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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 13, issue 9
Biogeosciences, 13, 2653-2673, 2016
https://doi.org/10.5194/bg-13-2653-2016
© 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, 2653-2673, 2016
https://doi.org/10.5194/bg-13-2653-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 04 May 2016

Research article | 04 May 2016

Dynamics of N2 fixation and fate of diazotroph-derived nitrogen in a low-nutrient, low-chlorophyll ecosystem: results from the VAHINE mesocosm experiment (New Caledonia)

Sophie Bonnet1,2, Hugo Berthelot1, Kendra Turk-Kubo3, Sarah Fawcett4,5, Eyal Rahav6,7, Stéphane L'Helguen8, and Ilana Berman-Frank6 Sophie Bonnet et al.
  • 1IRD, Aix Marseille Université, CNRS/INSU, Université de Toulon, Mediterranean Institute of Oceanography (MIO) UM 110, 13288, Marseille, France
  • 2IRD, Aix Marseille Université, CNRS/INSU, Université de Toulon, Mediterranean Institute of Oceanography (MIO) UM 110, 101 Promenade R. Laroque, BPA5, 98848 Noumea CEDEX, New Caledonia
  • 3Ocean Sciences Department, University of California, Santa Cruz, California, USA
  • 4Department of Geosciences, M45 Guyot Hall, Princeton University, Princeton, New Jersey 08544, USA
  • 5Department of Oceanography, University of Cape Town, Rondebosch, 7701, South Africa
  • 6Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University, Ramat Gan, Israel
  • 7National Institute of Oceanography, Israel Oceanographic and Limnological Research, Haifa, Israel
  • 8Université de Brest, CNRS/IRD, UMR6539, Laboratoire des Sciences de l'Environnement Marin, OSU-IUEM, 29280 Plouzané, France

Abstract. N2 fixation rates were measured daily in large (∼ 50m3) mesocosms deployed in the tropical southwest Pacific coastal ocean (New Caledonia) to investigate the temporal variability in N2 fixation rates in relation with environmental parameters and study the fate of diazotroph-derived nitrogen (DDN) in a low-nutrient, low-chlorophyll ecosystem. The mesocosms were fertilized with  ∼ 0.8µM dissolved inorganic phosphorus (DIP) to stimulate diazotrophy. Bulk N2 fixation rates were replicable between the three mesocosms, averaged 18.5±1.1nmolNL−1d−1 over the 23 days, and increased by a factor of 2 during the second half of the experiment (days 15 to 23) to reach 27.3±1.0nmolNL−1d−1. These later rates measured after the DIP fertilization are higher than the upper range reported for the global ocean. During the 23 days of the experiment, N2 fixation rates were positively correlated with seawater temperature, primary production, bacterial production, standing stocks of particulate organic carbon (POC), nitrogen (PON) and phosphorus (POP), and alkaline phosphatase activity, and negatively correlated with DIP concentrations, DIP turnover time, nitrate, and dissolved organic nitrogen and phosphorus concentrations. The fate of DDN was investigated during a bloom of the unicellular diazotroph UCYN-C that occurred during the second half of the experiment. Quantification of diazotrophs in the sediment traps indicates that ∼ 10% of UCYN-C from the water column was exported daily to the traps, representing as much as 22.4±5.5% of the total POC exported at the height of the UCYN-C bloom. This export was mainly due to the aggregation of small (5.7±0.8µm) UCYN-C cells into large (100–500µm) aggregates. During the same time period, a DDN transfer experiment based on high-resolution nanometer-scale secondary ion mass spectrometry (nanoSIMS) coupled with 15N2 isotopic labeling revealed that 16±6% of the DDN was released to the dissolved pool and 21±4% was transferred to non-diazotrophic plankton, mainly picoplankton (18±4%) followed by diatoms (3±2%). This is consistent with the observed dramatic increase in picoplankton and diatom abundances, primary production, bacterial production, and standing stocks of POC, PON, and POP in the mesocosms during the second half of the experiment. These results offer insights into the fate of DDN during a bloom of UCYN-C in low-nutrient, low-chlorophyll ecosystems.

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N2 fixation rates were measured daily in ~ 50 m3 mesocosms deployed in New Caledonia to investigate the high-frequency dynamics of diazotrophy and the fate of diazotroph-derived nitrogen (DDN) oligotrophic ecosystems. ~ 10 % of UCYN-C from the water column were exported daily to the traps, representing as much as 22.4 ± 5.5 % of the total POC exported at the height of the UCYN-C bloom. 16 ± 6 % of the DDN was released to the dissolved pool and 21 ± 4 % was transferred to non-diazotrophic plankton.
N2 fixation rates were measured daily in ~ 50 m3 mesocosms deployed in New Caledonia to...
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