bulk organic matter and protein δ15N linked across ocean regions and plankton sizes.•Low range of trophic positions for plankton size classes between 40 and >2000µm.•Isotope flow up the food web suggest unaccounted impact of diazotrophy.•New parameters to compare trophic structure based on amino acid δ15N.