Dinitrogen fixation and dissolved organic nitrogen fueled primary production and particulate export during the VAHINE mesocosm experiment (New Caledonia lagoon)

Abstract. In the oligotrophic ocean characterized by nitrate (NO₃⁻) depletion in surface waters, dinitrogen (N₂) fixation and dissolved organic nitrogen (DON) can represent significant nitrogen (N) sources for the ecosystem. In this study, we deployed large in situ mesocosms in New Caledonia in order to investigate (1) the contribution of N₂ fixation and DON use to primary production (PP) and particle export and (2) the fate of the freshly produced particulate organic N (PON), i.e., whether it is preferentially accumulated and recycled in the water column or exported out of the system. The mesocosms were fertilized with phosphate (PO₄^3-) in order to prevent phosphorus (P) limitation and promote N₂ fixation. The diazotrophic community was dominated by diatom–diazotroph associations (DDAs) during the first part of the experiment for 10 days (P1) followed by the unicellular N₂-fixing cyanobacteria UCYN-C for the last 9 days (P2) of the experiment. N₂ fixation rates averaged 9.8 ± 4.0 and 27.7 ± 8.6 nmol L⁻¹ d⁻¹ during P1 and P2, respectively. NO₃⁻ concentrations (< 0.04 μmol L⁻¹) in the mesocosms were a negligible source of N, indicating that N₂ fixation was the main driver of new production throughout the experiment. The contribution of N₂ fixation to PP was not significantly different (p > 0.05) during P1 (9.0 ± 3.3 %) and P2 (12.6 ± 6.1 %). However, the e ratio that quantifies the efficiency of a system to export particulate organic carbon (POC_export) compared to PP (e ratio = POC_export/PP) was significantly higher (p < 0.05) during P2 (39.7 ± 24.9 %) than during P1 (23.9 ± 20.2 %), indicating that the production sustained by UCYN-C was more efficient at promoting C export than the production sustained by DDAs. During P1, PON was stable and the total amount of N provided by N₂ fixation (0.10 ± 0.02 μmol L⁻¹) was not significantly different (p > 0.05) from the total amount of PON exported (0.10 ± 0.04 μmol L⁻¹), suggesting a rapid and probably direct export of the recently fixed N₂ by the DDAs. During P2, both PON concentrations and PON export increased in the mesocosms by a factor 1.5–2. Unlike in P1, this PON production was not totally explained by the new N provided by N₂ fixation. The use of DON, whose concentrations decreased significantly (p < 0.05) from 5.3 ± 0.5 μmol L⁻¹ to 4.4 ± 0.5 μmol L⁻¹, appeared to be the missing N source. The DON consumption (~ 0.9 μmol L⁻¹) during P2 is higher than the total amount of new N brought by N₂ fixation (~ 0.25 μmol L⁻¹) during the same period. These results suggest that while DDAs mainly rely on N₂ fixation for their N requirements, both N₂ fixation and DON can be significant N sources for primary production and particulate export following UCYN-C blooms in the New Caledonia lagoon and by extension in the N-limited oceans where similar events are likely to occur.