The OUTPACE (Oligotrophy to UlTraoligotrophy PACific Experiment) project aims to give a zonal characterization of the biogeochemical functioning and biological diversity of the South West Pacific toward a gradient of macro- and micro-nutrients availability, and produces a detailed study of the biological production and its subsequent fate in 3 contrasting sites, with a specific emphasis on the production sustained by dinitrogen (N2) fixation. The field campaign laid done in February-March 2015, will involve more than 50 scientists from France and other countries. This project is funded by ANR, INSU-LEFE, GOPS, IRD.
The functioning and productivity of oligotrophic systems, and the balance between production and mineralization in these oceanic areas, is still the subject of much debate. Once thought to be biological deserts, recent research has shown that these systems could largely contribute to the global ocean organic carbon (C) export. It is important to thoroughly understand the biogeochemical functioning of these vast ecosystems in order to characterize their evolution in the context of climate alteration. This project aims to give a zonal characterization of the biogeochemical functioning and biological diversity of the SW Pacific toward a gradient of macro- and micro-nutrients availability, and produces a detailed study of the biological production and its subsequent fate in 3 contrasting sites, with a specific emphasis on the production sustained by dinitrogen (N2) fixation. We will determine whether organic matter production, re-mineralization and export processes are different at these sites, and if so, determine how these differences are related with the diversity/functioning of N2-fixing organisms in the planktonic community. Comparing different sites, along a zonal gradient of variable nutrient availability, should provide us with a new insight for identifying and understanding the fundamental interactions between marine biogeochemistry and ecosystems. We will focus on several current issues of interest regarding the coupling/uncoupling between carbon and nutrient (N,P,Si,Fe) delivery and removal processes in the surface layer. These processes control the planktonic community structure and functioning and, ultimately, the ability of the ocean to biologically sequester C. We shall describe the functioning of each distinct ecosystem under physical conditions with new approaches combining satellite localization and drifters for Lagrangian strategy, and, finally, examine the biogeochemical role of diazotrophs in each system, using experimental and modelling approaches.
The scientific objectives will be achieved through the realization of 6 complementary Tasks (including promotion of Science, Task 6) involving international collaborations between physicists, biologists and biogeochemists with expertises ranging from marine physics, chemistry, optics, biogeochemistry, microbiology, molecular ecology, genetics, and modelling. The overall strategy of the project will combine experimental and modelling approaches. The experimental part is based on the realization of a 45-days oceanographic cruise onboard the R/V L’Atalante in the SW Pacific in 2015. The cruise will consist in 18 short duration (8 h) stations (SD stations) and 3 long duration (7 d) stations (LD stations) distributed along a zonal transect (19° S) in the SW Pacific ocean. During the SD stations, a characterization of the biogeochemistry and biological diversity from the North of New Caledonia to the western border of the South Pacific gyre will be performed (Task 1). The LD stations will allow to perform a process studies in contrasting oligotrophic and N2 fixation conditions following a Lagrangian approach for a 3D characterization of the upper water column (Task 2). During the LD stations, high frequency acquisition of physical, optical, biogeochemical and biological variables will be assessed. We will quantify primary / secondary production and their fate (Task 3), as well as mineralization and export of organic matter (Task 4) by measuring biogenic element (C,N,P,Si) stocks/fluxes and organisms responsibles for these fluxes (“Who does what ?”). Moreover, a multi-scale modelling approach is proposed that we plan to start from the beginning of the OUTPACE project (Task 5). Its objective is to complement the observations with the analyzed data in order to better understand the interactions between the biogeochemical cycles of the biogenic elements (predominantly the C cycle) and the dynamics of the planktonic trophic network in oligotrophic marine areas.