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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 13, issue 17 | Copyright

Special issue: Biogeochemical and biological response to a diazotroph bloom...

Biogeosciences, 13, 5103-5120, 2016
https://doi.org/10.5194/bg-13-5103-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 14 Sep 2016

Research article | 14 Sep 2016

Biogeochemical fluxes and fate of diazotroph-derived nitrogen in the food web after a phosphate enrichment: modeling of the VAHINE mesocosms experiment

Audrey Gimenez1, Melika Baklouti1, Sophie Bonnet1,2, and Thierry Moutin1 Audrey Gimenez et al.
  • 1Aix Marseille Université, Université de Toulon, CNRS, IRD, Mediterranean Institute of Oceanography (MIO), Marseille, France
  • 2Institut de Recherche pour le Développement (IRD), AMU/CNRS/INSU, Université de Toulon, Mediterranean Institute of Oceanography (MIO) UM110, 98848 Nouméa, New Caledonia

Abstract. The VAHINE mesocosm experiment in the oligotrophic waters of the Nouméa lagoon (New Caledonia), where high N2 fixation rates and abundant diazotroph organisms were observed, aimed to assess the role of the nitrogen input through N2 fixation in carbon production and export and to study the fate of diazotroph-derived nitrogen (DDN) throughout the planktonic food web. A 1-D vertical biogeochemical mechanistic model was used in addition to the in situ experiment to enrich our understanding of the dynamics of the planktonic ecosystem and the main biogeochemical carbon (C), nitrogen (N) and phosphate (P) fluxes. The mesocosms were intentionally enriched with  ∼ 0.8µmolL−1 of inorganic P to trigger the development of diazotrophs and amplify biogeochemical fluxes. Two simulations were run, one with and the other without the phosphate enrichment. In the P-enriched simulation, N2 fixation, primary production (PP) and C export increased by 201, 208 and 87%, respectively, consistent with the trends observed in the mesocosms (+124, +141 and +261% for N2 fixation, PP and C export, respectively). In total, 5–10 days were necessary to obtain an increase in primary and export productions after the dissolved inorganic phosphate (DIP) enrichment, thereby suggesting that classical methods (short-term microcosms experiments) used to quantify nutrient limitations of primary production may not be relevant. The model enabled us to monitor the fate of fixed N2 by providing the proportion of DDN in each compartment (inorganic and organic) of the model over time. At the end of the simulation (25 days), 43% of the DDN was found in the non-diazotroph organisms, 33% in diazotrophs, 16% in the dissolved organic nitrogen pool, 3% in the particulate detrital organic pool and 5% in traps, indicating that N2 fixation was of benefit to non-diazotrophic organisms and contributed to C export.

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In the context of the VAHINE mesocosm experiment in the Nouméa lagoon (New Caledonia), a 1-D vertical biogeochemical mechanistic model was used together with the in situ experiment to complement our comprehension of the planktonic ecosystem dynamics and the main biogeochemical carbon, nitrogen and phosphate fluxes. The model also showed the fate of fixed N2 by providing, over time, the proportion of diazotroph-derived nitrogen (DDN) in each compartment (mineral and organic) of the model.
In the context of the VAHINE mesocosm experiment in the Nouméa lagoon (New Caledonia), a 1-D...
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