N2 fixation rates were measured daily in large (∼ 50 m3) 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 ∼ 0.8 μM dissolved inorganic phosphorus (DIP) to stimulate diazotrophy. Bulk N2 fixation rates were replicable between the three mesocosms, averaged 18.5 ± 1.1 nmol NL-1 d-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 ± 1.0 nmol NL-1 d-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 ± 0.8 μm) UCYN-C cells into large (100-500 μm) 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.
Bibliographical noteFunding Information:
Funding for this research was provided by the Agence Nationale de la Recherche (ANR starting grant VAHINE ANR-13-JS06-0002), the INSU-LEFE-CYBER program, GOPS, and IRD. The authors thank the captain and crew of the R/V Alis. We acknowledge the SEOH diver service from Noumea, as well as the technical service of the IRD research center of Noumea for their helpful technical support together with C. Guieu, J.-M. Grisoni, and F. Louis for the mesocosm design and the useful advice. We thank François Robert, Smail Mostefaoui, and Rémi Duhamel from the French National Ion MicroProbe Facility hosted by the Museum National d'Histoire Naturelle (Paris) for providing nanoSIMS facilities and constant advice. We are grateful to Aude Barani and Gerald Gregori from the Regional Flow Cytometry Platform for Microbiology (PRECYM) of the Mediterranean Institute of Oceanography (MIO) for the flow cytometry analyses support. D. Sigman provided analytical support for the 15N measurements. Funding to I. B. Frank was provided through a collaborative grant with S. Bonnet from MOST, Israel, and the High Council for Science and Technology (HCST), France, as well as the German-Israeli Research Foundation (GIF), project number 1133-13.8/2011, and grant 2008048 from the United States-Israel Binational Science Foundation (BSF).
© Author(s) 2016.
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics
- Earth-Surface Processes