National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2008

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

Park, G.-H., K. Lee, R. Wanninkhof, J.-Z. Zhang, D.A. Hansell, and R.A. Feely

Limnol. Oceanogr., 53(5), 1697–1704, doi: 10.4319/lo.2008.53.5.1697 (2008)

Considerable drawdown of total dissolved inorganic carbon (CT) 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 CT and O2 were two to five times greater than those obtained using the mass balance of NO₃⁻. The remineralization rates obtained using the integrated rates of CT and NO₃⁻ accumulation and O₂ utilization for the upper thermocline waters (35–300-m depth) were in agreement with CT- and O₂-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, 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.

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