Introduction to the project VAHINE: VAriability of vertical and tropHIc transfer of diazotroph derived N in the south wEst Pacific

Abstract. On the global scale, N₂ fixation provides the major external source of reactive nitrogen to the surface ocean, surpassing atmospheric and riverine inputs, and sustains  ∼  50 % of new primary production in oligotrophic environments. The main goal of the VAriability of vertical and tropHIc transfer of diazotroph derived N in the south wEst Pacific (VAHINE) project was to study the fate of nitrogen newly fixed by diazotrophs (or diazotroph-derived nitrogen) in oceanic food webs, and how it impacts heterotrophic bacteria, phytoplankton and zooplankton dynamics, stocks and fluxes of biogenic elements and particle export. Three large-volume ( ∼  50 m³) mesocosms were deployed in a tropical oligotrophic ecosystem (the New Caledonia lagoon, south-eastern Pacific) and intentionally fertilized with  ∼  0.8 µM of dissolved inorganic phosphorus (DIP) to stimulate diazotrophy and follow subsequent ecosystem changes. VAHINE was a multidisciplinary project involving close collaborations between biogeochemists, molecular ecologist, chemists, marine opticians and modellers. This introductory paper describes in detail the scientific objectives of the project as well as the implementation plan: the mesocosm description and deployment, the selection of the study site (New Caledonian lagoon), and the logistical and sampling strategy. The main hydrological and biogeochemical conditions of the study site before the mesocosm deployment and during the experiment itself are described, and a general overview of the papers published in this special issue is presented.