Nitrate uptake and diffusive nitrate supply in the Central Atlantic

The latitudinal variation (35°S to 28°N) in the rate of diffusive nitrate supply across the thermocline and the associated variation in the uptake rate of nitrate and ammonium in the Central Atlantic was studied. The calculated diffusive nitrate flux showed a sharp latitudinal gradient, with the lowest nitrate supply (0.00037 μmol m^-3 d^-1) in the South Atlantic subtropical gyre and the highest values (23.5 μmol m^-3 d^-1) between the Equator and 15°N. The uptake rate of nitrate was inhibited at high irradiance at most stations. Both nitrate and ammonium uptake rates were lowest (about 3 and 10 μmol m^-3 d^-1, respectively) at the southern end of the transect and increased (about 20 and 55 μmol m^-3 d^-1, respectively) towards the Equator, with this increase being much greater for ammonium than for nitrate uptake. The f-ratio was highest (≃0.4) just south of the Equator and lowest (≃0.03) at the southern end of the transect. The slope between total uptake rate of dissolved inorganic nitrogen and gross primary production, calculated from O₂-based measurements, in surface waters (4.72 ± 1.54) was somewhat lower, but not significantly so, than the expected C/N ratio of 6.6. The average uptake rate of nitrate did not differ significantly from the average estimated diffusive supply of nitrate to the biogenic layer over the Central Atlantic. However, the nitrate uptake rate increased as the 1/3 power of the diffusive nitrate flux to the biogenic layer. As a result, nitrate uptake far exceeded (by up to 100-fold) the nitrate flux to the biogenic layer in the stations where the supply of nitrate was lowest. The excess nitrate uptake averaged 0.65 ± 0.24 mmol NO₃ m^-2 d^-1 (range, 0.05-1.9 mmol NO₃ m^-2 d^-1), which must be supplied through atmospheric deposition and other perturbation events. This excess nitrate uptake is relatively large compared to the diffusive supply in the most unproductive areas, where external nitrate inputs fuel the new production. In contrast, these sources of nitrate are far less significant where high diffusive fluxes suffice to maintain high nitrate uptake rates.