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 N-2-fixation play a critical role in supporting oceanic primary production and influencing global biogeochemical cycles. Numerous studies have documented significant N-2-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 N-15 in, and contribution of diazotroph N (N-D) 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 N-15 within the Western Atlantic and across the basin. We found that N-2-fixation is a key component in supporting both primary and secondary productions throughout the STNA. N-D contribution was generally higher for suspended particles than for mesozooplankton, and we observed a high N-D contribution to suspended particles over large spatial scales in the western and central STNA. Mesozooplankton N-D 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 mu m) generally incorporated more N-D than smaller mesozooplankton (250-10001 mu m), and thus may exert a stronger influence on an N-D movement within the water column. Spatial and vertical patterns of variation in mesozooplankton delta N-15 also suggest either broad geographical differences in an N-D contribution to secondary production, or temporal variations in N-D incorporation via mesozooplankton communities. (C) 2011 Elsevier Ltd. All rights reserved.