LIGHT/VERTICAL MIXING The prevailing role of light in the Southern Ocean was inferred from the weak vertical stability of surface waters and the strong winds which result in deep mixing of phytoplankton and a low time-integrated irradiance for the cells. Mitchell et al. (1991) predicted that for a minimum loss rate and typical conditions of stratification and irradiance, phytoplankton in the ACC would not utilize more than 10% of the available macronutrients. Revisiting Sverdrup's concept of critical depth, Nelson & Smith (1991) found that both the marginal ice zone and the open waters of the ACC provide favorable irradiance-mixing regimes only for the initiation and early development of phytoplankton blooms. On the contrary the highest chlorophyll levels that can be sustained in summer in open waters not stabilized by melt water should not exceed 1.0 µg L -1 in the Weddell and Scotia Seas and probably less in areas that experience stronger winds. OBEX 1 : Microbial community growth

SILICON Jacques (1983) called for the possibility of silicon limitation of Antarctic diatoms. Nelson & Tréguer (1992) summarized available silicon kinetic and pointed out that Southern Ocean diatoms seemed to exhibit a very low affinity (i.e. high KS and Kµ values). However the first tracer experiments which were done in the Ross Sea Fragilariopsis curta bloom using 30 Si did not support fully the previous results. Nelson & Tréguer (1992) concluded that there was only weak substrate limitation of Si(OH) 4 uptake rate in the western Ross Sea and that significant Si limitation in other subsystems of the Southern Ocean would be possible only if their diatom assemblages had much lower affinity for silicic acid than was observed in the Ross Sea. Caubert (1998) produced the first direct indication of that possibility during ANTARES 2, in the POOZ of the Indian sector ; using 30 Si, he demonstrated that diatom assemblages could exhibit very low affinity for silicic acid with a K S value poised to 27 µM. OBEX 1 : Microbial community growth

Representative results from 32 Si kinetic experiments (on the left) during AESOPS ( ) and distribution of K S values (on the right) (Nelson et al., 2001). OBEX 1 : Microbial community growth

IRON The role of Fe in the Southern Ocean has been evidenced first by De Baar et al. (1990) in the Weddell-Scotia area and then Martin et al. (1990) in the Ross Sea, by using shipboard culture enrichments. Fe plays a key role in structuring Southern Ocean ecosystems via the size-class dependent growth response to its availability. Using a DFOB metal complexation method, Timmermans et al. (2001) were able to evaluate a Kµ value of 1.12 nM Fediss for the large chain-forming diatom Chaetoceros dichaeta and a very low value of nM for the small Ch. brevis. OBEX 1 : Microbial community growth

IRON /SILICON Si(OH) 4 uptake kinetics in the PFZ (Australian sector) (Quéguiner, 2002) V max = 0.25 ± 0.05 d -1 K S = 30.7 ± 9.9 µM V (d -1 ) [Si(OH)] 4 (µM) V (d -1 ) V max = 0.24 ± 0.05 d -1 K S = 11.1 ± 6.4 µM 20 m 105 m PFZ OBEX 1 : Microbial community growth

Fe concentration (nM) Depth (m) 12 (Sedwick et al., 1999) (Quéguiner, in press 2002) V max = 0.25 ± 0.05 d -1 K S = 30.7 ± 9.9 µM V (d -1 ) [Si(OH)] 4 (µM) V (d -1 ) V max = 0.24 ± 0.05 d -1 K S = 11.1 ± 6.4 µM 20 m 105 m

OBEX 1 : Microbial community growth [Si(OH) 4 ] (µM) V (d -1 ) nM Fe nM Fe No Fe addition Control initial = t 0 day 3.5day day 9.51day 3.5day 6.5day day 3.5day 6.5day 9.5

OBEX 1 : Microbial community growth

IRON/LIGHT Iron is an essential element in the chlorophyll synthesis pathway. Hence in the Southern Ocean phytoplankton photoadaptation to low light levels (required by deep mixed layers) is directly linked to iron availability because chlorophyll synthesis is the major physiological mechanism for such an adaptation. The photochemical efficiency of photosystem II, assessed by the ratio of variable fluorescence to maximum fluorescence (Fv/Fm ratio) is known to be a proxy of physiological stress, including nitrogen-limitation and silicon-limitation, and in the typical HNLC area of the Southern Ocean it is strongly related to iron availability (Boyd et al., 2001). OBEX 1 : Microbial community growth

Chlorophyll a (µg L -1 ) Total Micro- Nano- Pico- Day of experiment Response to Fe enrichment as modulated by available light during SOIREE (Gall et al., 2001) Deck experiment simulating light conditions for different mixed layer depths MLD = 40 mMLD = 65 mMLD = 100 m

Chlorophyll a (µg L -1 ) Day of experiment Mean MLD (40 m) light (25% I 0 ) High light (50% I 0 ) Control Mean MLD (90 m) light (9% I 0 ) High light (25% I 0 ) Control 47°S 54°S Response to Fe enrichment as modulated by available light in subantactic (47°S) and polar frontal (54°S) waters in the Australian sector of the Southern Ocean during the SAZ cruise (Australian S.O.JGOFS, R.V. Aurora Australis) (Boyd et al., 2001) + 5 nM Fe addition

OBEX 1 : Microbial community growth propositions of experiments Shelf area : enriched with Fe Perturbation experiments aimed at simulating Fe availability decrease and photoadapatation (use of DFOB to reduce Fe availability and following of natural phytoplankton growth under different light conditions (simulating variations of the MLD) to answer the question of the triggering factor of the bloom, Transect : Si/Fe natural variations kinetic experiments to assess the role of Fe in Si uptake parameter control, kinetic experiments to assess the role of Fe in NO 3 uptake parameter control. HNLC area “Classical” perturbation experiments aimed at simulating Fe availability increase and silicon limitation, addition of Fe at different concenrtations to assess the role of Fe in Si and NO 3 uptake parameter control.