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

Park, Geun-Ha, Kitack Lee, Rik Wanninkhof, Jia-Zhong Zhang, Dennis A. Hansell, Richard A. Feely

Limnol. Oceanogr., 53(5), 2008, 1697-1704 | DOI: 10.4319/lo.2008.53.5.1697

ABSTRACT: 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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