Shipboard studies were performed for testing the classical hypothesis that Antarctic phytoplankton suffers from a deficiency of Fe In a suite of 5 experiments over 8 to 12 d periods and encompassing different water masses (Weddell Sea water proper, Weddell-Scotia Confluence water, Scotia Sea water), and various plankton communities, biomass and dynamic spring/summer (ice) conditions, we always observed Fe to stimulate chlorophyll a synthesis and nutrient assimilation. In 3 out of 5 experiments there was an immediate response to added Fe, while in the other 2 expenments an effect was observed after 3 to 6 d. In 4 out of 5 experiments final particulate organic carbon (POC) levels were also higher in Fe-enriched cultures compared to controls. However the controls were also found to outgrow steadily typical chlorophyll a and POC levels found in ambient waters. This strongly suggests that the in situ Fe concentration in itself does not hamper build-up of high biomass levels. Extrapolation to the in situ ecosystem therefore suggests that, despite enhancement of phytoplankton growth, Fe is not the major factor controlling phytoplankton in the Weddell/Scotia Seas. Marginal sediments appear to supply adequate dissolved Fe for supporting at least minimum growth of phytoplankton. More remote sectors of the Southern Ocean might be more likely candidates for occasional limitation by Fe alone.

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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

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During leg 2 (November 1988–January 1989) of the European Polarstern study (EPOS) in the Weddell‐Scotia Confluence zone, we implemented a new, simple, rapid, and sensitive method based on 32Si (a β‐emitter in equilibrium with 32P) to determine biogenic silica (BSi) production. This method provides simultaneous determination of the production of organic P. Rates of BSi production ranged from 0.09 to 0.95 mmol Si m‒2 h‒1, and the BSi content of the photic layer ranged from 7 to 220 mmol m‒2. The Si: C production molar ratios varied between 0.08 (small cell density of diatoms in communities dominated by cryptophyceans and dinophyceans) and 0.81 (diatom‐dominated populations), while the C: P production ratios varied between 47 and 151 with an average of 105, in agreement with the Redfield ratio. We give here the first direct evidence that in the well‐mixed waters of the Scotia Sea highly silicified diatom blooms may occur and that unlike earlier descriptions, because of the intense grazing pressure of krill in the marginal ice zone (MIZ) of the Weddell Sea, the remaining siliceous phytoplankton can grow with only moderate Si production rates. We conclude that the contribution of the MIZ to the total BSi production of the Southern Ocean must not be overstated.