Large, non-Redfieldian drawdown of nutrients and carbon in the extratropical North Atlantic Ocean (46°N): Evidence for dinitrogen fixation?

Considerable drawdown of total dissolved inorganic carbon (C_T) and oversaturation of oxygen (O₂) within a cold (∼15°C) oligotrophic eddy in the extratropical North Atlantic Ocean (46°N, 20.5°W) indicate that, despite the absence of nitrate (NO₃ ^-), the eddy was highly productive. Estimates of net community production using the mass balances of C_T and O₂ were two to five times greater than those obtained using the mass balance of NO ₃^-. The remineralization rates obtained using the integrated rates of C_T and NO₃^- accumulation and O₂ utilization for the upper thermocline waters (35-300-m depth) were in agreement with C_T- and O₂-based net community production over the same period; however, all the estimates exceeded the NO ₃^--based net community production by a factor of two to five, pointing to a considerable accumulation of NO₃^- in the upper thermocline in excess of changes in the mixed-layer NO ₃^- inventory. The amount of this excess NO ₃^- suggests that a considerable fraction of the net community production was not supported by the mixed-layer NO₃ ^- inventory and that an external source of NO₃^- must be present. Of the various mechanisms that might explain the inequity between NO₃^- drawdown in the surface layer and NO ₃^- accumulation in the upper thermocline, N₂ fixation is the most viable yet surprising mechanism for producing such excess NO₃^- in this oligotrophic eddy. A significant fraction of net community production in oligotrophic extratropical waters could be supported by processes that are not fully explored or to date have been considered to be insignificant.