This research attempts to assess the role of the Southern Ocean as a CO2 source or sink on a regional and seasonal scale by implementing an integrated multidisciplinary approach combining in situ measurements, process studies under controlled laboratory conditions and digital computation. It studies all the processes which govern the carbon cycle in the Southern Ocean, ranging from CO2 exchanges at the sea/air interface to the accumulation of carbon on the ocean floor. It is based on a mechanistic scientific approach which aims at improving understanding of the production and of mineralisation of carbon biomass in surface water as well as its transport towards the deep sea. In particular the role of light and the availability of nutrients (large and small) will be studied to determine what is the dominant phytoplankton community, what are the dynamics of the associated food chain, and how the carbon is exported. This study will take place in areas dominated by various phytoplanktonic communities, i.e. the diatoms, the Phaeocystis colonies and the nanophytoplankton. The role of aggregates as carbon conveyors from the surface to the deep sea and the sediments will be investigated with special attention for the bacterial mineralisation of aggregates, and the production and development of barite as tracer. The usability of barite as a tracer of the exported carbon flux will be evaluated to improve understanding of the existing transfer functions. The knowledge acquired will then be integrated into a 1D physical-biogeochemical model which simulates the cycle of carbon and other related biogenic elements (N, Si, Fe, Mn, Ba) in the water column of the Southern Ocean over a seasonal cycle. Lastly, this study will contribute to the preparation of diagnostic and predictive models which will be used to evaluate the role of the Southern Ocean in a global perspective. OBJECTIVES The overall objective of this integrated study is to determine the role of the Southern Ocean as a CO2 source or sink on a seasonal and regional scale. The following specific questions must be answered: 1- What are the CO2 source and sink areas in the Southern Ocean, and how large is the CO2 flux between ocean and atmosphere? 2- What are the biological and physical mechanisms controlling the distribution of CO2 partial pressure (pCO2)? 3- What are the mechanisms governing carbon production and mineralisation in the surface water? 4- By which mechanisms do light, inorganic nutrients (nitrate, ammonium, silicate) and tracer elements (Fe, Mn) control the photosynthesis and the growth of the three key components of the phytoplankton of the Southern Ocean (diatoms, Phaeocystis, nanophytoplankton) ? 5- What mechanisms determine the fate of organic carbon in surface water and in particular the factors controlling the formation of aggregates derived from phytoplankton and their mineralisation by means of the micro-organisms which adhere to it ? 6- Which mechanisms determine the transformation of organic carbon during its transfer from surface water to seabed, and which control the decomposition and mineralisation of the aggregates in the first 500 m of the water column? 7- To what extent are geochemical tracers such as Ba-barite valid "proxies" of the export production? 8- To what extent does the mechanistic model CLIO-1D - SWAMCO, whose digital code derives from the project results, allow the evaluation of the CO2 sink - source role of the Southern Ocean on a seasonal and regional scale?