Abstract. In marine ecosystems, N2 fixation provides the predominant external source of nitrogen (N) (140 ± 50 Tg N yr−1), contributing more than atmospheric and riverine inputs to the N supply. Yet the fate and magnitude of the newly-fixed N, or diazotroph-derived N (hereafter named DDN) in marine ecosystems is poorly understood. Moreover, it remains unclear whether the DDN is preferentially directly exported out of the photic zone, recycled by the microbial loop, and/or transferred into larger organisms, subsequently enhancing indirect particle export. These questions were investigated in the framework of the VAHINE (VAriability of vertical and tropHIc transfer of diazotroph derived N in the south wEst Pacific) project. Triplicate large volume (~50 m3) mesocosms were deployed in the tropical South West Pacific coastal ocean (New Caledonia) to maintain a stable water-mass without disturbing ambient light and temperature conditions. The mesocosms were intentionally fertilized with ~0.8 μM dissolved inorganic phosphorus (DIP) at the start of the experiment to stimulate diazotrophy. A total of 47 stocks, fluxes, enzymatic activities and diversity parameters were measured daily inside and outside the mesocosms by the 40 scientists involved in the project. The experiment lasted for 23 days and was characterized by two distinct and successive diazotroph blooms: a dominance of diatom-diazotroph associations (DDAs) during the first half of the experiment (days 2–14) followed by a bloom of UCYN-C during the second half of the experiment (days 15–23). These conditions provided a unique opportunity to compare the DDN transfer and export efficiency associated with different diazotrophs. Here we summarize the major experimental and modelling results obtained during the project and described in the VAHINE Special issue, in particular those regarding the evolution of the main standing stocks, fluxes and biological characteristics over the 23-days experiment, the contribution of N2 fixation to export fluxes, the DDN released to dissolved pool and its transfer to the planktonic food web (bacteria, phytoplankton, zooplankton). We then apply our Eco3M modelling platform further to infer the fate of DDN in the ecosystem and role of N2 fixation on productivity, food web structure and carbon export. Recommendations for future work are finally provided in the conclusion section.
Decoupled taxonomic and ecological recoveries from the Permo-Triassic extinction
