Augustin Lafond
email: augustin.lafond@gmail.com

Karine Leblanc

Justine Legras

Veronique Cornet

Bernard Queguiner

Véronique Cornet

Bernard Quéguiner

The structure of diatom communities constrains biogeochemical properties in surface waters of the Southern Ocean (Kerguelen Plateau)

Keywords: Southern Ocean, Kerguelen Plateau, Diatoms, Community structure, Biological pump

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Introduction

The Southern Ocean (SO) is a major sink for anthropogenic CO2 and plays an important role in redistributing heat and nutrients at the global scale [START_REF] Sarmiento | High-latitude controls of thermocline nutrients and low latitude biological productivity[END_REF][START_REF] Sallée | Localized subduction of anthropogenic carbon dioxide in the Southern Hemisphere oceans[END_REF][START_REF] Takahashi | The Changing Carbon Cycle in the Southern Ocean[END_REF][START_REF] Frölicher | Dominance of the Southern Ocean in Anthropogenic Carbon and Heat Uptake in CMIP5 Models[END_REF]. It is also the largest High Nutrient Low Chlorophyll (HNLC) area, and as such its surface waters are characterized by low phytoplankton biomass, despite elevated macronutrient concentrations. This paradox has been solved in the early 1990s when dissolved iron was identified as a potentially strong limiting factor for primary production [START_REF] Martin | Glacial-interglacial CO2 change: The Iron Hypothesis[END_REF]; "The Iron Hypothesis"). In contrast to HNLC waters, highly productive, naturally iron fertilized blooms are observed in discrete areas of the SO, located downstream of island systems such as the Kerguelen Plateau [START_REF] Blain | Effect of natural iron fertilization on carbon sequestration in the Southern Ocean[END_REF], Crozet Islands [START_REF] Pollard | Southern Ocean deep-water carbon export enhanced by natural iron fertilization[END_REF] and South Georgia [START_REF] Whitehouse | Seasonal and annual change in seawater temperature, salinity, nutrient and chlorophyll a distributions around South Georgia, South Atlantic[END_REF][START_REF] Korb | Contrasting primary production regimes around South Georgia, Southern Ocean: large blooms versus high nutrient, low chlorophyll waters[END_REF][START_REF] Korb | Magnitude and maintenance of the phytoplankton bloom at South Georgia: a naturally iron-replete environment[END_REF]. In these so called "natural laboratories", blooms are dominated by siliceous diatoms, and C export is 2-3 times higher than in surrounding Felimited areas [START_REF] Morris | 234Th-derived particulate organic carbon export from an island-induced phytoplankton bloom in the Southern Ocean[END_REF][START_REF] Savoye | 234Th-based export fluxes during a natural iron fertilization experiment in the Southern Ocean (KEOPS)[END_REF][START_REF] Planchon | Carbon export in the naturally iron-fertilized Kerguelen area of the Southern Ocean based on the 234 Th approach[END_REF] which are typically dominated by autotrophic nanoplankton [START_REF] Kopczyńska | Annual and interannual variability in phytoplankton at a permanent station off Kerguelen Islands, Southern Ocean[END_REF][START_REF] Uitz | A phytoplankton class-specific primary production model applied to the Kerguelen Islands region (Southern Ocean)[END_REF][START_REF] Lasbleiz | Composition of diatom communities and their contribution to plankton biomass in the naturally ironfertilized region of Kerguelen in the Southern Ocean[END_REF]. Due to their large sizes, and the ballasting effect of their frustule, diatoms are classically considered as efficient vector of C export [START_REF] Buesseler | The decoupling of production and particulate export in the surface ocean[END_REF][START_REF] Armstrong | A new, mechanistic model for organic carbon fluxes in the ocean based on the quantitative association of POC with ballast minerals[END_REF].

Throughout the productive season, they progressively consume Si(OH)4 which becomes limiting in summer, leading to the termination of the bloom [START_REF] Boyd | Environmental factors controlling phytoplankton processes in the Southern Ocean[END_REF]Leblanc et al., 2005;[START_REF] Nelson | A seasonal progression of Si limitation in the Pacific sector of the Southern Ocean[END_REF][START_REF] Franck | Iron and silicic acid concentrations regulate Si uptake north and south of the Polar Frontal Zone in the Pacific Sector of the Southern Ocean[END_REF].

However, the discovery of High Biomass Low Export ecosystems dominated by diatoms [START_REF] Lam | High biomass, low export regimes in the Southern Ocean[END_REF], and later on, the recurrent observation of an inverse relationship between primary production and export efficiency in the Southern Ocean challenged our understanding of the carbon biological pump [START_REF] Maiti | An inverse relationship between production and export efficiency in the Southern Ocean[END_REF][START_REF] Henson | Drivers of Carbon Export Efficiency in the Global Ocean[END_REF]. Plankton community structure and species-specific properties can strongly influence the transfer of biogenic minerals (e.g. Si) and organic matter (e.g. C, N etc.) to the ocean interior [START_REF] Salter | Diatom resting spore ecology drives enhanced carbon export from a naturally iron-fertilized bloom in the Southern Ocean[END_REF][START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF][START_REF] Quéguiner | Iron fertilization and the structure of planktonic communities in high nutrient regions of the Southern Ocean[END_REF]Rembauville et al., 2015bRembauville et al., , 2016;;[START_REF] Tréguer | Influence of diatom diversity on the ocean biological carbon pump[END_REF][START_REF] Arteaga | Nutrient Controls on Export Production in the Southern Ocean[END_REF]. This is particularly true for diatoms which exhibit a wide range of size and biovolumes [START_REF] Leblanc | A global diatom database -abundance, biovolume and biomass in the world ocean[END_REF], as well as an incredible morphological diversity, with certain shape facilitating the formation of rapidly sinking aggregates [START_REF] Alldredge | Direct observations of the mass flocculation of diatom blooms: characteristics, settling velocities and formation of diatom aggregates[END_REF][START_REF] Alldredge | Preface: Aggregation in marine system[END_REF][START_REF] Jackson | Role of algal aggregation in vertical carbon export during SOIREE and in other low biomass environments[END_REF][START_REF] Burd | Particle Aggregation[END_REF][START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF]. Cellular Si:C quota, a key parameter controlling frustule density and sinking rate, varies widely among diatom species [START_REF] Durkin | Frustule-related gene transcription and the influence of diatom community composition on silica precipitation in an iron-limited environment[END_REF], in addition of being influenced by environmental parameters such as iron availability [START_REF] Hutchins | Iron-limited diatom growth and Si:N uptake ratios in a coastal upwelling regime[END_REF][START_REF] Takeda | Influence of iron availability on nutrient consumption ratio of diatoms in oceanic waters[END_REF]. Indeed, numerous iron-fertilization experiments observed higher Si:C and Si:N ratios in diatoms growing under iron-stress, which seemed to indicate the development of more heavily silicified frustules, although it has also be linked to a reduction in cellular organic matter contents [START_REF] Takeda | Influence of iron availability on nutrient consumption ratio of diatoms in oceanic waters[END_REF][START_REF] Twining | Element stoichiometries of individual plankton cells collected during the Southern Ocean Iron Experiment (SOFeX)[END_REF][START_REF] Hoffmann | Effects of iron on the elemental stoichiometry during EIFEX and in the diatoms Fragilariopsis kerguelensis and Chaetoceros dichaeta[END_REF][START_REF] Marchetti | Diatom elemental and morphological changes in response to iron limitation: a brief review with potential paleoceanographic applications[END_REF][START_REF] Baines | Elemental composition of equatorial Pacific diatoms exposed to additions of silicic acid and iron[END_REF]. Within a given species, life stages such as auxospores and resting forms can undergo considerable morphological and physiological changes compared to vegetative stages, e.g. heavily silicified resting spores are resistant to grazing [START_REF] Kuwata | Selection and viability after ingestion of vegetative cells, resting spores and resting cells of the marine diatom, Chaetoceros pseudocurvisetus, by two copepods[END_REF] and can contain up to 10 times more carbon [START_REF] Doucette | Thalassiosira antarctica: vegetative and resting stage chemical composition of an ice-related marine diatom[END_REF][START_REF] Kuwata | Ecophysiological characterization of two life forms, resting spores and resting cells, of a marine planktonic diatom, Chaetoceros pseudocurvisetus, formed under nutrient depletion[END_REF], which make them efficient vector to C export [START_REF] Salter | Diatom resting spore ecology drives enhanced carbon export from a naturally iron-fertilized bloom in the Southern Ocean[END_REF]Rembauville et al., 2015b). At the end, mortality pathways (apoptosis, viral lysis, parasite infection, grazing etc.) will further influence export modes and their efficiency [START_REF] Kagami | Parasitic chytrids: their effects on phytoplankton communities and food-web dynamics[END_REF][START_REF] Sherr | Heterotrophic dinoflagellates: a significant component of microzooplankton biomass and major grazers of diatoms in the sea[END_REF][START_REF] Durkin | Sinking phytoplankton associated with carbon flux in the Atlantic Ocean: Sinking phytoplankton and carbon flux[END_REF][START_REF] Kranzler | Silicon limitation facilitates virus infection and mortality of marine diatoms[END_REF]. Despite this apparent complexity, the link between species diversity and particles export is often overlooked. To date, two studies attempted to further refine the diatom functional group intro two subgroups occupying different niches in the water column: C-sinker/Group 1 comprise small, fast-growing lightly silicified and colony-forming diatoms that dominate iron-fertilized areas, while Sisinker/Group 2 are large, slow growing heavily silicified species that thrive in iron-limited areas [START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF][START_REF] Quéguiner | Iron fertilization and the structure of planktonic communities in high nutrient regions of the Southern Ocean[END_REF].

The Kerguelen region is a good place to study the ecological processes at play in the biological pump, since the area is characterized by contrasted ecosystems differently impacted by iron.

Due to its shallow bathymetry (< 700 m) and its great meridional extension, the Kerguelen Plateau acts as a natural barrier diverting both the trajectories of the Antarctic Circumpolar Current (ACC) and the Polar Front (PF) (Park et al., 2008b;[START_REF] Park | Polar Front around the Kerguelen Islands: An up-to-date determination and associated circulation of surface/subsurface waters[END_REF]. Interactions between water masses and the plateau leads to the upward transfer of iron via diapycnal mixing enhanced by internal wave activity [START_REF] Blain | Effect of natural iron fertilization on carbon sequestration in the Southern Ocean[END_REF]Park et al., 2008a), giving an annually recurrent phytoplankton bloom from October to February [START_REF] Mongin | Seasonality and scale of the Kerguelen plateau phytoplankton bloom: A remote sensing and modeling analysis of the influence of natural iron fertilization in the Southern Ocean[END_REF]. Past oceanographic surveys investigated the decline and the onset of the diatom bloom (KEOPS1 and KEOPS2 cruises, respectively), by describing diatom community composition (Armand et al., 2008b;[START_REF] Lasbleiz | Composition of diatom communities and their contribution to plankton biomass in the naturally ironfertilized region of Kerguelen in the Southern Ocean[END_REF], biogeochemical environments [START_REF] Lasbleiz | Pigments, elemental composition (C, N, P, and Si), and stoichiometry of particulate matter in the naturally iron fertilized region of Kerguelen in the Southern Ocean[END_REF], silica production [START_REF] Mosseri | Impact of iron on silicon utilization by diatoms in the Southern Ocean: A case study of Si/N cycle decoupling in a naturally iron-enriched area[END_REF][START_REF] Closset | Seasonal evolution of net and regenerated silica production around a natural Fe-fertilized area in the Southern Ocean estimated with Si isotopic approaches[END_REF] and associated carbon export [START_REF] Savoye | 234Th-based export fluxes during a natural iron fertilization experiment in the Southern Ocean (KEOPS)[END_REF][START_REF] Planchon | Carbon export in the naturally iron-fertilized Kerguelen area of the Southern Ocean based on the 234 Th approach[END_REF]Rembauville et al., 2015a).

In this context, the MOBYDICK cruise (Marine Ecosystem Biodiversity and Dynamics of Carbon around Kerguelen: an integrated view) took place in February and March 2018, i.e. at the very end of summer to witness the demise of the diatom bloom, an event that is much less described than the spring bloom, but that nonetheless plays a crucial role in the carbon biological pump. Main objectives of MOBYDICK were 1) to track carbon from its initial fixation at the surface to its channeling through the food web or export into the deep ocean; and 2) provide a detailed description of the diversity at each trophic level, from microbes to micronekton. In this study, we aim at describing diatom diversity and the composition of the bulk particulate matter, on and off the plateau. In particular, we focused our effort on providing a comprehensive description of diatom community structures, in terms of abundances, carbon biomass, physiological states, and silicification activity at multiple scales, i.e. from the community level to single-cell level. The following questions are addressed: How does iron influence diatom community structure during the demise of the Kerguelen bloom? How do diatoms influence the elemental composition and stoichiometry of the particulate matter? What are the ecological vectors driving Si and C export?

Methods

Sampling strategy

The MOBYDICK cruise was conducted south of the Kerguelen Islands in the Indian sector of the Southern Ocean (SO), aboard the French R/V Marion Dufresne II in late austral summer (18 February-27 March 2018). In this study, coinciding with the demise of the diatom bloom, a total of four stations were investigated with repeated visits at some of them (Figure 1, Table 1). Two reference stations, M2 (sampled thrice) and M4 (sampled twice), were respectively defined for the Fe-enriched plateau and the HNLC off-plateau area. Two intermediate stations, M1 and M3 were additionally sampled, once and twice respectively. Bottom depths spanned from 520 m at M2 to 4 730 m at M4 (Table 1). According to [START_REF] Pauthenet | Seasonal Meandering of the Polar Front Upstream of the Kerguelen Plateau[END_REF], station M3 was located within the Polar Frontal Zone (PFZ), whereas the other stations were located in the Antarctic zone (AAZ), with M1 and M4 both situated very close to the PF (Figure 1).

Hydrology

Seawater collection and vertical profiles of water column properties were made from an array of 24 bottles (12 L) mounted on a rosette equipped with a Seabird SBE-911 plus CTD unit, which also included a fluorometer for detection of chlorophyll a (Chl a), and a PAR (photosynthetically available radiation) sensor (LI-COR instrument). The mixed layer depth (MLD) was defined as the depth where the potential density equaled the potential density at 10 m + 0.03 kg m -3 (de Boyer [START_REF] De Boyer Montégut | Mixed layer depth over the global ocean: An examination of profile data and a profile-based climatology[END_REF]. The depth of the euphotic zone (Ze) was defined as the depth where 1 % of the surface irradiance remained. Discrete water samples were collected at 11 depths down to 500 m at the plateau station M2 and down to 1000 m at the other stations.

Inorganic nutrients

Samples were collected in 125 mL HDPE bottles directly from the Niskin bottles. The samples were then filtered on 0.45 μm cellulose acetate filter and preserved with 100 µL HgCl2 (4g L -1 ) in 20 mL vials, which were then stored in the dark at room temperature until analysis in the laboratory. For NO3 -, PO4 3-, and H4SiO4, the preserved samples were analyzed in the laboratory with a segmented flow analyzer (Skalar) equipped with colorimetric detection using the classical methods described in [START_REF] Aminot | Dosage automatique des nutriments dans les eaux marines : méthodes en flux continu[END_REF]. The accuracy of the methods was assessed using reference material (Certipur, Merck). The precisions were in the range of 1-4 %, and the limit of detection was 0.02 μM for NO3 -, 0.03 μM for PO4 3-and 0.05 µM for H4SiO4 [START_REF] Blain | Distributions and stoichiometry of dissolved nitrogen and phosphorus in the iron-fertilized region near Kerguelen (Southern Ocean)[END_REF].

Pigments

For pigment measurements, 2.32 L water samples were filtered onto Whatman GF/F filters and stored in liquid nitrogen until analysis. Back at the laboratory, filters were extracted in 100 % methanol, disrupted by sonication, and clarified by filtration (GF/F Whatman) after 2 h. Samples were analyzed within 24 h using High Performance Liquid Chromatography on an Agilent Technologies HPLC 1200 system quipped with a diode array detector following [START_REF] Ras | Spatial variability of phytoplankton pigment distributions in the Subtropical South Pacific Ocean: comparison between in situ and predicted data[END_REF].

Particulate organic carbon (POC) and nitrogen (PON)

From the rosette sampling, 4 L of seawater were filtered onto precalcinated (450°C, 24h) 25 mm Whatman GF/F filters. Filters were then dried in an oven (40°C, 24h) and stored in a glass vial until analysis. After fumigation with pure HCl (10 h) to dissolve remaining inorganic carbon, analysis of C and N was made with a CHN elemental analyzer calibrated with acetanilide (Rembauville et al., 2016).

Biogenic silica stock (BSi)

Between 1.5 and 2 L Niskin samples were filtered through 0.4 μm Nucleopore polycarbonate filters. Filters were then rinsed with 0.2 μm filtered seawater, folded in quarters, placed in Petri dishes and dried overnight at 60°C. Filters were kept at room temperature until analyses at the laboratory. The extraction of silicon from biogenic particle phase was performed following the [START_REF] Ragueneau | A new method for the measurement of biogenic silica in suspended matter of coastal waters: using Si:Al ratios to correct for the mineral interference[END_REF] triple NaOH/HF extraction procedure.

Plankton counting

Prior to counting, plankton diversity was investigated in few representative samples using a Phenom Pro scanning electron microscope (SEM) in order to resolve the taxonomic list of species at the best possible level for the study area. For SEM analysis, water samples were filtered (50 mL) onto polycarbonate filters (0.8 μm pore size, Whatman), and stored in Millipore Petri Slides at room temperature with no further treatments. Pictures were taken at magnification up to x 25,000.

For plankton counting, seawater samples collected from the same casts and bottles as particulate Si samples were preserved in 250 mL amber glass bottles with 1.6 mL acidified Lugol solution before being stored in the dark at 4°C until analysis. For each station and visit, 8 samples from the surface down to 250 m were counted, giving a total number of 64 samples analyzed. Counting was performed at the laboratory following [START_REF] Utermöhl | Neue Wege in der quantitativen Erfassung des Plankton.(Mit besonderer Berücksichtigung des Ultraplanktons.): Mit 4 Abbildungen im Text[END_REF] using a Nikon TE-200 inverted microscope at magnification X 200. Counted volume varied between 50 and 100 mL depending on cell concentration. Every planktonic cell examined was counted and identified at the lowest possible taxonomic level. For diatoms, we counted separately cells that were intact (e.g. complete frustule with chloroplasts), empty (e.g. complete frustule devoid of internal content), broken (e.g. frustule with a clean break at the girdle band junction or partly dissolved), or crunched (e.g. frustule with a jagged break outside of the girdle junction; see supplementary Figure S1 for illustrating pictures), as recommended by [START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF]. Many mortality pathways can explain the presence of empty and broken frustules [START_REF] Tréguer | Influence of diatom diversity on the ocean biological carbon pump[END_REF], whereas crunched frustules are likely due to handling by copepod mandibles [START_REF] Assmy | Mechanisms determining species dominance in a phytoplankton bloom induced by the iron fertilization experiment EisenEx in the Southern Ocean[END_REF]. Due to the low magnification, taxonomic identification to the species level was occasionally difficult and necessitated categorizing diatom species to the genus or taxa groupings in the following manner: Chaetoceros spp. Hyalochaete resting spores (CRS) were not differentiated into species or morphotypes but were counted separately from the vegetative cells; Fragilariopsis separanda and Fragilariopsis rhombica were grouped as Fragilariopsis separanda/rhombica; most of the small (< 10 μm) and intermediate (10-50 μm) unidentified centric diatoms were grouped as Actinocyclus spp./Thalassiosira spp., after we identified these two genera as the dominant ones, based on SEM images. For some taxa exhibiting a wide range of sizes, size classes were defined to better estimate their carbon biomass. Finally, for a few species, we also distinguished different morphological forms and life stages (Figure S2). In Odontella weissflogii, we counted separately vegetative cells (rectangular, lightly silicified, with 4 processes located on each valve), resting cells (more rounded and silicified, with two processes on each valve), and resting spores (oval shape, heavily silicified, with very short processes) [START_REF] Hoban | Biddulphioid diatoms: Resting spores in antarctic Eucampia and Odontella[END_REF]. In Eucampia antarctica, many different morphological forms were observed: a typical form (i.e. heavily silicified, asymmetric cells that form coiling chains), a H-shape form, a flat form without horns, and some intermediate forms. Finally, three forms of Chaetoceros atlanticus were distinguished: a colonial form with cells having long and straight setae, a single cell form with sigmoidal thickened setae, and a 'bulbosum' form characterized by a single cell enriched with lipids and with short and inflated setae. Although non-exhaustive, a detailed list of taxa is presented in supplementary Table S1, along with linear measurements used to estimate biovolumes and carbon biomasses.

Estimation of the diatom C biomass

Specific C biomass was assessed only for diatoms, following the methodology described in [START_REF] Cornet-Barthaux | Biovolume and biomass estimates of key diatoms in the Southern Ocean[END_REF]. When possible, cell dimensions were determined from at least 20 representative cells for each species/taxon and for each sample. When the third dimension was not measurable due to the cell's position on the slide (e.g. the transapical axis of Fragilariopsis species, or the pervalvar axis of numerous centric), the hidden dimension (hd) was estimated by using mean dimension ratios from other comparable samples where hd was measured. For a few taxa (e.g. centrics such as Asteromphalus spp.), hd was not measured in any sample. In these cases, we used the visible to hidden dimension ratio documented for the same genera in European standards [START_REF] Afnor | EN 16695. Water Quality -Guidance on the estimation of phytoplankton biovolume[END_REF]. For each measured cell, a biovolume was estimated using linear dimensions in the appropriate geometric formula reflecting the cell shape [START_REF] Afnor | EN 16695. Water Quality -Guidance on the estimation of phytoplankton biovolume[END_REF]. A mean biovolume was then estimated for each taxon and converted to carbon content per cell according to the corrected equation of Eppley [START_REF] Smayda | From phytoplankton to biomass[END_REF] as follows:

Log10 C biomass (pg) = 0.76 Log10 (cell volume (µm 3 )) -0.352 CRS carbon content was estimated using the volume-to-carbon relationship of 0.039 pmol C μm -3 established from the resting spore of Chaetoceros pseudocurvisetus [START_REF] Kuwata | Ecophysiological characterization of two life forms, resting spores and resting cells, of a marine planktonic diatom, Chaetoceros pseudocurvisetus, formed under nutrient depletion[END_REF]. C content per cell (pg cell -1 ) was multiplied by cell abundance (cells L -1 ) to derive total carbon biomass per taxon, expressed in units of μg C L -1 . Total diatom carbon biomass was then compared to chemically determined POC concentrations to yield a percentage contribution of diatoms to the total POC fraction.

Diatom-specific contribution to silica deposition

Taxon-specific silica production was quantified using the fluorescent dye PDMPO. Leblanc and Hutchins (2005) have shown that the dye is incorporated in a nearly constant ratio with biogenic silica during the synthesis of the frustule, allowing semi-quantification of newly deposited Si at the specific level using epifluorescence microscopy. However, [START_REF] Mcnair | Quantifying diatom silicification with the fluorescent dye, PDMPO[END_REF] observed a linear decrease in the BSi : PDMPO incorporation ratio when ambient silicic acid concentration fall below 3 μmol L -1 . Hence, it is important to keep in mind that for a given PDMPO fluorescence, the amount of BSi synthetized might be higher at stations where Si(OH)4 reached 3 μmol L -1 . For each station/visit, seawater samples were collected at 5 different depths corresponding to different levels of PAR (50%, 25%, 4%, 1% and 0% of surface irradiance). For each sample, 150 mL of seawater were spiked onboard with 0.125 μM PDMPO (final concentration). Samples were then placed in deck incubators cooled by running sea surface water and fitted with blue plastic optical filters to simulate the light attenuation of the corresponding depth. After 24 h, samples were filtered onto 0.6 μm black Nucleopore polycarbonate filters. Filters were directly mounted onto permanent glass slides and stored in the dark at -20°C until analysis. Back at the laboratory, microscope slides were observed under a Zeiss Axio Observer Z1 inverted fluorescence microscope equipped with an Xcite 120LED source and a CDD black and white AxioCam-506 Mono (6 megapixel) camera fitted with a filter cube (Ex: 350/50; BS 400 LP; Em: 535/50). For each taxon, fluorescent cells were identified and counted, and we differentiated between valves and girdle bands, since those two parts of the frustule do not silicify at the same stage during the cell cycle, and also do not have the same area nor degree of silicification (i.e. the amount of BSi deposited per unit surface), and should thus not be compared directly. In each sample, a representative number of images was taken for each taxon (at least 20 images for the most common taxa). PDMPO fluorescence intensity was then quantified using a custom-made IMAGE J routine on original TIFF images. This routine needs to be changed depending on the format of images and associated information and can be made available upon request to the corresponding author.

The taxon-specific contributions to silica production were estimated by multiplying the number of fluorescent cells by the mean fluorescence per cell. The total fluorescence per taxon was expressed in AU (arbitrary unit) L -1 . Based on the images we also estimated the degree of silicification for the most active taxa by dividing individual cell fluorescence by its corresponding frustule surface.

Statistical analysis

Multivariate analyses were performed on total diatom abundances (i.e. including both intact and detrital cell counts) using MATLAB, version 8.4 (MathWorks). The abundances were Log(x+1) transformed to decrease the importance of numerically dominant taxa. Bray-Curtis dissimilarities were then calculated between each pair of samples, and a cluster analysis was run using the complete linkage algorithm [START_REF] Legendre | Numerical Ecology[END_REF]). An arbitrary threshold was applied to separate samples into compact clusters. A non-metric multidimensional scaling (NMDS) was also performed on total diatom abundances. The NMDS displays the relative similarity between samples as the distance between points in a twodimensional space. Hence, tightly grouped samples are more similar than widespread data points, thereby providing a visual representation of community differences. A comprehensive list of the taxa included in these two analyses is presented in supplementary Table S2.

Results

Hydrographic context

The mixed layer depth (MLD) at the first visit did not exhibit large variations between stations, ranging between 63-79 m (Table 1). M1 was weakly stratified and exhibited the shallowest MLD. Following a large storm event (10 March), the MLD deepened slightly at M2-3 (80 m), and more importantly at M3-3 and M4-2 (both at 96 m). The depth of the euphotic zone (Ze) was shallower above the plateau reference station (58-64 m) compared to the other stations (89-105 m).

Average vertical profiles of Chl a concentrations calculated from several visits at a same station are presented in Figure 2.a. Phytoplankton biomass was low at the 4 stations and mostly distributed in the upper 125 m, with the highest concentrations (up to 0.4 ± 0.2 μg L -1 ) reached at the plateau reference station M2. Station M3 and M4, located on the western side of the Kerguelen Plateau, presented the lowest Chl a concentrations (< 0.2 μg L -1 ) whereas M1 presented intermediate concentrations (0.3-0.4 μg L -1 ) in the upper 75 m. The third visit at M2 was marked by an increase in Chl a (up to 0.6 μg L -1 at 50 m, data not shown) following the storm event, explaining the larger standard deviation at this site.

Average profiles of silicic acid, nitrate, phosphate, and ammonium concentrations are presented in Figure 2.b-e. Silicic acid concentrations were depleted in the upper 75 m, especially at station M2 and M3, ranging between 1.7-3.1 μmol L -1 . In comparison, moderate to elevated silicic acid concentrations were observed at M4 (up to 4.8 μmol L -1 within the upper 75 m) and M1 (up to 6.9 μmol L -1 within the upper 50 m). Nitrate and phosphate concentrations were replete in the upper 75 m and ranged between 21.5-25.1 μmol L -1 and 1.5-1.9 μmol L -1 , respectively. Small differences were observed between stations and visits, as evidenced by the low standard deviations. Maximum NH4 + concentrations ranged between 0.9-1.3 μmol L -1 for the 4 stations, and were usually located in sub-surface. Highest concentrations were measured at M2-3, where POC and PON concentrations reached 6.2 and 1.0 μmol L -1 at 28 m, respectively. This station was also characterized by a large increase in POC and PON concentrations at 450 m, likely reflecting an organic material resuspension event after the storm. Apart from M2-3, no large differences were observed at the surface, with POC and PON varying respectively between 3.3-5.5 μmol L -1 and 0.6-0.9 μmol L -1 , while at the last sampled depths they varied between 0.5-1.9 and 0.1-0.2.

Depth-integrated standing stocks

BSi, POC, PON, and Chl a concentrations were integrated between 0 and 200 m (Figure 4; see supplementary Figure S3 for integrated concentrations between 200-500 m), independently of the mixed-layer depth because significant biogenic and organic particulate material was measured between 100 and 200 m (Figure 3). Integrated BSi ranged between 134-219 mmol m -2 at M1/M2/M3, and a decreasing trend was observed during the survey. In line with vertical profiles, BSi standing stocks were larger at M4 (up to more than twice) where it reached 493 mmol m -2 at the first visit, decreasing to 339 mmol m -2 at the second visit. POC and PON standing stocks ranged between 448-756 mmol m -2 and 73-108 mmol m -2 respectively, with maximum values measured at M2-1 and lowest values measured at M3-3.

Contrary to BSi, no large variations were observed between stations. A decreasing trend was observed between the first and the second visit at M2/M3/M4, but integrated stocks increased after the storm event at the plateau reference station M2 (3 rd visit). Chl a standing stocks did not vary a lot spatially, especially between M1/M3/M4 where it ranged between 20-33 mg m -2 . Above the plateau, it varied between 30-53 mg m -2 , with the largest increase observed during the last visit, after the storm event.

Stoichiometry of particulate matter

The elemental ratios within the upper mixed layer (0-MLD) and the subsurface layer (MLD-200 m) are presented for the 8 visits at the 4 stations (Figure 5) to highlight some spatial and/or temporal patterns over the study area. Within the ML, the most striking feature was the elevated Si:C and Si:N ratios at M4 (both visits), with average values ranging between 0.54-0.67 and 3.42-4.05, respectively. At the other stations, Si:C (0.14-0.20) and Si:N (0.78-1.16) ratios were much closer to the empirical 0.13 and 1.12 values reported by [START_REF] Brzezinski | The Si:C:N ratio of marine diatoms: interspecific variability and the effect of some environmental variables[END_REF] for diatoms grown in nutrient-replete medium. Below the MLD, Si:C and Si:N ratios increased rapidly at every station with average values ranging respectively between 0.35-1.06 and 2.13-6.58, with M4-1 still displaying the highest ratios. The average C:N ratios were close to the 6.62 Redfield empirical value. In contrast to Si:C and Si:N, no large variations were observed between the 0-MLD (average: 5. 71-6.32) and MLD-200 m layers (6.02-8.38).

Diatom community structures: abundances, carbon biomasses, and silica deposition activity

Diatom contribution to particulate organic carbon (POC)

Absolute diatom carbon biomass and its relative contribution to POC are presented in Figure 6. Above the plateau (M2), diatom carbon biomass increased progressively between the first and last visits, on average from 10.2 ± 3.1 μg C L -1 (17 ± 6 % of POC; n=4) to 30.7 ± 2.4 μg C L -1 (43 ± 6 % of POC) within the upper 75 m. The highest diatom C biomass measured throughout the survey was reached during the last visit at M2 (34.1 μg C L -1 ; 69 m), representing 52 % of POC biomass. At the first visit, diatom C biomass was homogeneous down to 100 m due to the accumulation of large Eucampia antarctica cells at this depth, while during the two following visits, it decreased rapidly below the MLD, where on average only 3.5 μg C L -1 (16 % POC) and 2.0 μg C L -1 (10 % POC) remained, respectively. It is interesting to notice the low but quasi constant diatom carbon biomass within the 100-200 m layer at the last two visits, which contrasts with the other stations. At the off-plateau station M4, diatom C biomass was stable within the upper 100 m at the two visits, averaging 10.5 ± 1.8 μg C L -1 (24 ± 6 % of POC; n=8). Lowest C biomass were measured at M3, where it averaged 2.1 ± 0.6 μg C L -1 (6 ± 2 % POC; n=8) at the two visits. These low biomasses can be explained by both lower abundances and the presence of diatoms having small biovolumes. In particular, the genus Fragilariopsis spp.

was mostly composed of small F. pseudonana < 10 μm at M3, whereas we counted numerous large F. kerguelensis at M1 and M4. Finally, diatom C biomass at M1 was homogeneous in the upper mixed layer (9.6 ± 0.3 μg C L -1 ; 16 ± 1 % of POC), then decreased progressively with depth towards quasi non detectable values below 100 m.

Absolute diatom abundances

The vertical profiles of diatom abundances with the contribution of full, empty, broken, and crunched frustules are presented in Figure 7. Overall, total diatom abundances peaked at 50 m and then decreased progressively with depth, except at station M2 where higher abundances were observed below the MLD, and at M3-3 where abundances were homogeneous in the 50-200 m layer. In the upper 75-100 m, abundances were higher at stations M1 and M4, ranging between 9.9 to 16.8 x 10 4 cells L -1 , with the highest number of cells reached at M4-2 (50 m). In contrast, lower abundances were measured at M2 in the upper 75 m, although we observed a progressive increase between the first and last visit, from 3.2 ± 0.9 x 10 4 cells L -1 (n=4) to 5.8 ± 0.3 x 10 4 cells L -1 . The contribution of intact cells to total diatom abundances decreased rapidly below 75 m at M1/M2, whereas it declined significantly only below 100 m at M3/M4 (see also Figure S4). Over the study area, it was homogeneous in the surface layer, with an average contribution of 66 ± 8 % (n=31). Above the plateau, numerous intact cells were still observed in the deepest samples located between 175-200 m (up to 48 % at M2-2), while only 3-12 % of diatoms were still intact at the other stations. The average percentage of empty and broken cells increased with depth from respectively 24 ± 5 % and 3 ± 2 % at the surface (n = 8) to 55 ± 9 % and 19 ± 9 % in the deepest samples. The percentage of crunched diatoms, which we consider as indicative of mesozooplankton grazing, did not exhibit either any obvious vertical or spatial trends (5 ± 3 %; n=64). Fragilariopsis kerguelensis (13 ± 5 %), Chaetoceros spp. (13 ± 7 %; mostly as small Chaetoceros sp. Hyalochaete), and Actinocyclus/Thalassiosira spp. (10 ± 5 %). Deep communities at stations M3/M4 formed a distinct cluster and were dominated by a mix of Fragilariopsis kerguelensis (37 ± 9 %; n=16), Chaetoceros atlanticus (23 ± 13 %), Pseudo-nitzschia spp. (13 ± 5 %), and Actinocyclus/Thalassiosira spp. (11 ± 3 %), while the contribution of CRS was minor at these sites.

Taxon-specific contribution to total diatom abundances

When looking at the taxon-specific contribution to intact and detrital cells (see supplementary Figure S5-8), some trends emerged. The only species that was almost exclusively observed intact at all sites within the mixed layer was Cylindrotheca closterium reflecting its possible recent growth and good physiological state at the time of sampling. Below the MLD, Chaetoceros atlanticus and the other Chaetoceros species (including both vegetative and resting stages) were the main contributors to intact cells, at M3/M4 and M2/M1 respectively, while Fragilariopsis kerguelensis cells were mostly detrital (i.e. empty, broken or crunched).

Crunched frustules were mainly associated with the pennate Fragilariopsis kerguelensis (42 ± 21 %; n=64) and the centric genera Actinocyclus/Thalassiosira spp. (24 ± 15 %), suggesting a selective grazing.

Taxon-specific contribution to diatom carbon biomass

Taxon-specific contribution to diatom carbon biomass is presented in Figure 9. At the reference plateau station M2, diatom carbon biomass was dominated within the upper 75 m (i.e. corresponding to the MLD) by the large centric species Corethron inerme, whose average contribution to biomass increased from 59 ± 3 % (1 st visit, n=4) to 83 ± 5 % (3 rd visit). Important contributors were also Guinardia cylindrus at the first visit (19 ± 5 %; n=4), followed by species belonging to Proboscia/Rhizosolenia genera (mostly P. alata and R. chunii) whose contribution was 10 ± 4 % (n=12) at the three visits. The taxa Actinocyclus/Thalassiosira spp., Thalassionema nitzschioides, Pseudo-nitszchia spp., and Cylindrotheca closterium which were key contributors to diatom abundances did not contribute significantly to C biomass, due to their low biovolumes. Below the MLD, C biomass was largely dominated by Eucampia antarctica at the first visit (58 ± 11 %; n=4). Its contribution rapidly decreased at the following visits (11 ± 9 %; 3 rd visit), being replaced by CRS (27 ± 18 %) and Chaetoceros spp. (23 ± 17 %; mostly the small Chaetoceros sp. Hyalochaete). Apart from M2, the community structures at the other stations appeared more diverse. The large centric species Proboscia/Rhizosolenia spp. contributed significantly to carbon biomass throughout the water column, except below 100 m at M1 (mostly P. inerme and R. chunii; 18 ± 11 %, n=35). Species belonging to Actinocyclus/Thalassiosira genera were also important contributors all over the water column (21 ± 10 %; n=40). Although a vertical structure was less obvious compared with diatom abundances (Figure 8), Chaetoceros atlanticus contributed significantly to C biomass below the MLD at M3/M4 (26 ± 15 %; n=18), while Chaetoceros spp. (mostly the small Chaetoceros sp. Hyalochaete; 22 ± 12 %; n=5) and CRS (22 ± 13 %) were the main contributors at M1 below 100 m. Interestingly, one can notice the much lower contribution of Fragilariopsis kerguelensis to C biomass in comparison with diatom abundances, which can be linked to its high percentage of detrital frustules, especially below the mixed layer, as well as to their small C content per cell (149 pg C cell -1 ; see supplementary Table S1). Although it had one of the smallest biomasses (8-25 pg C cell -1 ), Cylindrotheca closterium contributed on average to 17 ± 11 % of diatom C (n= 8) in the upper 100 m at M3, due to elevated abundances. Corethron spp. contributed significantly to C biomass at some depths at station M1/M3/M4, but unlike M2, it belonged mainly to the species C. pennatum.

Taxon-specific contribution to biogenic silica deposition

The contribution of each taxon to biogenic silica deposition is presented in Figure 10. The diversity appears smaller than in the previous figures for several reasons: first, species identification is harder in epifluorescence than in bright-field microscopy and leads to downgrading of the upper taxonomic level; second, because less species were actively silicifying in a given sample relative to the total number of taxa, meaning that some species were not active at the time of the incubation even though present as intact cells.

Above the plateau station M2, total PDMPO fluorescence increased progressively between the first and the last visit from a maximum of 4.1 x 10 6 AU L -1 (M2-1; 19 m) to 12.1 x 10 6 AU L -1 (M2-3; 9 m). This increasing silicification activity was associated with the development of Corethron inerme whose average contribution to total silicification was low at M2-1 (13 ± 10 %; n = 3), but increased to 52 ± 9 % (n = 4) and 47 ± 6 % (n = 4) at M2-2 and M2-3, respectively.

Proboscia/Rhizosolenia spp. (mostly P. alata) were also actively silicifying, with an average contribution of 39 ± 16 % (n = 11) to PDMPO fluorescence throughout the survey.

Fragilariopsis spp. cells (likely including F. kerguelensis, although identification to the species level was difficult) were actively silicifying at this station, albeit as a smaller contributor (14 ± 10 %; n = 11). At M2, we also incubated samples that were collected at 100 and 125 m (data not shown), but silicification activity was almost null, and we did not observe any labelled Eucampia antarctica or CRS, although they were important contributors to both abundances and C biomass at those depths. The lowest total PDMPO fluorescence was measured at M3-1 (< 1.0 x 10 6 AU L -1 ) where silicification activity was almost constant throughout the upper 100 m. Very few taxa contributed to biogenic silica deposition, which was dominated by the activity of Fragilariopsis spp. (48 ± 17 %; n = 4) and Actinocyclus/Thalassiosira spp. centric cells (44 ± 15 %; n = 4). By contrast, the highest total PDMPO fluorescence was measured at M1, reaching 25.0 x 10 6 AU L -1 at the surface before decreasing progressively with depth. At this station, Fragilariopsis spp. and Actinocyclus/Thalassiosira spp. were the most important contributors down to 90 m (respectively 31 ± 18 %, and 30 ± 19 %; n = 4). Actively silicifying Corethron pennatum were also observed at the surface (47 % at 13 m), whereas Rhizosolenia 2015) experimental results, which showed that the BSi : PDMPO incorporation ratio was higher at [Si(OH)4] > 3 μmol L -1 , it is plausible that relative differences between M1/M4 ([Si(OH)4] > 3 μmol L -1 within the mixed layer) and M2/M3 ([Si(OH)4] < 3 μmol L -1 ) in terms of total PDMPO fluorescence would be even higher if converted to new BSi.

Species-specific properties driving diatom elemental stoichiometry

Diatom Si:C ratio

Si:C ratios based on calculated diatom carbon biomass (defined hereafter as Si:Cdiatom) instead of total POC are presented in Figure 11. Within the ML, Si:Cdiatom ratios were well above the 0.13 value reported by [START_REF] Brzezinski | The Si:C:N ratio of marine diatoms: interspecific variability and the effect of some environmental variables[END_REF] with the highest values measured at M3/M4 (range: 2.4-3.6), intermediate values at M1 (1.2-1.3), and the lowest ones at M2 (range: 0.4-0.9).

Above the plateau, a decreasing trend was observed throughout the survey period. Overall, such elevated Si:Cdiatom ratios in the upper layer can be explained by several factors including the presence of detrital frustules, a high silicification degree, or a low carbon content per cell.

Below the ML, Si:Cdiatom ratios increased drastically with average values between 3.7-20.8, evidencing the preferential recycling of soft organic matter over BSi dissolution. At 200 m, where a significant fraction of diatom organic matter is likely to have been remineralized, Si:Cdiatom were still much lower at M2 in comparison with stations located outside the plateau.

Diatom silicification degree

The degree of silicification (i.e the quantity of BSi deposited per surface unit) was semiquantitatively estimated by normalizing the PDMPO fluorescence of each labelled cell with its labelled surface area (expressed in Arbitrary fluorescence Unit per µm -2 ). Figure 12 shows the degree of silicification for the most important contributors to BSi deposition. Small (< 10 µm) Actinocyclus/Thalassiosira spp. were less silicified at the iron-fertilized station M2 during the first two visits (Mann-Whitney U test, p < 0.05), whereas no significant differences were observed between the other stations. This was even more evident for the 10-30 μm size fraction characterized by a mean silicification degree equal to 0.51 ± 0.30 AU μm -2 at M2-1/M2-2, which was 3-4 times lower than at other stations (1.46-2.38 AU μm -2 ) but tended to increase during the successive visits. Diatoms belonging to the genus Fragilariopsis (small and large) were the most heavily silicified of all actively silicifying species. However, statistical analysis showed their degree of silicification was significantly lower at M2 (range: 2.44-2.70 AU μm -2 and 1.40-2.12 AU μm -2 for the size-classes <10 and 10-30 μm, respectively) in comparison with M1/M4 (range: 3.34-5.39 AU μm -2 and 3.91-7.40 AU μm -2 ). Actively silicifying Corethron inerme were mostly observed at station M2 (Figure 10). Girdle bands of C. inerme were weakly silicified (mean: 0.70 ± 0.31 AU μm -2 ; n = 133). Half-valves of C. inerme were more silicified with an average silicification degree equal to 2.70 ± 1.19 AU μm -2 (n = 54).

Labelled girdle bands of Proboscia alata were also numerous at M2/M4. Again, the degree of silicification was significantly lower at the plateau station M2 for the three visits (1.58 ± 0.46 AU μm -2 , n = 111) compared with the reference HNLC station M4-1 (2.87 ± 0.85 AU μm -2 , n = 7). Although not a dominant contributor to BSi deposition, we found that Chaetoceros atlanticus, an important contributor to both abundances and carbon biomass at M3/M4, was weakly silicified (1.04 ± 0.78 AU μm -2 , n = 71).

Diatom species-specific physiological state below the mixed layer

A detrital:intact cell ratio (defined hereafter as D:I ratio) averaged over the deep MLD-250 m layer was calculated to determine the physiological state of the cells exported out of the mixed layer (Table 2). Following Rembauville et al. (2015b), we used the threshold values of 2 (representing species mainly observed as detrital cells) and 0.5 (representing species mainly observed as intact cells). Among the main contributors to total diatom abundances below the ML (Figure 8), diatom taxa displaying D:I ratios > 2 were in decreasing order of importance:

Fragilariopsis kerguelensis, Pseudo-nitzschia spp., Actinocyclus/Thalassiosira spp., and Chaetoceros atlanticus, indicating those taxa were mostly empty, broken or crunched below the MLD. Instead, Chaetoceros spp. (mostly a small Chaetoceros sp. Hyalochaete) and CRS displayed D:I ratios < 0.5 at M2, meaning they were preferentially found intact below the ML.

Eucampia antarctica displayed ratios in between the thresholds < 2 and > 0.5 at M2-1 where it was an important contributor to diatom abundances, meaning this species was equally represented by full and empty cells. Finally, some taxon that were numerically important at the surface were few in number below the MLD (e.g. Fragilariopsis spp. such as F. pseudonana, Corethron inerme, Cylindrotheca closterium), suggesting they were not efficient vectors for both Si and C export.

Discussion

The MOBYDICK results presented above make it possible to draw the major characteristics of the final stage of the seasonal bloom around the Kerguelen Plateau by illustrating the relationships between the structure of its diatom communities and its regional biogeochemistry. While past oceanographic surveys (KEOPS1: January/February 2005; KEOPS2: October/November 2011) identified the physical mechanisms leading to the vertical input of iron-rich deep waters which are responsible for the recurrent plateau bloom development [START_REF] Blain | Effect of natural iron fertilization on carbon sequestration in the Southern Ocean[END_REF]Park et al., 2008a), the MOBYDICK project focused on the study of the relationships that exist between the oceanic biodiversity and the operation of the biological carbon pump. In this paper we specifically address the question of the elemental composition and stoichiometry of particulate matter and how these parameters may be driven by diatom community composition and species-specific properties (e.g. physiological state, life stages, carbon content, silicification degree etc.), above the Fe-fertilized plateau and in offshore waters.

Biogeochemical environments at the final stage of the Kerguelen bloom

At the beginning of our survey (February 26 th ), the siliceous biomass at station M2 located above the Kerguelen Plateau had mainly accumulated within a deep silica maximum (DSM) located at 100 m (1.7 μmol L -1 ), before progressively receding at the following visits. An opposite trend was observed in the upper 75 m corresponding to the mixed layer (ML), where BSi slightly increased from 0.6 to 0.8 μmol L -1 , while a three-fold diatom C biomass increase (10.2-30.7 μg C L -1 ) occurred following the development of a population of large Corethron inerme. However, these concentrations were fairly low by comparison to diatom biomasses measured earlier in the season both for Si (up to 21.3 μmol L -1 ; [START_REF] Mosseri | Impact of iron on silicon utilization by diatoms in the Southern Ocean: A case study of Si/N cycle decoupling in a naturally iron-enriched area[END_REF] and C (up to 100 μg C L -1 ; Armand et al., 2008b). Contrary to BSi, Chl a/POC/PON standing stocks did not show large variations between on-and off-plateau stations, which can be attributed to the decaying stage of the yearly bloom. Silicic acid concentrations were depleted (1.2 μmol L -1 at the surface) whereas nitrate and phosphate levels were still elevated, which confirms the high H4SiO4 uptake affinity of diatoms previously mentioned by [START_REF] Mosseri | Impact of iron on silicon utilization by diatoms in the Southern Ocean: A case study of Si/N cycle decoupling in a naturally iron-enriched area[END_REF] over the Kerguelen Plateau. Inputs of H4SiO4 through vertical mixing enhanced by internal waves (Park et al., 2008a), as well as BSi dissolution evidenced by [START_REF] Closset | Seasonal evolution of net and regenerated silica production around a natural Fe-fertilized area in the Southern Ocean estimated with Si isotopic approaches[END_REF] could have sustained this persisting diatom community. In addition, a large storm event on March 10 th with wind speeds up to 50 knots for two days led to a deepening of the MLD during the last visits at M2/M3/M4 (Table 1), enabling a pulse input of H4SiO4 in the surface layer. This input must have been limited and, although diatom growth was stimulated, we observed a rapid emergence of Si limitation signs on the plateau where H4SiO4 concentrations and Si:N ratios were at the lowest: (1) SEM images as well as PDMPO single-cell analysis revealed that Corethron inerme frustules were very weakly silicified, with collapsed frustules in most images, (2) germination experiments performed on Odontella weissflogii resting spores (Leblanc et al., in prep.) incubated under two different treatments (light + Si + P vs. light only) indicated that Si addition was the main factor triggering spore germination.

On the western side of the Kerguelen Plateau, it is unlikely that station M3 and M4 were influenced by iron fertilization from the plateau, due to the eastward flow associated with the PF (Park et al., 2008b;2014;[START_REF] Park | Polar Front around the Kerguelen Islands: An up-to-date determination and associated circulation of surface/subsurface waters[END_REF]. These two stations were marked by an important deepening of the MLD following the storm (from 69 to 96 m) but Ze was always deeper (from 95 to 101 m) suggesting that diatoms were not prone to light limitation in this area. However, stations M3 and M4 exhibited contrasted biogeochemical environments. At M3, silicic acid concentrations were low at the surface (2.4 μmol L -1 ), suggesting a strong H4SiO4 limitation since Southern Ocean diatoms have long been shown to exhibit very high KS values for H4SiO4 under HNLC conditions [START_REF] Jacques | Some ecophysiological aspects of the Antarctic phytoplankton[END_REF][START_REF] Nelson | Role of silicon as a limiting nutrient to Antarctic diatoms: evidence from kinetic studies in the Ross Sea ice-edge zone[END_REF][START_REF] Franck | Iron and silicic acid concentrations regulate Si uptake north and south of the Polar Frontal Zone in the Pacific Sector of the Southern Ocean[END_REF][START_REF] Nelson | A seasonal progression of Si limitation in the Pacific sector of the Southern Ocean[END_REF][START_REF] Quéguiner | Biogenic silica production in the Australian sector of the Subantarctic Zone of the Southern Ocean in late summer 1998[END_REF][START_REF] Mosseri | Impact of iron on silicon utilization by diatoms in the Southern Ocean: A case study of Si/N cycle decoupling in a naturally iron-enriched area[END_REF]. As a matter of evidence, diatom abundances decreased sharply throughout the survey period while silicification activity as well as diatom contribution to C (average in the ML: 5 % POC) were the lowest recorded during the cruise. This station has been extensively studied during past oceanographic surveys (e.g. KEOPS1 & 2) and corresponds to the time series station KERFIX [START_REF] Jeandel | KERFIX, a time-series station in the Southern Ocean: a presentation[END_REF]. These studies reported very low diatom productivity year-round in these waters with C biomass rarely exceeding 3 μg C L -1 [START_REF] Kopczyńska | Annual and interannual variability in phytoplankton at a permanent station off Kerguelen Islands, Southern Ocean[END_REF][START_REF] Lasbleiz | Composition of diatom communities and their contribution to plankton biomass in the naturally ironfertilized region of Kerguelen in the Southern Ocean[END_REF], and phytoplankton abundances largely dominated by naked pico-and nanoflagellates [START_REF] Kopczyńska | Annual and interannual variability in phytoplankton at a permanent station off Kerguelen Islands, Southern Ocean[END_REF]. According to [START_REF] Pauthenet | Seasonal Meandering of the Polar Front Upstream of the Kerguelen Plateau[END_REF], the zonal position of the PF is highly variable throughout the productive season on the westernside of the plateau and is located further south by up to 4° latitude in March compared to September. Hence, it isolates M3 from the southern Antarctic surface water (AASW), giving one potential explanation to the lower diatom contribution in this area, since flagellates are known to outnumber diatoms north of the PF [START_REF] Kopczynska | Phytoplankton species composition and abundance in the Indian sector of the Antarctic Ocean[END_REF]. In a study investigating the whole microplankton community structure across the Subantarctic and Polar Front, Rembauville et al. (2016) also showed that dinoflagellates and ciliates dominated microplankton POC in the PFZ, while diatoms dominated in the AAZ. Unexpectedly, we measured the highest siliceous biomass at the reference HNLC station M4, where BSi concentrations reached 3.2 μmol L -1 at the DSM (1 st visit), then started to decrease shortly after. Intermediate silicic acid concentrations were measured (≈ 4.5 μmol L -1 ) that appeared however sufficient for diatoms to grow since they were actively silicifying within the upper 100 m (Figure 10). Elevated BSi concentrations in HNLC waters were also reported in a similar HNLC environment earlier in the season, reaching 6.5 μmol L -1 at the end of January [START_REF] Mosseri | Impact of iron on silicon utilization by diatoms in the Southern Ocean: A case study of Si/N cycle decoupling in a naturally iron-enriched area[END_REF]. Heavily silicified species (e.g.

Fragilariopsis kerguelensis)

were particularly abundant at M4-1, which likely drove BSi towards elevated values. Hence, our results show that diatoms can maintain a significant Si biomass in HNLC waters, even exceeding those measured above the plateau at the end of the productive period.

On the eastern side of the Kerguelen Plateau, M1 was likely not limited by nutrients since we did not observe a DSM characterizing nutrient depleted waters and H4SiO4 concentrations were the highest (range: 6.6-6.9 μmol L -1 ) we have recorded. By contrast with M3 and M4, iron could have been supplied to this station by the transport of Fe-rich waters that circulate from the Kerguelen-Heard shelf basin into the deep basin east of the Kerguelen Islands [START_REF] Van Beek | Radium isotopes to investigate the water mass pathways on the Kerguelen Plateau (Southern Ocean)[END_REF][START_REF] Park | Polar Front around the Kerguelen Islands: An up-to-date determination and associated circulation of surface/subsurface waters[END_REF]. The ML was weakly stratified, which could have promoted nutrient diffusive input from the deep reservoir. Diatoms in this region were the most actively silicifying, yet they did not reach a significant Si biomass (BSi < 1 μmol L -1 ). Grazing could have prevented biomass accumulation, which is substantiated by elevated concentrations of pheopigments at this station (Irion et al., submitted).

All our observations agree as to the presence of a final phase of the seasonal phytoplankton development on all of the stations studied. However, differences are apparent in the factors controlling the emerging biogeochemical properties. The geographic location in relation to the plateau but also in relation to the PF (via the influence of the AASW) leads us to consider a modulation of these properties by the degree of limitation by iron and/or H4SiO4 and the intensity of grazing, both having direct effects on the structure of phytoplankton communities and their physiological state as will be discussed in the following sections.

4.2

The influence of diatom communities on C:N:Si elemental ratios [START_REF] Redfield | The biological control of chemical factors in the environment[END_REF] stated that "the environment not only determines the conditions under which life exists, but the organisms influence the conditions prevailing in their environment". Later on, [START_REF] Redfield | The influence of organisms on the composition of seawater[END_REF] provided the bases for reflecting upon the influence of the physiology of organisms on the composition of seawater, giving rise to the so-called 'Redfield-Ketchum-Richards (RKR) ratio' C/N/P = 106/16/1. Towards the end of the 90's the emphasis was shifted to the role of the environment on the stoichiometry of particulate organic matter and the influence of iron availability on diatom stoichiometry has been widely recognized and elevated at the paradigm level [START_REF] Hutchins | Iron-limited diatom growth and Si:N uptake ratios in a coastal upwelling regime[END_REF][START_REF] Takeda | Influence of iron availability on nutrient consumption ratio of diatoms in oceanic waters[END_REF][START_REF] Hoffmann | Effects of iron on the elemental stoichiometry during EIFEX and in the diatoms Fragilariopsis kerguelensis and Chaetoceros dichaeta[END_REF][START_REF] Baines | Causes and biogeochemical implications of regional differences in silicification of marine diatoms[END_REF]. Following this paradigm, diatoms growing in Fe-depleted HNLC waters should have higher (2-3 times) Si:C and Si:N ratios when compared to some empirical values (Si:C = 0.13, Si:N = 1.12) determined by [START_REF] Brzezinski | The Si:C:N ratio of marine diatoms: interspecific variability and the effect of some environmental variables[END_REF], which is supposed to indicate the development of more heavily silicified diatoms. Nevertheless, lower Si content relative to C and N under high iron conditions can be linked to either an increase in cellular organic matter or a decrease in cellular BSi concentration, highlighting the species-specific response to iron enrichment [START_REF] Takeda | Influence of iron availability on nutrient consumption ratio of diatoms in oceanic waters[END_REF][START_REF] Twining | Element stoichiometries of individual plankton cells collected during the Southern Ocean Iron Experiment (SOFeX)[END_REF][START_REF] Hoffmann | Effects of iron on the elemental stoichiometry during EIFEX and in the diatoms Fragilariopsis kerguelensis and Chaetoceros dichaeta[END_REF][START_REF] Baines | Elemental composition of equatorial Pacific diatoms exposed to additions of silicic acid and iron[END_REF].

Finally, a few studies pointed out that elemental ratios are primarily driven by taxonomic composition of resident diatom communities rather than a short term physiological response to iron addition [START_REF] Hoffmann | Different reactions of Southern Ocean phytoplankton size classes to iron fertilization[END_REF][START_REF] Baines | Causes and biogeochemical implications of regional differences in silicification of marine diatoms[END_REF][START_REF] Closset | Seasonal evolution of net and regenerated silica production around a natural Fe-fertilized area in the Southern Ocean estimated with Si isotopic approaches[END_REF][START_REF] Lasbleiz | Pigments, elemental composition (C, N, P, and Si), and stoichiometry of particulate matter in the naturally iron fertilized region of Kerguelen in the Southern Ocean[END_REF], again highlighting the primary role played by phytoplankton community structures in driving the biogeochemical cycles.

In our study, mixed layer Si:C and Si:N ratios in the bulk particulate matter (Figure 5) were close to the empirical values of [START_REF] Brzezinski | The Si:C:N ratio of marine diatoms: interspecific variability and the effect of some environmental variables[END_REF] at M1/M2/M3 (0.11-0.20 and 0.65-1.16, respectively) while they were significantly higher at the HNLC reference station M4 (0.52-0.59 and 3.36-3.66, respectively), which at first sight could indicate the presence of more heavily silicified diatoms. However, care should be taken when drawing conclusions about the diatom silicification degree based solely on particulate matter ratios. Indeed, these ratios result from the net effect of species-specific Si:C and Si:N composition [START_REF] Sackett | Taxon-specific responses of Southern Ocean diatoms to Fe enrichment revealed by synchrotron radiation FTIR microspectroscopy[END_REF], subsequent species-specific mortality pathways, diatom contribution to particulate organic matter and the presence of detrital particulate Si.

Further insight can be gained by looking at a Si:Cdiatom ratio (Figure 11) based on calculated diatom carbon biomass rather than the bulk POC measurements, which include both autotrophic and heterotrophic biomass as well as phytodetritus and faecal pellets. Since other organisms bearing siliceous skeleton like rhizarians or silicoflagellates were negligible compared with diatoms in our study, this ratio is more representative of diatom elemental Si:C ratios although it is still subject to overestimation because it includes detrital diatom fragments within BSi (average: 33 ± 7 % of broken/empty/crunched frustules without considering smaller unidentifiable fragments). In the ML, Si:Cdiatom ratios (> 0.4) were well above the empirical value of 0.13 [START_REF] Brzezinski | The Si:C:N ratio of marine diatoms: interspecific variability and the effect of some environmental variables[END_REF] at all stations. Such elevated ratios in the study area may be driven by the presence of detrital fragments of heavily silicified species (see supplementary Figure S6), such as those coming from Fragilariopsis cells at M1/M3/M4 and Thalassionema nitzschioides at M2. It is also plausible that Southern Ocean diatoms have Si:C ratios higher than Brzezinski's empirical value, which was obtained from temperate and tropical species cultivated under nutrient-replete conditions. Indeed, recent studies indicate that polar diatoms Si:C ratios could reach values up to 0.87 [START_REF] Lomas | Pumped Up by the Cold: Elemental Quotas and Stoichiometry of Cold-Water Diatoms[END_REF] and Southern Ocean species could be 11-fold more silicified than diatoms cultured in nutrient replete conditions [START_REF] Baines | Causes and biogeochemical implications of regional differences in silicification of marine diatoms[END_REF]. This is to be compared with the results of [START_REF] Quéguiner | Biogenic silica production rates and particulate organic matter distribution in the Atlantic sector of the Southern Ocean during austral spring 1992[END_REF] who reported Si:C production ratios ranging between 0.54-1.19 at the onset of a diatom bloom in the polar frontal zone of the Atlantic sector. During KEOPS, Mosseri et al. (2008) also measured higher Si:C and Si:NO3 elemental uptake ratios in HNLC waters in comparison with Fe-fertilized waters, which they suggested was attributable to the development of more heavily silicified diatoms.

The Si:Cdiatom ratios outside the plateau (average: 1.2-3.6) were much higher than over the plateau (average: 0.4-0.9). Based on PDMPO single-cell analysis (Figure 12), our data suggest that these differences are related to Southern Ocean diatom specificities. Within a given taxon (e.g. Fragilariopsis spp., Actinocyclus/Thalassiosira spp.), active diatoms were more heavily silicified outside the plateau at M1/M4, which supports the classical view that iron limitation leads to higher silicification degree and Si:C ratios [START_REF] Hutchins | Iron-limited diatom growth and Si:N uptake ratios in a coastal upwelling regime[END_REF][START_REF] Takeda | Influence of iron availability on nutrient consumption ratio of diatoms in oceanic waters[END_REF][START_REF] Hoffmann | Effects of iron on the elemental stoichiometry during EIFEX and in the diatoms Fragilariopsis kerguelensis and Chaetoceros dichaeta[END_REF][START_REF] Baines | Causes and biogeochemical implications of regional differences in silicification of marine diatoms[END_REF]. However, the elevated Si:Cdiatom ratios at M3 were also most likely driven by community differences, with the presence of smaller diatoms (e.g.

Fragilariopsis pseudonana) known to be heavily silicified and with low C content due to their small biovolume, a characteristic which would therefore be added to the physiological response to iron stress [START_REF] Marchetti | Diatom elemental and morphological changes in response to iron limitation: a brief review with potential paleoceanographic applications[END_REF]. Hence, regardless of the effects of iron on diatom physiology, the community composition itself strongly drives community silicification and elemental ratios. Heavily silicified Fragilariopsis frustules were numerous outside the plateau, while they were absent within the ML at M2 where we observed many weakly silicified Corethron inerme frustules. Discrepancies between elemental ratios in the bulk particulate matter and in diatoms can be attributed to the contribution of non-diatom components to total POC. As an illustration of this, the very low diatom contribution to total POC at M3 tended to attenuate the diatom signal, which resulted in low Si:C and low Si:N ratios in the bulk particulate matter, similar to those measured above the plateau, although diatom elemental ratios were much more elevated at M3. The combination of our measurements integrating biogeochemical analyses, fine microscopic observations of the state of diatoms, and their silicification activity measured by the PDMPO method then suggest that the higher diatom elemental ratios outside the plateau were likely related to the combined effects of both taxonomic composition and a direct physiological response to iron stress, with key taxa e.g. Fragilariopsis spp. driving ratios toward elevated values. The response to iron stress is therefore a complex process integrating both the selection of dominant species which are particularly silicified and an ecophysiological response via the slowing down of their cell cycles.

Ecological dynamics and the seasonal phytoplankton succession in the

Kerguelen area

Our statistical analysis revealed that depth was the main factor shaping diatom community structure, with one community roughly located within the upper mixed layer while a different one was systematically found within the pycnocline discontinuity (Figure 8). Spatial heterogeneity acted as a second factor separating on-versus off-plateau stations, hence, highlighting the role played by particular biogeochemical environments in structuring diatom communities [START_REF] Lasbleiz | Pigments, elemental composition (C, N, P, and Si), and stoichiometry of particulate matter in the naturally iron fertilized region of Kerguelen in the Southern Ocean[END_REF][START_REF] Lasbleiz | Composition of diatom communities and their contribution to plankton biomass in the naturally ironfertilized region of Kerguelen in the Southern Ocean[END_REF].

Above the plateau, the mixed layer community was characterized by the presence of the large centric species Corethron inerme and Guinardia cylindrus (1 st visit only), in association with Thalassionema nitzschioides, Cylindrotheca closterium, and the centric genera Actinocyclus/Thalassiosira spp. Despite depleted H4SiO4 concentrations, C. inerme was actively growing as evidenced by an increase of its numerical abundance, its silicification activity, and its contribution to diatom C biomass. Interestingly, this species has a small surface to volume ratio [START_REF] Cornet-Barthaux | Biovolume and biomass estimates of key diatoms in the Southern Ocean[END_REF], which would make it at first sight poorly adapted to low H4SiO4 environment. Based on microcosm experiment, [START_REF] Brzezinski | Co-limitation of diatoms by iron and silicic acid in the equatorial Pacific[END_REF] showed that Fe and Si were associated with two different biogeochemical pathways with high dissolved Fe increasing growth rates and organic matter production, especially of large diatoms, while H4SiO4 availability would regulate diatom Si content. Our observations support the hypothesis that the strong decoupling between organic matter production and silicification in diatoms [START_REF] Paasche | Silicon and the ecology of marine plankton diatoms. I. Thalassiosira pseudonana (Cyclotella nana) grown in a chemostat with silicate as limiting nutrient[END_REF][START_REF] Brzezinski | Silicon availability and cell-cycle progression in marine diatoms[END_REF][START_REF] Mcnair | Diatom populations in an upwelling environment decrease silica content to avoid growth limitation[END_REF] may allow the development of large diatoms above the iron fertilized plateau, despite low H4SiO4 concentrations. By contrast, large and heavily silicified Eucampia antarctica cells were observed within the pycnocline gradient in addition to small Chaetoceros spp. Hyalochaete vegetative stages and CRS; with the former species overwhelming diatom C biomass and likely driving the formation of the DSM. During KEOPS, at the end of the chlorophyll seasonal peak, a diatom community shift was observed, from one dominated by Chaetoceros spp.

Hyalochaete to a new one dominated by E. antarctica in the upper 100 m (Armand et al., 2008b). Hence, put together, these data suggest a late seasonal stage marked by the replacement of E. antarctica by Corethron inerme in the upper mixed layer, E. antarctica persisting below the MLD until late February before progressively disappearing. By early March, at the very end of the growing season, small Chaetoceros spp. Hyalochaete and their CRS became the main contributors to both abundances and carbon biomass in the deep layer. CRS were previously observed below the pycnocline in spring [START_REF] Lasbleiz | Composition of diatom communities and their contribution to plankton biomass in the naturally ironfertilized region of Kerguelen in the Southern Ocean[END_REF] and midsummer (Armand et al., 2008b), which supports the hypothesis of their year-round presence, enabling the species to bloom first with the onset of favorable conditions in spring [START_REF] Lasbleiz | Composition of diatom communities and their contribution to plankton biomass in the naturally ironfertilized region of Kerguelen in the Southern Ocean[END_REF]. Resting spore formation is a strategy known to overcome unfavorable conditions such as low light levels [START_REF] Mcquoid | Diatom resting stages[END_REF], macronutrients (Kuwata andTakahashi, 1990, 1999;[START_REF] Oku | Resting spore formation and phosphorus composition of the marine diatom Chaetoceros pseudocurvisetus under various nutrient conditions[END_REF], or iron limitation [START_REF] Sugie | Resting spore formation in the marine diatom Thalassiosira nordenskioeldii under iron-and nitrogen-limited conditions[END_REF]. A recent study also showed that spores can be produced in nutrient-replete environments when a population reaches a threshold density and starts a process of self-removal from the water column [START_REF] Pelusi | Densitydependent mechanisms regulate spore formation in the diatom Chaetoceros socialis[END_REF]. In our study, H4SiO4 depletion characterizing end-summer conditions is the most likely factor involved in triggering resting spore formation.

Outside the fertilized plateau, the remote diatom communities of the ML were very close with a dominance by Fragilariopsis species, Cylindrotheca closterium, and centric species belonging to the genera Actinocyclus and Thalassiosira. A common trait between off-plateau stations was the presence of Fragilariopsis kerguelensis below the ML, but its relative contribution was higher at M3 and M4, which supports that it is considered as an indicator of HNLC systems (Armand et al., 2008b;[START_REF] Lasbleiz | Composition of diatom communities and their contribution to plankton biomass in the naturally ironfertilized region of Kerguelen in the Southern Ocean[END_REF]. By contrast, within the pycnocline gradient, the two western diatom communities clustered separately from the eastern off-plateau community. The main differences were related to the dominance of Chaetoceros atlanticus on the western side, while it was virtually absent on the eastern side and replaced by CRS. The presence of the latter, known to characterize iron-fertilized systems [START_REF] Abelmann | Extensive phytoplankton blooms in the Atlantic sector of the glacial Southern Ocean[END_REF]Armand et al., 2008b;[START_REF] Lasbleiz | Composition of diatom communities and their contribution to plankton biomass in the naturally ironfertilized region of Kerguelen in the Southern Ocean[END_REF]Rembauville et al., 2016), tends to confirm that station M1 was at least partly influenced by the nearby plateau.

Three hypotheses can explain the presence of distinct diatom communities located either within the upper mixed layer or the pycnocline gradient: (1) the development of an active community supported by a deep nutricline, with diatoms adapted to low light levels [START_REF] Crombet | Deep silicon maxima in the stratified oligotrophic Mediterranean Sea[END_REF][START_REF] Kemp | High diatom production and export in stratified waters -A potential negative feedback to global warming[END_REF][START_REF] Quéguiner | Iron fertilization and the structure of planktonic communities in high nutrient regions of the Southern Ocean[END_REF], (2) the differential sinking and accumulation of diatoms from the overlying mixed layer [START_REF] Quéguiner | Biogeochemical dynamics and the silicon cycle in the Atlantic sector of the Southern Ocean during austral spring 1992[END_REF][START_REF] Uitz | A phytoplankton class-specific primary production model applied to the Kerguelen Islands region (Southern Ocean)[END_REF], or (3) the selective grazing/parasitism. In a study similar to ours, investigating the link between microplankton community and particulate matter stoichiometry in the Kerguelen-Crozet region, Rembauville et al. (2016) suggested that high Si(OH)4 diffusive fluxes were likely to sustain silicification in the transition layer (i.e. defined as the interface between the stratified ocean interior and the highly turbulent surface mixed layer). Our incubation experiments with the fluorescent dye PDMPO showed on the contrary that silicification activity within the pycnocline gradient (≥ 100 m) was almost null, suggesting that deeper cells were not produced in situ, but rather originated from the overlying mixed layer. In particular, Eucampia antarctica cells and CRS observed above the plateau were not silicifying and likely corresponded to a remnant population that accumulated at the base of the mixed layer prior to the cruise. The relatively low percentage of crunched frustules (5 ± 3 %) suggests that selective grazing had little impact on shaping communities, while parasites could have controlled population dynamics prior to the cruise since elevated relative abundances of free Syndiniales spores was suggested by environmental sequencing [START_REF] Sassenhagen | Protist Interactions and Community Structure During Early Autumn in the Kerguelen Region (Southern Ocean)[END_REF].

Hence, we argue that the density gradient acted as a strong physical barrier to sinking with variable impacts on species. Light species (e.g. Cylindrotheca closterium, Corethron inerme, small Fragilariopsis spp.) were efficiently recycled in the ML and could not bypass the density gradient, whereas denser ones (e.g. Fragilariopsis kerguelensis, Eucampia antarctica, Chaetoceros atlanticus, CRS etc.) rapidly settled out of the mixed layer and accumulated in the pycnocline, leading to a vertical structure of diatom communities.

Differential impact of diatom species on Si/C export

Past studies in naturally and artificially Fe-fertilized systems have shown the central role played by phytoplankton community structure in carbon vs. silica export, advocating the need for a species-centered approach to understand the fine dynamics of biogeochemical fluxes [START_REF] Smetacek | The role of grazing in structuring Southern Ocean pelagic ecosystems and biogeochemical cycles[END_REF][START_REF] Salter | Diatom resting spore ecology drives enhanced carbon export from a naturally iron-fertilized bloom in the Southern Ocean[END_REF][START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF][START_REF] Quéguiner | Iron fertilization and the structure of planktonic communities in high nutrient regions of the Southern Ocean[END_REF]Rembauville et al., 2015bRembauville et al., , 2016)). While [START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF] coined the terms "Si sinkers" and "C sinkers" to characterize the roles of individual species in the coupling/uncoupling of Si and C biogeochemical cycles, [START_REF] Quéguiner | Iron fertilization and the structure of planktonic communities in high nutrient regions of the Southern Ocean[END_REF] distinguished two types of more or less silicified diatom communities on the basis of their different ecological niches marked by a different seasonality.

In the present study, heavily silicified Fragilariopsis kerguelensis were mostly observed as empty frustules below the mixed layer (D:I ratio >> 2), which explains their low contribution to C biomass despite their high numerical abundances all over the study area. This species was identified as a strong Si sinker during EIFEX artificial fertilization experiment by [START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF] who reported a steady and downward flux of empty F. kerguelensis frustules, inside and outside the Fe-fertilized patch, associated with low-level, quasi-constant mortality. Indeed, F. kerguelensis frustules are thick and heavily silicified (Si:N ratios > 4:1) [START_REF] Hamm | Architecture and material properties of diatom shells provide effective mechanical protection[END_REF][START_REF] Hoffmann | Effects of iron on the elemental stoichiometry during EIFEX and in the diatoms Fragilariopsis kerguelensis and Chaetoceros dichaeta[END_REF][START_REF] Wilken | Diatom frustules show increased mechanical strength and altered valve morphology under iron limitation[END_REF], which has led to the hypothesis of a coevolution with copepods to withstand grazing pressure (the "arms race" - [START_REF] Smetacek | The role of grazing in structuring Southern Ocean pelagic ecosystems and biogeochemical cycles[END_REF]. Unexpectedly and contrary to the conceptual approaches of [START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF] and [START_REF] Quéguiner | Iron fertilization and the structure of planktonic communities in high nutrient regions of the Southern Ocean[END_REF], we observed many crunched Fragilariopsis kerguelensis frustules (average per station: 9-20 %) as well as copepod faecal pellets containing fragmented frustules (see supplementary Figure S9) which indicates that Fragilariopsis cells had been processed by copepod mandibles. In addition, we also frequently observed whole F. kerguelensis cells inside phaeodarians belonging to the genus Protocystis spp. (e.g. P. swirei, P. harstoni, P. tridens; see supplementary Figure S9), which is consistent with the results of [START_REF] González | Distribution and abundance of minipellets around the Antarctic peninsula. Implications for protistan feeding behaviour[END_REF]

who found

Fragilariopsis as the most frequent diatom genus present in phaeodarian minipellets in spring in the Weddell-Scotia confluence. Selective grazing on F. kerguelensis likely increased the number of detrital cells, enhancing the sinking of Si relative to C. The preferential exportation of Si over other nutrients by thick-shelled empty frustules then strongly fuels the Si pump [START_REF] Dugdale | The role of a silicate pump in driving new production[END_REF], explaining the extremely high Si:C and Si:N ratios we report below the MLD. Our hypothesis is supported by a significant correlation between BSi:POC and D:I ratios (Spearman rank correlation: n = 64, ρ = 0.67, p < 0.05), and the much higher residual NO3 -(> 21 μmol L -1 ) relative to H4SiO4 (range: 1.7-6.9 μmol L -1 ) concentrations in the surface layer.

Outside the plateau, the Si pump was obviously more efficient due to (1) the higher proportion of F. kerguelensis below the ML, and (2) the physiological response to iron stress further increasing their silicification degree. Our results are similar to those from Rembauville et al. (2016) who showed that Southern Ocean transition layers (i.e. the pycnocline gradient) are key areas of C and Si decoupling where empty diatom frustules mainly from F. kerguelensis and Pseudo-nitzschia accumulate. During austral summer, at the peak of the plateau bloom development, [START_REF] Mosseri | Impact of iron on silicon utilization by diatoms in the Southern Ocean: A case study of Si/N cycle decoupling in a naturally iron-enriched area[END_REF] also demonstrated that both the Fe-fertilized plateau and surrounding Fe-depleted waters behaved as Si pumps but based upon different mechanisms with contrasted intensities. In the HNLC area, the Si pump was mainly driven by the development of more heavily silicified diatoms, whereas above the Kerguelen Plateau, the Si pump was mainly driven by the preferential degradation of soft organic matter over BSi dissolution. Apart from F. kerguelensis, several taxa such as Actinocyclus/Thalassiosira spp., Pseudo-nitzschia spp., and Chaetoceros atlanticus displayed D:I ratios > 2, although their influence on the Si pump was probably weaker. By contrast, Chaetoceros spp. vegetative stages and CRS displayed D:I ratios < 0.5, indicating that many cells were still intact below the mixed layer. Heavily silicified CRS can contain up to 10 times more carbon than the vegetative forms [START_REF] Kuwata | Ecophysiological characterization of two life forms, resting spores and resting cells, of a marine planktonic diatom, Chaetoceros pseudocurvisetus, formed under nutrient depletion[END_REF], have no vacuole and are rich in lipids and carbohydrates [START_REF] Doucette | Thalassiosira antarctica: vegetative and resting stage chemical composition of an ice-related marine diatom[END_REF][START_REF] Kuwata | Ecophysiological characterization of two life forms, resting spores and resting cells, of a marine planktonic diatom, Chaetoceros pseudocurvisetus, formed under nutrient depletion[END_REF], which make them efficient vectors for C and Si export.

Above the plateau, CRS were abundant below the ML (Figure 8) and were associated with a quasi constant C biomass between 100 and 200 m (Figure 6), suggesting they efficiently bypassed the pycnocline discontinuity and carried C to deeper layers. These results are in line with sediment trap and surface-sediment core studies, which have reported CRS as one of the main vector of carbon export in naturally iron-fertilized systems, in particular over the Kerguelen Plateau and downstream of South Georgia where they can contribute to 42-52 % of the annual POC fluxes (Armand et al., 2008a;Rembauville et al., 2015bRembauville et al., , 2016[START_REF] Rembauville | The role of diatom resting spores in pelagic-benthic coupling in the Southern Ocean[END_REF]. In our study, Eucampia antarctica D:I ratio fell between the thresholds >0.5 and <2 at M2-1, meaning the species was almost equally represented by intact and detrital cells below the ML.

Nevertheless, it was likely an important vector of carbon export, since most of the diatoms sampled by the Bottle-Net device belonged to this species (47 % within the 100-450 m layer; Leblanc et al., in prep). During the naturally iron-fertilized Crozet bloom (CROZEX project), a significant positive relationship was reported between POC fluxes and E. antarctica by sediment traps deployed at 2000 m, highlighting the importance of this species for carbon export [START_REF] Salter | Diatom resting spore ecology drives enhanced carbon export from a naturally iron-fertilized bloom in the Southern Ocean[END_REF]. Hence, our data show that some species were able to efficiently bypass the pycnocline and to carry C/Si to the ocean twilight zone. Through their physiological state, life stages, and selective grazing by micro/mesozooplankton, they can influence the efficiency of the export, as well as the properties of the sinking particles.

Conclusion

Our ability to predict the future role of the ocean in modulating the global C cycle and ultimately controlling the evolution of global climate depends on our understanding of the functioning of the biological carbon pump. The latter still presents major gaps, especially in the knowledge of the relationships that exist between the oceanic biodiversity and the efficiency of carbon transfer to deep layers, which is partly reflected by the relative utilization of associated elements, in particular N and Si in the Southern Ocean. During the MOBYDICK expedition, south of the Kerguelen Islands, we studied the processes linking diatom diversity with biogeochemistry, at several contrasted sites differently impacted by iron availability.

Outside the iron-fertilized plateau, iron stress led to the selection of more heavily silicified taxa while a direct response to iron stress further increased the silicification degree of these selected diatoms. This affected the properties of the bulk particulate matter, which was characterized by higher elemental Si:C and Si:N ratios when compared to the plateau. Upper mixed layer diatom communities were clearly different between on-and off-plateau stations.

Above the plateau, large and weakly silicified Corethron inerme were able to grow despite low silicic acid concentrations (< 2.7 μmol L -1 ) characteristic of the end stage of phytoplankton development, accompanied with Actinocyclus/Thalassiosira spp. and the pennate species Thalassionema nitzschioides and Cylindrotheca closterium. In HNLC waters, the diatom community was different and mainly composed of Fragilariopsis species, Cylindrotheca closterium, and the centric genera Actinocyclus/Thalassiosira spp. Beside horizontal spatial heterogeneity, depth turned out as an important parameter shaping diatom communities, since a deep and inactive community associated with the pycnocline gradient was systematically observed, carrying organic matter (C, N) and biogenic minerals (Si) below the mixed layer. In particular, outside the plateau, the heavily silicified species Fragilariopsis kerguelensis carried mostly Si since it was largely found as detrital frustules, this being partly caused by a strong grazing pressure by micro-and mesozooplankton. Conversely, above the plateau, many Eucampia antarctica and Chaetoceros spp. cells (resting spores and vegetative stages) were still intact below the mixed layer, hence carrying both Si and C to the deeper layers.

Our study highlights the importance of species-specific properties (e.g. morphology, silicification degree, carbon content, etc.), along with their physiological state (here related to iron availability), their life stages, and entire ecosystem trophodynamics including selective grazing in determining the efficiency of the export and the biogeochemical properties of exported particles, as emphasized by [START_REF] Tréguer | Influence of diatom diversity on the ocean biological carbon pump[END_REF]. In the context of a changing Southern Ocean (e.g. greening, acidification, etc.), we advocate for the need to develop more studies following a species centered approach, as well as assessing trophic interactions through trophodynamics (e.g. predation, parasitism, saprophytism, etc.) in order to finely resolved biogeochemical fluxes and their control mechanisms. This should help to extract synthetic descriptors to better constrain more complex biogeochemical models aimed at simulating more closely the biological pump of carbon under the perspective of global climate change. The black dashed lines represent the empirical values of Si:C (0.13), and Si:N (1.12) for nutrient-replete diatoms reported by [START_REF] Brzezinski | The Si:C:N ratio of marine diatoms: interspecific variability and the effect of some environmental variables[END_REF], as well as the typical value of C:N (6.6) reported by [START_REF] Redfield | The influence of organisms on the composition of seawater[END_REF].

Figures

Tables

3. 2

 2 Elemental composition (Si, N, C) and stoichiometry of particulate matter 3.2.1 Vertical distribution of particulate matter BSi, POC, and PON vertical distributions are presented in Figure 3. Stations M2, M3, and M4 were characterized by a marked deep silica maximum (DSM) at the first visit that progressively diminished throughout the survey, whereas no DSM was observed at M1. At M2, the DSM was located at 100 m where BSi reached 1.7 μmol L -1 at the first visit, decreasing to 0.7 μmol L -1 at the last visit (18 days later), while an increasing trend was observed in the upper 75 m. At M3, BSi concentrations reached 2.0 μmol L -1 at the DSM located at 125 m, which deepened down to 150-175 m at the next visit (15 days later). The highest siliceous biomass was measured at station M4, located upstream of the Fe-fertilized plateau, where BSi reached 3.2 μmol L -1 at the DSM (75 m) during the first visit. At M1, Si biomass decreased regularly with depth, and never exceeded 1.0 μmol L -1 . Throughout the study area, BSi concentrations were still elevated down to 350-500 m, varying between 0.3 and 0.8 μmol L -1 , with the highest values measured at M2. POC and PON vertical distribution differed from BSi, with a decreasing trend with depth.

  Both a cluster analysis and a non-metric multidimensional scaling analysis (Figure8.a and b) performed on total abundances at all sites and depths revealed a strong vertical and spatial structure of diatom communities. The first clustering factor is based on depth, clearly separating samples collected between 0 and 75/100 m (depending on the station) from samples collected below, between 100/125 and 250 m. Secondly, within surface samples, M2 is isolated from the other three stations which cluster together, while at depth two different clusters (M1/M2 vs M3/M4) are identified. The stress value of the NMDS (0.11) indicates a fair model-data fit. The NMDS plot confirms that the four clusters are different from each other in terms of their taxonomic composition, with the first axis separating diatom communities according to their depth, while the second axis highlights the spatial heterogeneity. Taxon-specific contribution to total diatom abundances is presented in Figure 8.c. In the upper 75 m, diatom community structure above the plateau reference station M2 was clearly distinct from that observed at the other sites. At the first visit, diatoms were dominated by Actinocyclus/Thalassiosira spp. (29 ± 3 %; n=4), Thalassionema nitzschioides (17 ± 3 %), Guinardia cylindrus (13 ± 1 %), Corethron spp. (only C. inerme; 12 ± 1 %), and Cylindrotheca closterium (7 ± 4 %). A temporal evolution was observed at this station, with the third visit marked by the increasing contribution of C. inerme (27 ± 3 %), T. nitzschioides (28 ± 5 %) and Cylindrotheca closterium (15 ± 3 %), while Actinocyclus/Thalassiosira spp. and G. cylindrus decreased. As shown by the cluster analysis, diatom communities in the upper 75-100 m layer were similar between M1/M3/M4 and were dominated by a mix of Cylindrotheca closterium (23 ± 14 %), Actinocyclus/Thalassiosira spp. (20 ± 5 %), and Fragilariopsis kerguelensis (16 ± 10 %). Below the MLD, the diatom community at M2-1 was characterized by the presence of large and heavily silicified cells of Eucampia antarctica (13 ± 6 %; n=4) which almost disappeared at the following visits. Excluding this species, deep diatom communities at M2/M1 were similar and dominated by a mix of CRS (27 ± 10 %; n=17), Pseudo-nitzschia spp. (15 ± 9 %),

  spp. and Chaetoceros spp. were observed deeper, contributing respectively to 21 % at 60 m and 10 % at 90 m. At the reference HNLC station M4, total PDMPO fluorescence was close to M1, reaching 21.0 x 10 6 AU L -1 at M4-1 (29 m). This station was also the most diverse in terms of silicifying community. Indeed, throughout the two visits (n = 8), many different taxa were producing new frustules, including Fragilariopsis spp. (30 ± 12 %), Actinocyclus/Thalassiosira spp. (21 ± 10 %), Proboscia/Rhizosolenia spp. (16 ± 13 %), Corethron pennatum (9 ± 11 %), Chaetoceros dichaeta (5 ± 5 %), C. atlanticus (4 ± 4 %), and Membraneis spp. (4 ± 6 %). Based on McNair et al. (

Figure 1 .

 1 Figure 1. Location of the Kerguelen Plateau in the Indian sector of the Southern Ocean and detailed map of the satellite-derived surface chlorophyll a concentration (MODIS level 3 product) averaged over March 2018. The four stations sampled during MOBYDICK are indicated. Contour lines represent the bathymetry (m). The dotted line represents the position of the Polar Front (PF) according to Pauthenet et al. (2018).

Figure 2 .

 2 Figure 2. Average profiles of Chl a (a) and nutrients (b-e) from several visits at the four stations. 999

Figure 4 .

 4 Figure 4. Integrated BSi, POC, PON, and Chl a concentrations between 0 and 200 m at the four stations.

Figure 5 .

 5 Figure 5. BSi:POC, BSi:PON, and POC:PON molar ratios based on the bulk of particulate matter between 0-MLD (green dots) and MLD-200 m (blue dots) at the four stations.

Figure 6 .

 6 Figure 6. Absolute diatom C biomass and its contribution to POC. Absolute diatom carbon biomass (green area) and other sources of POC (brown area). The sum of the green and brown areas gives the total POC. Data are expressed in μg C L -1 . Blue dots indicate the diatom contribution to total POC (in percent). The dashed line indicates the location of the mixed layer depth (MLD).

Figure 7 .

 7 Figure 7. Vertical distribution of intact and detrital (i.e. empty, broken, and crunched) frustule abundances. The dashed line indicates the location of the MLD.

Figure 8 .

 8 Figure 8. Community structure and taxon-specific contribution to total diatom abundances. (a) Cluster analysis performed on log (x+1) abundances. The dendrogram displays the similarity relationship among samples in terms of their taxonomic composition. (b) Nonmetric multidimensional scaling (NMDS) ordination plot. Stations are coloured according to the cluster they belong to. (c) Taxon-specific contribution to total diatom abundances. Total abundances include both intact and detrital cells. Taxa are mutually exclusive, e.g. Chaetoceros spp. include all Chaetoceros species, excluding C. atlanticus and CRS. The dashed line indicates the location of the MLD.

Figure 9 .

 9 Figure 9. Taxon-specific contribution to diatom carbon biomass. The dashed line indicates the location of the MLD.

Figure 10 .

 10 Figure 10. Community silicification activity and taxon-specific contribution to biogenic silica deposition, based on PDMPO staining. White dots correspond to the total PDMPO fluorescence (AU x 10 6 L -1 ), i.e. the sum of each taxa absolute fluorescence. Collection depths were adjusted to correspond to simulated irradiance depths, which were equal to 50 %, 25 %, 4 %, and 1 % of the surface PAR. Samples collected deeper and incubated in the dark did not fluoresce and therefore are not presented. Colour code and taxa presented are intentionally different from Figure 8 and 9 to show taxa

Figure 11 .

 11 Figure 11. Vertical profiles of Si:Cdiatom ratios between 0-MLD (left panel) and MLD-200 m (right panel). This ratio is based on calculated diatom carbon biomass rather than the bulk POC measurements.

Figure 12 .

 12 Figure 12. Silicification degree estimated through PDMPO staining. Taxa presented are the most actively silicifying ones. The bottom and top edges of the box indicate the 25th and 75th percentiles, respectively. The line inside the box and the asterisk represent the median and the mean, respectively, and were calculated based on the data

Figure S1 .Figure S2 .

 S1S2 Figure S1. Selection of diatom images showing crunched frustules, which we considered as indicative of mesozooplankton grazing. (A) Asteromphalus hookeri, (B) Fragilariopsis kerguelensis, (C) Thalassiosira lentiginosa, (D) Azpeitia tabularis.

Figure S3 .

 S3 Figure S3. Integrated BSi, POC, PON, and Chl a concentrations between 200 and 500 m at the four stations.

Figure S9 .

 S9 Figure S9. Selection of images evidencing grazing of the heavily silicified species Fagilariopsis kerguelensis by micro-and mesozooplankton, a diatom species traditionally considered as resistant to grazing. (A-D) Fecal pellets containing recognizable F. kerguelensis remains. Protocystis harstoni feeding on a whole chain (E) or solitary frustules (F) of F. kerguelensis. Black arrows indicate F. kerguelensis frustules.

Table 1 .

 1 Metadata and hydrographical conditions at stations sampled during the MOBYDICK

	expedition.
	Station	Latitude/Longitude	Date	Bottom depth (m)	MLD (m)	Ze 1% (m)
	M4-1 M4-2	52.60°S, 67.20°E	01/03/2018 12/03/2018	4 731	69 96	95 101
	M3-1 M3-3	50.68°S, 68.06°E	04/03/2018 19/03/2018	1730	74 96	93 105
	M2-1		26/02/2018		79	64
	M2-2	50.62°S, 72.00°E	06/03/2018	520	73	61
	M2-3		16/03/2018		80	58
	M1	49.85°S, 74.90°E	09/03/2018	2 723	63

Table 2 .

 2 Average detrital:intact frustules ratios (D:I) below the MLD. Number in brackets correspond to the standard deviations. "Ab" corresponds to the relative abundance averaged below the MLD. Bold numbers represent numerically dominant taxa below the mixed layer depth, i.e. with a relative contribution to total diatom abundances higher than 10 %.

	Species and genera grouping	M4-1 D:I	Ab	M4-2 D:I	Ab	M3-1 D:I	Ab	M3-3 D:I	Ab	M2-1 D:I	Ab	M2-2 D:I	Ab	M2-3 D:I	Ab	M1 D:I	Ab
	Fragilariopsis kerguelensis	58.6 (71.3)	41	51.1 (35.3)	39	64.5 (55.7)	27	221.8 (57.5)	36	63.6 (79.5)	4	78.6 (84.7)	16	113.4 (81.7)	15 134.2 (126.8) 17
	Chaetoceros resting spore	0.3 (0.3)	0	0.3 (0.3)	1	1.1 (1.5)	3	1.4 (0.3)	4	0.5 (0.1)	20	0.4 (0)	25	0.7 (0.1)	25	7.5 (1.4)	32
	Pseudo-nitzschia spp.	34.2 (44.2)	17	74.6 (97.8)	16	6.2 (4.5)	8	9.4 (3.2)	8	13.3 (3.9)	28	11.1 (3.7)	13	12.7 (2.7)	11	20.4 (13.3)
	Actinocyclus/Thalassiosira spp.	24.4 (28.3)	13	26.8 (18.2)	13	20.4 (14.6)	10	28 (8.5)	8	10.2 (8.1)	5	50.1 (72.4)	11	29.4 (19.9)	13	24.2 (17.7)	13
	Chaetoceros atlanticus	2.2 (2.6)	10	3.4 (3.6)	19	3.3 (0.3)	31	8.7 (2.7)	29	-(-)	0	1.2 (0.7)	0	2 (1.7)	0	1 (0.5)
	Chaetoceros spp.	1.9 (2.2)	3	0.5 (0.3)	2	0.8 (0.6)	2	0.7 (0.2)	3	0.6 (0.1)	14	0.4 (0.1)	17	0.5 (0.1)	15	1.1 (0.7)
	Pennate spp.	18.7 (25)	5	67 (97.3)	4	23.9 (30.6)	12	17.3 (16.5)	4	24.9 (24.9)	4	30.8 (23)	5	31 (28.3)	6	5.7 (5.3)
	Thalassionema nitzschioides	1.2 (0.7)	0	2.3 (1.5)	0	2.6 (1.8)	1	6.4 (3.5)	2	2 (0.9)	6	4.2 (1.2)	5	6.6 (1.2)	6	11 (8.2)
	Eucampia antarctica	-(-)	0	-(-)	0	0.1 (0.1)	0	0.4 (0.2)	0	1.5 (1.6)	13	2.2 (2.3)	2	1.2 (0.9)	1	17.1 (24.1)
	Fragilariopsis spp.	12.9 (20)	4	18.4 (8)	1	11 (13.4)	3	62 (-)	2	28.1 (24.4)	1	51.6 (25.9)	2	59.8 (56.9)	3	8.7 (6.8)
	Corethron inerme	2.6 (3.7)	0	1.3 (0.3)	0	1.3 (1.1)	0	1 (-)	0	-(-)	2	16.8 (-)	1	28.2 (22.3)	1	3.7 (-)
	Cylindrotheca closterium	0.9 (1.4)	1	1 (1.4)	0	0.4 (0.5)	2	0.1 (0.1)	0	-(-)	0	0.1 (0.2)	0	0 (0)	0	0 (0)
	Proboscia/Rhizosolenia spp.	2.6 (2.4)	3	4.1 (2.1)	2	2.8 (2.3)	1	3.9 (0.9)	1	5.4 (2.7)	0	5.6 (0.9)	1	5.4 (2.8)	1	9.6 (7.2)
	Centric spp.	11.8 (14.5)	1	12.3 (14.5)	1	11.6 (17.1)	1	12 (10.9)	1	15.8 (2.1)	1	19.7 (9.5)	1	13.6 (16.4)	2	13.6 (10.5)
	Guinardia cylindrus	6 (7.8)	0	1.7 (1.2)	0	3 (2.8)	0	3 (2.8)	0	8.3 (7.4)	1	6.9 (4.7)	1	6.6 (5.1)	0	1 (1.4)
	Odontella weissflogii	-(-)	0	-(-)	0	-(-)	0	-(-)	0	3.2 (3.7)	1	1.6 (1.3)	0	0.7 (0.2)	0	0.8 (0.9)

Table S1 .

 S1 Range and mean (in parentheses when given together) values of linear measurements, biovolume, and carbon biomass of major diatom taxa observed during MOBYDICK. N = number of observations; D = diameter; AA = apical axis; TA = transapical axis; PA = pervalvar axis; CV = coefficient of variation. Shapes were used to estimate volume from linear dimensions: RB = rectangular box; C = cylinder; EC = elliptic cylinder; EC + 4H = elliptic cylinder + 4 horns; S = spindle; CU = cuboid; PP = prism on parallelogram base.

	Rhizosolenia chunii	724	3.8-47.2 (16)			9.6-171.9 (34.8)	C	12393	175	574
	Rhizosolenia polydactyla f. polydactyla	37	21.4-47.1 (38)			286.2-1433 (632.5)	C	754488		13032
	Thalassiosira lentiginosa	69	38.4-90.2 (59.6)			4.8-10.5 (7.4)	C	25227		985
	Species and genera Cylindrotheca closterium (robust)	N 1027	D	AA 5.7-41.7 (12.6)	TA 0.9-6 (2.3)	PA	Shape S	Biovolume 200	CV 158	Cell carbon 25
	groupings Cylindrotheca closterium (thin)	302	(μm)	(μm) 4.6-12.9 (6.7)	(μm) 1.4-7.3 (2.2)	(μm)	S	(μm 3 ) 44	(%)	(pg cell -1 ) 8
	Actinocyclus octonarius Fragilariopsis kerguelensis	32 1300	41.4-122.7 (57.3)	10.8-92.9 (32.2)	6.3-15 (10.3)	3.7-15.3 (7.2)	C EC	91364 2111	132	2619 149
	Actinocyclus / Thalassiosira spp. Fragilariopsis pseudonana	1332 417	7.1-49.6 (14.3)	1.5-13.5 (6.5)		4-23.4 (9.3) 1.4-9.4 (3)	C EC	2865 39	237	189 7
	Azpeitia tabularis Fragilariopsis rhombica	25 68	34.4-82.1 (48.3)	10.4-35.5 (15.9)	6.3-12.2 (9.3)		C EC	69309 614		2123 58
	Chaetoceros atlanticum (chain) Fragilariopsis rhombica/separanda	148 44		7.1-29.9 (17.4) 15.7-44.6 (24)	6.3-17.4 (8.3)	6.5-27.7 (16.1)	EC EC	2874 1227	102	189 99
	Chaetoceros atlanticum, (sigmoïd) Membraneis sp.	942 86		7-31.5 (15.2) 38.1-310.4 (110.4)	7.5-12.2 (10.2) 6.5-100.8 (36.6)	4.9-35.2 (10.9)	EC EC	1489 87413	125	115 2533
	Chaetoceros atlanticum (bulbosum) Pseudo-nitzschia cf lineola	234 666		8.3-24.1 (15.3) 22.1-150.7 (88.7)	9.8-19.2 (13.1) 2.1-6.4 (3.8)	6.8-19.3 (12) 2.2-5 (3.9)	EC PP	1933 481		140 49
	Chaetoceros castracanaei Pseudo-nitzschia cf heimii	71 172		8.5-16.6 (12.8) 63.6-150.8 (90.2)	6.5-9.8 (8.6) 5-9.8 (6.7)	5.4-20.2 (11.6) 4.9-10 (6.4)	EC PP	1098 1329		91 105
	Chaetoceros criophilus Thalassionema nitzschioides	19 1096		9.3-61.4 (16.2) 9.5-131.3 (26.2)	2.1-9.9 (4.1)	9.4-74.5 (16.9) 1.7-11.6 (4.2)	EC CU	10378 695	338 217	501 64
	Chaetoceros dichaeta Thalassiothrix sp.	100 19		14.1-45.8 (23.5) 744.4-1505.9 (1106.8)	3.4-18.7 (9.8)	9.4-30.7 (15.6)	EC CU	4987 139005	103	287 3603
	Chaetoceros cf neglectus	130		2.6-12.3 (5.8)		2.1-9.7 (4.4)	EC	92		14
	Chaetoceros resting spores	100		4.2-14.7 (6.4)		3.3-7.4 (4.6)	EC	94		44
	Chaetoceros sp. (small Hyalochaete)	93		5.4-18.3 (11.8)	4.8-8.9 (7)		EC	632		60
	Corethron inerme	1571	12.9-43 (30.2)			33.3-197.4 (98.5)	C	62874		1972
	Corethron pennatum (syn: C. criophilum)	67	6-75.9 (35.9)			16.4-434.5 (166.5)	C	316627	136	6736
	Dactyliosolen antarctica	79	14.9-71.7 (37.2)			34.4-215.6 (119.1)	C	148697		3793
	Eucampia antarctica (typical)	263		21.1-62.9 (51.2)	13.9-20.4 (17.3)	17.8-41 (29.2)	EC	20384		838
	Eucampia antarctica (H shape)	216		15.6-52.8 (22.4)	14.4-19.3 (16.3)	16.8-51.7 (25.7)	EC +4H	8702		439
	Guinardia cylindrus	316	6.6-29.1 (15.9)			16.2-122.9 (54.7)	C	12429		575
	Leptocylindrus danicus	51	5.1-13.9 (8.4)			24.8-160.8 (59.5)	C	4083	118	247
	Odontella weissflogii resting cell	61		24.2-43 (31.8)		39.9-57.5 (48.2)	EC	25546		994
	Odontella weissflogii resting spore	11		31.2-60.7 (52.8)		22.5-49 (45.2)	EC	75156		2258
	Proboscia alata	181	8.9-45.8 (20.9)			238.3-1183 (644.6)	C	236232		5392
	Proboscia inermis (winter form)	149	6.8-25.1 (11.2)			26-1070 (129)	C	18354	192	773
	Proboscia inermis	56	8.4-35.7 (18.8)			180.6-909 (522.6)	C	189895	113	4567
	Rhizosolenia antennata/hebetata f. semispina	25	7.8-38.3 (16.1)			221.7-1433.4 (749.7)	C	211809	165	4963

Table S2 .

 S2 List of the taxa included in statistical analyses (i.e. hierarchical clustering and NMDS).

	Proboscia inermis	(F.Castracane) R.W.Jordan & R.Ligowski (1991)
	Diatom taxa Pseudo-nitzschia heimii	Reference Manguin (1957)
	Actinocyclus Thalassiosira complex Pseudo-nitzschia lineola	Ehrenberg (1837) -Cleve (1873) (Cleve) Hasle (1993)
	Actinocyclus octonarius Rhizosolenia spp.	Ehrenberg (1837) Brightwell (1858)
	Asteromphalus spp. Rhizosolenia antennata	Ehrenberg (1844) (Ehrenberg) N.E.Brown (1920)
	Azpeitia tabularis Rhizosolenia chunii	Fryxell and Sims (1986) Karsten (1905)
	Centric Rhizosolenia curvata	-Zacharias (1905)
	Chaetoceros spp. Rhizosolenia polydactyla	Ehrenberg (1844) Castracane (1886)
	Chaetoceros atlanticus Rhizosolenia simplex	Cleve (1873) Karsten (1905)
	Chaetoceros castracanei Thalassionema nitzschioides	Karsten (1905) (Grunow) Mereschkowsky (1902)
	Chaetoceros convolutus Thalassiosira lentiginosa	Castracane (1886) (Janisch) Fryxell (1977)
	Chaetoceros criophylus Thalassiosira tumida	Castracane (1886) (Janisch) Hasle (1971)
	Chaetoceros decipiens Thalassiothrix antarctica	Cleve (1873) Schimper and Karsten (1905)
	Chaetoceros dichaeta Pennate	Ehrenberg (1844) -
	Chaetoceros neglectus	Karsten (1905)
	Chaetoceros spp. resting spore	-
	Corethron inerme	Karsten (1905)
	Corethron pennatum	(Grunow) Ostenfeld (1909)
	Coscinodiscus spp.	Ehrenberg (1839)
	Coscinodiscus centralis	Ehrenberg (1839)
	Cylindrotheca closterium	(Ehrenberg) Reimann & J.C.Lewin (1964)
	Dactyliosolen antarcticus	Castracane (1886)
	Dactyliosolen tenuijunctus	(Manguin) Hasle (1975)
	Eucampia antarctica	(Castracane) Mangin (1915)
	Fragilariopsis kerguelensis	(O'Meara) Hustedt (1952)
	Fragilariopsis pseudonana	(Hasle) Hasle (1993)
	Fragilariopsis rhombica	(O'Meara) Hustedt (1952)
	Guinardia cylindrus	(Cleve) Hasle (1996)
	Haslea trompii	(Cleve) Simonsen (1974)
	Leptocylindrus danicus	Cleve (1889)
	Leptocylindrus mediterraneaus	(H.Peragallo) Hasle (1975)
	Membraneis spp.	Paddock (1988)
	Navicula spp.	Bory (1822)
	Odontella weiisflogii	(Grunow) Grunow (1884)
	Plagiotropis sp.	Pfitzer (1871)
	Pleurosigma spp.	Smith (1852)
	Proboscia alata	(Brightwell) Sundström (1986)

Acknowledgements

We thank B. Quéguiner, the PI of the MOBYDICK project, for providing us the opportunity to participate to this cruise, the chief scientist I. Obernosterer and the captain and crew of the R/V Marion Dufresne for their enthusiasm and support aboard during the MOBYDICK-THEMISTO cruise (https://doi.org/10.17600/18000403). This work was supported by the French oceanographic fleet ("Flotte océanographique française"), the French ANR ("Agence Nationale de la Recherche", AAPG 2017 program, MOBYDICK Project number : ANR-17-CE01-0013), and the French Research program of INSU-CNRS LEFE/CYBER ("Les enveloppes fluides et l'environnement" -"Cycles biogéochimiques, environnement et ressources").

F, et al. 2014. Surface currents and upwelling in Kerguelen Plateau regions. Biogeosciences Discuss 11(5): 6845-6876. doi: 10.5194/bgd-11-6845-2014

The structure of diatom communities constrains biogeochemical properties in surface waters of the Southern Ocean (Kerguelen Plateau) Augustin Lafond 1 , Karine Leblanc 1 , Justine Legras 1 , Véronique Cornet 1 , Bernard Quéguiner 1 1 Aix-Marseille University, Université de Toulon, CNRS, IRD, MIO, UM 110, 13288, Campus de Luminy, 163 avenue de Luminy, 13288 Marseille, France Correspondence: augustin.lafond@gmail.com

Conflict of interest

We declare that all authors of this manuscript do not have any conflict of interest.

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