Basin-scale distributions of stable nitrogen isotopes in the subtropical North Atlantic Ocean: Contribution of diazotroph nitrogen to particulate organic matter and mesozooplankton

New N inputs via biological N2-fixation play a critical role in supporting oceanic primary production and influencing global biogeochemical cycles. Numerous studies have documented significant N2-fixation rates in the North Atlantic, but relatively little is known of the pathways and fluxes of new N through planktonic food webs. Here, we report the natural abundance of 15N in, and contribution of diazotroph N (ND) inputs to, suspended particles and mesozooplankton collected along two transects in the subtropical North Atlantic Ocean (STNA). Samples were collected in April–May 2000 along the two main transects to evaluate spatial trends of 15N within the Western Atlantic and across the basin. We found that N2-fixation is a key component in supporting both primary and secondary productions throughout the STNA. ND contribution was generally higher for suspended particles than for mesozooplankton, and we observed a high ND contribution to suspended particles over large spatial scales in the western and central STNA. Mesozooplankton ND incorporation suggests that diazotroph production supports oceanic food webs over longer timescales (e.g., weeks to months) than that of particle turnover (e.g., days). Larger mesozooplankton (1000–4000 μm) generally incorporated more ND than smaller mesozooplankton (250–1000 μm), and thus may exert a stronger influence on an ND movement within the water column. Spatial and vertical patterns of variation in mesozooplankton δ15N also suggest either broad geographical differences in an ND contribution to secondary production, or temporal variations in ND incorporation via mesozooplankton communities.