Claire Li

Valentina Krachmalnicoff

Patrick Bouchon

Julien Jaeck

Nathalie Bardou

Riad Haidar
email: riad.haidar@onera.fr

Yannick De Wilde
email: yannick.dewilde@espci.fr

Riad Haïdar

Near-Field and Far-Field Thermal Emission of an Individual Patch Nanoantenna

Keywords: numbers: 44.40.+a, 68.37.Uv, 42.70.Km

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Controlling the emission and absorption properties of surfaces and finite sized objects at infrared (IR) wavelengths is important for applications such as radiative cooling [1], thermal extraction [2,3], designing absorbers for detection [4][5][6][7][8], molecular sensing [9] or thermophotovoltaic energy conversion [10]. Another realm of interest is the production of novel IR sources with innovative features: spatial or spectral selectivity [11][12][13][14], subwavelength (sub-λ) dimensions [15][16][17] and modulation capabilities [18,19]. While thermal sources with no particular surface structuration are quasi-isotropic and spectrally broad in the far field, the surface patterning of materials with surface polaritons results in narrow diffraction lobes [14,20], and, conversely, makes them perfectly absorbing for waves with a given incidence angle and wavelength [21]. Similar thermal antenna effects can be achieved over a broad spectral band with thin films structures [22]. Another promising way to achieve spectral [11][12][13] and angular [23,24] control of thermal emission is through metasurfaces.

Thermal sources based on metasurfaces made of sub-λ resonators generally involve the periodization of a unit cell containing one or several nanoantennas into an array. Diffraction as well as coupling between neighboring antennas are generally superimposed on the intrinsic optical response of the unit cell [25]. Measurements on nanoantennas ensembles also suffer from inhomogeneous broadening and statistical effects. Although it is essential to characterize a metasurface down to the unit cell level to provide precise knowledge of the intrinsic optical response, it remains challenging due to the sub-λ dimensions of the object which call for high spatial and spectral resolutions in conjunction with enough detection sensitivity to address an individual nanoantenna.

In this letter, we demonstrate experimentally emission spectrum measurement and super-resolved mapping of a single nanoantenna, sub-λ in the infrared spectral range of interest (6-12 µm), based on the detection of its thermal radiation. Using far-field spectroscopy and thermal radiation scanning tun-neling microscopy (TRSTM) [26], we reveal both the spectral resonances of an individual metal-insulator-metal (MIM) resonator excited by thermal fluctuations and the spatial distribution of the local density of states (LDOS) at sub-λ scale. A spatial modulation spectroscopy method is proposed to extract the far-field thermal radiation of a sub-λ MIM resonator from the huge and spatially extended background thermal radiation. A comparative analysis is performed between experimental results on MIM nanoantennas and numerical simulations. Usually for non-dispersive insulators, only one fundamental resonance is expected but here we show that we find multiple solutions.

The investigated MIM nanoantennas are nanoresonators composed of a stack of a 200 nm-thick gold layer, a 180 nmthick SiO 2 layer and a 50 nm-thick gold patch of sub-λ width w as depicted in Fig. 1(a). They sustain Fabry-Perot resonances for the gap plasmon trapped below the patch, giving rise to strong local field enhancement as illustrated by the finite-domain time-difference (FDTD) calculations (Lumerical Solutions) of the electric field intensity |E| 2 at resonance in Fig. 1(b). The phase-matching condition ruling the resonance wavelengths λ res is given by [27]:

λ res = 2n eff w m -φr π (1)
with n eff the real part of the effective index ñeff = n eff +ik eff of the mode of a sub-λ metallic waveguide [28], m the mode order and φ r a phase correction factor. The geometrydependant resonance enables straightforward tunability for a wide spectral range [29][30][31][32]. Based on Eq. 1, we tune the fundamental resonant wavelength (m = 1) around λ res = 7 µm where n eff varies smoothly by changing the nanoantennas widths w from 2 µm to 2.4 µm. Square-shaped patches are chosen to realize narrowband omnidirectional unpolarized emitters in the mid-IR [5]. A scanning electron microscope (SEM) image of one of the fabricated nanoantenna is depicted in Fig. 1(c). Unlike previously reported investigations [33][34][35], the studies of individual nanoantennas presented here use the thermal emission of the sample as an internal source of electromagnetic field. It finds its origin in fluctuating thermal currents that excite both propagating and evanescent modes over a wide range of wavelengths, allowing to perform near-field and far-field measurements. While other studies of thermal emission from nanostructures rely either on periodic arrays [13] or on extended nanostructures in one direction [15], the technique we introduce is sensitive enough to extract the radiation of a nanoantenna that is both isolated and sub-λ in all directions. To investigate the far-field thermal radiation spectrum of an individual IR nanoantenna, a major difficulty arises from the necessity to eliminate the dominant background thermal radiation. We bypass this issue by means of a spatial modulation spectroscopy method inspired from reported measurements at visible or near-IR wavelengths of the extinction by a single nanostructure spatially modulated in the Gaussian intensity profile of a focused laser beam [36][37][38]. However, in our case, the background thermal radiation is spatially extended around the MIM nanoantenna, thus requiring the introduction of a new paradigm to apply spatial modulation. Here, we take advantage instead of the dependence of the detected far-field thermal radiation signal with respect to the position of the nanoantenna projection on a detector in a confocal geometry while the background thermal radiation is spatially independent.

The spectroscopy set-up based on spatial modulation is shown in Fig. 2(a). A FTIR is placed in the infinity region of an IR microscope. The sample is heated at temperature T sample with a heating plate. The microscope uses a Cassegrain objective to collect the thermal radiation from the sample surface. A zinc selenide (ZnSe) lens at the exit of the FTIR spectrometer focuses the collected thermal radiation on the active area of a nitrogen-cooled mercury cadmium telluride (MCT) IR detector whose small size (250 × 250 µm 2 ) produces a spatial selection. The IR detector captures the radiation from a ∼ 30 µm-sized region of the sample, hereafter called "confocal region". The IR thermal radiation from a single nanoantenna produces a spot in the image focal plane of the ZnSe lens with a diameter smaller but comparable with the size of the detector active area. The sample and the heating plate are installed on a piezoelectric translation stage allowing fine control of the nanoantenna position in the xy-plane. The thermal radiation signal from a single nanoantenna is practically undetectable as it is superimposed with a much more intense and spectrally distinct background thermal radiation signal which acts as a spectrally dependent offset. The emission cross-section of a 2 µm-wide nanoantenna is calculated by FDTD numerical simulations to be 18 µm 2 at resonance which is only a few percent of the confocal region area. The power emitted by the nanoantenna scales with its emission cross-section. The fraction radiated towards the Cassegrain objective is typically ∼ 1 nW and represents at maximum about 5% of of the thermal background as detailed in the Supplementary Material [39].

However, only the signal from the single nanoantenna exhibits a spatial dependence with respect to translations of the sample in the xy-plane while the background is spatially uniform. In fact, the signal coming from the nanoantenna is maximum when the latter is perfectly at the center of the confocal region and monotonously decays when its position shifts towards the edges, following a behavior which can be approximated by a Gaussian. This light fall-off effect of the signal from the nanoantenna as a function of its position is the keystone of the spatial modulation method that we have developed to extract the sole nanoantenna contribution from the thermal radiation signal measured by the IR detector in spite of the huge and dominant background.

We use a function generator to drive the piezoelectric stage with a sine modulation at frequency Ω, displacing the sample periodically with an amplitude of a few micrometers along The nanoantenna is at the center of the confocal region, i.e. optically conjugated with the center of the IR detector active region when S MIM 1 vanishes between the two lobes in Fig. 2(b) while at the same time S MIM 2 exhibits a pronounced extremum in Fig. 2(c). This enables fine positioning of the MIM antenna with sub-λ accuracy within the confocal region despite being based on a purely far-field and therefore diffraction-limited optical measurement (see Supplemental Material [39]). Note that a cross-section through a lobe of S MIM 1 or S MIM 2 along the unmodulated direction y directly reflects the behavior of the detector response to a sub-λ source. This quantity, represented in Fig. 2(d) together with a Gaussian fit whose full width at half maximum is 17 µm, would not be accessible without modulation due to the background thermal radiation. We emphasize that the spatial variations reported here are similar to the ones observed with spatial modulation of a nano-object in a focused laser beam [36][37][38], but in our case they reflect the light fall-off effect of a sub-λ-sized source of thermal radiation within the confocal region.

The spectral measurements of the far-field thermal radiation of a nanoantenna at T sample = 433 K were carried out by positionning it at an extremum of S MIM 1 to obtain maximum lock-in signal. We operate the FTIR in step scan mode [44] and the Fourier transform of the interferogram of S MIM 1 yields a spectrum that we correct from the instrumental response (i.e. detector sensitivity, optics transmission, absorption) by dividing it with the spectrum of a blackbody at T sample measured by the set-up with a chopper. Emission spectra measurements performed on single MIM nanoantennas for different widths w and normalized to unity are depicted in Fig. 3(a). As expected, a resonant peak is observed at λ res 7 µm but another is also present at 11 µm with both resonances exhibiting a red-shift with w.

The effective emission cross-section σ em for MIM nanoantennas of varying widths w are calculated by FDTD simulations, normalized to unity and depicted in Fig. 3(b). Two resonances are found with a good agreement with the experimental spectra. A small shift of ∼ 100 nm in the position of the two resonances is observed between the measured and simulated spectra. The differences are attributed to the accuracy of the electron beam lithography used to pattern the square patches which produces errors of tens of nanometers on their nominal widths as verified by scanning electron microscopy. Another source of discrepancy is the robustness of the material models used in simulations (see Supplemental Material [39]) with the real and imaginary parts of the refractive index affecting the resonance position and amplitude respectively. Consequently, both experimental and calculated emission spectra of the investigated MIM antennas exhibit not only one narrowband peak near 7 µm but also another resonance near 11 µm. The latter is an unexpected result which cannot be ascribed to an harmonic of the fundamental mode. It originates from the strong variations of the permittivity of SiO 2 in the spectral range between 8 µm and 12 µm thus affecting the Fabry-Perot resonance conditions for the gap plasmon. We illustrate in Fig. 3(c) the phase-matching condition for the fundamental mode by plotting 2n eff (λ)w for various w. As φ r is small enough to be neglected, we have 2n eff (λ)w = λ according to Eq. 1. For a given value of w, three intersections with the dashed line λ are found which correspond to the solutions λ res . Only two resonances are observed experimentally and theoretically in Figs. 3(a) and 3(b). The third resonance near 9.5 µm is suppressed because it is concomitant with strong absorption, as shown by the pronounced maximum of k eff plotted in Fig. 3(c). These material losses hinder the optical confinement inside the nanoantenna cavity at resonance. Indeed, as they increase, the peak emission decreases and broadens until it is completely suppressed, which is typical for a Fabry-Perot with intracavity losses [45]. Figure 3(d) displays the spatial maps of the mode distribution under the patch calculated at the two resonances. They exhibit an identical spatial distribution and confirm that the fundamental mode of the MIM nanoantenna is excited at both wavelengths.

Remarkably, we have also been able to get an insight about the spatial distribution of the fundamental mode at sub-λ scale by mapping the near-field thermal radiation at the surface of a MIM nanoantenna with the scattering tip of a TRSTM. The instrument relies on the thermal excitation of the electromagnetic modes and on the direct relation which exists between the local electromagnetic energy density and the LDOS [26,46], a quantity which is not directly accessible with nearfield probes using either directional external sources to excite the electromagnetic modes of the sample [33,34] or thermovoltage measurements induced by a heat flux [47]. Note that the MIM antenna geometry is unfavorable for near-field measurements with a TRSTM. The latter is a surface probe while most of the electromagnetic energy of the thermally excited modes in a MIM nanoantenna is confined under the gold patch in a buried region inaccessible to the probe (Fig. 1(b)). This situation is dramatically different from other studies aimed at probing thermally excited surface polaritons at the interface between a material and air [26,44,[48][49][50], and has limited us here to the realization of wavelength-integrated measurements.

The isothermal surface of the heated sample radiates a field whose local electromagnetic energy density is the LDOS multiplied by the average energy of a quantum oscillator [26,44,46,[48][49][50][51]. The TRSTM uses a tungsten tip oscillating at a frequency Ω tip in intermittent contact with the sample along the z-direction and scatters the thermal radiation towards a mid-IR detector. A lock-in amplifier extracts the near-field thermal radiation signal by demodulating the detector signal at Ω tip or a higher harmonic, yielding a signal S NF n with n being the harmonic order [46]. The sample is raster scanned in the xy-plane and the recording of S NF n during the scan yields a super-resolved image with a spatial resolution ∼ 100 nm given by the curvature radius of the tip. The near-field optical image of S NF 2 using the full spectral width of the IR detector for a 2.4 µm-wide MIM nanoantenna heated at 403 K is represented in Fig. 4(a). As the two resonances observed in the far-field spectrum (Fig. 3(a-b)) share the same mode pattern, it enables signal enhancement for wavelength-integrated near-field imaging without having distinct modes overlapping in the resulting image.

The energy confined in the cavity formed by the dielectric layer leaks out from the discontinuities at the edges of the patch, leading to a pattern related to the resonance mode profile. The line scans across the patch edges (Fig. 4(c)) show that the TRSTM signal reveals maxima in all four corners of the patch. The image of Fig. 4(a) presents a slight asymmetry with a higher signal at the upper-right corner of the patch due to a small tilt of the TRSTM tip direction with respect to the perpendicular direction to the sample surface z.

We emphasize that other scanning near-field probes using scattering-tip [13,34] or photothermal induced resonance [52,53] techniques combined with an external laser excitation have revealed similar hot spots on nanostructures. While these techniques benefit from high incident field intensity and wavelength selectivity provided by tunable midinfrared lasers, the near-field images they produce reflect the distribution of the modes excited for a given direction and polarization of the laser excitation, which is different from the thermal excitation of all the modes performed with the TRSTM. For comparison with the experiments, FDTD simulations were performed to calculate the LDOS projected along z which is the direction expected to give the highest TRSTM signal provided that the tip is aligned along z [26]. The image (Fig. 4(b)) and corresponding line scan (Fig. 4(d)) are depicted for the first nanoantenna resonance. The calculated LDOS exhibits local maxima at the four corners of the square patch which is in good qualitative agreement with the TRSTM measurements of Figs. 4(a) and 4(c). The maps with subλ resolution provides useful information complementing the preceding spectral characterization. While the LDOS simulations shown in Fig. 4(b) and 4(d) are calculated without any tip, a detailed simulation of the TRSTM image would require to compute the signal radiated by the nanoantenna in the presence of the tungsten tip, whose position on the patch varies during the scan, and to take into account the demodulation process. The qualitative agreement observed between the calculated LDOS and the TRSTM maps suggests that the metallic tip of the TRSTM does not significantly perturb the near-field distribution around the MIM nanoantenna, except for the fact that it favors the detection along the z-axis. This may be due to the fact that it is scanned at the surface of the MIM nanoantenna, which does not disturb the region of high field confinement buried under the patch. However, a spectral shift of the resonances induced by the tip is not ruled out [44].

In summary, we have investigated the thermal radiation emitted by individual plasmonic nanoantennas. A method of spatial modulation has been implemented for background suppression in FTIR spectroscopy measurements of the far-field thermal radiation of sub-λ sized objects, exploiting the spatial dependence of the detected signal which shows a light falloff effect in a confocal geometry. The far-field thermal radia-tion spectra measured on individual MIM nanoantennas have shown that the fundamental mode of a nanoantenna can be excited at distinct wavelengths around a resonance of the effective index of the plasmonic cavity. Using thermal radiation scanning tunneling microscopy, we have imaged the thermal near-fields at the surface of a MIM nanoantenna, and revealed the spatial distribution of its fundamental mode. The experimental methods used in this work can be readily applied to other nanostructures in the mid-IR. Notably, the FTIR spectroscopic approach combined with spatial modulation which we have developed could be used not only to retrieve the far-field thermal emission spectrum of a nanoantenna, but it could also be combined with an external broadband IR source for the farfield characterization of any kind of deeply sub-wavelength nanostructures or nanomaterials by FTIR spectroscopy.

This work received financial support from Labex WIFI (ANR-10-LABX-24 and ANR-10-IDEX-0001-02 PSL*) and ANR-16-CE09-0012 (CarISOVERRE). The authors are grateful to Karl Joulain (Institut Pprime) for enlightening discussions.

2 Figure 1 .

 21 Figure 1. (color online). (a) Schematic of a MIM nanoantenna. (b) Calculated intensity of a nanoantenna at y = 0 at resonance. (c) SEM image.

Figure 2 .

 2 Figure 2. (color online). (a) Schematic of the spatial modulation spectroscopy set-up. (b-c) Three-dimensional plots of the raster scan of a spatially modulated nanoantenna in the x-direction, highlighting the spatial dependence of signal amplitude with respect to the nanoantenna position in the sample plane. The two-dimensional projection is represented in the xy-plane at the bottom. (d) Line section of the normalized central lobe ofS MIM 2 along y (black data points) and Gaussian fit (red line), showing the signal decay as the nanoantenna is displaced from the center toward the edges of the confocal region.

  and 2Ω is thus expected to suppress the spatially independent background, allowing to retrieve the first and second harmonics of the thermal radiation of the sole nanoantenna, S MIM 1 and S MIM 2 respectively. These two quantities are integrated over the spectral range of the IR detector. Their magnitude depends on the mean nanoantenna position with respect to the confocal region and also on the nanoantenna effective emission cross-section.The spatial variations of S MIM 1 and S MIM 2 measured when raster scanning the nanoantenna in the xy-plane are presented in Fig. 2(b) and (c). Due to the expected behavior of the signal emitted by the single nanoantenna within the confocal region, S MIM 1 and S MIM 2 qualitatively reflect the spatial dependencies of the first and second derivative of a Gaussian-like function with respect to the direction of modulation x. The measured images of S MIM 1 and S MIM 2 show lobes of alternating signs in the x-direction.

Figure 3 .

 3 Figure 3. (color online). (a) Experimental emission spectra for a single nanoantenna for different widths w. (b) Calculated effective emission cross-sections. (c) Phase-matching condition for the fundamental mode 2neff(λ)w = λ and effective extinction coefficient keff. (d) Calculated intensity maps inside the cavity of a MIM nanoantenna (dashed outline) for the two resonances denoted by the markers in (b). The white scale bars correspond to 1 µm.

Figure 4 .

 4 Figure 4. (color online). (a) Near-field optical distribution and (c) corresponding line scans across the top and bottom edges as indicated by the dashed lines. (b) Calculated LDOS map at resonance and (d) line scan across the edge. The white scale bars correspond to 1 µm.


Marine Lasbleiz
email: marine.lasbleiz@gmail.com

Karine Leblanc

Leanne K Armand

Urania Christaki

Cl Ément Georges

Ingrid Obernosterer

Bernard Qu Éguiner

Gary King

Composition of diatom communities and their contribution to plankton biomass in the naturally iron-fertilized region of Kerguelen in the Southern Ocean

Keywords: diatoms, carbon biomass, plankton community structure, natural iron fertilization, Southern Ocean

The phenology of the naturally iron-induced phytoplankton blooms east of the Kerguelen Islands was investigated by documenting the spatial and temporal variation of phytoplankton community structure concurrently with biogeochemical observations.

INTRODUCTION

Several decades of careful oceanographic research has allowed the identification of key biogeochemical processes influencing control on the biological carbon (C) pump (e.g. [START_REF] Legendre | Hydrodynamical singularities as controls of recycled versus export production in oceans[END_REF][START_REF] Nelson | Vertical budgets for organic carbon and biogenic silica in the Pacific sector of the Southern Ocean, 1996-1998[END_REF][START_REF] Smetacek | Deep carbon export from a Southern Ocean iron-fertilized diatom bloom[END_REF]. The impact produced by such processes on C export to the deep ocean remains, however, insufficiently documented as a result of the limited number of particle flux studies. In this context, understanding the complex interactions between phytoplankton communities and oceanic biogeochemical cycles has been specifically highlighted as requiring more focused research [START_REF] Boyd | Diatom traits regulate Southern Ocean silica leakage[END_REF].

Diatoms are silicified phytoplankton, and are recognized as one of the main vectors of C export in the Southern Ocean (e.g. [START_REF] Buesseler | Particle export during the Southern Ocean iron experiment (SOFeX)[END_REF][START_REF] Romero | Marine diatoms as indicators of modern changes in oceanographic conditions[END_REF]Rigual-Hern ández et al. 2015a). Recent studies have begun to focus on the wide variation in diatom morphology, elemental composition, community structure and aggregate formation as additional factors in understanding C export efficiency [START_REF] Assmy | Thick-shelled, grazerprotected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF]Quéguiner 2013;[START_REF] Sackett | Taxon-specific responses of Southern Ocean diatoms to Fe enrichment revealed by synchrotron radiation FTIR microspectroscopy[END_REF]. These factors indeed influence the sinking rates of particles, their transfer to higher trophic levels and their remineralization in the water column (e.g. [START_REF] Armstrong | Settling velocity spectra and the ballast ratio hypothesis[END_REF][START_REF] Salter | Diatom resting spore ecology drives enhanced carbon export from a naturally ironfertilized bloom in the Southern Ocean[END_REF]Rigual-Hern ández et al. 2015b).

Diatoms are particularly responsive to the addition of iron in regions of the Southern Ocean characterized by high nutrient, low chlorophyll (HNLC) regimes (e.g. [START_REF] De Baar | Synthesis of iron fertilization experiments: From the Iron Age in the Age of Enlightenment[END_REF]. Extensive blooms of diatoms are recurrently observed east of Kerguelen Island due to the natural iron fertilization of the surface waters, located in the wake of the Antarctic Circumpolar Current [START_REF] Blain | Effect of natural iron fertilization on carbon sequestration in the Southern Ocean[END_REF][START_REF] Armand | Late summer diatom biomass and community structure on and around the naturally iron-fertilised Kerguelen Plateau in the Southern Ocean[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]). Therefore, the Kerguelen region represents a unique natural laboratory to study the impact of iron on the functioning of plankton communities and associated biogeochemical cycles. The first Kerguelen Ocean and Plateau Study (KEOPS) cruise (KEOPS 1) was conducted in this region in late summer (January-February 2005) and demonstrated enhanced C export efficiency of the naturally iron-fertilized system situated within a typical HNLC region [START_REF] Blain | Effect of natural iron fertilization on carbon sequestration in the Southern Ocean[END_REF]. Plankton community composition was clearly different between the iron-fertilized and the HNLC areas [START_REF] Armand | Late summer diatom biomass and community structure on and around the naturally iron-fertilised Kerguelen Plateau in the Southern Ocean[END_REF][START_REF] Carlotti | Zooplankton community structure, biomass and role in carbon fluxes during the second half of a phytoplankton bloom in the eastern sector of the Kerguelen Shelf (January-February 2005)[END_REF][START_REF] Christaki | Microbial food web structure in a naturally iron-fertilized area in the Southern Ocean (Kerguelen Plateau). Deep Sea Res Part II[END_REF][START_REF] West | Major differences of bacterial diversity and activity inside and outside of a natural iron-fertilized phytoplankton bloom in the Southern Ocean[END_REF]). These community-scale observations, across and within the studied locations, confirmed the strong link between plankton community composition and their related biogeochemical cycles.

The present study is focused on the early bloom stages of the naturally iron-fertilized Kerguelen Plateau system (October-November 2011, KEOPS 2), and therefore is placed in the context of the previous study that examined the bloom pinnacle and decline (KEOPS 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]. During the KEOPS 2 program, the biogeochemical drivers underpinning plankton community structure (from viruses to mesozooplankton) and succession, and the consequent impact on C export efficiency were investigated from many angles (e.g. [START_REF] Christaki | Microbial food web dynamics during spring phytoplankton blooms in the naturally iron-fertilized Kerguelen area (Southern Ocean)[END_REF][START_REF] Carlotti | Mesozooplankton structure and functioning during the onset of the Kerguelen phytoplankton bloom during the KEOPS2 survey[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][START_REF] Landa | Shifts in bacterial community composition associated with increased carbon cycling in a mosaic of phytoplankton blooms[END_REF]). In the present paper, surface diatom community composition and contribution to plankton community C biomass were considered by addressing the spatial differences between contrasted regions in terms of iron availability and mesoscale activity. Complementary nethaul and conductivity-temperature-depth (CTD) rosette samplings were conducted across the surveyed region to get the most realistic vertical picture of diatom community composition at the highest possible taxonomic level. The objectives were to (i) understand the phenology of the eastern Kerguelen blooms by comparing the spatial and temporal variation of phytoplankton community structure with concurrent biogeochemical observations, (ii) explore the island mass effect (i.e. iron availability, proximity of the coast) on diatom community composition by comparing observations in an HNLC off-plateau area, and (iii) discuss the potential impact of surface phytoplankton community structure on C export.

MATERIALS AND METHODS

Study area

The KEOPS 2 cruise was conducted in the south Indian Ocean sector of the Southern Ocean, principally to the east of Kerguelen Plateau (KP), aboard the French R/V Marion Dufresne II in austral spring (10 October-20 November 2011). The KP acts as a natural barrier diverting both the trajectory of the Antarctic Circumpolar Current (ACC) and the trajectory of the Polar Front (PF) (Park et al. 2008a;Fig. 1). Interactions between the plateau and hydrodynamic features result in natural iron inputs to surface waters to the east of the KP, which sustain large annual blooms contrasting with the HNLC surrounding waters of the Southern Ocean [START_REF] Blain | Effect of natural iron fertilization on carbon sequestration in the Southern Ocean[END_REF]Park et al. 2014a;Quérou é et al. 2015).

Composite satellite images (produced by F. Nencioli, KEOPS2 website: http://keops2.obs-vlfr.fr/) showed that the 2011 Kerguelen bloom developed rapidly from mid-October with a large spatial heterogeneity leading to a mosaic of biogeochemical environments reflected in variations in surface water productivity [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] Cavagna | Production regime and associated N cycling in the vicinity of Kerguelen Island, Southern Ocean[END_REF] and particulate matter [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].

In this study, a total of eight stations were investigated (Fig. 1): seven stations representing iron fertilized bloomaffected regions to the northeast (E and F-L stations) and southeast (KEOPS 1 repeat station A3-2), and one station representing surrounding HNLC conditions off the plateau (R-2). The latter station was characterized by low biomass and primary production (Table 1) induced by iron limitation [START_REF] Qu Érou É F | High variability in dissolved iron concentrations in the vicinity of the Kerguelen Islands (Southern Ocean)[END_REF]. Conversely, the stations situated in the eastern bloom within the Polar Front zone (F-L), the southeastern KP bloom (A3-2) and the eastern flank of the Kerguelen Islands (E-4W) were characterized by high particulate matter concentrations (chlorophyll a, biogenic silica), and high primary production and biogenic silica production (Table 1). The onset of the bloom was offset locally inside the meander of the PF in the northeastern KP region, as shown by intermediate biomass levels, and moderate primary and biogenic silica productions (Table 1). This area, corresponding to a quasi-stagnant feature (Park et al. 2014b), was sampled temporally (E-1, E-3, E-4E and E-5) as part of a pseudo-Lagrangian survey. A drifter-derived velocity field (Park et al. 2014b) suggested, however, that E-4E corresponds to a distinct physical feature from the other three E stations. Therefore, in this study, E-4E was excluded from the time-series (E-1, E-3, E-5).

Diatom sampling and enumeration

For each station, samples were collected from both vertical net hauls and CTD rosette casts providing, respectively, qualitative and quantitative information on the diatom community composition. in a Sedgwick-Rafter 1 mm × 1 mm gridded cell and observed under a Nikon TS100 inverted microscope. Qualitative assessment of phytoplankton community composition was made using four abundance categories: (i) dominant, when a given species was observed in all fields of view; (ii) common, when a species was observed in most fields of view (∼50-80%); (iii) minor, when a species was observed less than 15 times in a sample; and (iv) rare, when a species was observed once or twice in a sample.

Net haul sampling

CTD rosette cast sampling

Seawater samples were collected from a 12 L bottle CTD rosette equipped with a Seabird SBE 911-plus CTD unit. Samples were taken at four to six depths relative to varying photosynthetically active radiation (PAR; 75-0%; Table 1). Aliquots of 250 mL were preserved with 0.8 mL acidified Lugol's solution in amber glass bottles. Bottles were then stored in the dark at 4 • C until analysis.

Diatom counts were made following the Uterm öhl method [START_REF] Hasle | Using the inverted microscope[END_REF]. For the HNLC reference station (R-2) with low chlorophyll concentrations [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], a 100 mL seawater sample was settled for 48 h in a settling chamber, whereas for the other stations presenting higher cell abundances only 10-25 mL samples were settled for 24 h. After settling, samples were analyzed with a Nikon Eclipse TE2000-E inverted microscope equipped with phase contrast, at ×100, ×200 or ×400 magnification. Each complete diatom cell was identified to the lowest taxonomic level using taxonomic concepts detailed in [START_REF] Hasle | Marine diatoms[END_REF] while taxonomic listings (Supplementary Table S1) follow accepted names listed in the World Reg-ister of Marine Species (WoRMS: http://www.marinespecies.org/). Several groupings at the genus level were made due to the difficulty of separating some species at ×400 magnification. All small Chaetoceros (Hyalochaete) species were grouped together although the dominant contributors were Chaetoceros debilis and Chaetoceros curvisetus. Fragilariopsis rhombica and Fragilariopsis separanda species could not be differentiated under inverted microscopy and were grouped under the term 'Fragilariopsis rhombica/separanda'. For the same reasons, the Tropidoneis group encompasses Membraneis and Plagiotropsis species as well as Haslea trompii. Thalassionema nitzschioides was represented by three varieties: mainly T. nitzschioides var. nitzschioides, accompanied with T. nitzschioides var. parva and T. nitzschioides var. lanceolata. The term 'centric spp. (<25 μm)' corresponds to an assemblage of small centric diatoms composed principally of several Thalassiosira species and Actinocyclus spp. Live cells and empty frustules were identified from all preserved CTD samples. Frustules with no chloroplasts visible to the eye during observation were considered as empty. This category does not include broken cells and fragments, neither of which were tallied.

Specific diatom C biomass was assessed following the methodology of Cornet-Barthaux, Armand and Qu éguiner (2007). Cell dimensions were determined from at least 10 representative images of each species from all samples using Nis-Element AR 2.30 software (measurement precision <1.0 μm). When not measurable due to the cell's position on the slide (e.g. the transapical axis of Fragilariopsis kerguelensis or the pervalvar axis of numerous centrics), the missing dimensions were calculated alternatively from the visible to hidden dimensions ratio by guest on September 19, 2016 http://femsec.oxfordjournals.org/ Downloaded from Table 1. Depth list of discrete water samples within the water column taken from CTD casts with their corresponding light levels expressed in terms of the percentage photosynthetically active radiation (PAR) as compared with the surface. Each station is associated with the corresponding mixed layer depth (MLD) and integrated biogeochemical properties reflecting phytoplankton productivity within the euphotic layer (i.e. layer from the surface to the depth to which 1% of surface PAR remains). Chl a, chlorophyll a; BSi, biogenic silica; GCP; gross community production; ρSi, the silica production rates. deduced from the shape of the cell [START_REF] Olenina | Biovolumes and Size-Classes of Phytoplankton in the Baltic Sea[END_REF][START_REF] Leblanc | A global diatom databaseabundance, biovolume and biomass in the world ocean[END_REF] When possible the ratio was also estimated from other comparable samples in which the missing dimension was occasionally measurable. Cellular biovolume was estimated from the mean of linear dimensions using the appropriate geometric formulae reflecting the cell shape [START_REF] Hillebrand | Biovolume calculation for pelagic and benthic microalgae[END_REF][START_REF] Sun | Geometric models for calculating cell biovolume and surface area for phytoplankton[END_REF][START_REF] Leblanc | A global diatom databaseabundance, biovolume and biomass in the world ocean[END_REF]. The C content of diatoms was calculated from cellular biovolume according to the corrected equation of Eppley [START_REF] Smayda | From Phytoplankton to Biomass[END_REF] as applied in Cornet-Barthaux, Armand and Qu éguiner (2007).

Station PAR Depth MLD Chl a a BSi a ρSi b GCP c (%) (m) (m) (mg m -2 ) (mmol m -2 ) (mmol m -2 d -1 ) (mmol m -2 d -

Sampling and enumeration of plankton organisms other than diatoms

Seawater samples were collected at a minimum of 10 depths from the surface to 900 m at each station by CTD rosette to as-sess the C biomass contribution of major functional plankton groups other than diatoms from pico-to micro-sized organisms. The term 'functional plankton group' refers to conceptual groupings of plankton species with common biogeochemical and ecological attributes.

The abundance of heterotrophic bacteria (sensus stricto Bacteria and Archaea), heterotrophic nanoflagellates (HNF), picoand nanophytoplankton (including Synechococcus, nano-and picoeukaryotes) was determined using a FACSCalibur flow cytometer [START_REF] Marie | Enumeration of phytoplankton, bacteria and viruses in marine samples[END_REF][START_REF] Christaki | Optimized routine flow cytometric enumeration of heterotrophic flagellates using SYBR Green I[END_REF]. For heterotrophic bacteria, the biomass conversion factors used were 12.4 fg C cell -1 [START_REF] Fukuda | Direct determination of carbon and nitrogen contents of natural bacterial assemblages in marine environments[END_REF]. For HNF, in order to establish the size of cells of the three cytometric populations identified during this study, cells from each population were sorted with a FACSAria cell sorter (BD Biosciences). The sorted cells (1000 to 3000 cells per sample) were collected on Nucleopore filters (0.2 μm pore size, 25 mm diameter), and examined using a Zeiss AX10 microscope at ×1000 magnification. The mean biovolume of each cytometric population was calculated based on the linear dimensions of the cells and the mean C biomass (3.26 pg C cell -1 ) was estimated from a conversion factor of 183 fg C μm -3 [START_REF] Caron | The contribution of microorganisms to particulate carbon and nitrogen in surface waters of the Sargasso Sea near Bermuda. Deep Sea Res Part I[END_REF]. For Synechococcus, picoand nano-phytoplankton, conversion factors of the identified groups were established during KEOPS 1 mission according to [START_REF] Veldhuis | Application of flow cytometry in marine phytoplankton research: current applications and future perspectives[END_REF] with the application of fractionated filtration steps (10, 8, 5, 3, 2, 1 and 0.6 μm). C biomasses were calculated using the empirical relationship between biovolume and cell C content (Eq. 1) of [START_REF] Verity | Relationships between cell volume and the carbon and nitrogen content of marine photosynthetic nanoplankton[END_REF]:

0.433 × V 0.86 (1)
where V is biovolume in μm 3 and cell C content is expressed in pg C cell -1 . This resulted in 0.34, 0.86 and 3.81 pg C cell -1 for Synechococcus, picoeukaryotes and nanophytoplankton (<10 μm), respectively. These values are within the range of biomasses reported in a recent study in the Sargasso Sea (0.337 and 3.02-3.34 for Synechococcus and eukaryotic algae, respectively; [START_REF] Casey | Changes in partitioning of carbon amongst photosythethic pico-and nano-plankton groups in the Sargasso Sea in response to changes in the North Atlantic Oscillation[END_REF].

Ciliates and dinoflagellates were counted in 500 mL samples fixed in acid Lugol's solution and were identified based on their morphology at the lowest possible taxonomic level [START_REF] Christaki | Microzooplankton community associated with phytoplankton blooms in the naturally iron-fertilized Kerguelen area (Southern Ocean)[END_REF]. Ciliates and dinoflagellates were further divided into six size groups (<20, 20-40, 40-60, 60-80, 80-100 and >100 μm). Linear dimensions were measured at ×400 magnification using an image analyzer with a camera mounted on the microscope (Nikon Eclipse TE2000-S). Cell biovolume was calculated assuming the nearest geometrical shape. For these two taxa, a minimum of 10 cells (for rare species) and a maximum of 300 cells (for the most abundant species) were measured. Biovolume was then converted to C biomass based on a C:biovolume factor of 0.19 pg C μm -3 [START_REF] Putt | An experimentally determined carbon:volume ratio for marine 'oligotrichous' ciliates from estuarine and coastal waters[END_REF] for ciliates and from the following equation (Eq. 2) for heterotrophic dinoflagellates (Menden-Deuer and Lessard 2000):

0.216 × V 0.939 (2)
where V is biovolume in μm 3 and cell C content is expressed in pg C cell -1 .

Copepod nauplii, acantharians, foraminifera and radiolarians were counted in the same samples prepared for diatom counts in order to determine their approximate contribution to the total C biomass as compared with diatoms. We acknowledge that the sample volumes used were too small to obtain an accurate estimate of their respective abundances. Therefore, they were not included in the estimate of the total C biomass. We recommend for future studies sampling volumes and preservation techniques attuned to the appropriate estimation of these four plankton groups from the water column.

Surface water partitioning

For the purposes of comparing community carbon structure across the study stations, C biomass calculations for each plankton functional group were integrated across two distinct layers of the water column, following [START_REF] Buesseler | Shedding light on processes that control particle export and flux attenuation in the twilight zone of the open ocean[END_REF] approach. The first layer corresponds to the euphotic layer: limited by the euphotic depth (Ze), traditionally defined as the depth to which 1% of surface PAR remains. The second, subeuphotic layer underlies the euphotic layer and extends 100 m below the Ze.

RESULTS

Diatom cell abundance and biomass vertical profiles

The vertical profiles of diatom abundance demonstrating the contribution of live cells, empty frustules and C biomass estimates obtained from CTD rosette sampling are presented in Fig. 2 (detailed results are provided in Supplementary Table S2). Live cell abundances and C biomass estimates typically decreased with light attenuation through the water column, with the exception of the profiles recovered from stations R-2 and E-1. The lowest live cell abundance (1.5 ± 0.2 × 10 4 cells L -1 ) andC biomass (3.0 ± 0.3 μg C L -1 ) were encountered at the HNLC reference station, R-2 (Fig. 2d). These minimum values contrast starkly with the mixed-layer maximum live cell abundances, ranging from 128 × 10 4 to 293 × 10 4 cells L -1 , and the highest C biomasses varying between 69.3 and 274 μg C L -1 , derived from the highly productive stations A3-2, E-4W and F-L (Fig. 2a-c). In the PF meander stations (Fig. 2e-h), low live cell abundances and C biomasses were recorded during the two first visits (E-1 and E-3, 5.9 ± 2.2 × 10 4 cells L -1 , 20.0 ± 5.5 μg C L -1 ). At E-5, estimated live cell abundances and C biomass increased to 14.0 ± 8.7 × 10 4 cells L -1 and 33.3 ± 15.4 μg C L -1 . Compared with the three PF meander stations E-1, E-3 and E-5, the station E-4E located further east had higher live cell abundances and C biomass (31.8 ± 15.0 × 10 4 cells L -1 and 40.2 ± 20.1 μg C L -1 ).

The proportion of empty frustules increased with depth at all stations, varying on average from 6 ± 7% in the surface (75% PAR) to 24 ± 13% below the Ze. Within the euphotic layer, the lowest percentages of empty frustules (mean 3 ± 2%) were observed at the high productivity stations (A3-2, E-4W and F-L) and E-4E, whilst the highest percentages (mean 20 ± 7%) were found at stations R-2, E-1, E-3 and E-5 (Fig. 2; Supplementary Table S2).

Diatom community composition

Comparison of CTD rosette vs net haul samples

The comparison between CTD rosette and net haul results (Fig. 3) illustrates how the two sampling techniques complement the vertical picture of the diatom community structure. Net haul sampling highlighted the presence of large diatoms (e.g. Corethron pennatum, Dactyliosolen antarctica, Fragilariopsis kerguelensis and Rhizosolenia spp.), which were rare in CTD rosette samples. As a relevant example, the large diatom C. pennatum dominated the diatom assemblages in net haul samples at R-2, E-1 and E-5, but only represented less than 1% at R-2 and 5% at E-1 and E-5 of the diatom cell abundance over the water column when enumerated from CTD rosette samples. Conversely, CTD rosette sampling allowed the capture of small diatoms (e.g. centric spp. (<25 μm), Thalassionema nitzschioides), that were not retained by the 35 μm-mesh phytoplankton net. This difference was more pronounced at the station F-L where the small centrics (Thalassiosira-like), mainly present in the form of single cells rather than chain colonies as observed everywhere else over the KP region, were absent from net haul samples while they dominated cell abundances in Niskin bottles. The analysis of CTD rosette and net haul results also provided evidence that the diatom assemblages sampled from Niskin bottles were dominated (i.e. when a given species contributed to >10% of the diatom cell abundance) by fewer species/genera than were observed in the concentrated net haul samples. Common or minor species in net haul samples usually represented <1% of the total diatom cell abundance in Niskin bottles. All these observations were strong arguments for combining net haul and CTD rosette results to better meet the objectives of this study. As CTD rosette

h) E-4E 0 2 0 4 0 6 0 0 2 0 4 0 6 0 g) E-5 0 1 0 2 0 3 0 0 2 0 4 0 6 0 0 1 0 2 0 3 0 f) E-3 0 1 0 2 0 3 0 Biomass (μg C L -1 )
Abundance of diatom cells (Cells x 10 4 L -1 ) sampling provides a picture of the diatom community structure at several discrete layers over the water column, all subsequent specific diatom observations (live and empty cells) and associated C biomass estimates were based on the latter sampling methodology (detailed results are provided in Supplementary Table S2). Quantification of the proportion of diatom C biomass that was collected from net haul sampling was not undertaken in this study. Net haul results were used qualitatively to complement the vertical view of living diatom composition and community.

PAR (%) e) E-1 f) E-3 (g) E-5 (h) E-4E (

Specific composition of the live diatom communities

CTD rosette results from the eight studied stations indicate that the relative composition of the live diatom communities remained quite homogeneous throughout the upper water column (Fig. 4).

The two stations A3-2 and E-4W showed community similarities with an overwhelming dominance of Chaetoceros (Hyalochaete) spp. (mean 87 ± 3% of the live cells and 55 ± 6% of the C biomass), and major contributions from the small cen-tric spp. (<25 μm; mean 5 ± 2% of the live cells and 18 ± 8% of the C biomass) and Pseudo-nitzschia spp. (mean 3 ± 1% of the live cells and 7 ± 3% of the C biomass).

At the station F-L, live cell abundances and C biomasses were dominated by the small centric spp. (<25 μm; respective means 54 ± 13 and 65 ± 4%) and Chaetoceros (Hyalochaete) spp. (respective means 31 ± 12 and 9 ± 5%). Station F-L was also characterized by a substantial proportion of large Thalassiosira spp. (>25 μm) and Pseudo-nitzschia spp., comprising 6 and 7%, respectively, of the total C biomass.

At all three highly productive stations, net haul results revealed major contributions of the large-sized diatoms F. kerguelensis, Eucampia antarctica and C. pennatum (Fig. 3). Station F-L was also characterized by an elevated live cell abundance of Odontella weissflogii in the upper 100 m.

At the HNLC reference station R-2, the live cell abundance was dominated by a group of pennate diatoms: F. kerguelensis (mean 18 ± 2%), Fragilariopsis rhombica/separanda (mean 8 ± 4%), Fragilariopsis pseudonana (mean 7 ± 4%) and T. nitzschioides (mean 21 ± 3%). Even though centric diatoms only represented 33% of the live cell abundance throughout the water column, they Large centrics (Thalassiosira-like) largely dominated diatom C biomass (mean 72 ± 6%). The main contributors to C biomass from 75 to 0.3% PAR were Rhizosolenia spp. (mean 22 ± 5%), the Tropidoneis group (mean 9 ± 3%), C. pennatum (mean 6 ± 4%) and large centric diatoms (mean 22 ± 4%) predominantly representative of Thalassiosira spp. (including among others T. lentiginosa and T. tumida) as net haul results suggest. Among pennate diatoms, only F. kerguelensis significantly contributed to the C biomass where its contribution averaged 12 ± 5% over the water column.

A3-2 E-4W F-L R-2 E-1 E-3 E-5 E-4E A3-2 E-4W F-L R-2 E-1 E-3 E-5 E-4E
At the three PF meander stations sampled temporally (E-1, E-3 and E-5), the same diatom species were observed but their specific contributions to live cell abundance and C biomass changed over time (Fig. 4e-g). The first two visits (E-1 and E-3) were characterized by diatom assemblages with equally distributed species, which shifted towards a system dominated by few species (E-5). At the initial visit (E-1), the main contributors to the live cell abundance were F. kerguelensis (mean 18 ± 9%), T. nitzschioides (mean 21 ± 2%), Chaetoceros (Hyalochaete) spp. (mean 22 ± 3%) and O. weissflogii (mean 7 ± 2%). The temporal evolution of specific contributions to live cell abundance from the first (E-1) to the last visit (E-5) was illustrated by (i) the noticeable increase in the Chaetoceros (Hyalochaete) spp. contribution (reaching a mean of 36 ± 7%), (ii) the progressive decrease in F. kerguelensis and O. weissflogii contributions (reaching respective means of 6 ± 4 and >1%), and (iii) the constant contribution of T. nitzschioides (mean 17 ± 3%). Net haul results captured similar temporal evolution in the diatom community composition and highlighted the major contribution of the large diatom species C. pennatum and F. kerguelensis when compared with live cell abundance in the upper 100 m (Fig. 3). The C biomasses were dominated by C. pennatum over the water column (mean 42 ± 9%) at the three PF meander stations E-1, E-3 and E-5 (Fig. 4e-g). From E-1 to E-5, species-specific contributions to C biomass followed the same temporal evolution with those of the live cell abundance over the water column by showing increasing contributions of Chaetoceros (Hyalochaete) spp. (from 0.8 ± 0.1 to 10 ± 2%) and decreasing contributions of F. kerguelensis and O. weissflogii (respectively from 13 ± 6 to 3 ± 2% and from 11 ± 6 to 2 ± 1%).

CTD rosette results demonstrated that station E-4E was clearly differentiated from the other PF meander stations E-1, E-3 and E-5, as exemplified by the dominance of Chaetoceros (Hyalochaete) spp., Pseudo-nitzschia spp. and Thalassiosira spp. (<25 μm) (highest to lowest abundances; Fig. 4h). Moreover, net haul observations revealed a substantial occurrence of the large chain-forming diatoms D. antarctica and E. antarctica, and the narrow and elongated diatom Thalassiothrix antarctica (Fig. 3). Large diatoms such as large Thalassiosira spp. (>25 μm), C. pennatum, E. antarctica and species belonging to the Tropidoneis group (Fig. 4h) dominated the C biomass at station E-4E, even though in CTD samples these diatoms were present in lower abundances.

Chaetoceros resting spores were only found at the low light levels of 0.01% PAR at F-L, E-3 and E-5 and 0% PAR at station A3-2, contributing to <1% of the live cell abundance and total C biomass (Fig. 4).

Specific contribution to empty frustule abundance

Empty frustule abundances across all three highly productive stations (A3-2, E-4W, F-L) were mainly composed by the same diatom species: T. nitzschioides, the small centric spp. (<25 μm), F. kerguelensis, Chaetoceros (Hyalochaete) spp. and Pseudo-nitzschia spp. (highest to lowest abundances; Fig. 5a-c). Only station F-L was characterized by a large contribution of empty E. antarctica (24 ± 7%) cells within the euphotic layer, whilst below the mixed layer, small centric diatoms dominated the empty cell abundance (54%). Empty frustules of Chaetoceros (Hyalochaete) spp. were only found below the euphotic layer at A3-2 (14 ± 1%) and F-L (14%). At station R-2, empty frustules were primarily represented by T. nitzschioides (mean 19 ± 6%), small unidentified centric diatoms (<25 μm; mean 14 ± 9%) and three Fragilariopsis species through the water column. Fragilariopsis rhombica/separanda (11 ± 7%) and F. pseudonana (13 ± 2%) were major contributors to the empty frustule abundance across 75 to 0.3% PAR, whereas F. kerguelensis' contribution reached an average 30 ± 3% from 25 to 0.3% PAR (Fig. 5d). Elevated contributions to the empty frustule abundance were observed for Rhizosolenia spp. only at 75% PAR (17% in comparison with 3 ± 2% between 25 and 0.3% PAR).

At the three PF meander stations E-1, E-3 and E-5 sampled temporally, empty frustule abundances were dominated by T. nitzschioides (mean 31 ± 12%), F. kerguelensis (mean 24 ± 10%) and Thalassiosira spp. (<25 μm; mean 12 ± 6%) (Fig. 5e-g). Of these three stations, only E-1 exhibited a dissimilar empty frustule diatom composition at 75% PAR, with the dominance of F. rhombica/separanda (21%), Navicula spp. (15%), the Tropidoneis group (13%), O. weissflogii (15%) and Chaetoceros atlanticum (6%).

Station E-4E was furthermore differentiated from the three other E stations located in the meander of the PF by a distinct, heterogeneous species-specific contribution to the empty frustule abundance despite no apparent physical and biogeochemical heterogeneity (Fig. 5h). Surprisingly, at 75% PAR all the empty frustules observed corresponded to Thalassiosira spp. (<25 μm). From 25 to 0.01% PAR, the Thalassiosira spp. contribution was decreased to a tenth of the 75% PAR value (<11%) whereas T. nitzschioides and F. kerguelensis were found to be the major contributors in the underlying deep waters (29 ± 7 and 19 ± 6%, respectively). Unique elevated abundances in the empty frustules of F. rhombica/separanda (33%) and E. antarctica (20%) were observed at 25 and 1% PAR, respectively.

Contribution of functional plankton groups to total C biomass

In order to evaluate the potential contribution of diatoms to the total C biomass in the water column, the C biomass of planktonic groups (from pico-to micro-sized organisms) was estimated within the euphotic and subeuphotic layers from the eight studied stations (Fig. 6; detailed results are provided in Supplementary Table S3). Microscope observations revealed that copepod nauplii, acantharians, foraminifera and radiolarians were minor contributors to the total C biomass (approximatively <2%; data not shown) at each studied site. The C biomass of autotrophs dominated the euphotic and subeuphotic layers at all stations, accounting on average for 83 ± 6% of total C biomass. Nanoeukaryotes and diatoms were the most important C contributors, but their contributions varied from one site to another. At the highly productive stations (A3-2, E-4W and F-L; Fig. 6a-c), diatoms largely dominated total C biomass in the top 200 m, representing on average 70 ± 4 and 58 ± 1% in the euphotic and subeuphotic layers, respectively. Nanoeukaryotes increased their C biomass contri-butions in the subeuphotic layer (mean 22 ± 6%) in comparison with the euphotic zone (mean 15 ± 6%). In the euphotic layer of the HNLC station (R-2; Fig. 6d) autotrophic nanoeukaryotes dominated the C biomass (65%). Picoeukaryotes (9%), heterotrophic bacteria (8%) and diatoms (7%) provided relatively equal proportions as the next dominant C biomass contributors in the euphotic zone. Below, in the subeuphotic layer at R-2, the autotrophic nanoeukaryote contribution decreased to 40%, while silicoflagellates, diatoms and heterotrophic bacteria by guest on September 19, 2016 http://femsec.oxfordjournals.org/ Downloaded from each contributed 13-14% (combined total 40%) to the total C biomass.

At the time series stations E-1, E-3 and E-5, the C biomass contributions were dominated by autotrophic nanoeukaryotes in the euphotic layer (representing 59, 52 and 44% of the total C biomass, respectively; Fig. 6e-g). However, between stations E-1 and E-5, the autotrophic nanoeukaryote contribution to the total C biomass was eroded by increasing diatom C biomass input (from 25% at E-1 to 38% at E-5). Notably, diatom contributions to C biomass were clearly higher in the subeuphotic layer (mean 41 ± 4%) than in the euphotic layer (mean 27 ± 3%) during the first two repeat station visits (E-1, E-3) and were then found to be of equivalent proportions across both layers at station E-5 (mean 36 ± 2%). At the remaining station located in the meander of the PF (E-4E), the contribution of diatoms to C biomass (48%) in the euphotic layer was clearly higher than the autotrophic nanoeukaryote contribution (29%, Fig. 6h). Station E-4E also stood out from the three other stations (E-1, E-3 and E-5) with higher C biomass contributions from dinoflagellates in both layers (7%). Finally, in the subeuphotic layer, all E stations located in the meander of the PF were characterized by higher heterotrophic community (i.e. bacteria, nanoflagellates and dinoflagellates) C biomass contributions (mean 21 ± 5%) than the values estimated from this community in the euphotic zone (mean 15 ± 4%; Fig. 6e-h; Supplementary Table S3).

DISCUSSION

Phenology of the eastern Kerguelen blooms

Early spring bloom in the northeastern Kerguelen area

In early spring, the eastern Kerguelen plateau oceanographic region was characterized by a mosaic of biogeochemical environments as reflected, for example, in the particulate matter distribution [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]), primary production (Cavagna et al. 2015) and biogenic silica production [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]. Together these observations suggest a dynamic region, host to diverse phytoplankton communities at differing stages of development, as similarly reported in other naturally iron-fertilized systems of the Southern Ocean [START_REF] Pollard | The Crozet Natural Iron Bloom and Export Experiment (CROZEX). Deep Sea Res Part II[END_REF][START_REF] Tarling | DISCOVERY 2010: Spatial and temporal variability in a dynamic polar ecosystem[END_REF].

The pseudo-Lagrangian study in the meander of the PF exemplifies the evolving bloom community in response to complex mesoscale conditions. The two first visits (E-1 and E-3) were associated with non-blooming conditions [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]; Table 1), the dominance of nano-and picophytoplankton communities (Fig. 6), and a high proportion of empty frustules throughout the water column (Fig. 2). These non-blooming conditions were attributed to the instability of the mixed layer [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] as suggested by the CTD vertical density profiles (Fig. 2 in [START_REF] Laurenceau-Cornec | The relative importance of phytoplankton aggregates and zooplankton fecal pellets to carbon export: insights from free-drifting sediment trap deployments in naturally iron-fertilised waters near the Kerguelen Plateau[END_REF]. In our study, the observation of an elevated relative contribution of diatoms with respect to C biomass in the subeuphotic layer at E-1 and E-3 (Fig. 6) suggests that a short-lived diatom bloom was terminated prior to the first sampling event (E-1) as a response to adverse light-mixing conditions [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]. Station E-1 was notably characterized by the presence of a secondary pycnocline around 130 m depth, suggestive of episodic deep-water mixing that could have rapidly transferred blooming diatom cells from the surface waters to unfavorable light conditions at depth.

In contrast, higher particulate matter concentrations and enhanced BSi production fluxes [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]) observed during the last visit to the PF meander (E-5) were evidence of the initiation of the main spring bloom. Our results from this last visit also signal higher diatom-sourced contributions to C biomass (Fig. 6), a three-fold increase in cell abundance and a visibly decreased proportion of empty frustules (Fig. 2) in the euphotic layer compared with the pre-bloom station observations. The ability of diatoms to rapidly accumulate in the surface waters as soon as light conditions were sufficient to trigger a phytoplankton bloom (E-5) was evident from our temporal survey. Somewhat expectedly, Chaetoceros (Hyalochaete) species revealed a remarkable increase in their relative contribution to the total live cell abundance (Fig. 4e-g) in response to favorable conditions. At the highly productive stations (A3-2, E-4W and F-L; Fig. 4a-c), where a more advanced bloom stage was evidenced [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], Chaetoceros (Hyalochaete) spp. and small centrics (mainly Thalassiosira spp.), largely dominated the phytoplankton communities. Combined, these bloom state observations imply the plankton community structure in the meander of the PF was evolving from a nanoeukaryote-dominated (unstable, non-bloom condition) to diatom-dominated (stable, bloom condition) communities, primarily composed of Chaetoceros (Hyalochaete) spp. This scenario is consistent with observations at the PF meander station E-4E, located further east compared with the other E stations (Fig. 1). Here, Chaetoceros (Hyalochaete) spp. were one of the major contributors to a two-fold increased diatom cell abundance, and BSi concentrations and silica production fluxes were slightly raised [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]. Combined, these observations suggest that station E-4E was at a slightly more advanced stage of bloom development in comparison with station E-5. Light availability has previously been shown to play an important role in controlling the phytoplankton growth and community composition in several regions of the Southern Ocean (e.g. [START_REF] Wright | Phytoplankton community structure and stocks in the East Antarctic marginal ice zone (BROKE survey, January-March 1996) determined by CHEM-TAX analysis of HPLC pigment signatures[END_REF][START_REF] Boyd | Control of phytoplankton growth by iron supply and irradiance in the subantarctic Southern Ocean: Experimental results from the SAZ Project[END_REF][START_REF] Moore | Iron-light interactions during the CROZet natural iron bloom and EXport experiment (CROZEX) I: Phytoplankton growth and photophysiology[END_REF]. As a relevant comparison with our study, mesoscale structures were responsible for delaying the phytoplankton bloom development in the naturally iron fertilized Crozet region [START_REF] Read | Sub-mesoscale structure and the development of an eddy in the Subantarctic Front north of the Crozet Islands[END_REF][START_REF] Venables | Physical conditions controlling the development of a regular phytoplankton bloom north of the Crozet Plateau, Southern Ocean[END_REF].

The complementary use of the CTD rosette and net haul samplings has highlighted the presence of large diatoms (e.g. C. pennatum, F. kerguelensis), which were in lower abundance in Niskin bottles than in net haul samples (Fig. 3). The same observation was made from KEOPS 1 sampling, where the large and heavily silicified F. kerguelensis and the giant T. antarctica (Armand et al. 2008) were dominant in net-hauled community compositions but notably absent from Niskin-sampled communities. Differences between the two sampling techniques (i.e. dissimilar phytoplankton cell size sampling bias, total water volumes and integrative versus discrete sampling) are the obvious reasons for the divergent community observations. However, such observation differences can also imply that two distinct diatom communities were present over the Kerguelen region in spring: one that was homogeneously distributed in the mixed layer, while another (mainly composed of very large diatoms) probably occupied discrete micro-layers heterogeneously distributed in the upper 200 m (Qu éguiner, [START_REF] Qu Éguiner | High primary production and vertical export of carbon over the Kerguelen Plateau as a consequence of natural iron fertilization in a high-nutrient[END_REF]. Such discretelayered, large-diatom species populations are far more likely to be efficiently sampled from net hauls than Niskin samples because of the former instrument's integrated sampling ability of the top 100 m surface waters. Across the same water column, CTD rosette sampling only subsamples discrete depths unlikely to encounter such micro-scale biological variation systematically. Our observations have illustrated the importance of complementary water column sampling (CTD rosette and net haul) in providing the most realistic vertical picture of natural by guest on September 19, 2016 http://femsec.oxfordjournals.org/ Downloaded from diatom assemblages. Large-celled and/or long-chained species, even present in low abundances, can be major contributors to diatom C biomass, as observed for C. pennatum at the three PF meander stations, E-1, E-3 and E-5 (Fig. 4e-g).

Seasonal evolution of the southeastern KP bloom

The phytoplankton community of the southeastern Kerguelen Plateau (KP) bloom (station A3) was investigated in early spring (October-November 2011; KEOPS 2) and in late summer (January-February 2005; KEOPS 1). Although separated temporally, the two KEOPS programs enable a picture of the Kerguelen diatom bloom's evolution to be discerned. Species belonging to the genus Chaetoceros (Hyalochaete) play a significant role in the annual KP bloom. These small, chained diatoms (exemplified by the dominant occurrence of C. debilis) dominated the early spring phytoplankton bloom during KEOPS 2 and were similarly reported in high abundances during KEOPS 1 (i.e. mid-January; [START_REF] Armand | Late summer diatom biomass and community structure on and around the naturally iron-fertilised Kerguelen Plateau in the Southern Ocean[END_REF], signifying the species' potential ability to persist continuously through summer. From mid-January to early February, the phytoplankton community evolved to an almost monospecific community of Eucampia antarctica v. antarctica in the surface waters [START_REF] Armand | Late summer diatom biomass and community structure on and around the naturally iron-fertilised Kerguelen Plateau in the Southern Ocean[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]. During this period, [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] provided evidence for the formation of a deep silica maximum (DSM) at ∼100 m that was attributed to E. antarctica and Chaetoceros resting spores [START_REF] Armand | Late summer diatom biomass and community structure on and around the naturally iron-fertilised Kerguelen Plateau in the Southern Ocean[END_REF].

Qu éguiner (2013) described a conceptual scheme for phytoplankton seasonality in naturally iron-fertilized systems. According to this scheme, differing diatom groups (e.g. lightly silicified species versus grazer-resistant species) would thrive in distinct niches in the water column due to different limiting factors (e.g. light availability, nutrient limitation, grazing pressure and other mortality processes). By comparison with our observations, Chaetoceros (Hyalochaete) spp. closely correspond to Qu éguiner's (2013) lightly silicified diatoms, which develop rapidly over the mixed layer in early spring once light conditions become favorable and would be sustained until summer. Meanwhile, our observations of the presence of E. antarctica aligns with the category of grazer-resistant diatoms, where low growth rates and heavily silicified frustules progressively developed in discrete water column layers sometimes leading to the formation of a DSM (Qu éguiner 2013).

The presence of Chaetoceros resting spores at the end of the productive period was attributed to a survival strategy in response to the depletion of silicic acid and iron stocks in the surface waters [START_REF] Armand | Late summer diatom biomass and community structure on and around the naturally iron-fertilised Kerguelen Plateau in the Southern Ocean[END_REF]. Laboratory experiments have commonly associated the formation of Chaetoceros resting spores with nutrient depletion (e.g. [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][START_REF] Oku | Resting spore formation of the marine planktonic diatom Chaetoceros anastomosans induced by high salinity and nitrogen depletion[END_REF]. Nevertheless, our results revealed that Chaetoceros resting spores were already present in early spring below the mixed layer, despite high macronutrient and iron concentrations in the surface waters (Table 1). This implies that Chaetoceros resting spore formation would appear to be primarily controlled by light availability in spring at the southeastern KP station, as has been previously assumed in the North Atlantic Ocean [START_REF] Rynearson | Major contribution of diatom resting spores to vertical flux in the sub-polar North Atlantic. Deep Sea Res Part[END_REF]. Interestingly, this observation could also support [START_REF] Armand | Late summer diatom biomass and community structure on and around the naturally iron-fertilised Kerguelen Plateau in the Southern Ocean[END_REF]'s hypothesis that the constant presence of Chaetoceros resting spores below the pycnocline would allow the sustenance of a Chaetoceros (Hyalochaete) spp. bloom throughout the entire productive period. The enhanced vertical mixing characterizing the southeastern KP (Park et al. 2008b) thus is highly likely to facilitate the continuous seeding of Chaetoceros resting spores to surface waters.

Comparison with previous artificial and natural iron-fertilization experiments

Diatom community composition at station R-2 fits with typical diatom assemblages of HNLC systems as described by Smetacek, [START_REF] Smetacek | The role of grazing in structuring Southern Ocean pelagic ecosystems and biogeochemical cycles[END_REF], being composed of larger and more heavily silicified species (e.g. Rhizosolenia spp., Fragilariopsis spp.), and/or species that possessed robust spines (e.g. C. pennatum), than species commonly found in iron-replete conditions (Kopczy ńska, [START_REF] Fiala | Seasonal and interannual variability of size-fractionated phytoplankton biomass and community structure at station Kerfix, off the Kerguelen Islands, Antarctica[END_REF][START_REF] Smetacek | The role of grazing in structuring Southern Ocean pelagic ecosystems and biogeochemical cycles[END_REF][START_REF] Poulton | Phytoplankton community composition around the Crozet Plateau, with emphasis on diatoms and Phaeocystis[END_REF]. HNLC areas have also been reported as hosting a background diatom assemblage composed of lightly silicified and/or small size species [START_REF] Smetacek | The role of grazing in structuring Southern Ocean pelagic ecosystems and biogeochemical cycles[END_REF], such as the Chaetoceros (Hyalochaete) spp. and small centric communities, which were also encountered at R-2 (Fig. 4e). A similarly structured plankton community was observed at the KERFIX station, located further south of the station R-2 (50 • 40 S, 68 • 25 E) at the limit between the HNLC area and the western Kerguelen Plateau. At this site, diatoms only ever contributed 11% C biomass between the years 1990 to 1995. Species analyses from this study indicate that F. kerguelensis and T. nitzschioides were the principal diatom contributors with other seasonally occurring species (Chaetoceros spp., Pseudo-nitzschia spp., F. pseudonana and C. pennatum;[START_REF] Fiala | Seasonal and interannual variability of size-fractionated phytoplankton biomass and community structure at station Kerfix, off the Kerguelen Islands, Antarctica[END_REF].

The dominant species found at the highly productive ironfertilized stations (A3-2, E-4W and F-L) belonged to the taxa groupings Chaetoceros (Hyalochaete) spp. and Thalassiosira spp. (Fig. 4a-c). These two taxa groups are commonly found in ironinduced blooms [START_REF] Smetacek | The role of grazing in structuring Southern Ocean pelagic ecosystems and biogeochemical cycles[END_REF] and their stimulation in mesoscale iron-enrichment experiments conducted in polar regions (Southern Ocean: [START_REF] Assmy | Mechanisms determining species dominance in a phytoplankton bloom induced by the iron fertilization experiment EisenEx in the Southern Ocean. Deep Sea Res Part I[END_REF][START_REF] Assmy | Thick-shelled, grazerprotected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF][START_REF] Boyd | Mesoscale iron enrichment experiments 1993-2005: Synthesis and future directions[END_REF]Subarctic Pacific: Tsuda et al. 2003) has been well documented. Of particular note, artificial iron enrichment in the South Atlantic produced a C. debilis cell abundances 150-fold higher compared with the initial abundances (EisenEx, [START_REF] Assmy | Mechanisms determining species dominance in a phytoplankton bloom induced by the iron fertilization experiment EisenEx in the Southern Ocean. Deep Sea Res Part I[END_REF]). The KEOPS 1 and 2 results combined suggest that a similar expansive response in cell abundances in the Chaetoceros (Hyalochaete) spp. occurs in the naturally fertilized Kerguelen region between pre-bloom spring and the end of summer [START_REF] Armand | Late summer diatom biomass and community structure on and around the naturally iron-fertilised Kerguelen Plateau in the Southern Ocean[END_REF].

One major difference between the naturally iron-fertilized Kerguelen blooms and the previous Southern Ocean mesoscale iron-enrichment experiments was the lower contribution of the oceanic diatoms F. kerguelensis and Pseudo-nitzschia spp. to diatom C biomass (e.g. IronEx II, [START_REF] Coale | A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean[END_REF]SOIREE, Gall et al. 2001) as compared with the neritic Chaetoceros (Hyalochaete) spp. This notable difference stokes the uncertainty surrounding the validity of comparing naturally iron-fertilized blooms, usually located in the vicinity of islands and possibly viewed as extensions of coastal blooms, with iron-induced blooms in the open-ocean [START_REF] Smetacek | The next generation of iron fertilization experiments in the Southern Ocean[END_REF][START_REF] Salter | Diatom resting spore ecology drives enhanced carbon export from a naturally ironfertilized bloom in the Southern Ocean[END_REF]Quéguiner 2013). In our study, the influence of the KP on the phytoplankton community composition is clear from the high contribution of the Chaetoceros (Hyalochaete) spp. to cell abundance and C biomass in the northeast Kerguelen blooms (Fig. 4). This phytoplankton response is considered to be influenced by the northward lateral advection of waters coming from the plateau across the Heard and McDonald Islands to the northeast Kerguelen region (Park et al. 2008a(Park et al. , 2014b;;[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] Bowie | Iron budgets for three distinct biogeochemical sites around the Kerguelen Archipelago (Southern Ocean) during the natural fertilisation study, KEOPS-2[END_REF]. In our easternmost sampled station (Station F-L) the additional effect of the northeastward drift of the coastal waters driven by the Subantarctic surface water flow [START_REF] Blain | A biogeochemical study of the island mass effect in the context of the iron hypothesis: Kerguelen Islands, Southern Ocean. Deep Sea Res Part I[END_REF][START_REF] Bucciarelli | Iron and manganese in the wake of the Kerguelen Islands (Southern Ocean)[END_REF]d'Ovidio et al. 2015) undoubtedly modified the phytoplankton community. The hydrographic properties by guest on September 19, 2016 http://femsec.oxfordjournals.org/ Downloaded from of the eastern KP region have been shown to generate complex interactions between water masses with distinct origins and biogeochemical characteristics (Park et al. 2014b). The notable presence of typical open-ocean species in the northeastern Kerguelen blooms (e.g. F. kerguelensis at station F-L or T. antarctica and D. antarctica at station E-4E; Fig. 3) provides evidence of phytoplankton community compositions that are not the result of coastal bloom extensions into the Southern Ocean. The influence of the open-ocean on the northeastern Kerguelen blooms was also supported by the mesozooplankton community composition, where, for example, high abundances of oceanic species such as the copepods Ctenocalanus citer (typical of oceanic Subantarctic and Antarctic waters) and Scolecithricella minor (cosmopolitan species; [START_REF] Carlotti | Mesozooplankton structure and functioning during the onset of the Kerguelen phytoplankton bloom during the KEOPS2 survey[END_REF] were observed. Phytoplankton community composition of the naturally ironfertilized Kerguelen blooms are therefore the result of complex interactions (e.g. interspecific competition, top-down control) between plankton communities originated from both coastal and open-ocean waters. Our observations would explain the noticed discrepancy with previous mesoscale iron-enrichment experiments.

Potential contribution of surface phytoplankton community structure on C export

When comparing C biomass across functional planktonic groups (Fig. 6) with concurrent biogeochemical observations, our results confirmed that diatoms largely dominated C biomasses from the surface to 200 m within the natural iron-fertilized blooms (i.e. the stabilized, final bloom conditions observed at A3-2, E-4W and F-L). These observations suggest that surface diatom communities of the iron-fertilized systems can be considered a potentially valuable vector of C export to the deep ocean. This idea is in accordance with sediment trap results from the stations A3-2 and F-L, where particle fluxes around 100 and 200 m were mainly mediated through diatom-dominated phytodetrital and fecal aggregates [START_REF] Laurenceau-Cornec | The relative importance of phytoplankton aggregates and zooplankton fecal pellets to carbon export: insights from free-drifting sediment trap deployments in naturally iron-fertilised waters near the Kerguelen Plateau[END_REF]. Unfortunately, the composition of aggregates in the latter study was not specifically determined so the diatoms responsible for particle fluxes in the upper 200 m were not identified. However, our observations by light microscopy from CTD rosette samples suggested numerous aggregates of Chaetoceros (Hyalochaete) spp. at the A3-2 and E-4W stations, as well as abundant fecal pellets (50-100 μm) composed of small centric diatoms (<25 μm, including Thalassiosira spp.) at station F-L (data not shown).

Comparison with the composition of the diatom assemblage collected in a sediment trap around 290 m at A3-2 station (Rembauville et al. 2015) over the KP reveals a striking feature: diatom species dominating the exported material below 200 m did not correspond to the dominant Chaetoceros (Hyalochaete) spp. in the surface waters. Total diatom flux was mainly mediated by empty frustules of the small centric diatoms (<25 μm, including Thalassiosira spp.) and the heavily silicified diatoms T. nitzschioides and F. kerguelensis [START_REF] Rembauville | Export fluxes in a naturally iron-fertilized area of the Southern Ocean -Part 2: Importance of diatom resting spores and faecal pellets for export[END_REF] that dominated empty frustule abundance in the upper 200 m (Fig. 5). This noticeable feature suggests that these heavily silicified diatoms were more efficiently exported than the lightly silicified Chaetoceros (Hyalochaete) spp. cells, likely due to the species-specific variability in their cell properties (morphology, life cycle, elemental composition, aggregate formation and potential rapid remineralization) and interactions with higher trophic levels [START_REF] Assmy | Thick-shelled, grazerprotected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF]Quéguiner 2013;[START_REF] Laurenceau-Cornec | The relative importance of phytoplankton aggregates and zooplankton fecal pellets to carbon export: insights from free-drifting sediment trap deployments in naturally iron-fertilised waters near the Kerguelen Plateau[END_REF]. During the CROZEX natural iron-fertilization study [START_REF] Pollard | The Crozet Natural Iron Bloom and Export Experiment (CROZEX). Deep Sea Res Part II[END_REF], selective export of large and heavily silicified diatoms (i.e. E. antarctica resting spores) was similarly observed as a result of the bloom conditions north of the Crozet Plateau [START_REF] Salter | Diatom resting spore ecology drives enhanced carbon export from a naturally ironfertilized bloom in the Southern Ocean[END_REF].

Interestingly, this feature closely aligns with two crucial biogeochemical features characterizing the KP bloom in early spring (as interpreted from station A3-2). First, the unexpected low C export efficiency reported below 200 m [START_REF] Planchon | Carbon export in the naturally iron-fertilized Kerguelen area of the Southern Ocean based on the 234 Th approach[END_REF] despite the high accumulation of C and BSi biomass in the surface layers [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]) could be related to the retention of the dominant Chaetoceros (Hyalochaete) spp. cells (>55% of the C biomass) in the surface waters. Second, the KP spring bloom was assumed to behave as a silicon pump via the observation of unusually high silicon to carbon ratios in the upper 200 m (i.e. bulk particulate matter and uptake rates; [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] and in exported material around 290 m [START_REF] Rembauville | Export fluxes in a naturally iron-fertilized area of the Southern Ocean -Part 2: Importance of diatom resting spores and faecal pellets for export[END_REF]. Such findings are inevitable given that empty frustules of typical heavily silicified species (T. nitzschioides and F. kerguelensis; e.g. [START_REF] Assmy | Mechanisms determining species dominance in a phytoplankton bloom induced by the iron fertilization experiment EisenEx in the Southern Ocean. Deep Sea Res Part I[END_REF]) would be preferentially exported over the dominant Chaetoceros (Hyalochaete) spp. live cells.

All these observations combined clearly illustrate the close link between oceanic biogeochemical cycles and phytoplankton communities at the species-specific level. This emphasizes the need to go further than the diatom functional group concept and to focus on biological interactions at the individual species level in order to better understand the key factors controlling the fate of iron-induced blooms from surface waters to the seafloor [START_REF] Laurenceau-Cornec | The relative importance of phytoplankton aggregates and zooplankton fecal pellets to carbon export: insights from free-drifting sediment trap deployments in naturally iron-fertilised waters near the Kerguelen Plateau[END_REF][START_REF] Rembauville | Export fluxes in a naturally iron-fertilized area of the Southern Ocean -Part 2: Importance of diatom resting spores and faecal pellets for export[END_REF]Rigual-Hern ández et al. 2015a).

CONCLUSION

Spatial differences in the plankton community structures were tightly linked to the evolving stage of phytoplankton development dependent on the physical and chemical characteristics of the study areas. The iron-limited HNLC area (R-2) was typically characterized by autotrophic nanoeukaryote-dominated communities with low C biomass contributions from large and highly silicified diatom species (e.g. Fragilariopsis spp., Rhizosolenia spp., Corethron pennatum). Despite iron-replete stocks, comparable plankton communities were reported in the meander of the PF at the beginning of the mission (Stations E-1 and E-3) where non-blooming conditions were previously reported. The start of a well-established spring bloom in this area (Stations E-4E and E-5) was characterized by a remarkable increase in the C biomass contribution of diatoms, and especially those of Chaetoceros (Hyalochaete) spp. This genus together with an assemblage of small centric diatoms (mainly composed of Thalassiosira spp.) overwhelmingly dominated the well-established spring blooms at three different sites over the eastern KP area (Stations A3-2, E-4W and F-L).

Our observations confirmed that diatoms largely dominated C biomass in the naturally iron-fertilized surface waters, suggesting their potential for exporting C to the deep ocean compared with HNLC systems. Despite the high contribution of Chaetoceros (Hyalochaete) spp. to the northeastern Kerguelen blooms, the notable presence of typical open-ocean species implied that the phytoplankton community composition would be the result of complex interactions (e.g. interspecific competition, top-down control) between plankton communities originated from both coastal and open-ocean waters.

Combining the KEOPS 1 and 2 mission data sets from the southeastern KP station (station A3-2) revealed the sustained dominance of Chaetoceros (Hyalochaete) spp. throughout the by guest on September 19, 2016 http://femsec.oxfordjournals.org/ Downloaded from productive period (from mid-November to mid-January). We hypothesize that this sustained presence and dominance is due to the continuous seeding of Chaetoceros resting spores from below the pycnocline up into the surface waters. Regardless of their input and continued surface development, the dominant Chaetoceros (Hyalochaete) cells were found to be less efficiently exported than diatom species with low abundances in early spring. Such knowledge highlights the key role of speciesspecific cell properties and trophic interactions on matter export efficiency. Our study emphasizes the importance of exploring the specific composition of natural diatom assemblages in relation to biogeochemical parameters to better understand the functioning of marine biogeochemical cycles. We believe our results call for continued exploration of spatio-temporal scale responses appropriate to the dynamics of biological processes (e.g. silicification at the cellular scale, biological compartment interactions) while considering key environmental factors shaping phytoplankton community ecology and structure.

Figure 1 .

 1 Figure 1. Location of the eight sampling stations (a) and MODIS-Aqua satellite (CLS-CNES) images of surface chlorophyll a concentration (Chl a) representative of phytoplankton bloom stages at the sampling time of each station (b-f). R-2 is the HNLC reference station. F-L is located in the eastern bloom within the Polar Front zone. The stations A3-2 and E-4W are respectively the reference southeastern Kerguelen Plateau bloom and the reference eastern flank of the Kerguelen Island. The stations E-1, E-3, and E-5 correspond to a time series of the recirculation system in the meander of the Polar Front. E-4E is located further east compared with the three latter stations and was associated to a distinct physical feature (Park et al. 2014b). The dotted line represents the approximate location of the southern branch of the Polar Front and the white arrow corresponds to the Antarctic Circumpolar Current core (ACC Core) (from Park et al. 2014b).

  by guest on September 19, 2016 http://femsec.oxfordjournals.org

Figure 2 .

 2 Figure 2. Vertical distributions of live cell abundance (in gray), empty frustule abundance (in white) and carbon biomass (black dots) of diatoms at each studied station. The dashed line indicates the approximate location of the mixed layer depth (MLD). A second discontinuity was identified at E-1 from the density CTD profile.

  by guest on September 19, 2016 http://femsec.oxfordjournals.orgdominant, c: common, m: minor, r: rare Specific contribution to live cell abundance (%)

Figure 3 .

 3 Figure 3. Qualitative assessment of diatom community composition from net hauls in the upper 100 m and comparison with the CTD rosette results averaged at all depths sampled over the water column. Only the main diatom species or genera observed over the study area are reported in this figure.

Figure 4 .Figure 5 .

 45 Figure 4. Vertical distributions of the relative contributions of major diatom species (as a percentage) to live cell abundance and carbon biomass at each studied station. The asterisk in front of a given light level indicates the presence of Chaetoceros resting spores. 'Other species' corresponds to low cell abundance and biomass species (≤1% of contribution). The dashed line indicates the approximate location of the mixed layer depth (MLD) and the second discontinuity observed at E-1.

Figure 6 .

 6 Figure 6. Relative contribution of each plankton group to carbon biomass (%) at the eight studied stations within the euphotic layer and the subeuphotic layer (defined as the layer extending 100 m below the euphotic depth, Ze). The color code ensures easy identification of autotrophic and heterotrophic organisms: blue-green colors are associated to autotrophic organisms while heterotrophic organisms are represented in red-orange.

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ACKNOWLEDGEMENTS

We thank the KEOPS coordinator Prof. St éphane Blain, Captain Bernard Lassiette and crew of the R/V Marion Dufresne II for their support aboard. We acknowledge the assistance of V éronique Cornet-Barthaux (MIO) for phytoplankton sampling. MODIS (NASA) satellite ocean-color images courtesy of Francesco d'Ovidio (UPMC, Univ. Paris), produced by Ssalto/Duacs and CLS with support from CNES. We thank the two anonymous referees for their constructive comments.

FUNDING

This work was supported by the French Research program of INSU-CNRS LEFE-CYBER (Les enveloppes fluides et l'environnement -Cycles biog éochimiques, environnement et ressources); the French ANR (Agence Nationale de la Recherche, SIMI-6 program); the French CNES (Centre National d' Études Spatiales); the French Polar Institute IPEV (Institut Polaire Paul-Emile Victor); an Endeavour Research Fellowship [grant to M.L.]; and an Australian Antarctic Division science support grant [AAS 3214 to L.A.].

SUPPLEMENTARY DATA

Supplementary data are available at FEMSEC online.

Conflict of interest. None declared.