Séminaire
Nitrogen fixation by diazotrophs is the dominant process maintaining the ocean’s fertility against persistent loss of fixed N in low-oxygen environments. However, the geographical distribution and ecological controls on N2 fixation are difficult to constrain with limited in-situ measurements. We present convergent estimates of N2 fixation from a new inverse biogeochemical model and a prognostic ocean model. Our results demonstrate (1) strong spatial variability in the N:P ratio of exported organic matter, (2) this N:P variability greatly increases the global N2-fixation rate, (3) new N supports up to 50% of export in subtropical gyres, (4) spatially-decoupled patterns of N2 fixation and denitrification, (5) current-era N sources and sinks largely balance on multidecadal timescales, and (6) a role for top-down grazing control in shaping the global patterns of N2 fixation. Together these results give a clearer picture of the marine N cycle, critical for understanding the response of marine ecosystems to ongoing anthropogenic perturbations.
olivier.aumont@ird.fr