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Expected Results

lundi 12 mai 2014

Expected results

The OUTPACE project is organized on three main objectives involving collaborations between physicists, biologists and biogeochemists with specialities ranging from marine optics to modelling. The final goal is to obtain a better representation of the interactions between planktonic organisms and the cycle of biogenic elements, considering a variety of scales, from single-cell processes to the whole SW Pacific ocean. The main objectives of this research are the following:

1) To give a zonal description of the biogeochemistry and biological diversity of the SW Pacific during the strongest stratified period (austral summer)
The zonal characterization of stations from the North of New Caledonia to the western border of the South Pacific gyre will provide a physical and biogeochemical data set which will be implemented into the CYBER data base.
This will provide an accurate estimate of nutrient availability during pronounced oligotrophic conditions, enabling the quantification of mineral, organic, dissolved and particulate pools of biogenic elements over the whole water column and the penetration of anthropogenic CO2. Such data will be used to validate multi-element biogeochemical models for the SW Pacific at the large scale of the general circulation. A detailed description of the planktonic communities will be assessed by investigating biological diversity (using classical taxonomic and molecular tools). In this way, the key roles of some of the functional groups in the biogeochemical cycles, such as nitrogen-fixing organisms, will be acknowledged.

2) To study the production and fate of organic matter of 3 contrasting oligotrophic environments with particular emphasis on nitrogen fixation
High frequency acquisition of physical, optical, biogeochemical and biological variables will be assessed over long duration (3 of 6 days long) stations (LD) that represent contrasting oligotrophic conditions (from oligotrophy to ultra-oligotrophy) and N2 fixation conditons (from high to low N2 fixation rates). This sampling strategy will make it possible to identify the impact of small scale dynamics on primary productivity, by providing new estimates for the vertical diffusion coefficients. The importance of nitrogen fixation in new nitrogen input and the control of this flux by nutrient availability will be assessed (in situ and microcosm studies). We will study primary and secondary production as well as mineralisation and export of organic matter by measuring biogenic element (C,N,P,Si) fluxes and focusing on the major functional groups responsible for these fluxes using bulk and single-cell approaches. Finally, physiological characteristics of microorganisms will be studied at the scale of species or functional groups that play key roles in biogenic element fluxes, and particularly N2-fixing organisms (diazotrophs). Identifying the precise biogeochemical role of diazotrophs is of major importance as they will probably play an increasing role in the future ocean due to their sensitivity to global warming. It is thus critical to quantify their contribution to C sequestration in the present ocean, as well as their ability to sustain higher trophic levels of the marine food web. The results should provide a better understanding of the fundamental interactions between biogeochemical cycles and the structure of the planktonic food web in oligotrophic marine environments.

3) To obtain a satisfactory representation of the main biogeochemical fluxes (C, N, P, Si) and the dynamics of the planktonic trophic network, both in situ and by using microcosm experiments
The data obtained during the OUTPACE experiment will be rapidly available to the international scientific community to validate biogeochemical models (circulation-ecosystems models), whose overall objectives are to understand and/or predict the interactions between environmental climate changes and the marine ecosystems. The data will also be used for modelling on a finer scale in the frame of the present project, particularly in order to obtain a better representation of the process of diazotrophy in the biological C pump. A biogeochemical model relevant for SW Pacific waters has already been developed during the VAHINE project. It is based on a previous version of a muti-element (C, N, P) model from bacteria to mesozooplankton implemented in the Eco3M modelling tool, in which the diazotroph Trichodesmium spp. has been added. The introduction of other unicellular diazotrophs is currently under development (Post Doc ANR VAHINE). The objective is to obtain a better representation of the C fluxes and associated biogenic elements in the surface layer of oligotrophic marine environments.