BELCANTO II focuses on the role of the Southern Ocean in global change and is submitted by BELCANTO (BELgian research on Carbon uptake in the ANTarctic Ocean) an existing interdisciplinary network of biologists, geochemists, and physical and ecological modelers. The objective is to further develop geochemical proxies and numerical tools for assessing and understanding the present-day functioning of the CO2 biological pump in the iron-limited Southern Ocean and predicting its evolution in response to scenarios of increasing atmospheric CO2. The research methodology will involve and combine collection of historical and new field data, laboratory process-level studies and numerical work in order to improve our understanding of the mechanisms controlling the production of key bloom-forming phytoplankton groups of the Southern Ocean (diatoms and Phaeocystis), their sinking rate and biodegradation when exported in the mesopelagic zone (100-1000 m). Biological process and proxies investigations will be conducted under laboratory-controlled conditions on cultures of key Antarctic phytoplankton (Fragilaria kerguelensis, Chaetoceros brevis, Phaeocystis colonies and free-living cells) and bacteria grown in sub-nanomolar-iron Southern Ocean waters and at low temperature (1 - 4∞C). Biological studies will focus on the light and multiple nutrient (Fe/NO3/NH4/Si) regulation of phytoplankton growth and sinking rate; on the carbon and iron control of bacterial degradation of phytoplankton-derived material including the mineralisation of nitrogen and biogenic silica. Proxies investigations will focus on the use of Ba/Sr, f-ratio and isotopic signatures of C, N, Si for tracing phytoplankton and diatom export production and will involve measurements of Ba/Sr uptake and isotopic fractionation by cultured phytoplankton for different growth and decay conditions. On this basis the relevance of each proxy for tracing export production will be assessed and algorithms will be developed for estimating export production from field measurements. Fieldwork will concentrate on the collection of new data on surface pCO2 and proxies measurements on suspended matter, collected particles in sediment traps and sediment records. These data will be included in a data base collecting existing data on pCO2, chlorophyll a, diatom/Phaeocystis distributions, primary production and relevant proxies in order to reconstruct distribution fields of atmospheric carbon uptake, bloom formation and export production in the contemporary ocean. Numerical work will involve the upgrading of the biogeochemical model SWAMCO describing C, N, P, Si and Fe cycling through aggregated biological (diatoms, nano-pico-phytoplankton, heterotrophic nanoflagellates, microzooplancton, bacteria) of the surface layer and the subsequent export production at its lower boundary. Upgrading will include the addition of Phaeocystis colonies as explicit state variable and the implementation of a simple description of the phytoplankton-aggregation process. Parameterisation will be obtained from process-level results. A simplified version will be derived from model analysis for further coupling with a fine grid (30-50km) version of a three-dimensional ice-ocean model. The application domain will cover the region southward of 30∞S. Initial and boundary conditions for all state variables and atmospheric CO2 will be derived from existing global atmosphere-ocean models. Atmospheric iron supply will be described as direct dust input or upon snow-ice melting. Model runs will be conducted under present-day climatological forcing. Validation will be performed by comparison of predictions with in situ observations (sea ice concentration, temperature, salinity), reconstructed fields of surface chlorophyll a and primary production derived from SEAWIFS, diatom/Phaeocystis distributions, and exported production estimated by inverse modelling. Finally climate change prospective scenarios with doubling atmospheric CO2 will be conducted with initial/boundary physical and biogeochemical conditions and atmospheric forcing obtained from OBCMís runs. Results will be analysed with respect to regional and seasonal changes of CO2 sources and sinks. The resulting changes of sea-ice cover, mixed-layer depth and surface circulation will be described and their causes will be investigated by means of sensitivity experiments. Finally, the impact of the resulting changing physical conditions on the behaviour of the biological pump will be assessed.