Importance of N₂ fixation vs. nitrate eddy diffusion along a latitudinal transect in the Atlantic Ocean

We present ocean, basin-scale simultaneous measurements of N₂-fixation, nitrate diffusion, and primary production along a south–north transect in the Atlantic Ocean crossing three biogeographic provinces: the south subtropical Atlantic (SSA; ~ 31°S–12°S), the equatorial Atlantic (EA; ~ 12°S–16°N), and the north subtropical Atlantic (NSA, ~ 16°N–9°N) in April–May 2008. N₂-fixation and primary production were measured as 15N₂ and 14C uptake, respectively. Dissipation rates of turbulent kinetic energy (ε) were measured with a microstructure profiler. The vertical input of nitrate through eddy diffusion was calculated from the product of diffusivity, derived from ε, and the gradient of nanomolar nitrate concentration across the base of the euphotic zone. The mean N₂-fixation rate in EA was 56 ± 49 μmol N m-2 d-1, whereas SSA and NSA had much lower values (~ 10 µmol N m-2 d-1). Because of the large spatial variability in nitrate diffusion (34 ± 50, 405 ± 888, and 844 ± 1258 µmol N m-2 d-1 in SSA, EA, and NSA, respectively), the contribution of N₂-fixation to new production in the SSA, EA, and NSA was 44% ± 30%, 22% ± 19%, and 2% ± 2%, respectively. The differences between SSA and NSA in the contribution of N2 fixation were partly due to the contrasting seasonal forcing in each hemisphere, which likely affected both N₂ fixation rates and vertical nitrate diffusion. The variability in the nitrogen budget of the Atlantic subtropical gyres was unexpectedly high and largely uncoupled from relatively constant phytoplankton standing stocks and primary production rates.