Biological Productivity Regime and Associated N Cycling Printer-friendly Version Interactive Discussion Biological Productivity Regime and Associated N Cycling in the Vicinity of Kerguelen Island Area, Southern Ocean Bgd Biological Productivity Regime and Associated N Cycling Printer-friendly Versio

This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract Although the Southern Ocean is considered a High Nutrient Low Chlorophyll area (HNLC), massive and recurrent blooms are observed over and downstream the Ker-guelen Plateau. This mosaic of blooms is triggered by a higher iron supply resulting from the interaction between the Antarctic Circumpolar Current and the local 5 bathymetry. Net primary production, N-uptake (NO − 3 and NH + 4), and nitrification rates were measured at 8 stations in austral spring 2011 (October–November) during the KEOPS2 cruise in the Kerguelen area. Iron fertilization stimulates primary production, with integrated net primary production and growth rates much higher in the fertilized areas (up to 315 mmol C m −2 d −1 and up to 0.31 d −1 , respectively) compared to the HNLC 10 reference site (12 mmol C m −2 d −1 and 0.06 d −1 , respectively). Primary production is mainly sustained by nitrate uptake, with f ratio (corresponding to NO − 3 uptake/(NO − 3 uptake + NH + 4 uptake)) lying in the upper end of the observations for the Southern Ocean (up to 0.9). Unexpectedly, we report unprecedented rates of nitrification (up to ∼ 3 mmol C m −2 d −1 , with ∼ 90 % of them < 1 mmol C m −2 d −1). It appears that nitrate 15 is assimilated in the upper part of the mixed layer (coinciding with the euphotic layer) and regenerated in the lower parts. We suggest that such high contribution of nitrifi-cation to nitrate assimilation is driven by (i) a deep mixed layer, extending well below the euphotic layer, allowing nitrifiers to compete with phytoplankton for the assimilation of ammonium, (ii) extremely high rates of primary production for the Southern Ocean, 20 stimulating the release of dissolved organic matter, and (iii) an efficient food web, allowing the reprocessing of organic N and the retention of nitrogen into the dissolved phase through ammonium, the substrate for nitrification.


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Tables Figures is assimilated in the upper part of the mixed layer (coinciding with the euphotic layer) and regenerated in the lower parts.We suggest that such high contribution of nitrification to nitrate assimilation is driven by (i) a deep mixed layer, extending well below the euphotic layer, allowing nitrifiers to compete with phytoplankton for the assimilation of ammonium, (ii) extremely high rates of primary production for the Southern Ocean, stimulating the release of dissolved organic matter, and (iii) an efficient food web, allowing the reprocessing of organic N and the retention of nitrogen into the dissolved phase through ammonium, the substrate for nitrification.

Introduction
The Southern Ocean is considered a High Nutrient Low Chlorophyll (HNLC) system due to a combination of several factors, including upwelling of nutrient-rich waters south of the Antarctic Polar Front and, iron (Fe) and light co-limitation for phytoplankton growth (Martin, 1990;Falkowski et al., 1998;Sarmiento et al., 2004).The Southern Ocean is also recognized as a major hot spot for gas exchanges accounting for up to 20 % of the global ocean CO 2 uptake (Takahashi et al., 2009).The concern regarding ongoing climate change has triggered a great interest for this part of the global ocean (Le Quéré et al., 2007).
Since the Iron Hypothesis highlighted by Martin (1990) the last two decades have witnessed an exponential research interest on iron availability and associated biological processes.The aim was to explore the role of iron in enhancing the biological pump and the subsequent increase in the drawdown of atmospheric CO 2 .Numerous on-site artificial Fe-enrichment experiments (see Boyd et al., 2007) were conducted in situ to explore the fertilization effect of iron (de Baar et al., 2005, 208;Boyd et al., 2007;Smetacek et al., 2012;Quéguiner, 2013).In parallel, several naturally Fe-enriched sites were studied (e.g., Boyd et al., 2007) to understand the mechanisms driving such iron fertilization and their impacts on biogeochemical systems without external forcing, i.e. without artificial Fe-enrichment (Blain et al., 2007(Blain et al., , 2008;;Pollard et al., 2007Pollard et al., , 2009;;Bowie et al., 2011).The KEOPS project, carried out in the Kerguelen area, is dedicated to the understanding of the functioning of a naturally iron-fertilized system over a large and seasonal scale.For that purpose a first cruise was completed during austral summer 2005 (KEOPS1, February-March; Blain et al., 2007) and a second cruise was completed during austral spring 2011 (KEOPS2, October-November 2011).
Through its crustal and seawater interface, the Kerguelen area (composed of Kerguelen Island, the Kerguelen Plateau and Heard Island in the south-east) enriches the surrounding waters with macronutrients and trace elements.It also strongly impacts the local oceanic physical dynamics due to its Island Mass Effect (IME; Doty and Introduction

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Full Oguri , 1956).The interactions between geostrophic water flow disturbances due to the presence of the island, the tidal activity over the plateau, and the strong winds characterizing the area produce a complex hydrodynamical environment, resulting from the generation of internal waves, eddies, jets, and Ekman pumping (Heywood et al., 1990;Park et al., 2008;Gille et al., 2014).These characteristics are key factors inducing the fertilization and the associated higher productivities around the Kerguelen Island area as compared to the HNLC surrounding waters (Blain et al., 2001(Blain et al., , 2007;;Fig. 1).Moreover, a third condition impacting productivity is the proximity of the Polar Front (PF), which trajectory is steered by the bottom topography (Park et al., 2014).Such frontal system by itself offers peculiar conditions inducing significant biological activity (de Baar et al., 1995).
Here we report net primary production at 8 stations visited during KEOPS2, representing the variability encountered over the Kerguelen plateau area and downstream, together with N-uptake (NO − 3 and NH + 4 ) and nitrification rates.Regarding the nitrogen cycle, studies conducted during KEOPS1 showed a decoupling of the seasonal use of nitrate and silicic acid stocks over the Kerguelen Plateau (Mosseri et al., 2008), although Si : NO 3 assimilation ratios were close to 1, in accordance with what is expected for non-limiting iron conditions (Takeda, 1998;Hutchins and Bruland, 1998).Mosseri et al. (2008) attributed this peculiar decoupling to the capacity of diatoms to grow on an ammonium stock resulting from a high heterotrophic activity at the end of the productive period, thus highlighting the importance of nitrogen recycling in the surface layer.In this context, the objectives of the studies undertaken during KEOPS2 were: 1. to evaluate and compare the C-and N-assimilation rates during austral spring over the Kerguelen Plateau, in the wake of Kerguelen Island, and at a reference HNLC site, 2. to assess the importance of recycling processes in the supply of phytoplankton nitrogen in the surface layer, Introduction

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Materials and methods
The KEOPS 2 expedition took place in the Indian sector of the Southern Ocean during the early spring of 2011 (September to November) on board R/V Marion Dufresne.
We sampled 8 sites for process studies (Fig. 1), including a reference station (R2) in a HNLC off-plateau region located south-west of Kerguelen Islands; 3 stations within the phytoplankton plume (A3-2, above the Plateau; E4W on the Plateau margin and F-L north of the Polar Front to the east of Kerguelen; and 4 stations (E1, E3, E4E, E5) in a complex recirculation system of a stationary meander confined by the Polar Front.At these 8 sites we performed isotopic-tracer incubations (24 h) to assess the following rates: -C-fixation rates (hereafter defined as net primary production) by adding 13 C-DIC into incubation bottle, and subsequent measurement of the incorporation of 13 C into biomass.
-Ammonium and nitrate uptake rates by adding either 15 N-NH -Nitrification rates by adding 15 N-NO − 3 into incubation bottle, the 14 N isotopic dilution of nitrate by the oxidation of both ammonium and nitrite by nitrifiers is measured.
The surface water was sampled at 7 to 8 depths corresponding to 75, 45, 25, 16, 4, 1, 0,3, 0,01 % of the surface photosynthetically active radiation (PAR), using Niskin bottles mounted on a rosette fitted with a PAR sensor.For each light level, two acid-cleaned 1 L polycarbonate incubation bottles were filled with seawater.The two bottles were spiked with 200 µmol L −1 of 13 C-HCO Introduction

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Full calculated taking into account the original nitrate and ammonium concentrations in order to achieve concentration increments < 10 %.Incubation bottles were then placed into on-deck incubators, cooled with circulating surface seawater, and wrapped in neutral density screens simulating the photometric depths.Incubation experiments were stopped after 24 h.This relatively long incubation time implies that we probably underestimated uptake rates in case of Eq. (1) release of 15 N and 13 C to the dissolved organic pool during the course of the experiment (Bronk et al., 1994;Laws et al., 2002) and Eq. ( 2) ammonification, which would result in diluting the 15 N-spiked ammonium pool with 14 N.However, the choice of long incubation times was expected to increase sensitivity in order to detect nitrification, and to enable the comparison with Si uptake and dissolution experiments (Closset et al., 2014) carried out over similar 24 h periods.Both at the initial and final incubation times, subsamples for nitrate (10 mL) and ammonium (2 × 20 mL) were directly measured with continuous flow analysis (Aminot and Kérouel, 2007) and the fluorometric method (Holmes et al., 1999), respectively.A further additional 10 mL was sampled from the 15 N-NO − 3 bottle, filtered using 0.2 µm Acrodisc filters (Sartorius) and stored at −20 • C, for later nitrate isotope analysis.The remaining seawater was filtered on pre-combusted (450 • C) glass fiber filters (Sartorius, MGF, nominal porosity 0.7 µm, 25 mm diameter).Filters were placed in pre-combusted scintillation vials, dried at 50 • C and stored in the dark at room temperature until further analysis at the laboratory.
Particulate organic nitrogen (PON) and particulate organic carbon (POC) concentrations along with their 15 N and 13 C abundances (atom 13 C and 15 N %) were analysed via an elemental analyser-isotope ratio mass spectrometer (EA-IRMS) using the method described in Savoye et al. (2004).Atom 15 N % for nitrate was measured using the den-Introduction

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Full C-assimilation and ammonium uptake rates (ρNH + 4 ) were estimated from the equations of Dugdale and Goering (1967), the following is for C-fixation: where V is the specific C assimilation rate (in d −1 ), NPP net primary production (in µmol ), atom % 13 C the measured abundances minus the natural abundances, and tf and ti refer to the final and initial time of incubation, respectively.Except for nitrate for which initial abundances were measured, the initial abundances for both DIC and NH + 4 were calculated by taking into account the spike addition and isotopic abundance.
The nitrate uptake rate (ρNO − 3 ) was corrected for the isotope dilution effect during incubation and assessed with the equation provided by Nelson and Goering (1977a, b): Tables Figures

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Full The modelled calculated nitrification rates were screened for consistency with observed evolutions of nitrate concentrations over the duration of the incubation experiment and with measured nitrate uptake rates.The difference between nitrate uptake and nitrification should be compatible with the change in nitrate concentration over the duration of the experiment, taking into account a 10 % precision (relative SD) on rates and concentrations measurements.When the rates given by the model were incompatible with concentration evolution, it was considered as an outlier and discarded from the dataset.We point out that measuring nitrification rates under conditions of high ambient nitrate is methodologically challenging.Indeed high nitrate concentration implies (i) a poor sensitivity, which we estimate at 0.26 µmol L −1 d −1 by taking into account how much nitrification is needed to change the isotopic composition of the nitrate pool by 2sd, and (ii) a poor precision (±0.26 µmol L −1 d −1 ; estimated by taking into account the precision on the isotopic measurement; 2sd).Such values for sensitivity and precision of the nitrification rates exceed most of the nitrification rates reported to date for the open ocean (Ward, 2007).

Results
Except for the station (F-L) north of the Polar Front, the different water masses and associated biogeochemical properties were characteristic of the Antarctic Zone: a relatively warmer mixed layer at the surface and the remnant of the winter mixed layer in subsurface (temperature minimum layer; Park et al., 2008).South of the Polar Front, macro-nutrients (PO 3− 4 , NO − 3 , Si(OH) 4 ) were present at high and relatively uniform concentrations between stations (Christaki et al., 2014;Closset et al., 2014), with average nitrate concentration of ∼ 25 µmol L −1 in the mixed layer.In agreement with a progressive nutrient consumption along the meridional water transfer across the Antarctic Circumpolar Current (Sigman et al., 1999;Sarmiento et al., 2004), the station north of the Polar Front (F-L) presented higher surface temperatures (with an increase of ∼ 1-2 Full  (Christaki et al., 2014).Both particulate organic C and N concentrations (POC and PON) increased from the HNLC reference station (R2) to the meander stations (E stations), and then to the bloom station, over the Kerguelen Southeast Plateau and at the margin (A3-2, E4W) and North of the Polar Front (F-L) (Fig. 2).The high concentrations observed at station E4W are due to the interaction between the meander zone and the plateau at the margin as shown by Lasbleiz et al. (2014).Overall, this spatial distribution of phytoplankton biomass is in agreement with satellite chl a observations (Fig. 1).
A positive relationship is apparent between biomass (µmol L −1 ) and assimilation rates (µmol L −1 d −1 ), both for carbon and nitrogen (Fig. 2).Such relationship also holds between biomass and specific uptake rate (V ; d −1 ; growth rate) and doubling time (Td = ln(2)/V ; in days).The bloom over the Kerguelen Plateau (A3-2 and E4W) and north of the Polar Front (F-L) presented higher assimilation rates per unit of biomass (µmol C and N L −1 ) than the stations in the HNLC area (R2) and in the meander (E stations).
In general, vertical profiles of net primary production followed light availability in the mixed layer, with a sharp decrease below the euphotic layer depth (Ze; 1 % surface PAR depth) (Fig. 3).The primary production activity occurs well below Ze, although at much lower rates than in the upper levels of the surface layer.This is particularly obvious for the most productive stations; primary production rates are still 0.36, 0.37, and 0.12 µmol CL −1 at the 0.3 % surface PAR level, as compared to rates of 11.9, 8.3, and 6.2 µmol CL −1 at the surface, respectively at stations PF, E4W, and A3-2.At the 0.01 % surface PAR levels values of primary production are close to 0 for the least productive sites but are still significant at PF, E4W, and A3-2 stations (respectively 0.12, 0.06, and 0.08 µmol C L −1 ).Vertical profiles of N-assimilation rates closely paralleled the vertical evolution of primary production, for both NO ergetically demanding than ammonium, because nitrate has to be first reduced into ammonium, an ATP demanding processes (Mulholland and Lomas, 2007).In most of the stations, nitrate was still preferentially assimilated over ammonium (Fig. 4a), resulting in high f ratio defined as NO − 3 uptake/(NO − 3 uptake + NH + 4 uptake) ratio according to Dugdale and Goering (1967).The f ratios in the euphotic layer increased with Nuptake rates (NO − 3 + NH + 4 ), from ∼ 0.4 in the less productive HNLC reference station to ∼ 0.9 north of the Polar Front.
Unexpectedly, some nitrification rates were significant in the mixed layer, reaching values up to (Fig. 3).In general, nitrification rates reached maxima near the bottom of the euphotic layer (Fig. 3).

High primary production in naturally iron-fertilized blooms
At the most productive stations of the different blooms, primary production still persists, but a much lower rates, at levels far deeper than the 1 % surface PAR depth, traditionally regarded as the lower limit of the euphotic layer.Such an observation is not unusual and this has been recently discussed by Marra et al. (2014).
For this reason, we have calculated the vertically-integrated primary production rates are calculated from the surface down to the mixed layer depth.The spring phytoplankton blooms developing over and downstream the Kerguelen Southeast Plateau exhibit very high rates of net primary production (up to 315 mmol C m −2 d −1 ;) as compared to the adjacent HNLC area (∼ 13 mmol C m −2 ) (Fig. 5).Our value of primary production at Several massive blooms occur in the core of the Antarctic Circumpolar Current (Fig. 1): (i) over the Kerguelen Southeast Plateau, remarkably constrained by the bathymetry, (ii) in a plume extending eastward through the interaction between the Polar Front jet, crossing the plateau in a narrow mid-depth channel just to the south of the Kerguelen Island, and the rise bordering the basin to the north, (iii) to the easternmost part of the study area in a zone of retroflection of the Polar Front and of eddy mixing between Antarctic and Sub-Antarctic Surface Waters (see Park et al., 2014).This complex distribution is in agreement with an input of iron from the interaction between the iron-deficient Antarctic Circumpolar Current and the local bathymetry (Blain et al., 2007;Sokolov and Rintoul, 2007).Trace metals, and iron in particular, are required for many important cellular processes such as photosynthesis (including photo-adaptation), respiration, and nitrate reduction.Iron-enrichment experiments ranging from bottle incubations to large scale open-ocean amendment studies have demonstrated that iron supply stimulates phytoplankton growth in the Southern Ocean (Martin, 1990;de Baar et al., 1990de Baar et al., , 2005;;Boyd et al., 2007).Primary production up to < 130 mmol C m −2 d −1 has been reported in these iron-fertilized patches (Gall et al., 2001;Gervais et al., 2002;Coale et al., 2004;Smetacek et al., 2012).Naturally iron fertilized systems usually present higher rates of primary production, similar to the KEOPS study: e. given by Arrigo et al. (2008) for the most productive areas of the Seasonal Ice Zone.
Notwithstanding the complex interplay between biogeochemical and physical processes in this large-scale natural iron-fertilization experiment, we do observe a strong, relatively linear, simple relationship between the amount of biomass (µmol C and NL −1 ), and both, assimilation rates (µmol C and NL −1 d −1 ) and specific growth rates (d −1 ) (Fig. 2).When biomass increases, primary production per unit of biomass also increases.Such a trend cannot be explained by variations in macro-nutrient concentrations or temperature in the mixed layer.Indeed, the maximum difference in temperature was less than 2 • C, and the relative uniform and high macro-nutrient concentrations (Christaki et al., 2014;Closset et al., 2014) prevented any Si, P or N limitation for phytoplankton growth (e.g., Sarthou et al., 2005, and reference therein).The shortest doubling times for POC (2 days) correspond to stations F-L, A3-2 and E4W (Fig. 2e).Given the range of surface temperatures of these stations, the minimum doubling time is however shorter, on the order of 1 day, according to the growth-temperature relationship of Eppley (1972).This is indicative that some limitation occurs.Limitation by available light cannot ruled out since the euphotic layer could be shallower than the mixed layer, probably due to self-shading by the developing phytoplankton (Fig. 3).A higher supply of iron and trace metals over the Kerguelen Plateau and north of the Polar Front stands, however, as the most important cause for the observed differences in primary production (from ∼ 0 to 9 µmol CL −1 d −1 ) and C growth rate (from < 0.1 and 0.3 d −1 ) between stations (Martin, 1990;de Baar et al., 2005;Boyd et al., 2007;Blain et al., 2007; Fig. 2).Over the plateau iron is supplied through the interaction of the bottom waters with sediments and volcanic rocks at low bathymetry.By this process, subsurface water is becoming enriched in iron, which subsequently can be efficiently supplied to the mixed layer via vertical mixing, both in winter and spring (Blain et al., 2007, in prep;Park et al., 2008).At the Polar Front iron fertilization results from the previously

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Full described enrichment over the plateau and the spreading of this enrichment eastward through the additional interaction between the Polar Front jet and local bathymetry (Mongin et al., 2008;Sanial et al., 2014).This mode of supply can be compared with the Polar Front associated bloom in the Atlantic sector, where iron is supplied by lateral advection from the Patagonian shelves and islands in the Scotia Ridge area (de Baar et al., 1995) or by direct lithogenic aerosol deposits (Quéguiner et al., 1997).Similar increases in growth rate driven by the degree of iron fertilization (Fig. 2) have been reported earlier for the Southern Ocean (e.g., the growth rate increase from < 0.1 to 0.5 d −1 in the SOIREE fertilized patch; Gall et al., 2001).
Net primary production over the Kerguelen Plateau measured during summer (January-February, KEOPS 1; Mosseri et al., 2008), reached values close to ∼ 80 mmol C m −2 d −1 .This was lower than the estimates for spring in the present study (∼ 200 mmol C m −2 d −1 ) (Fig. 5).Such seasonal pattern is in agreement with the seasonal evolution observed from satellite chl a observations.These show that the bloom starts in early November, reaches its maximum level in late November-Early December, and collapses in January-February (Mongin et al., 2008;Blain et al., 2013).Contrasting with the spring situation studied in the present study, in summer there is no relationship between biomass and primary production or growth rate (Fig. 6).In early February, primary production and growth rate decreased rapidly from about 14 down to 2 µmol CL −1 d −1 , and from ∼ 0.3 down to 0.01 d −1 , respectively.Associated with these changes, the mixed layer evolved from an autotrophic system, typical for spring, to a heterotrophic situation where respiration exceeds photosynthesis (Lefèvre et al., 2008;Christaki et al., 2014).With still nitrate and phosphate at non-limiting concentration, a top-down control of the mortality of phytoplankton cells (mostly diatoms; George et al., 2014;Sackett et al., 2014) is the most likely cause of this change (Carlotti et al., 2008;Brussard et al., 2008;Sarthou et al., 2008).Late summer Si limitation of diatom growth cannot be ruled out too, since measured silicic acid concentrations (∼ 1 µmol L −1 ) are significantly below measured half saturation constants for Si-uptake (5-15 µmol L −1 ; Mosseri et al., 2008).However, Fv/Fm ratios over the Introduction

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Full plateau remained high indicating the phytoplankton community was relieved from nutrient stress, including iron (Timmermans et al., 2008).
To conclude, the resulting increase in integrated primary production (up to 8 times; Fig. 5) between fertilized (Plateau and Polar Front) and unfertilized areas (HNLC reference site) is similar to the largest increase reported for artificial iron addition experiments worldwide (Boyd et al., 2007; up to 10 times).Despite the much higher primary production for the fertilized area in comparison with the HNLC area, the export of organic matter remains relatively small during spring and summer (Savoye et al., 2008;Planchon et al., 2014).Low export efficiency suggests that the products of primary production are mainly recycled within the mixed layer by an efficient microbial loop (Sarthou et al., 2008;Obernosterer et al., 2008;Christaki et al., 2014;Malit et al., 2014).The carbon sequestration efficiency induced by iron-fertilization remains, therefore, relatively low (Blain et al., 2007(Blain et al., , 2014b)).

Upper Ocean N cycling: high nitrification rates in deep productive mixed layer
Except for the HNLC reference station, nitrate uptake was higher than ammonium uptake in the euphotic layer (Fig. 4a).The f ratio increases with measured N-uptake rates (NO ), from ∼ 0.4 in the less productive HNLC reference station to ∼ 0.9 north of the Polar Front.The latter value lies in the upper end for the Southern Ocean, and is indicative of a nearly complete NO − 3 -based primary production (Savoye et al., 2004; and references therein).Below the euphotic layer, ammonium is preferentially assimilated over nitrate, with f ratios ranging between 0.1 and 0.5 (not shown).Assimilation of nitrate is more energetically demanding than ammonium and should be more dependent on light.Once nitrate has been transported into the cell, it has to be further reduced into ammonium before it gets assimilated (Mulholland and Lomas, 2007).At the end of summer, the f ratio over the plateau decreases rapidly, from 0.6 to 0.2, indicating that the system evolved from a nitrate-based to an ammonium-based primary production (Mosseri et al., 2008;P. Raimbault, unpub. results 2014).Such evolution Introduction

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Full is in agreement with an increase in ammonium availability in a decaying bloom.We acknowledge that f ratios can be overestimated, since it is likely that also organic nitrogen compounds are incorporated by phytoplankton (Bronk et al., 2007).However, we note that urea uptake, a proxy for dissolved organic N assimilation, is usually relatively low south of the Polar Front, where dissolved inorganic N is abundant (Waldron et al., 1995;Sambrotto et al., 2000;Savoye et al., 2004).Except for the HNLC reference station, the euphotic layer depth is relatively constant between stations while mixed layer depth varies significantly.The latter is generally deeper, more variable and extends more significantly below the euphotic layer over the Kerguelen plateau and at the HNLC reference station.Such spatial segregation of light distribution in the mixed layer, more or less expressed between stations, has profound consequences on the cycling of nitrogen.Nitrification rates are high (up to 3 µmol L ) at low light intensities, and insignificant (below our detection limit) at high light intensities (Fig. 4b).Such a distribution fits supports the fact that nitrification is known to be inhibited by light (Hagopian and Riley, 1998;Ward, 2007) even though if in a recent compilation, Yool et al. ( 2007) observed no clear relationship in the euphotic layer with depth.The review of Ward (2007) shows that highest nitrification rates occur near the bottom of the euphotic layer.At this depth light intensity is reduced, phytoplankton is light limited (Fig. 2), and nitrifiers can compete with phytoplankton for ammonium.Indeed, the energy yield of ammonium oxidation is low (Hagopian and Riley, 1998) and in case there is enough light to sustain primary productivity and associated nutrient-assimilation, nitrifiers are unlikely to out-compete phytoplankton.
In the mixed layer the extinction of light with depth promotes nitrate assimilation in the upper parts (euphotic layer) and nitrification in the lower parts (Figs. 2 and 4b).
Since a deep mixed layer is a common feature in the Southern Ocean, iron fertilization should boost nitrification by stimulating the release of dissolved organic matter in this area, originated by higher phytoplankton growth (Malits et al., 2014).In turn the microbial foodweb is stimulated and will thus process organic matter more efficiently Introduction

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Full  (Obernosterer et al., 2008;Christaki et al., 2014;Malits et al., 2014).The effect of viruses is also enhanced (Malits et al., 2014), increasing bacterial mortality and allowing the retention of organic matter within the mixed layer.These conditions combined with grazing (Brussard et al., 2008;Sarthou et al., 2008) likely promote the release of ammonium from remineralization of organic N and excretion by zooplankton.Ammonium is the substrate for nitrification and larger substrate availability will likely be favorable for the development of an efficient nitrifier community.
In this study, the nitrification rates are well above the maximum rates reported for the open ocean, up to 0.75 µmol L −1 d −1 in the Peru Upwelling (Lipschultz et al., 1991;Ward, 2007), and for the Southern Ocean (< 0. Olson, 1981;Bianchi et al., 1997).The available Southern Ocean nitrification data pertains to fall and winter, seasons during which primary production and remineralization are expected to be low.Such high nitrification rates are likely to be driven by: (i) very high rates of primary production (Sect.4.1), similar to the most productive areas in the ocean; (ii) the indirect iron-driven effect on the ability of the microbial foodweb to process organic matter (Malits et al., 2014;Christaki et al., 2014); and (iii) a shallower euphotic layer compared to the mixed layer, allowing nitrifiers to compete with phytoplankton for the assimilation of ammonium in the depth interval between the bottom of the euphotic layer and the mixed layer.This is further confirmed by Fripiat et al. (in prep)  ally considered as a new nutrient, with little regeneration in the mixed layer.However, several recent studies highlight that nitrification rates in the surface waters at the global scale has been underestimated (Yool et al., 2007;Ward, 2007;Clark et al., 2008).The Southern Ocean is still generally presented as an exception.Indeed, reported nitrification rates are low (Olson, 1981;Bianchi et al., 1998), but these studies, as described before, are not representative of the bloom condition.Instead, we show that nitrification efficiently regenerates nitrate over and downstream the Kerguelen Plateau.Iron fertilization by the plateau likely promotes higher nitrification rates than in the HNLC area.However, deep mixed layers are widespread in the Southern Ocean.It appears likely that the decoupling between nitrate assimilation and nitrification with depth in the mixed layer, is in some degree an ubiquitous condition during the vegetative season.
High mixed layer nitrification is also in agreement with (i) the measured low carbon export efficiency (NPP/ 234 Th-export; Savoye et al., 2008;Planchon et al., 2014), and (ii) the low nitrate seasonal depletion (Mosseri et al., 2008) despite high nitrate assimilation rates.Significant nitrification can also explain why silicic acid is depleted (with high biogenic silica production/dissolution ratio) but not nitrate (Mosseri et al., 2008;Closset et al., 2014).Si : NO 3 assimilation ratios were close to 1 (Mosseri et al., 2008;Closset et al., 2014), in accordance with what is expected for non-limiting iron conditions (Takeda, 1998;Hutchins and Bruland, 1998).Without nitrification, nitrate will be depleted as silicic acid.The hypothesis puts forward in Mosseri et al. (2008), attributing this peculiar decoupling to the capacity of diatoms to grow on an ammonium stock resulting from a high heterotrophic activity, is unlikely.Ammonium assimilation rates are much lower than nitrate and nitrification efficiently competes with phytoplankton for ammonium.

Conclusions
This study further confirms the impact of iron-fertilization on primary production in the Southern Ocean.Fertilized areas present much higher integrated primary production Introduction

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Full ) and carbon growth rates (up to 0.31 d −1 ) than unfertilized HNLC areas (12 mmol C m −2 d −1 ; 0.06 d −1 , respectively).Primary production in the euphotic layer of the fertilized areas is mainly sustained by nitrate uptake (f ratio up to 0.9).In the unfertilized areas, the contribution of ammonium to primary production increases, with f ratio down to 0.1.
Unexpectedly, nitrification rates are high (up to ∼ 3 µmol L ) and mirrors nitrate uptake rates with depth.Nitrate is mainly assimilated in the upper parts of the deep mixed layer, corresponding to the euphotic layer (between 100 and 1 % PAR), and regenerated below.Such high nitrification rates could be explained by (i) deep mixed layer depth, encompassing the euphotic layer, inducing a spatial segregation within the mixed layer of light, (ii) unprecedented rates of primary production, corresponding to one of the most productive place in the open ocean, stimulating the release of dissolved organic matter and secondary primary production, (iii) an efficient microbial foodweb, reprocessing efficiently organic N into ammonium, the substrate of nitrification.These conditions should be favorable for the development of an efficient nitrifier community.

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Full Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | lation was observed in the mixed layer all across the study area, but concentrations remained lower than 0

−
3 and NH+ 4 uptake.The decrease in NH + 4 assimilation rates with depth was less severe than that for NO − 3 .Assimilation of nitrate is more en-Discussion Paper | Discussion Paper | Discussion Paper | the HNLC station falls within the range of values reported for such environments.It is in the same order of magnitude as the value of ∼ 20 mmol C m −2 reported for the HNLC reference site of CROZEX in austral spring by Seeyave et al. (2007).In the Pacific sector of the Southern Ocean, Hiscock et al. (2003) report values ranging between 13 Discussion Paper | Discussion Paper | Discussion Paper | to 93 mmol C m −2 d −1 all over the seasonal cycle in an area extending from 54 to 72 • S spanning the Sub-Antarctic Zone, the Polar Frontal Zone, to the ice-edge in the wake of the retreating sea-ice.For the Atlantic sector, such low primary production rates, down to 25 mmol C m −2 are also reported by Jochem et al. (1995) in the Permanently Open Ocean Zone, south of the Polar Front, during austral spring.Finally, at a broader spatial scale, Arrigo et al. (2008) give values ranging from 4-6 mmol C m −2 in winter to 25-33 mmol C m −2 at the peak of the spring bloom in their so-called pelagic zone of the Southern Ocean.
g., up to ∼ 275 mmol C m −2 d −1 over the South Georgia plateau (Korb et al., 2005); and up to ∼ 250 mmol C m −2 d −1 within the fast-flowing, iron-rich jet of the Polar Front in the Atlantic sector (Jochem et al., 1995) and in the iron-fertilized plume Discussion Paper | Discussion Paper | Discussion Paper | of Crozet Islands (Seeyave et al., 2007).The production rates reported here for the Kerguelen plume then fell within the highest rates measured for the Southern Ocean and compare most favorably with the maximum values of 117 and 133 mmol C m −2 d −1 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | who simulated, over the Kerguelen Plateau, the change from October to February in the concentration of the fixed N pools and their isotopic composition (δ 15 N and δ 18 O), and show that nitrification can contribute between 40 and 80 % to the seasonal nitrate assimilation.Such findings have profound consequences on the concept of new production (Dudgale and Goering, 1967; Eppley and Petterson, 1979).New production is functioning on the supply of what it is called "new nutrients".New nutrients are distantly produced, supplied into the mixed layer by vertical mixing (both in winter and in summer), and assimilated during the vegetative season.To balance this supply, new production has to be equal to the production exported to depth, stripping out nutrients from the mixed layer.Especially in the Southern Ocean, with the ventilation of nutrient-rich deep waters, nitrate is gener-Discussion Paper | Discussion Paper | Discussion Paper |

Figure 1 .
Figure 1.KEOPS2 study area from October to November 2011.Chl a (color scale), surface velocity fields (arrows), the polar front (PF, black line), and the position of the different stations (black circles).Stations for which we performed incubations are represented by the white circles: the bloom stations over the Kerguelen plateau (E4-W and A3-2) and North of the Polar Front (F-L); and the meander stations (E stations).The reference HNLC station (R2) is not shown as it is out of the area of the map (Long.East 66.69, Lat.South 50.39).Note that the Chl a concentrations correspond to the last week of the KEOPS2 cruise.Courtesy of Y. Park and colleagues.