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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 13, issue 16
Biogeosciences, 13, 4645-4657, 2016
https://doi.org/10.5194/bg-13-4645-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

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

Biogeosciences, 13, 4645-4657, 2016
https://doi.org/10.5194/bg-13-4645-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 18 Aug 2016

Research article | 18 Aug 2016

Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the VAHINE mesocosm experiments

Angela N. Knapp1, Sarah E. Fawcett2,3, Alfredo Martínez-Garcia4, Nathalie Leblond5, Thierry Moutin5, and Sophie Bonnet5 Angela N. Knapp et al.
  • 1Earth, Ocean, and Atmospheric Science Department, Florida State University, 117 N Woodward AVE, Tallahassee, FL, 32306, USA
  • 2Department of Geosciences, Guyot Hall, Princeton University, Princeton, NJ 08544, USA
  • 3Department of Oceanography, University of Cape Town, Rondebosch, 7701, South Africa
  • 4Max Plank Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
  • 5Mediterranean Institute of Oceanography (MIO), IRD/CNRS/Aix-Marseille University, IRD Noumea, 101 Promenade R. Laroque, BPA5, 98848 Noumea Cedex, France

Abstract. In a coastal lagoon with a shallow, 25m water column off the southwest coast of New Caledonia, large-volume ( ∼ 50m3) mesocosm experiments were undertaken to track the fate of newly fixed nitrogen (N). The mesocosms were intentionally fertilized with 0.8µM dissolved inorganic phosphorus to stimulate diazotrophy. N isotopic evidence indicates that the dominant source of N fueling export production shifted from subsurface nitrate (NO3) assimilated prior to the start of the 23-day experiments to N2 fixation by the end of the experiments. While the δ15N of the sinking particulate N (PNsink) flux changed during the experiments, the δ15N of the suspended PN (PNsusp) and dissolved organic N (DON) pools did not. This is consistent with previous observations that the δ15N of surface ocean N pools is less responsive than that of PNsink to changes in the dominant source of new N to surface waters. In spite of the absence of detectable NO3 in the mesocosms, the δ15N of PNsink indicated that NO3 continued to fuel a significant fraction of export production (20 to 60%) throughout the 23-day experiments, with N2 fixation dominating export after about 2 weeks. The low rates of organic N export during the first 14 days were largely supported by NO3, and phytoplankton abundance data suggest that sinking material primarily comprised large diatoms. Concurrent molecular and taxonomic studies indicate that the diazotroph community was dominated by diatom–diazotroph assemblages (DDAs) at this time. However, these DDAs represented a minor fraction (<5%) of the total diatom community and contributed very little new N via N2 fixation; they were thus not important for driving export production, either directly or indirectly. The unicellular cyanobacterial diazotroph, a Cyanothece-like UCYN-C, proliferated during the last phase of the experiments when N2 fixation, primary production, and the flux of PNsink increased significantly, and δ15N budgets reflected a predominantly diazotrophic source of N fueling export. At this time, the export flux itself was likely dominated by the non-diazotrophic diatom, Cylindrotheca closterium, along with lesser contributions from other eukaryotic phytoplankton and aggregated UCYN-C cells, as well as fecal pellets from zooplankton. Despite comprising a small fraction of the total biomass, UCYN-C was largely responsible for driving export production during the last  ∼ 10 days of the experiments both directly ( ∼ 5 to 22% of PNsink) and through the rapid transfer of its newly fixed N to other phytoplankton; we infer that this newly fixed N was transferred rapidly through the dissolved N (including DON) and PNsusp pools. This inference reconciles previous observations of invariant oligotrophic surface ocean DON concentrations and δ15N with incubation studies showing that diazotrophs can release a significant fraction of their newly fixed N as some form of DON.

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The goal of this manuscript was to track the fate of newly fixed nitrogen (N) in large volume mesocosms in the coastal waters of New Caledonia. We used a N isotope ("δ15N") budget and found a shift in the δ15N of sinking particulate N over the 23-day experiment, indicating that nitrate supported export production at the beginning of the experiment, but that nitrogen fixation supported export at the end. We infer that nitrogen fixation supported export production by a release of dissolved N.
The goal of this manuscript was to track the fate of newly fixed nitrogen (N) in large volume...
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