Dynamics of N<sub>2</sub> fixation and fate of diazotroph-derived nitrogen in a low-nutrient, low-chlorophyll ecosystem: results from the VAHINE mesocosm experiment (New Caledonia)

Abstract

Abstract. N 2 fixation rates were measured daily in large (∼ 50 m 3 ) mesocosms deployed in the tropical southwest Pacific coastal ocean (New Caledonia) to investigate the temporal variability in N 2 fixation rates in relation with environmental parameters and study the fate of diazotrophderived 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 N 2 fixation rates were replicable between the three mesocosms, averaged 18.5 ± 1.1 nmol N L −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 N L −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, N 2 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 nanometerscale secondary ion mass spectrometry (nanoSIMS) coupled with 15 N 2 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

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

“…5b. Moreover, the presence of large (100-500 µm) UCYN-C aggregates in the mesocosms facilitated their export into the traps where UCYN-C accounted for up to 22.4 ± 4.0 % of total C export at the height of their extensive development (Bonnet et al, 2016a). This indicates that UCYN-C cannot only contribute to direct export but promote indirect export.…”

“…The magnitude of this release (10 to 80 %) is still under debate in the scientific community (Glibert and Bronk, 1994;Konno et al, 2010;Benavides et al, 2013b, a) and seems to depend on the physiological state of the cells (Berthelot et al, 2015a) as well as on exogenous factors such as viral lysis (Hewson et al, 2004) or sloppy feeding (O'Neil et al, 1996). Nevertheless, recent methods coupling 15 N 2 isotopic labeling, cell sorting by flow cytometry and high-resolution nanometer-scale secondary ion mass spectrometry (nanoSIMS) analyses enable the quantification of the DDN transfer from diazotrophs to specific groups of nondiazotrophic phytoplankton and bacteria, indicating that the DDN released in the dissolved pool is available and actively used by surrounding non-diazotrophic communities (Bonnet et al, 2016a).…”

“…Complementary field approaches were used during the VAHINE project including a δ15 N budget to assess the dominant source of N (from NO − 3 and/or N 2 fixation) fueling export production throughout the experiment (Knapp et al, 2016). Bonnet et al (2016a) explored the fate of DDN on shorter timescales, investigating the relative contribution of each diazotroph phylotype to direct C export and quantifying the DDN release and its subsequent transfer to different groups of plankton by using nanoSIMS. In the present study, we developed a 1-D vertical biogeochemical model including the representation of Trichodesmium and UCYN diazotrophs of Group C (UCYN-C), which developed extensively during the mesocosm experiment .…”

Contribution and pathways of diazotroph-derived nitrogen to zooplankton during the VAHINE mesocosm experiment in the oligotrophic New Caledonia lagoon

“…The water column was well mixed in the mesocosms as temperature and salinity were homogeneous with depth over the course of the experiment (Bonnet et al, 2016b). Prior to the DIP fertilization on day 4 (hereafter called P0), DIP concentrations in the mesocosms ranged from 0.02 to 0.05 µmol L −1 (Berthelot et al, 2015b).…”

“…VAHINE was designed specifically to investigate the fate of DDN in the ecosystem, i.e. its transfer to the planktonic food web and its contribution to export production (Bonnet et al, 2016b). Here, we present the zooplankton component of the VAHINE program.…”