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)

Abstract. 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 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, 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.