VAHINE - Fate of N2 fixation in the Ocean

CoordinationSophie Bonnet (Senior scientist IRD)


The aim of the VAHINE project is to study the fate of fixed nitrogen in the oceanic pelagic food web and its potential impact on carbon export. The field campaign of VAHINE took place in the South West Pacific (New Caledonia) in 2013, involving 45 scientists from France, Israel, Germany and USA.


VAHINE was funded by ANR (ANR Jeune chercheur S. Bonnet), CNRS INSU-LEFE, GOPS, and IRD.


The scientific results of VAHINE are published in 16 articles in a Special issue in Biogeosciences


Biogeochemical and biological response to a diazotroph bloom in a low-nutrient, low-chlorophyll ecosystem : results from the VAHINE mesocosms experiment


Editor(s) : S. Bonnet, D. G. Capone, F. Lacan, and E. Marañón



Abstract : The goal of this special issue is to present the knowledge regarding the fate of N2 fixation in the oligotrophic ocean based on the large data set acquired during the VAHINE experiment. VAHINE was a multidisciplinary project involving close collaborations between biogeochemists, molecular ecologist, chemists, marine opticians and modellers. 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. 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. During VAHINE, triplicate large volume (∼  50 m3) mesocosms were deployed in the tropical south-west Pacific coastal ocean (New Caledonia). 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 special issue details the major experimental and modelling results obtained during the project regarding the evolution of the main standing stocks, fluxes, plankton diversity and activity 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). Finally, a synthesis study attempts to summarize the knowledge and the unresolved questions regarding the fate of DDN in the ocean to synthetize and link the major experimental and modelling results obtained during the project. It reconciles the diverse and complementary methodological approaches used in this study to answer the scientific questions of the VAHINE project.