ALBERT

Notes: 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 km 2) can experience severe macronutrient depletion paralleling intense diatom-dominated phytoplankton blooming (maximum 〉 8 mg Chl a m−3) at the ice edge. In the Seasonal Ice Zone (SIZ, 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 (Chl a maximum around 4 mg m−3). A shift from new production to a regenerated production regime has been demonstrated during spring, along with the key role played by protozoans in controlling high ammonium concentrations (maximum 〉 2 μM) in the surface layers. The well-mixed Permanently Open Ocean Zone (POOZ, 14 M km 2) 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 〈 10 μM could help limit the phytoplankton growth. Although favourable conditions have been demonstrated for the initiation of blooms in spring in the Antarctic Circumpolar Current, it appears that critical-depth/ mixing-depth relationships control maximum chlorophyll a concentrations 〈 1 μg l−1 during summer. The POOZ is usually not influenced directly by euphausiids, except for the Scotia Sea and Drake Passage where migrations of krill from the adjacent SIZ are usual. Mesoscale eddies are typical of the Polar Front Zone (FPZ, 3 M km 2): significant increases in phytoplankton biomass have been reported in this frontal area (maximum Chl a = 2 mg m−3). Food web and biogeochemical cycles in this sub-system are poorly documented. The question of limitation of the primary production by eolian-transported trace-metals in these different sub-systems is still a matter of debate, although clear iron limitation has been evidenced for offshore waters of the Ross Sea.