Marine Ecosystem Biodiversity and Dynamics of Carbon around Kerguelen: an integrated view – MOBYDICK

The Southern Ocean is an oceanic region largely under-sampled in view of the severe logistical constraints inherent in the organization of oceanographic cruises in this remote area, heavily impacted by adverse weather conditions. However, national and international programs presently recognize this region as a major component of the Earth system undergoing rapid transitions due to global change. With regards to the CO2 problem, part of the response of the Earth system is controlled by the oceanic biological carbon pump, which is responsible for establishing the pCO2 gradient between atmosphere and surface waters, while being also the sole geological–scale carbon sequestration mechanism. Our ability to predict the future role of the ocean for the global C cycle and ultimately for climate depends on our understanding of its functioning which still presents major gaps, especially in the knowledge of the relationships that exist between the oceanic biodiversity and the operation of the biological carbon pump (both in terms of intensity and efficiency). If we want to firstly predict and, on the other hand, lay the observation groundwork for the underway changes affecting oceanic ecosystems, it is essential to achieve an in-depth understanding of the processes linking biogeochemistry and biodiversity in an end–to–end approach, i.e., from prokaryotes to apex predators. This is the general challenging goal of MOBYDICK. The project will tackle this issue by a combined investigation of the biological carbon pump and the end-to-end structure of the pelagic food web. The main objective is to trace the key element C from its initial fixation, its cycling within, and transfer across trophic levels, and its final export towards the ocean interior. The study will be conducted in an area of the Southern Ocean where the biogeochemical dynamics have been well documented, thanks to French national initiatives, the Kerguelen area, but where the link with biodiversity has not still received a real attention. The project has been divided into three parts: 1) utilization of historical data, 2) collaboration with other projects in the Southern Ocean addressing biodiversity issues and which will be reanalyzed in the light of MODYDICK questions, and 3) the MOBYDICK oceanographic campaign. The latter will be the major milestone of the project dedicated to study the food web structure from microbes to nekton, by using complementary biogeochemical and biological techniques, at study sites with contrasting production regimes. We will use a combination of up–to–date methods (“omic”, isotopic techniques, microscopy/image analyses, in situ sensors incl. acoustics, laser–based size spectrum acquisition and imagery), as well as multiple sampling tools (rosette, bottle–net, plankton nets, mesopelagic trawl), which have never been associated until now. The strength of this proposal resides in the complementarity of the consortium members, as well as the internationally–recognized expertise of each partner in the study region. For several years now, after the era of the JGOFS and GLOBEC programs, and during the development of the IMBER program, now included in the Future Earth approach, the international community has stressed the need to study the coupling between biogeochemical cycles and biodiversity. What MOBYDICK proposes is, to our knowledge, the first ambitious implementation of such an approach. Conceived in a context of strong international collaboration, it will provide a detailed description of the biological diversity and associated C flows of entire food webs at 2 contrasting sites of the Southern Ocean. From this information, we will extract synthetic descriptors of biodiversity in relation to the functioning of the biological pump, which will be valuable to develop future complex modelling approaches as well as to provide practical information for the design of follow-up studies, which will have to be developed in other oceanic regions.