N2 fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)

Mathieu Caffin, Thierry Moutin, Rachel Ann Foster, Pascale Bouruet-Aubertot, Andrea Michelangelo Doglioli, Hugo Berthelot, Cecile Guieu, Olivier Grosso, Sandra Helias-Nunige, Nathalie Leblond, Audrey Gimenez, Anne Alexandra Petrenko, Alain De Verneil, Sophie Bonnet

    Research output: Contribution to journalArticle

    Abstract

    We performed nitrogen (N) budgets in the photic layer of three contrasting stations representing different trophic conditions in the western tropical South Pacific (WTSP) Ocean during austral summer conditions (February-March 2015). Using a Lagrangian strategy, we sampled the same water mass for the entire duration of each long-duration (5 days) station, allowing us to consider only vertical exchanges for the budgets. We quantified all major vertical N fluxes both entering (N2 fixation, nitrate turbulent diffusion, atmospheric deposition) and leaving the photic layer (particulate N export). The three stations were characterized by a strong nitracline and contrasted deep chlorophyll maximum depths, which were lower in the oligotrophic Melanesian archipelago (MA, stations LD A and LD B) than in the ultra-oligotrophic waters of the South Pacific Gyre (SPG, station LD C). N2fixation rates were extremely high at both LD A (593g±51gμmol N m2d1) and LD B (706g±302 N m2d1) and the diazotroph community was dominated by Trichodesmium N2 fixation rates were lower (59g±16μmol N m2d1 at LD C, and the diazotroph community was dominated by unicellular N2-fixing cyanobacteria (UCYN). At all stations, N2 fixation was the major source of new N > 90%) before atmospheric deposition and upward nitrate fluxes induced by turbulence. N2 fixation contributed circa 13-18% of primary production in the MA region and 3% in the SPG water and sustained nearly all new primary production at all stations. The e ratio (e ratiog = particulate carbon exportg/primary production) was maximum at LD A (9.7%) and was higher than the e ratio in most studied oligotrophic regions (<5%), indicating a high efficiency of the WTSP to export carbon relative to primary production. The direct export of diazotrophs assessed by qPCR of the nifH gene in sediment traps represented up to 30.6% of the PC export at LD A, while their contribution was 5 and < 0.1% at LD B and LD C, respectively. At the three studied stations, the sum of all N input to the photic layer exceeded the N output through organic matter export. This disequilibrium leading to N accumulation in the upper layer appears as a characteristic of the WTSP during the summer season.

    Original languageEnglish (US)
    Pages (from-to)2565-2585
    Number of pages21
    JournalBiogeosciences
    Volume15
    Issue number8
    DOIs
    StatePublished - May 2 2018

    Fingerprint

    Pacific Ocean
    fixation
    primary productivity
    atmospheric deposition
    primary production
    particulates
    nitrates
    Trichodesmium
    nitrogen-fixing bacteria
    duration
    carbon
    summer
    Cyanobacteria
    nitrate
    trophic conditions
    water
    turbulent diffusion
    station
    South Pacific Ocean
    organic matter

    ASJC Scopus subject areas

    • Ecology, Evolution, Behavior and Systematics
    • Earth-Surface Processes

    Cite this

    Caffin, M., Moutin, T., Ann Foster, R., Bouruet-Aubertot, P., Michelangelo Doglioli, A., Berthelot, H., ... Bonnet, S. (2018). N2 fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise). Biogeosciences, 15(8), 2565-2585. https://doi.org/10.5194/bg-15-2565-2018

    N2 fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise). / Caffin, Mathieu; Moutin, Thierry; Ann Foster, Rachel; Bouruet-Aubertot, Pascale; Michelangelo Doglioli, Andrea; Berthelot, Hugo; Guieu, Cecile; Grosso, Olivier; Helias-Nunige, Sandra; Leblond, Nathalie; Gimenez, Audrey; Alexandra Petrenko, Anne; De Verneil, Alain; Bonnet, Sophie.

    In: Biogeosciences, Vol. 15, No. 8, 02.05.2018, p. 2565-2585.

    Research output: Contribution to journalArticle

    Caffin, M, Moutin, T, Ann Foster, R, Bouruet-Aubertot, P, Michelangelo Doglioli, A, Berthelot, H, Guieu, C, Grosso, O, Helias-Nunige, S, Leblond, N, Gimenez, A, Alexandra Petrenko, A, De Verneil, A & Bonnet, S 2018, 'N2 fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)', Biogeosciences, vol. 15, no. 8, pp. 2565-2585. https://doi.org/10.5194/bg-15-2565-2018
    Caffin M, Moutin T, Ann Foster R, Bouruet-Aubertot P, Michelangelo Doglioli A, Berthelot H et al. N2 fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise). Biogeosciences. 2018 May 2;15(8):2565-2585. https://doi.org/10.5194/bg-15-2565-2018
    Caffin, Mathieu ; Moutin, Thierry ; Ann Foster, Rachel ; Bouruet-Aubertot, Pascale ; Michelangelo Doglioli, Andrea ; Berthelot, Hugo ; Guieu, Cecile ; Grosso, Olivier ; Helias-Nunige, Sandra ; Leblond, Nathalie ; Gimenez, Audrey ; Alexandra Petrenko, Anne ; De Verneil, Alain ; Bonnet, Sophie. / N2 fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise). In: Biogeosciences. 2018 ; Vol. 15, No. 8. pp. 2565-2585.
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    AU - Michelangelo Doglioli, Andrea

    AU - Berthelot, Hugo

    AU - Guieu, Cecile

    AU - Grosso, Olivier

    AU - Helias-Nunige, Sandra

    AU - Leblond, Nathalie

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