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

Abstract

Considerable drawdown of total dissolved inorganic carbon (C-T) and oversaturation of oxygen (O-2) within a cold (similar to 15 degrees C) oligotrophic eddy in the extratropical North Atlantic Ocean (46 degrees N, 20.5 degrees 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 O-2 were two to five times greater than those obtained using the mass balance of NO3-. The remineralization rates obtained using the integrated rates of C-T and NO3- accumulation and O-2 utilization for the upper thermocline waters (35-300-m depth) were in agreement with C-T- and O-2-based net community production over the same period; however, all the estimates exceeded the NO3--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 NO3- inventory. The amount of this excess NO3- 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 NO3- accumulation in the upper thermocline, N-2 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 waters could be supported by processes that are not fully explored or to date have been considered to be insignificant.