Study on the dynamics of the planktonic microbial food web of the Southern Ocean in response to changing environmental conditions. OBJECTIVES To develop a predictive mathematical model of the functioning of the microbial ecosystem in the Southern Ocean in response to anticipated global climatic changes or local anthropic disturbances. The purpose of the model is to describe the cycles of carbon, nitrogen and silicon through the microbial food web of the surface waters of the Antarctic Ocean (that part of the Southern Ocean limited by the polar front) throughout the seasonal cycle. It is designed from the coupling of a 1D hydrodynamic model - which calculates the depth of the mixed layer of the water column from meteorological data - with a biological model of the dynamics of the first trophic levels - which incorporates different models representing the behaviour of phytoplankton, heterotrophic bacteria and herbivorous macro-invertebrates. In particular, construction of this model requires a more detailed knowledge of the physiology of "netplanktonic" diatoms (> 20 µm) and nanoplanktonic algae (< 20 µm), on the one hand, and factors controlling the grazing of protozoans on phytoplankton and bacterioplankton, on the other.