Summary. Four major functional units have been identified in the Southern Ocean and the mechanisms that control the dynamics of nutrients and phytoplankton are detailed for the different sub-systems. The very productive Coastal and Continental Shelf Zone (CCSZ, 0.9 M kmZ) can experience severe macronutrient depletion paralleling intense diatom-dominated phytoplankton blooming (maximum> 8 mg ChI a m -3) at the ice edge. In the Seasonal Ice Zone (SI Z, 16 M km 2 ), dramatic variations in the hydrological structure occur in surface waters during the spring to summer retreat of the pack-ice, changing from a well-mixed system to a stratified one within the reaches of the ice edge. Grazing activity of euphausiids limits phytoplankton biomass to a moderate level (ChI a maximum around 4 mgm-3 ). A shift from new production to a regem;rated production regime has been demonstrated during spring, along with the key role played by protozoans in controlling high ammonium concentrations (maximum > 2 .uM) in the surface layers. The well-mixed Permanently Open Ocean Zone (POOZ, 14 M kmZ) is characterised by variable N/Si ratios in surface waters along a north-south transect: at the northern border of the POOZ (N/Si = 0.25) silicate concentrations as low as