Carbon-Based Estimate of Nitrogen Fixation-Derived Net Community Production in N-Depleted Ocean Gyres

Type Article
Date 2018-08
Language English
Author(s) Ko Young HoORCID1, Lee KitackORCID1, Takahashi TaroORCID2, Karl David M.3, Kang Sung-Ho4, Lee Eunil5
Affiliation(s) 1 : Pohang Univ Sci & Technol, Div Environm Sci & Engn, Pohang, South Korea.
2 : Columbia Univ, Lamont Doherty Earth Observ, Palisades, NY USA.
3 : Univ Hawaii Manoa, Daniel K Inouye Ctr Microbial Oceanog Res & Educ, Honolulu, HI 96822 USA.
4 : KIOST, Korea Polar Res Inst, Div Polar Ocean Sci, Incheon, South Korea.
5 : Korea Hydrog & Oceanog Agcy, Ocean Res Div, Busan, South Korea.
Source Global Biogeochemical Cycles (0886-6236) (Amer Geophysical Union), 2018-08 , Vol. 32 , N. 8 , P. 1241-1252
DOI 10.1029/2017GB005634
WOS© Times Cited 9
Keyword(s) net community production, nitrogen fixation, seasonal carbon drawdown, oligotrophic ocean gyre

Accurate estimation of net community production (NCP) in the ocean is important for determining the future trend for carbon dioxide concentrations in the atmosphere and thus for understanding the global carbon cycle and climate change. Most methods for measuring NCP rely on analysis of dissolved fixed inorganic nitrogen species (N), which are believed to be limiting factors for NCP. However, in the vast areas of the ocean gyres only low levels of N are available for phytoplankton during much of the year. In this study the NCP was estimated by summing the seasonal reduction in the concentration of dissolved inorganic carbon (C-T) in the surface mixed layer, corrected for changes associated with salinity variation, net air-sea CO2 flux, horizontal C advection, non-Redfield diffusive C and N fluxes (deviations from the C:N ratio of 7), and anthropogenic nitrogen deposition. The mixed layer reduction in C-T was calculated from an annual C-T cycle, deduced from comprehensive records of surface pCO(2) and total alkalinity, using an established thermodynamic model. This method yielded a value of 0.6 +/- 0.2Pg of C, which represents the NCP that occurred during the warming period (approximately 8 months) in the nitrate-depleted (<0.2 mu mol/kg) ocean. Our estimate is broadly consistent with the global N-2 fixation rate estimated using the N-15-based method and suggests that N-2 fixation by microorganisms is a major driver for this NCP.

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