Carbon and nitrogen uptake in the South Pacific Ocean: evidence for eﬀicient dinitrogen fixation and regenerated production leading to large accumulation of dissolved organic matter in nitrogen-depleted waters

A major goal of the BIOSOPE cruise on the R/V Atalante to the South

to estimate rates of carbon fixation, inorganic nitrogen uptake (including dinitrogen fixation), ammonium (NH 4 ) and nitrate (NO 3 ) regeneration, and dissolved organic nitrogen (DON) release resulting from both NH 4 and NO 3 uptake.The SPG had revealed the lowest rates of primary production (0.1 gC.m −2 .d−1 ), while rates were 7 to 20 fold higher around the Marquesas Islands and in the Chilean upwelling, respectively.In this very low productive area, most of primary production was sustained by active regeneration processes which fuelled up to 95% of the biological nitrogen demand.Since nitrification was very active in the surface layer and often balanced the biological demand of nitrate, dinitrogen fixation, although acting at low daily rate (≈1-2 nmoles l −1 d −1 ), sustained the main part of new production.Then, new production in the SPG (0.008±0.007 gC m −2 .d−1 ) was two orders of magnitude lower than this measured in the upwelling where it essentially sustained by nitrate (0.69±0.49gC.m −2 .d−1 ).In the whole investigated region, the percentage of nitrogen release as DON represented a large part of the inorganic nitrogen uptake (13-15% in average), and reaching 26-41 % in the SPG where the production of DON appeared to be a major part of the nitrogen cycle.Due to the lack of annual vertical mixing and very low lateral advection, the high release rates could explain the large accumulation of dissolved organic matter observed in the nitrogen-depleted and low productive waters of the South Pacific Gyre.

Introduction
The nitrogen cycle in the oceanic gyres has been studied since the pioneering work begun by Menzel and Ryther in the Sargasso Sea (Menzel and Ryther, 1960;Ryther and Menzel, 1961).Some years later, Dugdale and Goering (1967) defined new and regenerated nitrogen and, hence new production, fuelling by allochthonous N-sources (mainly NO − 3 ) supplied via diffusion from the nitracline.On the other hand, "regenerated" production is fuelled by autochthonous N-sources (mainly NH + 4 ), derived from biological processes (Harrison et al., 1987).The fraction of primary production based on new nutrients is termed the f -ratio (Eppley and Peterson, 1979) and, at steady state, accounts for the proportion of production available for export.Historically, measurements of the nitrogen cycle were primarily based on 15 N tracer techniques.New production estimates, computed in terms of carbon by using the f -ratio and primary production, has known considerable variability (Aufdenkampe et al., 2002), due to the inaccuracy in estimating f and NO − 3 assimilation (Priscu and Downes, 1985;Ward et al., 1989;Gentilhomme and Raimbault, 1994;Raimbault et al., 1999;Diaz and Raimbault, 2000;Aufdenkampe et al., 2001).Some of the main sources of error in classical 15 N uptake experiments are still being debated, like nitrogen regeneration and release of dissolved organic nitrogen (DON) which inspired several revisions of the concepts of new and regenerated production (Fernandez and Raimbault, 2007).Ammonium regeneration is the main source of regenerated nitrogen to the euphotic zone.By studying ammonium isotopic dilution (recycling of unlabeled substrate) during 15 N incubation experiments, Glibert et al. (1982), and Harrison et al. (1987) showed that ammonium regeneration can result in significant underestimations of regenerated production, which would also bias the assessment of the f ratio.Nitrification (the oxidation of NH 4 to NO 3 mediated by bacteria) is also an important variable, now not only considered responsible for the deep nitrate reservoir but also believed to provide a source of "in situ" regenerated nitrate at the base of the euphotic zone (where phytoplankton is light limited to compete for NH EGU view in Ward 2000).Eppley et al. (1990) and Eppley and Koeve (1990) found rapid production of nitrate at shallow depth (20 m) in the Atlantic.Until the last decade, nitrification was thought to be restricted to specific environments but it is now known that it is more widely distributed (Zehr and Ward, 2002).Ward et al. (1989) and Eppley and Renger (1986) pointed out the importance of this process to new production in the southern California Bight.In the permanently oligotrophic North pacific Dore and Karl (1996) found nitrification could supply 47-142% of nitrate assimilation, results equivalent to those found in the equatorial Pacific, where nitrification can fuel 20 to 100% of new production (Raimbault et al., 1999).Its role in providing regenerated nitrate in the euphotic zone has been confirmed by recent findings in the Atlantic as well as in the Mediterranean Sea, where nitrification can support 25 to 100% of new production (Diaz and Raimbault, 2000;Fernandez and Raimbault, 2007).Failure in estimating dissolved inorganic nitrogen (DIN) taken up by phytoplankton and released as DON represents another and not entirely resolved source of error in DIN uptake rates (Bronk et al. 1994;Slawyk and Raimbault 1995).However, the impact of such processes is still rarely accounted as a possible overestimation factor in new production calculations and new production models suffers from a general lack of data, probably related to schedule constraints and the labour intensive nature of the methods involved.Finally, oceanographers have traditionally viewed the upward eddy-diffusive flux of nitrate as the nearly exclusive source of new nitrogen supporting the export flux of biogenic particles in the open oceans.In fact, the ubiquitous pool of dinitrogen gas (N 2 ) dissolved in the sea can represent a significant source of new nitrogen and estimates of biological nitrogen fixation have been recently revised upward (Galloway et al., 1995;Gr über and Sarmiento, 1997;Capone and Carpenter, 1999).In the north subtropical and tropical Atlantic and Pacific oceans, it has been estimated that N 2 fixation is equivalent to 50-180% of the flux of nitrate into the photic zone (Capone et al., 2005), demonstrating that a large part of new primary production is fuelled by N 2 fixation rather than deep nitrate diffusing from deeper layer into the photic zone.While the large size classes (Trichodesmium and diatoms containing endosynbiotic Richelia) are thought to be re-

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sponsible for the vas majority of N 2 fixation, recent works pointed out the important contribution of nanoplanktonic N 2 -fixers (Zehr et al., 2001;Karl et al., 2002).These small diazotrophic organisms, while present at low cellular concentration, could sustain a great part of the new production under nitrate-depleted conditions (Montoya et al., 2004;Falcon et al., 2002;Garcia et al., 2006Garcia et al., , 2007)).By representing 60% of the global ocean's area, the subtropical open-ocean ecosystems are the largest coherent biomes of our planet and the biogeochemical processes they support have global consequences (Karl, 2002).These environments provide ideal ecological niche for the development of nitrogen-fixing organisms.However, all of the studies dedicated to the nutrient control of nitrogen fixation have concentrated so far on the northern hemisphere and there is extremely few data available on the southern hemisphere, which contains the largest ocean area of the global ocean.Thus, precise measurements of nitrogen utilization are crucial to correctly relating new production to export of particulate (Eppley and Peterson, 1979) and dissolved (Toggeweiler, 1989) organic matter to deep ocean, and to quantify the biological impact of the ocean on the global carbon cycle.At present, however, the magnitude of these key fluxes is under debate, especially in oligotrophic waters, which represent a great fraction of the global ocean.
As our understanding of the nitrogen cycle recently moves on, these facts lead to a new revision of the f ratio and to important implications for the annual budget of the carbon and nitrogen cycles.Methods are now available to investigate simultaneously in oligotrophic waters primary production, nitrogen uptake including dinitrogen fixation (Montoya et al., 1999) and nitrogen regeneration (Glibert et al., 1982;Raimbault et al., 1999), as well as organic matter release (Bronk and Glibert, 1995;Slawyk and Raimbault, 1995;Karl et al., 1998).In this context, the BIOSOPE (BIOgeochemistry and Optics South Pacific Experiment) cruise was scheduled to provide a complete data set of biogeochemical parameters in the South Pacific Ocean.The South Pacific Central Gyre has been described as the most oligotrophic zone in the world ocean (Claustre and Maritorena, 2003) with an extreme nutrient limitation.It is also one of the least studied areas of the Ocean (Daneri and Quinones, 2001).The 8000 km transect,

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stretching from the Marquesas Islands to the Chilean coast and crossing the centre of South Pacific Gyre, is an ideal area to study primary production and new production along an extreme trophic gradient.The present work focuses on the geographical distribution of photosynthetic carbon fixation, nitrogen assimilation (including dinitrogen fixation and release of dissolved organic nitrogen) and nitrogen regeneration in order to accurately estimate primary production and f ratio, e.g. the fraction sustained by "new" nitrogen.Additionally, it provides an objective methodological approach to the difficulty of estimating new production in oligotrophic systems.

Methods
This work was carried out on board the R/V Atalante in October-November 2004.Data were gathered during the BIOSOPE (BIOgeochemistry and Optics South Pacific experiment) cruise carried out in the southeast Pacific Ocean along a transect stretching from the Marquesas archipelago to the Chilean coasts (between 146.36 W and 72.49• W, Fig. 1).Twenty four short stations have been sampled along a 8000 km transect crossing different oceanic situations.The mesotrophic area associated to the plume of the Marquesas Island (141 • W-134 • W), the adjacent high nitrate -low chlorophyll waters ( 132• -123 • W), the ultra-oligotrophic waters associated with the central part of the south pacific gyre (123 • W-101 • W), the oligotrophic eastern side of the gyre (101 • W-81 • W) and the Chilean upwelling (80 • W-72 • W).In addition, six experimental sites were specifically investigated with long fixed stations (over 2-5 days), representing different trophic regimes sites : MAR=Marquesas archipelago (141,3 • W; 8.4 • S); HLN = High Nutrient Low Chlorophyll area east of the Marquesas islands (136.8 • W; 9 • S); GYR = centre of the South pacific gyre 114 • W, 26 • S); EGY = eastern border of the gyre (91.4 • W, 31.8 • S), UPW and UPX situated in the area of Chilean upwelling (73 • W-

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• S and 72.4 • W-34.5 • S).Station from the gyre has been selected from ocean color images as having the lowest surface chlorophyll concentration in the world ocean.

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Nutrient measurements were performed at every station of the grid.For nitrate and nitrite determination, samples were taken in 250 ml polyethylene flasks and analyzed on board immediately after sampling using a semi automatic Technicon Autonanal-yser® II according to Raimbault et al. (1990) for low-nitrate water (<500 nmoles.l−1 ; detection limit=3 nmoles.l−1 ) and Tr éguer and Lecorre (1975) for high-nitrate water (>500 nmoles.l−1 ; detection limit=0.05µmoles l −1 ).Ammonium concentrations (40 ml collected in 50 ml Schott glass bottle) were measured using the fluorometric method (Holmes et al., 1999;detection limit=0.005µmoles l −1 ).At each production station, rates of carbon fixation (primary production), nitrate and ammonium uptake as well dinitrogen fixation (diazotrophy) were measured by using a dual 13 C/ 15 N isotopic technique.For this purpose, three 580 ml samples were collected before sunrise at 6-7 standard depths between the surface and 1% light, and poured into acid-cleaned polycarbonate flasks.Bottles were rinsed after use with 10% HCl, then with distilled water from a Milli Q ion exchange unit.
In each bottle, bicarbonate sodium 13 C labelled (NaH 13 CO 3 -6 g 250 ml −1 deionized water −99 at % 13 C, EURISOTOP) was added in order to obtain ≈10% final enrichment (0.5 ml 580 ml −1 sea water).Then, the 15 N 2 gas (99 at % 15 N, EURISOTOP) was introduced to bottles equipped with a gas-tight septum (2 ml of gas 580 ml −1 sea water).We added a fixed quantity of 15 N 2 gas and calculated the enrichment of each bottle on the basis of its volume and the solubility of N 2 .We used the equations provided by Weiss (1970) to calculate the N 2 initial concentration, assuming equilibrium with the overlying atmosphere.The 15 N 2 enrichment in the incubation bottles ranged between 22% and 25%, for seawater temperature varying from 15

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Incubations were started immediately after tracer addition just before dawn.They were done on-deck in an incubator, which consisted of 6-7 opaque boxes, each with a light screen that allowed 50%, 25%, 15%, 8%, 4%, 1% and 0.3% of light penetration.The incubator was maintained at sea-surface temperature with pumped sea water.During the 5 experimental sites, incubations were performed in situ on a drifting rig at the same depth from which they were collected.After 24 h (dawn to dawn), final concentrations of NO − 3 and NH + 4 were measured and samples were filtered thereafter on precombusted ( 450• C) Whatman GF/F filters (25 mm in diameter) using a low vacuum pressure (<100 mm Hg).At the same time, filtrates from15 N-NH 4 experiments were recovered in Duran Schott flask and poisoned with 1 ml HgCl 2 (6 g l −1 ) in order to prevent bacterial activity during conservation.An aliquot of these filtrates (200 ml) was filtrated again on 0.2 µm Teflon membrane.Filtrates from 15 N-nitrate flasks were collected during in situ experiments, only.In this case 300 ml of < GF/F filtrate were refiltrated through 0.2 µm Teflon membrane and stored as indicated above.After filtration, filters were placed in pre-combusted glass tubes, dried during 24 h in a 60 • C oven and stored dried until laboratory analysis.These filters were used to determine the final 15 N / 13 C enrichment in the particulate organic matter as well as concentrations of particulate carbon and particulate nitrogen.
The dual isotopic enrichment analysis were performed on a Integra-CN mass spectrometer calibrated with glycin references every batch of 10-15 samples.The accuracy of our analytical system was also regularly verified using reference materials from the International Atomic Energy Agency (AIEA, Analytical Quality Control Services).The mean atom% 15 N does not vary between 0.2 and 10 µmoles N.Then, the low background of the system allows to analyse safely samples containing low nitrogen concentrations (0.4-1 µmole), values often observed in surface oligotrophic waters.The transport rate of 15 N-labelled dissolved inorganic nitrogen (DIN), from the DIN pool to the PON pool, i.e., the net DIN uptake (ρ net DIN in nmoles l −1 d −1 ) was computed, according to Dugdale and Wilkerson (1986), from Eq. (1): Where R PON and R DIN represent the 15 N atom % excess enrichment in the PON and DIN pools respectively, [PON] represents the final PON concentration and T represents the incubation period (in day).As mentioned after, to correct ammonium uptake rates for isotopic dilution, we made R DIN in Eq. ( 1) equal to the mean value between initial and final R NH 4 .According to these experimental conditions, the detection limit for nitrogen uptake, calculated from significant enrichment (0.014% in excess) and lowest particulate nitrogen (0.2 µmole N) is estimated from Eq. (1) to 0.12 nmol.l−1 .d−1 for nitrogen-fixation (mean R DIN ≈ 24%) and 0.03 nmol.l−1 .d−1 for nitrate and ammonium uptake in nutrient-depleted waters (R DIN ≈100%).Carbon fixation rates were calculated according to Slawyk and Collos (1984), with a time 0 enrichment of 1.113±0.005%(n=8).This time 0 value is little higher than the natural abundance for phytoplankton (1.089), certainly due to some residual traces of 13 C tracer.It should be noted that 13 C enrichment of samples was less problematic than 15 N enrichment, since inorganic carbon is assimilated by the whole phytoplankton, and excess values ranged from 0.3 to 3.6%.Data of 13 C fixation rates, e.g.primary production, corresponding to the mean of the three replicates, are expressed in µgC l −1 d −1 .Applying similar calculation than for nitrogen, the detection is limit is estimated to 0 Printer-friendly Version Interactive Discussion

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Filtrates <GF/F from 15 NH 4 incubations were used to measure the final 15 N enrichment 1) in the DIN pool and 2) in the <GF/F organic matter pool, as outlined by Raimbault et al. (1999).In this procedure, all forms of DIN are removed from the sample as (NH 4 ) 2 SO 4 , by successive diffusion and reduction processes.The first diffusion step allows to quantify the final 15 N enrichment of the ammonium pool, and to estimate isotope dilution of the tracer due to NH 4 regeneration.During the second diffusion, the 15 N enrichment of the nitrate pool in the ammonium filtrates allows to quantify the oxidation of ammonium to nitrate (nitrification).During the third diffusion, the 15 N enrichment was determined in the <GF/F fraction to estimate the rate of 15 N-NH 4 accumulation in the <GF/F organic matter (dissolved organic nitrogen + <GF/F particulate nitrogen).
The <0.2 µm filtrates from 15 N-NH 4 and 15 N-NO 3 experiments were used to measure 15 N enrichment in the <0.2 µm, fraction which only contents dissolved organic nitrogen (DON), e.g. to estimate loss of tracer in term of DON (ρ loss DIN ).Ammonium regeneration rates (rNH 4 in nmoles.l−1 .d−1 ) were estimated according to Laws (1984): Where [NH 4 ] I and [NH 4 ] F represent initial and final concentrations of ammonium during the incubation experiment.Ro NH4 and Rf NH4 are the initial and final excess enrichments in 15 N-NH 4 for the incubation period.Nitrification rates (ρ NIT in nmoles.l−1 .d−1 ) were computed according to Raimbault et al. (1999): Where R NO 3 is the 15 N atom% excess enrichment in the (NO ), calculated as following (Eq.4): Where R <GF/F and R NH 4 are the 15 N atom % excess enrichment of the <GF/F fraction and final ammonium pool respectively, and [PON <GF/F ] is the final particulate nitrogen less than GF/F.The measurement of 15 N abundance in the extracelullar DON pool (R DON ) allowed us to calculate the DIN (nitrate or ammonium) lost (ρ loss DIN ) as DON (Eq.5), calculated as outlined by Slawyk et al. (1998): During our study, nitrate and ammonium concentrations were often lower the detection limit (especially in the South Pacific Gyre) and it was experimentally impossible to reduce the tracer addition to the ideal level (<10% of ambient concentration).In this nutrient conditions, addition of tracer would violate the general assumption that addition of tracers does not disturb the steady-state of the system and may have involved a major perturbation of the nitrate and ammonium uptake (Allen et al., 1996;Harrison et al., 1996).But the use of kinetics parameters described by Harrison et al. (1996)

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allowed us to account for uptake rate enhancement according to the following equation given by Rees et al. (1999), Where N is nitrate or ammonium, ρN 0 is the original uptake rate (nmoles l −1 d −1 ), ρN H is the uptake rate adjusted for enhancement of tracer, N sp is ambient + tracer nutrient (nmoles), N A is the ambient nutrient and Ks is the half-saturation constant.In this case, N A was assumed to be 3 nmoles l −1 for nitrate and 5 nmoles l −1 for ammonium, corresponding to the detection limit of our analytical procedures.To quantify the affinity constant Ks, two kinetic studies were performed with 6 graduate additions of 15 Nlabeled substrate (Fig. 2).Monod equation [V=Vm.S/(Ks+S)] was assumed, where V is the uptake rate for substrate concentration S, Vm is the saturated uptake rate and K S , the affinity constant, e.g. the substrate concentration at half Vm.The kinetics constant K S , needed for the use of Rees et al.'s model, was derived from the Wolf plot linear transformation of S/V vs S. Ks obtained during the 4 experiments performed in nitrogen-depleted waters (12.9 and 15 nmoles.l−1 for nitrate and ammonium, respectively) were a little lower than those in other oceanic waters (around 25-30 nmoles.l−1 ) by Sahlsten (1987) and Harrison et al. (1996).In first approximation, we assumed that DIN losses as DON (ρ loss DIN ) and nitrification (ρ NIT ) could be activated in the same way as net uptake rates, and we applied to theses processes the same procedure of correction.
More, spiked nutrients additions in nutrient-poor waters could be also stimulated 13 C fixation.Then samples enriched with 15 N-N 2 , that no significantly change N 2 concentration in samples, were used to test the absence of interference on primary production caused by 15 N-tracer additions in oligotrophic waters (Fig. 3).The model II gives a regression coefficient of 0.98, revealing no significant stimulation of primary production by low nutrient addition, at least during the 24 h experiments.Since 13 C isotope is not still currently used for estimating marine productivity, especially in oligotrophic oceanic area, our results (PP 13 C) also offer the opportunity to perform an extensive comparison The depth of the euphotic zone (Ze), representing the depth where the irradiance is reduced to 1% of its surface value, was computed using the in situ TChla concentration profiles (see Ras et al., 2007) according the model developed by Morel and Maritorena (2001).Data throughout this paper are limited to the 0-200 m water column.

Results
A detailed description of the geographical distribution of nutrients and biomass during this study can be found in companion paper (Raimbault et al., 2007).Briefly, longitudinal distribution of surface nitrate showed minimal values (lower than the detection limit of 3 nmoles.l −1 ) between 125 • W and 95 • W, e.g. in the South Pacific gyre (SPG) .Other studied regions (Marquesas Islands and Chilean upwelling) showed significant nitrate concentrations (>0.5 µmoles.l−1 ) in surface.Chlorophyll biomass followed this general trend with very low values in the center of the SPG (0.023 µg.l −1 ) while levels reached 0.3 µg l −1 and 1 µg l −1 near the Marquesas Islands and in the Chilean upwelling, respectively.The photic layer located around 40-50 m in the Marquesas and upwelling regions deepened in the centre of the SPG, reaching 160 m between 120 • W and 105 • W, in the clearest natural water in the world (Morel et al., 2007).The incident solar radiation was more or less constant during the cruise (41± 7 Em −2 s −1 ) except during three very cloudy days (9 to 11 November 2004, e.g. from 120 • W to 117

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Primary production and nitrogen uptake rates described the same general distribution, responding to nutrient variation observed in the photic layer along the transect (Fig. 4).A large central area was characterized by very low rates surrounded with two small regions: the subequatorial Marquesas region (in the west) where primary production reached 10 µg C l −1 d −1 in surface, and a part of the Chilean upwelling in the east, which was the most productive area with primary production higher than 50 µgC l −1 d −1 .The most part of the transect (e.g. the South Pacific Gyre = SPG) was characterized by very low carbon fixation rates, less than 2 µg C l −1 d −1 between 130 and 95 • W. It would be noted that in this centre of the SPG, vertical variations of primary production were very weak, rates remaining more or less constant (≈1-2 µg C l −1 d −1 ) between the surface and the base of the photic layer.Nitrate and ammonium uptake rates followed the same general pattern than primary production.Nitrate uptake ranged between 100-500 nmoles l −1 d −1 in the upwelling and less than 5 nmoles l −1 d −1 in the SPG.Intermediate values were found near the Marquesas Islands ranging between 20 and 100 nmoles l −1 d −1 .Ammonium uptake rates (corrected from isotopic dilution) were always significantly higher than nitrate uptake rates.Upwelling presented again the highest uptake rates (up to 500 nmoles l −1 d −1 ), Marquesas Island the intermediate levels (up to 100 nmoles.l−1 d −1 ), and SPG very low rates (<40 nmoles.l−1 d −1 ).As for primary production, vertical distribution of ammonium uptake, as well as nitrate uptake were quite homogeneous in the SPG, with very low vertical variation and any clear surface or subsurface maximum.But, it would be noted, that contrary to primary production, noticeable rates of ammonium and nitrate uptake (>60 and > 5 nmoles l −1 d −1 , respectively) were detected until 100 m on the eastern edge of the SPG between 90 and 100 • W. Dinitrogen fixation rates showed a particular distribution, with rates always lower than ammonium and nitrate uptake.First, vertical extension of this biological process was more important in the west part (until 150 m) than in the eastern part of the investigated area.Second, nitrogen fixation was essentially located near the surface in the SPG where a clear surface maximum can be detected.Rates decreased rapidly with depth Introduction

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EGU and were <0.5 nmoles l −1 d −1 below 50 m.The geographical gradient was weak since maximum surface rates measured in the upwelling (3.6 nmoles.l−1 d −1 ) was only two fold higher than the maximum rates measured in the SPG (1.8 nmoles.l−1 d −1 ).The Marquesas Island was marked by intermediate nitrogen fixation (≈2 nmoles.l−1 d −1 ).Ammonium regeneration rates showed the same regional variations.This process was very active in surface water near the upwelling (>200 nmoles.l−1 .d−1 ), as well as around the Marquesas Islands with rates >100 nmoles.l−1 .d−1 until 135 • W. Surface values decreased to 20 nmoles.l−1 .d−1 in the SPG, but these rates remained more or less constant between surface and 200 m depth.As for ammonium uptake, a subsurface maximum (125 m) was detected around 90 • W on the east edge of the SPG.
The significant ammonium regeneration measured at all stations, whatever the trophic level, induced great variations of ammonium enrichment during the 24 h incubations.The Fig. 5 demonstrated that no taking into account isotopic dilution leads to strong underestimation of ammonium uptake always higher than 50%.More, any relationship was found between the magnitude of underestimation and the uptake rates, clearly indicating that estimations of isotopic dilution are absolutely required to correctly quantify ammonium uptake during 24 h incubation, even in oligotrophic waters.Nitrification showed similar pattern than ammonium regeneration indicated a tight coupling between the two processes although rates, always significant, were globally one order of magnitude lower than those of ammonium regeneration.Highest nitrification rates (>30 nmoles l −1 d −1 ) were observed in the upwelling with a westward extension until 90 • W. The subsurface patch of ammonium regeneration at 90 • W was also marked by active nitrification (up to 40 nmoles l −1 d −1 ).Marquesas region was characterized by relatively low nitrification rates (5-10 nmoles l −1 d −1 ), while lowest but detectable values (<5 nmoles l −1 d −1 ) were measured in the SPG.As observed for preceding biological processes, nitrification was again homogeneous over the water column without any significant maximum in the oligotrophic central region.Nitrate uptake compared to nitrification generally dominated in the upper layer.At the base of the photic layer, nitrification rates were 2 to 10 times higher than corresponding nitrate up-Introduction

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Interactive Discussion EGU take rates, especially in the upwelling area.However, both rates were quite similar over the photic layer in the SPG.A significant part of ammonium was assimilated in the fraction less than GF/F (Fig. 6).This <GF/F ammonium uptake was large in the upwelling region, with rates higher than 25 nmoles.l−1 d −1 and up to 170 nmoles l −1 d −1 , but also obviously significant in the SPG (10 to 20 nmoles l −1 d −1 ).A part of this <GF/F uptake was ultimately found in the <0.2 µm fraction, e.g. in the dissolved organic nitrogen pool, rates here named as ρ loss .Ammonium loss in term of DON was again highest in the upwelling area (50 to 100 nmoles l −1 d −1 ), but highly significant in the oligotrophic SPG (5 to 20 nmoles l −1 d −1 ).The lowest rates (<5 nmoles l −1 d −1 ) were measured in the west part of the section.The nitrate loss was lowest in the SPG (1 to 5 nmoles l −1 d −1 ) and highest in the surface waters of the upwelling (100 nmoles l −1 d −1 ).Some data in the SPG appeared close to zero, which may partly explained by the very low uptake rate and very low excess enrichment in the <0.2 µm DON pool (close to 0.01%).Loss of nitrate tended generally to rapidly decrease with depth as observed in the most productive region (Marquesas Islands and Chilean upwelling).The mean percentages of DIN loss showed important regional variations (Table 1).The lowest productive system (SPG) revealed the most important loss of recently nitrogen uptake (>20%), while in the other regions percentages were lower than 15%.It would be noted that percentages of N-nitrate loss were generally equivalent to those of N-ammonium loss, except around the Marquesas Islands (Mar and HLN sites).
Concerning the f ratio, several corrections have been applied to take into account of several processes rarely measured during nitrogen uptake experiments, as nitrification, DIN loss and nitrogen fixation.Nitrification (as a source of nitrate) can induce overestimations of new production by adding to the nitrate pool some NO − 3 coming from what is, actually, regenerated production (Dore and Karl 1996;Ward et al. 1989;Priscu and Downes 1985;Dugdale and Goering 1967).Then, the fraction of nitrate that was produced by nitrification (ρ NIT )was subtracted from total nitrate uptake (ρ NO 3 ) in order to truly assess the uptake of "new" nitrate as classically defined by Dugdale  and Goering (1967).As nitrogen fixation rates (ρ N 2 ) were available, they were included in calculation of new production (Pnew), calculated as following:

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When nitrification rates were higher than nitrate uptake (ρ NO 3 -ρ NIT<0 ), Pnew is estimated to be equal to ρ N 2 .According to this assumption, the f ratio can be calculated as followed: As ρ loss were not available for all stations (especially from 15 N-NO 3 experiments), gross uptake rates were not used for the calculation of the f ratio, which was estimated with the net uptake rates.Using data coming from in situ experiments during which both losses of N-nitrate and N-ammonium were measured, the model regression analysis (Sokal and Rohlf, 1995) gives a slope of 1.02 (Fig. 7).The slope was not significantly different from 1, indicating that inclusion of DIN loss does not change estimations of f ratios.To assess the influence of nitrification and N 2 -fixation in the magnitude of f ratio, we compare the f ratio calculating according Eq. ( 8) to those calculated without nitrification (f −NIT ) or without N 2 -fixation (f −N 2 ).The possible over-estimation due to the non inclusion of uptake of urea could not be evaluated, since flux of this organic nitrogen compound has not been investigated in this area.
The geographical variations of the different calculated f ratios (averaged over the photic layer) appear closely related to the distribution of the levels of primary productivity (Fig. 8), with highest values (>0.30) observed in the upwelling.Waters surrounding the Marquesas Islands were characterized by f ratio ranged between 0.1 and 0.2.In the SPG, f ratio was generally lower than 0.1, with very low values (0.02) found in the centre.The no inclusion of nitrification rates (f −NIT ) significantly increased f values in the SPG (0.08-0.15).In the other regions nitrification, although acting at significant rates, does not really modify mean f ratio.In the same manner, influence of nitrogen

Discussion
This study confirms previous satellite observation suggesting very low productivity in the SPG, e.g. in the clearest waters of the world ocean (Morel et al., 2007).Primary productivity (0.10 gC.m −2 .d−1 , Table 2) as well as new production (0.008 gC.m −2 .d−1 , Table 3) were the lowest rates ever measured in the ocean and can be considered as an end point for trophic level.Nixon (1995) assigned to oligotrophic, mesotrophic and eutrophic areas, annual carbon rates of <100, 100 to 300 and 300 to 500 g C.m −2 .y−1 , respectively.Following this criteria, and assuming that our measured rates remained constant over the year, Marquesas region appeared as mesotrophic (270 g C.m −2 .y−1 ), upwelling (660 g C.m −2 .y−1 ) as eutrophic and the SPG (36 g C.m −2 .y−1 ) as an "ultra" oligotrophic system.
The longitudinal variation of integrated rates over the euphotic layer, using the trapezoidal method, revealed some specific patterns (Table 2).For example, enhancement of primary production and nitrogen uptake was not as pronounced as enhancement of nutrient availability.Although integrated nitrate concentration increased by a factor of 100 and 300 from the oligotrophic zone (0.9 mmoles.m−2 ) to the Marquesas or to upwelling regions (128 mmoles.m−2 and up to 285 mmoles.m−2 , respectively), integrated carbon fixation rates increased only by 10 to 20 fold.Nitrogen fixation appeared more or less constant around 0.03-0.11mmoles.m−2 .d−1 , while integrated nitrate uptake increased 10 or 50 fold between the SPG (0.2 mmoles.m−2 .d−1 ) in the Marquesas region (2.9 mmoles.m−2 .d−1 ) or the upwelling (11 mmoles.m−2 .d−1 ).The C/N uptake ratios were always lower than the conventional 6.6 Redfield ratio (Table 2).Such low C/N uptake ratio may suggest that some nitrogen assimilation (especially ammonium) was due to heterotrophic organisms.The large ammonium uptake measured in the Introduction

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EGU <GF/F fraction (Table 1) seems to confirm this hypothesis.Assuming a C/N uptake by photosynthetic organisms close to 6.6, we can estimate the heterotrophic uptake rates of ammonium in the >GF/F fraction (ΣHρ NH 4 ).Except in the subequatorial site (MAR), this heterotrophic ammonium uptake could represent a large percentage (≈50%) of the gross ammonium uptake, which reached 70% in the SPG (Table 2).The importance of submicron heterotrophic organisms was here emphasized by the large amount of 15 N-NH 4 ultimately found after 24 h incubation in the <GF/F fraction (Table 1).The low efficiency of GF/F filters for PON retention compared to 0.2 µm membranes is now welldocumented in a variety of marine environments (Altabet, 1990;Libby and Wheeler, 1994;Slawyk and Raimbault, 1995;Raimbault et al., 2000;Fernandez et al., 2007) and has been confirmed during this study (Raimbault et al., 2007).Our experiments showed that the use of GF/F filters can result in severe underestimations of ammonium uptake under high productivity as well as under oligotrophic conditions.This possibility points out to a re-opening of the methodological debate on the use of GF/F and 0.2 µm filters for tracer addition experiments.But, since GF/F filters are able to correctly collect all particles containing chlorophyll, e.g.photosynthetic organisms (Chavez et al., 1995;Raimbault et al., 2007), this heterotrophic ammonium uptake has not to be included in the estimates of regenerated primary production.Consequently, the underestimation of gross ammonium uptake rates by filtration through GF/F filters should not have any consequences for f ratio estimation.Present data give no information on the possible nitrate uptake by the <GF/F fraction, but previous study in the Equatorial Pacific has shown that submicron particles passing through GF/F filters does not assimilate nitrate (Raimbault et al., 2000).
Due to the deviation of our C/N uptake relative to the Redfield ratio, there is not a good agreement between new production estimates obtained either from 13 C fixation rates multiplied by the independently calculated f -ratio or computed from direct measurements of new nitrogen multiplied by the 6.6 Redfield ratio (Table 3).The two estimates often disagree by 25-50%, with lowest (0.007-0.008 g C.m −2 .d

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The extremely low f ratio obtained in the SPG (0.05±0.03) confirms that the most entirely primary production, which maintained relatively significant rates over the 0-180 m water, was supported by regenerated nitrogen.This is indicated by the relatively high values of ammonium uptake (2.1 mmoles.m−2 .d−1 ) as well as by significant ammonium and nitrate regeneration rates, 3.5 and 0.76 mmoles.m−2 .d−1 , respectively (Table 2).
There are seldom measurements of NH 4 regeneration in the literature for oceanic waters but our results are in the range of the scarce available data (e.g.Bode et al. 2002;Raimbault et al., 1999;Fernandez and Raimbault, 2007).Ammonium regeneration showed high rates (up to 500 nmoles.l−1 .d−1 ) in the high productive Marquesas and upwelling regions (MAR and UPW), but significant activity was also measured in the SPG (10-20 nmoles.l−1 .d−1 ), rates sufficient to sustain the biological demand.This observation was confirmed by the picoplankton abundance which was strongly dominated by heterotrophic bacterioplankton along the whole transect (Grob et al., 2007).
Another important finding deals with the magnitude of the nitrification process in the euphotic layer.Although the potential significance of "regenerated" nitrate in the euphotic zone has been acknowledged for some time (Dugdale and Goering, 1967;Ward et al. 1989;Dore and Karl, 1996) and nitrification studies have been performed in a variety of marine environments (Ward et al., 1984, Codispoti and Christensen, 1985, Ward and Zafiriou, 1988), observations in the open ocean are rare.Nitrification rates presented here (Table 2) are in the range of previous observations (e.g. Ward and Zafiriou, 1988;Bianchi et al., 1994;Raimbault et al., 1999;Fernandez and Raimbault, 2007).Regeneration processes were active in the entire water column even in the oligotrophic waters of the SPG, and were largely able to sustain phytoplanktonic demand of nitrate in the photic layer (Table 2).Rates measured in the top 100 m suggest that about 80 to 100% of nitrate uptake in surface waters was supported by nitrification in the SPG.This result confirms the widely significant role of nitrification in the upper layers of oceanic waters, as previously found in the equatorial Pacific where 20 to 100% of total nitrate demand can be fueled by nitrification (Raimbault et al., 1999) and 142% at station ALOHA (Dore and Karl, 1996).A logical conclusion is the possibility of consid- Nowadays, the availability of high-precision isotope ratio mass spectrometers combined with sensitive field tracer method (Montoya et al., 1996) makes it feasible to carry out 15 N 2 -tracer incubations on unconcentrated natural water samples, with minimal disturbance of the system.But actually, such measurements of N 2 -fixation in oceanic waters are rare and data reported here are the first available for the South Pacific.Oceanic waters of south Pacific seem to be a favourable region for diazotrophy, which is observed at significant rates in surface throughout the transect.N 2 -fixation appeared the weakest but the most stable biological process along the 8000 km investigated area, in spite of large deviation in nutrients biomass and primary productivity.In the SPG, the N 2 -fixation was essentially located in the upper layer (0-50 m), where the irradiance is higher and atmospheric inputs, even very weak in this region are certainly more available.Unfortunately, the organisms responsible of this process remain unidentified in the present study.Regardless the absence of diazotrophic Trichodesmium populations during the cruise we can postulate that N 2 -fixation was certainly performed by nanoplanktonic organisms, recently revealed by new molecular biological techniques (Zehr et al., 1998(Zehr et al., , 2000)).Zehr et al. 2001) found small (2-3 µm in diameter) unicellular cyanobacteria expressing gene nifH in the subtropical North Pacific, associated with significant nitrogen accumulation (0.008-0.016 nmol L −1 h −1 ).The discovery of other potentially important marine diazotrophs than Trichodesmium is opening a new window on the importance of nitrogen fixation in the ocean.Due to their ability to fix N, these nanoplanktonic cyanobacteria can contribute substantially to the input of new nitrogen in to the nutrient-poor water, even if the rates of fixation measured are typically quite Introduction

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EGU low relative to the apparent N demand of the ecosystem.Our integrated values (30-910 µmol m −2 d −1 ) are on the same order of magnitude than the very scarce values actually available in the literature concerning dinitrogen fixation by nanoplankton, if we except the high value (3955 µmol m −2 d −1 ) found along the Australian a coasts by Montoya et al. (2004).Zehr et al. (2001) give an integrated of 92 µmol m −2 12 h −1 for the North Pacific.This value is a little higher than those measured by Montoya et al. (2004) in the same region (24-66 µmol m −2 d −1 ).Similar results have been found in the Southwest Pacific around the New Caledonia (Garcia et al., 2007), with nitrogen fixation in the <10 µm fraction, e.g.not associated with Trichodeminum populations, ranged from 40-300 µmol m −2 d −1 .Falcon et al. (2002), including night fixation found a daily range of 62-167 µmol m −2 d −1 for the north Atlantic.And recent equivalent rates (≈ 50 µmol m −2 d −1 ) were obtained in Mediterranean Sea during the oligotrophic summer period (Garcia et al., 2006), representing up to 40% of new production.N 2 -fixation sustained a large fraction of new production (up to 100%) in this oligotrophic environment and this nitrogen pool represents a potentially important nitrogen source for other organisms in the pelagic food web, especially in the oligotrophic waters where any input of other form of new nitrogen is excluded: the vertical flux of nitrate from below the thermocline is extremely low due to the deep of the thermocline; and potential atmospheric deposition of nitrogen and phosphorus is almost absent according to the aerosols measurements performed in this area by Jickells et al. (2005).Then, in agreement with observations made in the North Pacific Gyre where nitrogen fixation is a major source of newly fixed nitrogen to the photic layer (Karl et al., 1997), the N-deficiency SPG would provide an ideal ecological niche for the proliferation of N 2 -fixing unicellular cyanobacteria, even if our estimated rates were 6 fold lower than this expected from a biogeochemical model (120 mmoles m −2 .y−1 ) by Deutsch et al. (2007).
All along the investigated area, primary production appears quantitatively related to new production (Fig. 9), so that the measurement of total production based on photosynthetic assimilation can be used to provide preliminary estimates of new production, and use for modeling purposes.The model predicts that a threefold increase of total

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EGU production in oligotrophic waters would result in a tenfold increase of new production.Such a relationship between primary production and new production agrees well with data from Dugdale et al. (1992) and Raimbault et al. (1999) for the equatorial Pacific (until 16 • S), but were much lower (up to 10 fold) than those estimated by model of Eppley and Peterson (1979).More, in this very oligotrophic water and although strong nutrients gradient, we don't find an evident pattern between nitrate concentration and new production as suggested by Platt and Harrison (1985).When mean values of f ratio are plotted versus integrated primary productivity rates there is a positive logarithmic relationship (Fig. 10).The initial slope (0.0003) for new production <500 mg.m −2 .d−1 (≈200 mg.m −2 .y−1 ) is close this found by Dugdale et al. (1992) in the equatorial Pacific (0.00063±0.00036),but much lower than 0.0025 proposed by Eppley and Peterson (1979).Then the model proposed by Eppley and Peterson (1979) certainly fails for these oceanic waters, because nitrate uptake in the coastal region is distinct from that on oligotrophic waters.Only if suitable time scales are considered, the f ratio provides an indirect estimation of export rates of particulate organic matter toward the deep ocean.This concept is supported by the assumption that the input by advection of nitrate toward the surface should be balanced by the losses to the deep of particulate and dissolved matter.Sinking fluxes of POC (0.14-1.15 mgC.m −2 .d−1 ) and PON (0.04-0.2 mgN.m −2 .d−1 ) measuring in the SPG by sediment trap (J.C. Miquel, personal communication) appears much lower that our new production rates obtained with tracer uptake (7-8 mg ).Several reasons are often evoked to explain this discrepancy often observed.One of these is the vertical transport of dissolved organic matter (by vertical mixing or horizontal advection) which can exceed fluxes of sinking particles and thus appear as major fate of new production (Copin-Mont égut-and Avril, 1993;Peltzer and Hayward, 1996).According to Toggweiler (1989), the most realistic balance is obtained when half of the new production due to upwelled nutrient goes into a pool of dissolved organic compounds.In fact, our data demonstrated that a significant part of the DIN taken up was lost as dissolved organic nitrogen (DON) during the

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EGU 24 h incubation experiments.It is difficult to ascertain whether the 15 N-tracer detected in the DON pool was transferred solely via direct and active release from living phytoplankton cells.The 15 N excess enrichment in the DON pool may have also result from cell rupture by sloppy-feeding, and from cells lysis by viral infection (Bronk et al., 1994;Procter and Fuhrman, 1990).Pooling all of available data, DI 15 N ultimately found in the extracellular DON pool (ρ loss ) represented on average 13 to 15% for nitrate and ammonium, respectively.But values were often higher in the SPG, up to 46% in a subsurface layer.It appears that ρ net :ρ gross ratio were in the range of values found in literature, e.g. between the low loss rates (<15%) found in the equatorial Pacific (Raimbault et al., 1999;Raimbault et al., 2000) and the very high ones (74%) found in the Southern California Bight by Bronk and Glibert (1991).They are also in the same order of magnitude than those found in the Mediterranean Sea during spring (Diaz and Raimbault, 2000) and in the Atlantic Ocean (Fernandez and Raimbault, 2007).But, one should keep in mind that ρ loss does not represent the total flux of nitrogen from the particulate organic matter (PON) to the DON pool.The true DON release (ρ DON ) is greater than our measured loss of DIN (ρ loss ).Quantification of ρ DON depends on the 15 N enrichment in the intracellular DON (R DONi ).R DONi is not experimentally accessible, but it is related to the initial 15 N enrichment DIN pool (R DIN ) according the relation defined by Raimbault et al. (2000), Because of the dilution of the 15 N tracer by intracellular nitrogen during uptake R DIN /R DONi is higher than 1.Then, this relationship indicates that 1) the total fluxes of organic nitrogen from particulate matter to extracellular DON would be higher than the loss of tracer in the form of DON as measured here and 2) lower is the final enrichment of intracellular DON pool greater in the difference between DIN loss and DON release.Assuming phytoplanktonic growth rates calculating from the residence time of chlorophyll-containing particles (Raimbault et al., 2007), R DIN /R DONi ratio would be 3 to 6 fold higher in the SPG than in the productive regions (Marquesas Islands and

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Chilean upwelling).Consequently, we can hypothesize that the production of DON was proportionally much higher in the oligotrophic waters than in productive system.Similar data on dissolved organic carbon (DOC) release are not yet available for this region.However, DOC loss has been observed during field experiments with 14 C (Karl et al., 1998) and it is worthwhile to note that nitrogen loss rates obtained here were comparable to release rates found for recently fixed photosynthetic products during photosynthesis (Mague et al., 1980), e.g. for dissolved organic carbon (DOC).It is now well-known that under nutrient limitation, as during summer stratification, large amounts of dissolved organic matter can be produced and accumulated in the upper layer of temperate waters (Buttler et al., 1979;Maita and Yanada;1990;Libby and Wheeler, 1997), until they disappeared from the surface layer during destratification and winter mixing (Copin-Montegut and Avril, 1993;Carlson et al., 1994).This assumption is strengthened by the fact that a large concentration of carbon-rich dissolved organic matter (C/N=16-23) occurred between 125 • W and 95 • W (Raimbault et al., 2007), which appears inversely and exponentially related to the primary production (Fig. 11).Annual convection mixing is excluded in the SPG, and due to low horizontal advection, the dissolved organic matter released by primary producers can be accumulated for a long time, reaching levels never yet measured in oceanic waters.Only a small fraction of the primary production (4%) can be exported in term of DOC by turbulent diffusion (Raimbault et al., 2007).Therefore, in spite of low primary production and very low new production, the net biological effect of the SPG in the CO 2 trapping should be not as weak as expected.These results support the conclusion that the SPG system is locked into a strong grazing loop where primary producers are trapped into small-sized, low sinking rate-group of species acting in a very efficient regeneration loop.It is well known that small particles are more strongly retained in the euphotic layer of unproductive oligotrophic areas than in the euphotic layer of productive areas (Lohenz et al., 1992).Small phytoplankton cells are consistent with high grazing rate, e.g.favouring DOM release, since small cells sink slowly and are more likely to be grazed within the euphotic zone.

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Pacific Gyre, the distance from terrestrial sources of volatile compounds, the long residence time of the quiescent waters accumulated inside the anticyclonic circulation, and the depth of the nitracline are probably favorable to maintain an active microbial loop with a long residence time of organic matter, where new nitrogen is essentially brought by dinitrogen-fixation.As suggested by Thingstad et al. (1997), such microbial loop acting under some nutrient limitation can drive of large accumulation of carbon-rich dissolved organic matter due to over consumption of carbon (K ähler and Koeve, 2001), which is likely to the origin of this paradoxical property of the South Pacific gyre.

Conclusions
These results thus demonstrate the very unique character of the ultraoligotrophic South Pacific Gyre (SPG) and its borders, and lead to a number of questions.Primary productivity as well as new production was the lowest ever measured in the ocean, and can be considered as an end point for trophic level.A large part of new primary production was fuelled by N 2 -fixation, rather than deep nitrate diffusing from deeper layer into the photic zone.As a consequence f ratio appeared often close to zero, essentially sustained by nitrogen fixation acting at very low rate and leading to very weak particulate export.The productive system seemed to be maintained by an efficient loop of regeneration, active over the whole photic layer and leading to overconsumption of carbon.Then, in spite of very low primary productivity, the SPG could accumulate new carbon from the atmosphere in terms of dissolved organic carbon (DOC).According to the long-term fate of this accumulated of DOC, SPG could be a sink for atmospheric CO 2 .Fig. 9. Plot of integrated new production (ΣNew production) versus integrated primary production (Σprimary production).OLIPAC data are from Raimbault et al. (1999); WEC 88 data are from Dugdale et al., (1992).The power relationship is calculated from data of this study (dark points).Insert shows present data for primary production higher than 1000 mg C.m Where R DON and R DIN are the 15 N atom % excess enrichment of the extracellular DON and DIN pool respectively, and [DON] is the final extracellular DON concentration.The quantification of the ρ loss DIN offers the possibility to calculate the gross uptake rate ρ loss DIN as the sum of the net DIN uptake and the DIN loss: ρ gross DIN = ρ net DIN + ρ loss DIN

Fig. 5 .
Fig.5.Percentage of underestimation of ammonium uptake (% underestimation) when isotopic dilution is not included for calculating rates versus ammonium uptake rates no corrected from the isotopic dilution (uncor NH 4 uptake).

Fig. 7 .
Fig. 7.Comparison of f ratio calculating from gross nitrogen uptake (gross f ratio) and from net nitrogen uptake rates (net f ratio).The linear regression line was obtained from model II regression.

Fig. 10 .Fig. 11 .
Fig.10.Plot of mean f ratio (f ratio) versus integrated primary production (Σprimary production).Dashed curve is the best relationship calculated for all the data.Straight line is the linear relationship calculated for primary production lower than 500 mg C.m −2 .d−1 .
sensitive in the SPG.In this oligotrophic area, f −N 2 down to very low values (often close to 0) clearly indicating that nitrogen fixation was the main process providing new nitrogen in this upper layer.
current new production estimates if all nitrogen regeneration processes are not taken into account.In terms of integrated values over the photic layer, overestimation was only significant in the SPG, where rates of new nitrate assimilation (Pnew) were lower than expected, even close to zero.Consequently average values of f ratio was very low in the SPG and no zero values were essentially due to new production sustained by N 2 -fixation, only source of new nitrogen in the upper layer of the SPG.
The exceptional huge size of the South Introduction

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EGUpart by the French Research and Education Council.The Biosope cruise program was managed by H. Claustre and A. Sciandra.We thank the crew of the R/V Atalante for outstanding shipboard support operations.Data concerning euphotic layer depth and surface light intensity were kindly provided by H. Claustre.D. Taillez and C. Bournot are warmly thanked for their efficient help in CTD rosette management and data processing.J. C. Miquel provided sediment trap data, and T. Moutin provided data of primary production from14C experiments.Introduction Sokal, R. R. and Rohlf, F. J.: Biometry, The principles and practice of statistics in biological research, WH Freeman and Company, New-York, 1995.Thingstad, T. F., Hagstrom A., and Rassoulzadegan, F.: Accumulation of degradable DOC in surface waters: is it cause by malfunctionning microbial loop?, Limnol.Oceanogr., 42, 398-404, 1997.Introduction Zehr, J. P. and Ward, B. B.: Nitrogen cycling in the Ocean: New perspectives on processes and paradigms, Appl.Environ.Microbiol., 68, 3, 1015-1024, 2002.Zehr, J. P. and Ward, B. B.: Nitrogen cycling in the ocean: new Nitrogen cycling in the ocean: new perspectives on processes and paradigms, Appl.Environm.Microbial., 68, 1015-1024, 2002.Introduction

Table 1 .
Percentages (mean values integrated over the photic layer) of ammonium uptake in the <GF/F fraction (ρ <GF/F ) and loss in term of DON (ρ DON<0.2 µm ) from ammonium and nitrate relative to gross uptake rates (ρ gross =ρnet + ρ DON<0.2 µm ) for the five investigated oceanic regions.Values from nitrate values are available from only in situ profiles performed on the 5 experimental sites.