Abstract
Abstract. In marine ecosystems, biological 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, whether the DDN is preferentially and directly exported out of the photic zone, recycled by the microbial loop and/or transferred into larger organisms remains unclear. 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). 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 unicellular cyanobacterial lineage C (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-day 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 to further infer the fate of DDN in the ecosystem and the role of N2 fixation on productivity, food web structure and carbon export. Recommendations for future work are finally provided in the conclusion section.
Highlights
Atmospheric dinitrogen (N2) is the largest pool of nitrogen (N) on earth yet it is unavailable for most organisms that require N for growth
According to the model, N2 fixation fuelled 43.5 % of primary production (PP) and 8 % of bacterial production (BP) during the 23 days of the simulated experiment. This does not necessarily mean that non-diazotrophic autotrophs benefit more from the DDN compared to heterotrophs as the DDN was nearly distributed between autotrophs and heterotrophs. This higher effect on PP than on BP is derived from the fact that the diazotrophs themselves were strongly affected by their inability to fix N2 as suggested by the far lower abundance of UCYN-C in the NOFIX simulation compared to the REF one (Fig. 6)
The VAHINE project provided unique opportunities to study and compare the fate of N2 fixation associated with different diazotrophs in the marine environment
Summary
Atmospheric dinitrogen (N2) is the largest pool of nitrogen (N) on earth yet it is unavailable for most organisms that require N for growth. While the role and contribution of marine N2 fixation on biogeochemical cycles have been intensely investigated, a critical question that remains poorly studied is the fate of newly fixed N, or diazotroph-derived N (hereafter named DDN) in LNLC ecosystems (Mulholland, 2007). It remains unclear whether the DDN is preferentially exported directly out of the photic zone, recycled by the microbial loop and/or transferred into larger organisms, subsequently enhancing indirect particle export. We first summarize some of our knowledge regarding the fate of DDN in the ocean, describe the ongoing technical challenges to study this question, and describe and the specific objectives of the VAHINE project
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