Nitrate isotope anomalies reflect N₂ fixation in the Azores Front region (subtropical NE Atlantic)

The subtropical northeast Atlantic has previously been identified as a marine environment with an apparent imbalance between low nitrate supply to the surface and concurrent high export production. To better constrain the sources and fluxes of mixed layer nitrate and to assess the potential role of N 2 fixation in providing new nitrogen (N), we investigated the depth distribution of nitrate δ 15 N and δ 18 O at six stations across the Azores Front in the NE Atlantic. In addition, we measured the δ 15 N of dissolved organic N (DON) in surface waters and of sinking particulate N collected in sediment traps at 2000 m depth between 2003 and 2005 at Station KIEL276. The nitrate isotope profiles at the majority of the hydrographic stations displayed a decrease in the δ 15 N from depth toward low‐nitrate surface waters, concomitant with an increase in δ 18 O. Given that nitrate uptake by phytoplankton leads to a proportional increase in nitrate δ 15 N and δ 18 O, the observed surface water nitrate isotope anomalies (Δ(15;18) up to −6‰) indicate that nitrate assimilation is not the sole process controlling the isotopic composition of nitrate in the photic zone and implicate a significant addition of newly fixed N that is remineralized in surface and subsurface waters. Both the concentration of DON and its δ 15 N in surface water were spatially invariant, showing mean values of 4.7 ± 0.5 μ mol L −1 and 2.6 ± 0.4‰ (n = 35), respectively, supporting the conjecture of a mostly recalcitrant DON pool. The weighted biannual mean δ 15 N of sinking particulate N (1.8 ± 0.8‰, n = 33) was low with respect to thermocline nitrate. The anomalous dual nitrate isotope signatures together with the low δ 15 N of export production and elevated nitrate‐to‐phosphate ratios in surface and subsurface waters strongly suggest that N 2 fixation represents a substantive source of N in this part of the subtropical northeast Atlantic. Simple isotope mass balance suggests that, locally, N 2 fixation supplies between 56 and 259 mmol N m −2 a −1 for phytoplankton growth in the photic zone, accounting for up to ∼40% of the estimated export production.