Biogenic silica production rates and particulate organic matter distribution in the Atlantic sector of the Southern Ocean during austral spring 1992

Deep-Sea Research II 49 (2002) 1765–1786

Biogenic silica production rates and particulate organic matter distribution in the Atlantic sector of the Southern Ocean during austral spring 1992 B. Que! guinera,*, M.A. Brzezinskib a

Centre d’Oc!eanologie de Marseille, Laboratoire d’Oc!eanographie et de Biog!eochimie, Parc Scientifique et Technologique de Luminy, UMR CNRS 6535, Case 901, F-13288 Marseille Cedex 9, France b Department of Ecology, Evolution and Marine Biology, and the Marine Science Institute, University of California, Santa Barbara, CA 93106, USA

Abstract Several of the components of the silicon cycleForthosilicic acid (Si(OH)4), biogenic silica (BSi), and biogenic silica production rates (rSi)Fhave been investigated, together with the distribution of particulate organic carbon (POC), particulate organic nitrogen (PON) and carbon primary production (rC), on a series of transects across three subsystems in the Atlantic sector of the Southern Ocean (61W): the seasonal ice zone (SIZ), the permanently open ocean zone (POOZ), and the southern boundary of the polar frontal zone (PFZ). The study was conducted in Spring 1992 as part of the European SO-JGOFS cruise aboard the R.V. Polarstern. High BSi concentrations (maximum: 11.7 mmol Si l
1) were recorded in late November at the southern border of the PFZ. In contrast, no large BSi biomass was found in the other subsystems studied. In the SIZ, no diatom bloom was observed, despite a sea-ice retreat of 200 km during the study period, and BSi biomass never exceeded 0.6 mmol Si l
1. The POOZ also showed very low BSi biomass (o0.5 mmol Si l
1), and low BSi/POC molar ratios from the surface to 200 m (0.04–0.06 at 531S) suggest that this was an area where phytoplankton were not dominated by siliceous organisms. At the southern border of the PFZ, BSi/POC molar ratios were among the highest ever recorded in the surface waters of the Southern Ocean (maximum: 1.33). This could be a result of the presence of heavily silicified diatoms or also could reflect a more rapid recycling of POC as compared to BSi. High concentrations of BSi (>1.5 mmol Si l
1) extended well below the euphotic zone to 200 m depth between 491S and 511S, suggesting significant sedimentation of siliceous particles in that area. High values of rSi also were observed in the PFZ (29.6–60.7 mmol Si m
2 d
1, during the production maximum) indicating that this subsystem is important in the biogeochemical budget of the Southern Ocean. High depth-integrated rSi/rC (0.25–0.46) and BSi/ POC (0.53–0.85) in the PFZ imply the production of diatoms rich in silica compared to organic matter. The high rates of silica production observed in the PFZ support the recent hypothesis that the formation of the abyssal siliceous oozes that encircle much of Antartica form primarily as the result of high levels of silica production in surface waters rather than as a result of high rates of opal preservation as has been suggested in the past. r 2002 Elsevier Science Ltd. All rights reserved. Re´sume´ Les recherches pre! sente! es concernent la distribution de plusieurs parame" tres du cycle bioge! ochimique du siliciumFl’acide orthosilicique (Si(OH)4), la silice biog!enique (BSi) et les taux de production de silice biog!enique *Corresponding author. Tel.: +33-04-9182-9205; fax: +33-04-9182-1991. E-mail address: [email protected] (B. Qu!eguiner). 0967-0645/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 2 ) 0 0 0 1 1 - 5

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(rSi)Fainsi que du carbone organique particulaire (POC), de l’azote organique particulaire (PON) et de la production primaire (rC), sur plusieurs radiales successives traversant les trois sous-syste" mes du secteur Atlantique de l’Oce! an Austral a" la longitude 61W: la zone saisonni"ere des glaces (SIZ), la zone de l’oc!ean ouverte en permanence (POOZ) et la bordure sud de la zone polaire frontale (PFZ). L’!etude a e! t!e men!ee au printemps 1992 dans le cadre de l’op!eration European SO-JGOFS a" bord du navire oce! anographique Polarstern. A la fin du mois de novembre de fortes concentrations de BSi (maximum: 11.7 mmol Si l
1) ont ainsi e! t!e observ!ees au sud de la PFZ. Au contraire, les autres sous-syst"emes n’ont pas montr!e de fortes teneurs de BSi. Dans la SIZ, en particulier, aucun d!eveloppement de diatom!ees n’a pu e# tre mis en e! vidence malgr!e un retrait de la banquise sur 200 km pendant la p!eriode d’!etude et les concentrations de BSi n’ont jamais de! passe! 0.6 mmol Si l
1. Les teneurs de BSi sont aussi reste! es tre" s faibles dans la POOZ (o0.5 mmol Si l
1) et les faibles valeurs du rapport molaire BSi/POC entre la surface et 200 m de fond (0.04–0.06 a" 531S) am"enent a" penser que le phytoplancton de cette zone n’!etait pas domin!e par des organismes siliceux. Par contre, au sud de la PFZ, les rapports molaires BSi/POC qui ont e! te! e! value! s sont parmi les plus e! leve! s jamais enregistre! s dans l’Oc!ean Austral (maximum: 1.33). Cette observation peut e# tre interpr!et!ee comme refl!etant la pr!esence de diatom!ees fortement silicifi!ees ou bien comme r!esultant d’un recyclage plus rapide du POC par rapport a" la BSi. De fortes concentrations en BSi (>1.5 mmol Si l
1) ont aussi e! te! mises en e! vidence en profondeur, largement au-dessous de la zone euphotique, jusqu’"a 200 m de fond entre 491S et 511S, ce qui sugg"ere un ph!enom"ene de s!edimentation massive des particules siliceuses de cette zone. Les fortes valeurs de rSi observ!ees au sud de la PFZ (29.6–60.7 mmol Si m
2J
1 au maximum de production) indiquent que ce sous-syst"eme est particuli"erement important pour le bilan du silicium dans l’Oc!ean Austral. Les rapports int!egr!es sur la verticale rSi/rC (0.25 a" 0.46) et BSi/POC (0.53 a" 0.85) de la PFZ impliquent que les diatom!ees sont ici particuli"erement silicifi!ees. Enfin, les valeurs tr"es e! lev!ees de production de silice # siliceux qui entourent le biog!enique que nous avons observ!e confirment l’hypoth"ese r!ecente de la formation des d!epots continent Antarctique, formation lie! e a" des valeurs particulie" rement e! leve! es de production dans les eaux de surface et non pas a" une meilleur pr!eservation de l’opale abyssale dans cette r!egion.

1. Introduction The Southern Ocean is the major area of seafloor opal deposits in the World Ocean. However, the mechanisms leading to the formation of those sediments are still a matter of debate. Nelson et al. (1995) argued that especially high biogenic silica production rates (rSi) in surface waters and an unusually high burial efficiency for biogenic silica (BSi) are the two likely explanations. Nelson et al. (1995) favored the second hypothesis, with the caveat that direct measurements of BSi dissolution rates were scarce in the literature. Recently, Pondaven et al. (2000) have questioned the high efficiency of BSi preservation in the Indian sector of the Southern Ocean; using BSi production data, silicon and nitrogen seasonal depletion, sediment-trap flux measurements, and 230 Th-normalized burial rate in sediments they have concluded that the overall burial efficiency of BSi is in fact similar to the global mean of 2–5% (Calvert, 1983). Nelson et al. (2002) also have concluded that the opal preservation efficiency in

the Pacific sector is indistinguishable from the global average of 3%. The conclusions of Pondaven et al. (2000) and Nelson et al. (2002) are of particular importance because they offer the possibility of using sedimentary opal accumulation rates to reconstruct past palaeoceanographic BSi production rates in the surface waters. Iron availability also has been shown to control the Si:C ratio of diatoms (Hutchins & Bruland, 1998; Takeda, 1998). Recently, Que! guiner et al. (in preparation) and Franck et al. (2000) have added further complexity to the question of the control of particulate Si:C ratios by showing that iron availability can induce modifications in the silicic acid uptake parameters in some areas of the Southern Ocean. Only a few studies have directly examined the production of BSi in the different subsystems of the Southern Ocean, and most of these did not resolve seasonal patterns or variability (e.g., Nelson and Gordon, 1982; Que! guiner et al., 1991; Leynaert et al., 1993). One major exception is the work that has been conducted in the Ross

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Sea (Nelson and Smith, 1986; Nelson et al., 1991), where successive cruises have led to a reasonable understanding of the seasonal evolution of siliceous biomass within this area of the coastal and continental shelf zone (CCSZ) (Nelson et al., 1996). Also, the recent AESOPS cruises in the Pacific sector of the Southern Ocean provide new insights into the seasonal pattern of biogenic silica production within the different sub-systems of the Antarctic circumpolar current (ACC) (Brzezinski et al., 2001). In 1992, we had the opportunity to document the spring temporal variation in biogenic silica production in three different sub-systems of the Atlantic sector of the Southern Ocean: the seasonal ice zone (SIZ), the permanently open Ocean zone (POOZ), and the southern edge of the polar frontal zone (PFZ), referred to elsewhere as the polar front region (PFr) (Que! guiner et al., 1997). The study was conducted during the course of austral spring, and preliminary data concerned with the BSi distribution at the end of spring have already been published (Que! guiner et al., 1997). Here we present the complete temporal evolution of siliceous phytoplankton production during spring, including direct measurements of silicic acid uptake by the natural phytoplankton communities. In this paper we address the following questions: (1) How does the BSi content of surface waters vary during spring? (2) How do the different subsystems compare in terms of biogenic silica production? (3) Is it possible to derive an estimate of the contribution of the different sub-systems to the biogeochemical budget of silicon in the Southern Ocean?

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tion also was measured during the cruise, and results have been presented elsewhere (Jochem et al., 1995; Que! guiner et al., 1997). The Antarktis X/6-S.O. JGOFS cruise was principally devoted to the study of a latitudinal section on the 61W, between the pack-ice and the southern border of the Polar Front; i.e. between 591300 S and 471S (Fig. 1), during the period of sea-ice retreat. Results presented here come from transects 2 and 3 (11 stations, between 48 and 571S, from 12 to 22 October 1992), transect 5 (18 stations between 47 and 561S from 24 to 31 October 1992), and transect 11 (16 stations between 10 and 21 November 1992). This survey encompassed the three sub-systems mentioned above: the SIZ south of 541300 S, the POOZ between 541300 S and 501300 S, and the PFZ from 501300 S to the northernmost boundary (Que! guiner et al., 1997). During the cruise sea ice retreated from 551S to about 581S. Water samples were taken at six depths derived from PAR measurements (100%, 25%, 10%, 3%, 1%, and 0.1% of surface PAR) for every parameter; PAR profiles (400–700 nm) were obtained

2. Materials and methods The distributions of dissolved orthosilicic acid, particulate BSi, particulate organic carbon (POC), particulate organic nitrogen (PON), and rSi were measured between October and November 1992, during the Antarktis X/6-S.O. JGOFS cruise onboard R.V. Polarstern in the Atlantic sector of the Southern Ocean (for details of the cruise, see Smetacek et al., 1997). Carbon primary produc-

Fig. 1. Location of the study area during Antarktis X/6-S.O. JGOFS cruise (29 September–29 November 1992). The major limits of the sub-systems are indicated (PFZ: Polar Frontal Zone; POOZ: Permanently Open Ocean Zone; SIZ: Seasonal Ice Zone).

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using a LI-COR quantum meter (LI-193SA spherical quantum sensor connected to a LI-100 Data Logger). Three additional deeper levels were sampled for particulate matter and orthosilicic acid concentrations, at 100, 150, and 200 m. Orthosilicic acid concentrations were measured onboard ship. Samples were analyzed on an Autoanalyzer Technicon II using the method of Mullin and Riley (1965); precision: 70.05 mM. For chlorophyll a analysis, 0.5-l seawater samples were filtered on to Whatman GF/F filters (nominal cut-off size: 0.7 mm), transferred into dry Pyrex tubes and immediately frozen on board (
201C). Samples were later analyzed by the fluorimetric method of Yentsch and Menzel (1963), using a calibrated Turner 112 fluorometer; precision: 71%. For particulate silica analysis, 1-l seawater samples were filtered on to 0.4-mm Nuclepore polycarbonate filters. Filters were then oven dried (601C) on board, stored in plastic Petri dishes, and returned to the laboratory for further analysis. BSi was measured by the hot NaOH digestion method of Paasche (1973) modified by Nelson et al. (1989); blanks: 0.00670.005 mmol l
1, precision: 710% in the range 0–20 mmol l
1. For POC and PON analysis, 2.5-l seawater samples were filtered on to Whatman GF/F filters pre-combusted at 4501C. Filters were stored frozen (
201C) in glass boxes. After elimination of inorganic carbon by fuming with concentrated HCl, POC and PON concentrations were measured by a combustion method (Strickland and Parsons, 1972) using a Carlo Erba model N 1500 analyzer; blanks: 0.170.01 mmol POC l
1 and 0.0270.002 mmol PON l
1, precision: 710% in the range 0–14 mmol POC l
1 and 0–2 mmol PON l
1. For rSi measurements, samples were drawn in 1-l clean polycarbonate bottles covered with neutral density screens to simulate the light intensity of the sampling depths (see above). Samples were spiked with 11.5 ml of 1.7 mM 30 Na30 2 SiO3 solution or 13 ml of 1.6 mM Na2 SiO3 solution, resulting in an increase of the final ambient orthosilicic acid concentration of about 20 mM Si(OH)4. Due to the low Si(OH)4 concentrations measured at the northern stations (o4 mM

Si(OH)4), the tracer addition could have significantly increased the natural uptake rates, and this point will be discussed later. Samples were then placed in a deck incubator cooled by running sea surface water. After 24 h incubation, samples were filtered through 0.4-mm Nuclepore polycarbonate filters and stored like the particulate silica samples. Samples were analyzed using either the MAAS 6– 60 mass spectrometer of the Marine Science Institute (University of California Santa Barbara) using the method of Nelson and Goering (1977), or the mass spectrometer of the Institut Universitaire Europe!en de la Mer (Universite! de Bretagne Occidentale, Brest, France) using the method described by Caubert (1998). Particulate silica samples from the surface and the 0.1% light depth were also size-fractionated by filtering samples through 10 mm followed by 0.4 mm Nuclepore polycarbonate filters.

3. Results 3.1. Silicic acid The distribution of Si(OH)4 shows a general south–north decrease in concentration (Fig. 2). This general decrease is likely related to Ekman northern transport of Antarctic surface water (AASW) originating from continuous mixing of upper circumpolar deep water (UCDW) with surrounding water masses of the ACC (Veth et al., 1997), although there is evidence that the position of the silicic acid gradient is influenced significantly by in situ biological consumption of silicic acid (Dafner and Mordasova, 1994; Brzezinski et al., 2001). Indications of a seasonal decrease in silicic acid concentrations related to biological activity can be seen in the PFZ, and also in the POOZ from 521S to 541S, between transect 5 and transect 11 (Fig. 2b and c), a period coinciding with an increase in production and biomass as discussed later. At the southern edge of the study area, the steep silicic gradient observed in surface waters corresponded to the ACC-Weddell Gyre boundary front which Veth et al. (1997) positioned at 571S–581300 S during our cruise. That frontal area is characterized by the occurrence of eddies

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Fig. 2. Temporal evolution of orthosilicic acid distribution (mM): (a) transect 2+3 (12–22 October), (b) transect 5 (24–31 October), (c) transect 11 (10–21 November). The sea-ice extent is symbolized by the quadrangle on top right of the frame.

and meanders. The observations of Veth et al. (1997) during the Antarktis X/6 cruise also revealed the presence of an active meander in the

Polar Front, which is manifested as a bi-modal minimum in the Si(OH)4 distribution in the surface waters (Fig. 2).

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3.2. Chlorophyll a The temporal evolution of chlorophyll a was characterized by strongly different patterns in the southern and the northern parts of the study region (Fig. 3). In the PFZ, the phytoplankton biomass was already elevated at the beginning of the study period, with values quite homogeneously distributed over the upper 0–80 m (0.4–0.5 to >0.6 mg l
1). Chlorophyll a concentrations increased in the PFZ during the cruise, and a deep chlorophyll maximum (DCM) became established by the end of the study period (maximum >1.4 mg l
1 at 30–60 m depth depending on the location). Although we were not able to sample the POOZ adequately during transect 2+3 due to a storm event, the chlorophyll a distribution observed during transect 5 suggested a moderate phytoplankton development with highest values of >0.3 mg l
1. Afterwards, chlorophyll a levels declined and values o0.2 mg l
1 characterized the central POOZ during transect 11 (Fig. 3c); although higher values were observed at the northernmost POOZ station near the Polar Front. The SIZ also contained low phytoplankton biomass during transect 11 (Fig. 3c), with chlorophyll a concentrations close to 0.2 mg l
1 present in this zone down to 100 m. A very small increase of chlorophyll a concentrations (o0.25 mg l
1) was noticed in the vicinity of the receding ice edge at the end of the cruise.

3.3. Particulate organic matter The distribution of POC (Fig. 4) and PON (Fig. 5) strongly paralleled that of chlorophyll a, with the main seasonal feature being the development of the bloom in the PFZ. The main difference was observed at the end of the study period when high concentrations of both POC (>8 mmol C l
1) and PON (>1 mmol N l
1) were observed below the euphotic zone at depths exceeding 100 m at 501S, suggesting bloom sedimentation at that time. There was a moderate increase of POC and PON within the SIZ, especially at the ice edge, but permanent deep wind-induced mixing resulted in a deep entrainment and quite homogenous vertical

distribution of particulate organic matter as the ice edge moved away southward. 3.4. Biogenic silica BSi concentrations were high in the PFZ at the beginning of the study period, with values well above 1.0 mmol Si l
1 in the upper 100 m (Fig. 6a). As mentioned above for POC and PON, the high concentration layer of BSi extended to considerable depth near 501S at the end of the last transect (Fig. 6c), but contrary to POC and PON, this pattern was already established for BSi at the beginning of the first transect, suggesting early sedimentation of Si-rich material and decoupling of BSi versus POC and PON cycling in the surface layer. In the course of austral Spring BSi values increased attaining maximal values in the PFZ (at 49–501S) by the end of the study period where BSi concentrations >11 mmol Si l
1 were observed between 40 and 60 m depth (maximum value: 11.7 mmol Si l
1 at 491S). High values extended down to 200 m with concentrations well above 1 mmol Si l
1. In contrast, BSi concentrations were relatively low in the POOZ and the SIZ during all three transects. Initial values were moderate at the beginning of the first transect and quite homogeneously distributed over the surface layer (0.2– 0.3 mmol Si l
1 in the open waters of the POOZ as well as in the ice-covered waters of the SIZ). As sea ice retreated from 551S to 591S, the BSi increase was moderate in the SIZ, and maximum values were observed at 571S in an area of increased vertical stability corresponding to the ACCWeddell Gyre Boundary Front (see Veth et al., 1997). However, the maximum BSi value (0.66 mmol Si l
1) was far lower than the maximum value reached in the PFZ. In the POOZ, a slight increase in BSi occurred between the first and the second transect, with concentrations reaching >0.4 mmol Si l
1. The dominant change within the POOZ was the seasonal evolution of a north– south gradient of BSi starting at 521S associated with the development of the bloom in the PFZ. The size-fractionation of BSi (Fig. 7) clearly shows the major role of large (>10 mm) over small (o10 mm) siliceous phytoplankton in the bloom

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within the PFZ. The relative contribution of large and small fractions to BSi did not show any clear seasonal trend, and the major variations were more related to spatial heterogeneity between the three sub-systems. The >10 mm fraction accounted for, respectively, 64.7%711.4%, 75.7%75.5%, 89.9%73.9% of the surface BSi and 55.5%715.3%, 73.3%79.1%, 82.2%711.5% of the deep (0.1% light level) BSi, in the SIZ, the POOZ and the PFZ. The increased contribution of small phytoplankton towards the SIZ paralleled a shift from phytoplankton assemblages dominated by Corethron criophilum/inerme and Fragilariopsis kerguelensis in the PFZ to small pennate communities (Pseudonitzschia spp.) at the ice edge (see Bathman et al., 1997). 3.5. Elemental ratios of the particulate matter The elemental composition of the particulate matter also reflected the development of large diatoms in the PFZ. Si:C ratios increased from quite high values (0.09–0.37) at the beginning of the study to extremely high values by the last transect (Fig. 8), especially within the subsurface BSi maximum where the maximum Si:C ratio of 1.33 was observed . In contrast, Si:C ratios in the POOZ and the SIZ were similar to typical values for nutrient-replete diatoms (Brzezinski, 1985), and values did not show much change from transect 2+3 (Fig. 8a), when values ranged from 0.05 to 0.18, to transect 11 (Fig. 8c) when values between 0.03 and 0.22 were observed. C:N ratios (data not shown) followed the classical vertical increase (see Honjo and Manganini, 1993), and there was no clear seasonal pattern. The C:N ratio ranged from 6.1–22.0 to 6.1–15.1 in the POOZ and SIZ, and from 6.6–15.4 to 6.1–16.0 in the PFZ during the study period. 3.6. Biogenic silica production rates rSi increased in the PFZ during the study, whereas consistently low rates characterized the other subsystems (Fig. 9). Due to the storm event during the two first transects, we are not able to document precisely the distribution of rSi at the

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beginning of the cruise, but there are some indications of a slightly higher activity in the PFZ reaching up to 0.29 mmol Si l
1 d
1 in subsurface waters (30 m) at 491S whereas values p0.05 mmol Si l
1 d
1 were recorded in ice-covered waters of the SIZ. rSi remained quite low in the POOZ and the SIZ during all transects, barely exceeding 0.05 mmol Si l
1 d
1, with the exception of the surface waters at the ice edge during transect 11 where rSi values slightly exceeded 0.10 mmol Si l
1 d
1, indicative of moderate siliceous phytoplankton growth (Fig. 9c). The main feature of the change in silica production with time was the especially high values reached in the PFZ, where rSi rates up to 0.80 mmol Si l
1 d
1 were observed on transect 5 (Fig. 9b) with values reaching 1.08 mmol Si l
1 d
1 on transect 11 at 491S (Fig. 9c). These values are in the lower end of the range of the very high values measured by Nelson and Smith (1986) in a bloom following the receding ice edge of the Ross Sea and are nearly equivalent to those measured within an intense diatom bloom in the SIZ overlapping PFZ in the Pacific sector along 1701W by Brzezinski et al. (2001). It is interesting to note that, contrary to the distribution of BSi stocks, which exhibited sub-surface maxima, silica production rates were elevated at the surface and were quite homogeneously distributed in the 0–50 m layer. This further supports the idea of significant export of siliceous biomass while the bloom was still growing. As mentioned in the previous section, the isotope addition resulted in a large increase of orthosilicic acid (+20 mM) relative to the ambient concentration in the PFZ, so that the rSi values we measured need to be considered as potential maximum rates. However, we believe that the rates we measure are quite close to the true values for two reasons. Firstly, only a few locations located at the northern boundary of the study area exhibited low orthosilicic acid concentrations (i.e. o5 mM), and these stations were not the ones where the maximum rate were recorded. High Km values for orthosilicic acid (>10 mM) have been reported in the literature for Southern Ocean open-water diatoms (Jacques, 1983; Sommer, 1986, 1991; Caubert, 1998; Nelson et al., 2001) but some studies suggest Km values o5 mM

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Fig. 3. Temporal evolution of chlorophyll a distribution (mg l
1): (a) transect 2+3 (12–22 October), (b) transect 5 (24–31 October), (c) transect 11 (10–21 November). The sea-ice extent is symbolized by the quadrangle on top right of the frame.

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Fig. 4. Temporal evolution of particulate organic carbon distribution (mmol C l
1): (a) transect 2+3 (12–22 October), (b) transect 5 (24–31 October), (c) transect 11 (10–21 November). The sea-ice extent is symbolized by the quadrangle on top right of the frame.

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Fig. 5. Temporal evolution of particulate organic nitrogen distribution (mmol N l
1): (a) transect 2+3 (12–22 October), (b) transect 5 (24–31 October), (c) transect 11 (10–21 November). The sea-ice extent is symbolized by the quadrangle on top right of the frame.

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Fig. 6. Temporal evolution of biogenic silica distribution (mmol Si l
1): (a) transect 2+3 (12–22 October), (b) transect 5 (24–31 October), (c) transect 11 (10–21 November). The sea-ice extent is symbolized by the quadrangle on top right of the frame.

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PFZ

POOZ

SIZ

(a) 100

80 60 40

0 46

48

50

52

54

56

58

60

46

48

50

52

54

56

58

60

46

48

50

52

54

56

58

60

80 60

0.1 % I 0

(b) 100 100 % I 0

% biogenic silica in the > 10 µ m fraction

20

40 20 0

(c) 100

80 60 40 20 0

LATITUDE (˚S) Fig. 7. Temporal evolution of biogenic silica >10 mm size fraction at the surface (100% I0 ) and below the euphotic zone (0.1% I0) (a) transect 2+3 (12–22 October), (b) transect 5 (24–31 October), (c) transect 11 (10–21 November).

(Nelson and Tre! guer, 1992; Nelson et al., 2001), and Que! guiner et al. (in preperation) have observed an inverse relationship between Km values and Fe concentration in the PFZ. Secondly, the results of Que! guiner et al. (in preperation) and the high Fe concentrations in the PFZ (average concentration of dissolved Fe: 1.87 nM) measured . by Loscher et al. (1997), suggest that Km values were most probably low (o5 mM) during our study, i.e. below the in situ orthosilicic acid concentration. Hence we consider our measured

rSi values as characteristic of the spring PFZ in the study sector.

4. Discussion 4.1. The control of phytoplankton growth The limiting factors of phytoplankton development during Antarktis X/6 cruise have been reviewed in detail in previous studies (Jochem

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et al., 1995; De Baar et al., 1995, 1997; Que! guiner et al., 1997; Scharek et al., 1997). The absence of phytoplankton blooms in the POOZ and the SIZ has been related to the combination of low iron availability and deep mixing induced by the severe weather conditions prevailing during the cruise, whereas the PFZ diatom bloom development is thought to have been triggered by the combination of mixing relaxation and a supply of iron to the surface mixed layer.

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Depth-integrated values of the biogeochemical parameters are presented in Table 1. We have included the values of C primary production from Que! guiner et R R al. (1997), which have been used to derive rSi: rC elemental ratios. There is a strong contrast between the low values of integrated rates R R (R rSi and rC) as well as of integrated stocks R R ( BSi, POC, and PON) in the POOZ, SIZ versus the PFZ. The contrast extends to the relative stability of the parameters in the POOZ and the SIZ over the duration of the cruise, compared to the large increases in bothR standing R stocks and production rates in the PFZ. rSi: rC ratios ranged between 0.15 and 1.19, spanning a very large range. Such high values are not unusual for the Southern Ocean, as Nelson and Smith (1986) reported an average value of 0.57 in the Ross Sea. The very high maximum value of 1.19 was observed during transect 2+3 and corresponded to the extension at depth of the rSi maximum at 491S (see Fig. 9), which probably reflected the rapid sedimentation of living diatoms. R R In each subsystem rSi: rC ratios were always higher than the Si:C ratio of 0.13 observed for nutrient-replete diatoms (Brzezinski, 1985), which could be due to a variety of biogeochemical and physiological processes.

resulted in a decrease of the rSi: rC ratios from up to 0.70 down to 0.30, but had little influence on BSi:POC (range: 0.09–0.11) and BSi:PON (range: 0.73–0.95) biomass ratios (Table 3). The decrease of the rSi:rC ratio is consistent with the response of Fe-stressed diatoms to Fe additions (Hutchins and Bruland, 1998; Takeda, 1998; Watson et al., 2000). We do not believe that a change in Fe supply played a major role in altering Si:C uptake ratios, however, because only weak increases in Fe concentration were observed and they were restricted to the upper 10 m of the water column . at the ice edge (Loscher et al., 1997). The decrease of the rSi:rC ratio more likely reflects the growth of a mixed ‘‘green algae’’ population (Peeken, 1997), which was probably rapidly grazed by microheterotrophs of the nanoprotozooplankton (Becquevort, 1997), which prevented its signature from being reflected as changes in the BSi:POC biomass ratios. Jochem et al. (1995) also indicate that the contribution of microphytoplankton (o20 mm) to primary production was mostly negligible in the SIZ. The low, consistent BSi:POC ratio thus can be attributed to the persistence of small nano-sized diatoms (Brzezinski, 1985), which accounted for around 40% of the pigment biomass in the SIZ during the entire cruise (Peeken, 1997). These small diatoms were mainly typical ice algae species of the genus Pseudo-nitzschia (Bathman et al., 1997). BSi:POC and BSi:PON stock ratios in the SIZ thus do not show large variations from the typical diatom values, which suggests that diatoms were not severely iron-stressed and reinforces the idea of hydrodynamic control of phytoplankton growth in the SIZ. However, small diatoms are sometimes mentioned as having lower cellular Fe requirements than the larger diatoms (e.g. see Sunda and Huntsman, 1995, p. 200), and Fe availability also may have played a role in determining community structure in the SIZ.

4.3. The SIZ

4.4. The POOZ

Both rSi (range: 1.8–7.7 mmol Si m
2 d
1) and carbon production rates (range: 2.6–25.6 mmol C m
2 d
1) were consistently low in the SIZ (Table 2). During the cruise, the factor of four increase in rSi as compared to the 10-fold increase of rC

The spring evolution of particulate matter in the POOZ was quite similar to that of the SIZ. As shown in Table 2, rSi (range: 2.4–4.6 mmol Si m
2 d
1) and carbon production rates (range: 6.7–17.7 mmol C m
2 d
1) are the same order of

4.2. Silicon-carbon coupling in spring

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Fig. 8. Temporal evolution of elemental Si:C ratio distribution: (a) transect 2+3 (12–22 October), (b) transect 5 (24–31 October), (c) transect 11 (10–21 November). The sea-ice extent is symbolized by the quadrangle on top right of the frame.

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Fig. 9. Temporal evolution of biogenic silica production rates (nmol Si l
1 d
1): (a) transect 2+3 (12–22 October), (b) transect 5 (24– 31 October), (c) transect 11 (10–21 November). The sea-ice extent is symbolized by the quadrangle on top right of the frame.

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station #

location

∫ρSi (mmol m−2 d −1)

∫ρ C (mmol m−2 d −1)

∫ BSi (mmol m−2)

∫ POC (mmol m−2)

∫ PON (mmol m−2)

ρ Si:ρC (mol:mol)

Si:C (mol:mol)

Si:N (mol:mol)

C:N (mol:mol)

Transect 2+3 866/2 868/2 870/2 872/2 874/2 876/2 882/2 877/3 879/2

57˚ 43' 57˚ 00' 56˚ 00' 55˚ 01' 54˚ 00' 53˚ 01' 49˚ 59' 49˚ 00' 48˚ 01'

S S S S S S S S S

- n.d. 1.8 - n.d. 3.2 2.4 - n.d. - n.d. 16.5 13.3

- n.d. 2.6 - n.d. 6.5 15.6 - n.d. - n.d. 13.9 24.7

47.8 35.8 51.3 64.5 43.5 49.3 302.1 286.9 246.2

586.5 447.6 471.6 573.2 489.0 585.7 1013.6 1049.3 1037.6

40.7 44.1 56.1 72.4 62.0 68.6 140.9 134.2 127.7

- n.d. 0.70 - n.d. 0.48 0.16 - n.d. - n.d. 1.19 0.54

0.08 0.08 0.11 0.11 0.09 0.08 0.30 0.27 0.24

1.17 0.81 0.92 0.89 0.70 0.72 2.14 2.14 1.93

14.4 10.1 8.4 7.9 7.9 8.5 7.2 7.8 8.1

56˚ 00' 55˚ 00' 54˚ 00' 53˚ 00' 52˚ 00' 51˚ 00' 50˚ 00' 49˚ 00' 48˚ 00' 47˚ 00'

S S S S S S S S S S

3.8 4.0 4.9 2.5 6.9 4.1 24.6 45.4 12.6 22.7

15.8 15.7 16.3 10.6 16.1 27.6 24.7 127.9 - n.d. 100.1

66.8 54.1 71.9 44.0 132.5 105.5 449.9 719.6 353.4 657.3

615.3 532.2 569.2 610.4 617.5 950.4 996.6 1323.0 954.0 1253.9

74.0 66.4 67.4 71.2 85.9 92.9 132.1 190.1 125.8 170.9

0.24 0.25 0.30 0.23 0.43 0.15 1.00 0.35 - n.d. 0.23

0.11 0.10 0.13 0.07 0.21 0.11 0.45 0.54 0.37 0.52

0.90 0.81 1.07 0.62 1.54 1.14 3.41 3.79 2.81 3.85

8.3 8.0 8.4 8.6 7.2 10.2 7.5 7.0 7.6 7.3

Transect 5 887/3 891/1 893/2 895/2 897/2 899/2 901/3 903/2 905/2 907/3

B. Qu!eguiner, M.A. Brzezinski / Deep-Sea Research II 49 (2002) 1765–1786

Table 1 R R R R R Depth-integrated Si ( rSi) and C ( rC) production rates, Si ( BSi), C ( POC), and N ( PON) stocks, Si:C elemental production ratios (rSi:rC), Si:C, Si:N; and C:N elemental stock ratios measured during the three complete transects of Antarktis X/6-S.O. JGOFS cruise (29 September–29 November 1992). Shading indicates the position of the stations in the SIZ ( ), the POOZ ( )and the PFZ ( ). For clarity, only stations where complete profiles of biomass parameters (C, N, and Si) were performed are shown on the table. Primary production data are taken from Qu!eguiner et al. (1997). Within each transect, station occupations were in the same temporal sequence as in the table

Transect 11 59˚ 30' 59˚ 00' 58˚ 00' 57˚ 03' 56˚ 00' 55˚ 02' 54˚ 00' 53˚ 00' 52˚ 00' 51˚ 00' 50˚ 00' 49˚ 00' 48˚ 00' 47˚ 00'

S S S S S S S S S S S S S S

n.d.=not determined.

- n.d. 6.1 11.0 7.4 6.1 - n.d. 4.6 - n.d. 3.6 - n.d. 29.6 60.7 41.1 41.3

- n.d. 26.7 24.9 - n.d. 25.1 - n.d. 6.7 - n.d. 16.4 - n.d. 118.8 240.9 133.6 89.6

36.6 50.1 72.2 84.3 46.6 98.8 97.8 30.2 72.0 78.0 1231.2 1043.9 603.6 650.1

586.0 719.2 770.4 638.7 632.8 759.7 774.2 709.5 641.5 734.0 1447.5 1230.2 1201.6 1228.5

55.5 88.3 106.2 80.4 90.3 107.9 112.7 97.3 86.1 105.4 200.2 163.7 162.1 159.0

- n.d. 0.23 0.44 - n.d. 0.25 - n.d. 0.69 - n.d. 0.22 - n.d. 0.25 0.25 0.31 0.46

0.06 0.07 0.09 0.13 0.07 0.13 0.13 0.04 0.11 0.11 0.85 0.85 0.50 0.53

0.66 0.57 0.68 1.05 0.52 0.92 0.87 0.31 0.84 0.74 6.15 6.38 3.72 4.09

10.6 8.1 7.3 7.9 7.0 7.0 6.9 7.3 7.4 7.0 7.2 7.5 7.4 7.7

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930/2 931/2 934/2 941/2 943/2 945/2 947/2 949/3 951/2 953/2 956/2 960/2 964/2 969/2

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Table 2 R R R R R Temporal evolution of depth-integrated Si ( rSi) and C ( rC) production rates, Si ( BSi), C ( POC), and N ( PON) stocks, in the three subsystems (average value7standard error, except when only one or two measurements were made available). Stations where only BSi profiles were performed have been included in the calculation R R R R R rSi (mmol m
2 d
1) rC (mmol m
2 d
1) BSi (mmol m
2) POC (mmol m
2) PON (mmol m
2) Seasonal sea-ice zone (SIZ) 11–14 October 1.8–3.2 24–25 October 3.8–4.0 10–14 November 7.772.3

2.6–6.5 15.7–15.8 25.671.0

49.8711.8 61.476.6 64.8724.2

519.7770.3 532.2–615.3 684.5775.8

53.3714.3 66.4–74.0 88.1719.2

Permanently open ocean zone (POOZ) 15 October 2.4 26–27 October 4.671.8 15–17 November 3.6–4.6

15.6 17.777.1 6.7–16.4

105.57102.4 124.47101.6 69.5728.4

489.0–585.7 686.97177.0 714.8755.7

62.0–68.6 79.4712.1 100.4711.4

Polar frontal region 17–19 October 28–30 October 18–21 November

13.9–24.7 84.2753.4 145.7766.0

290.4733.6 453.67201.9 873.57265.0

1 033.5718.2 1 131.97183.8 1 277.07114.4

134.376.6 154.7730.9 171.2719.4

(PFZ) 13.3–16.5 26.3713.8 43.2712.9

Table 3 Temporal evolution of Si:C elemental production ratios (rSi:rC), Si:C, Si:N; and C:N elemental stock ratios, calculated from depthintegrated production and stocks, in the three subsystems (average value7standard error, except when only one or two measurements were made available rSi:rC (mol:mol)

Si:C (mol:mol)

Si:N (mol:mol)

C:N (mol:mol)

0.48–0.70 0.24–0.25 0.3070.12

0.1070.02 0.10–0.11 0.0970.03

0.9570.16 0.81–0.90 0.7370.21

10.273.0 8.0–8.3 8.071.3

Permanently open ocean zone (POOZ) 15 October 0.16 26–27 October 0.2870.12 15–17 November 0.22–0.69

0.08–0.09 0.1370.06 0.1070.04

0.70–0.72 1.0970.38 0.6970.26

7.9–8.5 8.671.2 7.170.3

Polar frontal region (PFZ) 17–19 October 28–30 October 18–21 November

0.2770.03 0.4770.08 0.6870.19

2.0770.12 3.4670.48 5.0871.37

7.770.5 7.470.3 7.570.2

Seasonal sea-ice zone (SIZ) 11–14 October 24–25 October 10–14 November

0.54–1.19 0.5370.41 0.3270.10

magnitude in the POOZ and the SIZ, but there is little evidence of temporal trends in any parameter in the POOZ. The lack of diatom development in the POOZ could be explained in terms of light availability during the cruise (Jochem et al., 1995) but also could reflect low Fe availability (De Baar et al., 1995).

4.5. The PFZ In the PFZ the temporal evolution of Si and C production followed a different pattern. Silica production rates increased to high values >1 mmol Si l
1 d
1 by transect 11, with integrated rates between 20 and 60 mmol Si m
2 d
1 at that time.

B. Qu!eguiner, M.A. Brzezinski / Deep-Sea Research II 49 (2002) 1765–1786

Very high rSi:rC ratios (up to 1.19) were measured at the beginning of the season, and they show a clear tendency to decline in the course of the bloom down to 0.32 at the end of the cruise (Table 3). The opposite trend characterized the stock ratio values, which increased from 0.27 up to 0.68. This enrichment of the particulate matter in silicon relative to carbon and nitrogen can be attributed to two different but converging factors. First, relatively high Fe levels were maintained in the PFZ during the spring (de Baar et al., 1995; . Loscher et al., 1997) and the observed decrease in rSi:rC could be explained by a community structure shift from heavily silicified diatom species such as Fr. kerguelensis initially (when ambient silicic acid was high) to lightly silicified and non-siliceous species later (when ambient silicic acid was depleted). Second, the larger increase in stock ratios reflects large differences in the regeneration rates of Si relative to C and N. A high preservation of BSi relative to organic matter in the euphotic zone has also been observed in a diatom bloom in the Pacific sector of the Southern Ocean (Brzezinski et al., 2001). High Si:C and Si:N ratios in the PFZ are consistent with the enhanced productivity by heavily silicified diatoms south of the Polar Front postulated by Pondaven et al. (2000) for POOZ phytoplankton. We point out that Pondaven’s hypothesis and the decoupling of Si and organic matter recycling suggested by our data are not mutually exclusive. The temporal evolution of the vertical distribution of BSi also reveals the possibility for rapid sinking of Si-enriched particulate matter in the PFZ which is consistent with the preferential export of silica over POC and PON. We interpret the subsurface biomass maxima (Chl a, POC, PON, BSi) that developed in the PFZ as indications of biogenic material sedimentation because they were decoupled from the production maxima that occurred at the surface. Care must be taken before ruling out completely in situ growth, which could reflect iron input from below, as rSi maximum extended quite deep especially at 491S (Fig. 9). However, rC data did not show any clear trend to deep maximum (Que! guiner et al., 1997), which tends to favor the effect of mass sedimentation in the PFZ as the

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main factor responsible for subsurface biomass maxima. 4.6. Spring blooms in the PFZ Table 4 summarizes the gross silica production estimates for the duration of the cruise in the different sub-systems sampled. The dominance of the PFZ is clear, with a spring production of 1.12 mol Si m
2 over a 37-day period. The data we obtained in the Atlantic sector are limited to spring production, and there has not been comparable measurements there during the other seasons. Even though primary production has been measured during the Polarstern cruise in December 1995–January 1996, no direct measurements of silicon uptake were obtained precluding the calculation of an annual gross biogenic silica production in the Atlantic sector. Data from the AESOPS cruises of the US S.O.JGOFS program along 1701W longitude (Brzezinski et al., 2001) allow a comparison of silica cycling between the Atlantic and Pacific sectors of the ACC. Brzezinski et al. (2001) report silica production in the Pacific Sector to be dominated by the POOZ and SIZ rather than the PFZ. The POOZ is not extensive in the Pacific sector along 1701W, where Brzezinski et al. worked, as sea ice extends to within 200 km of the PF during winter. Brzezinski et al. (2001) report that silica production in the region from the Polar Front (61.41S) to the northern edge of the seasonal sea ice (ca. 631S at that time) along 1701W longitude was 0.4 mol Si m
2for a 33.5 day period in October–November, i.e. the same time interval as we observed during Antarktis X/6. They report a higher average rate of 1.13 mol

Table 4 Spring biogenic silica production estimates in the Atlantic sector Sub-system

Gross biogenic silica production (mmol Si m
2)

Duration (days)

SIZ POOZ PFZ

173735 141737 11237418

36 35 37

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B. Qu!eguiner, M.A. Brzezinski / Deep-Sea Research II 49 (2002) 1765–1786

Si m
2 for the region between the PF and the northern edge of the seasonal sea ice during a 41 day period during December and early January, with maximal values in the SIZ rather than in the PFZ. The rate that they observed in December/ January is very similar to the value that we obtained earlier in the year in the Atlantic sector, but it occurs in a different sector. The temporal and spatial differences in silica production between the Atlantic and Pacific sectors cannot be explained on the basis of the limited data available, but the differences may arise due to higher overall annual production in the Atlantic sector, or they may simply reflect the interannual variability of the seasonal production maximum between sectors may be related to Fe supplyFfrom upstream shelf-sediment sources in the PFZ in the Atlantic sector versus fertilization in the SIZ (later) in the Pacific sector. Clearly much remains to be learned about the seasonal silica cycle in the different sectors of the Southern Ocean. One feature that the data from the Atlantic sector presented here shares with that of Brzezinski et al. (2001) is that the rates of silica production measured during both studies are significantly higher than have been estimated for the ACC in the past (0.4–0.8 mol Si m
2 yr
1, Nelson et al. 1995). These high production rates are supported by analyses of seasonal nutrient drawdown that show high rates of net silica production in the ACC (Sigmon et al., 2002; Pondaven et al., 2000). Our results also suggest that longitudinal heterogeneity has to be taken in account when deriving elemental budgets for the Southern Ocean; in that respect, it is important to keep in mind that perhaps the South Atlantic as well as the Pacific AESOPS areas may not be typical of the entire Southern Ocean, in that both receive unusually high iron inputs and thus sustain unusually high diatom production. Taken together, these data have helped inspire a revision of the probable mechanism leading to the formation of the extensive siliceous abyssal sediments of the Southern Ocean. Our results support the hypothesis of Pondaven et al. (2000) and Nelson et al. (2002), that the opal sediments of the Southern Ocean form because of high rates of silica production in surface waters rather than through enhanced opal

preservation efficiencies in this area as suggested in the past by Nelson et al. (1995).

5. Conclusion The results obtained during the Antarktis X/6 cruise clearly show the PFZ to be the major area of opal production and sedimentation in the Atlantic sector of the ACC during this study. Diatoms in the PFZ were producing silica at high rates relative to organic carbon early in the cruise, probably due to the particular diatom species found in the area, the effect of initial low iron availability, and changes in species composition in the course of spring. Interestingly, increases in the Si:C ratio of particulate matter during the cruise suggests a strong preferential recycling of organic matter over silica in surface waters as observed in other sectors of the ACC. Comparisons between the Atlantic and Pacific sectors show that the overall integrated gross biogenic silica production and biomass levels are similar in each sector, although the timing and location of diatom blooms within the ACC are different. The data support recent hypotheses that high rates of silica production in the surface waters of the ACC are a major factor in the formation of the extensive abyssal opal deposits beneath the ACC.

Acknowledgements We wish to thank the Captain and crew of R.V. Polarstern for their support. We would like to thank Laurence Pinturier, Annick Masson, Rudolph Corvaisier, and for technical assistance and help, respectively, with BSi measurements, POC/ PON and Chl a measurements, and 30Si mass spectrometry. Also the help of Pascal David and Laetitia Teissier, for sampling at sea, is gratefully acknowledged. Many thanks are also due to C. Veth and his team for running the CTD and water sampling. This work was supported by INSU/ CNRS and B. Que! guiner received financial support from the Minist"ere de l’Enseignement Supe!rieur et de la Recherche (post-doctoral grant). We would also like to thank two anonymous reviewers

B. Qu!eguiner, M.A. Brzezinski / Deep-Sea Research II 49 (2002) 1765–1786

for their critical comments which greatly improved the quality of the manuscript.

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Jochem, F.J., Mathot, S., Qu!eguiner, B., 1995. Size-fractionated primary production in the open Southern Ocean in austral Spring. Polar Biology 15, 381–392. Leynaert, A., Nelson, D.M., Qu!eguiner, B., Tr!eguer, P., 1993. The silica cycle in the Antarctic Ocean: is the Weddell Sea atypical? Marine Ecological Progress Series 96, 1–15. . Loscher, B.M., De Baar, H.J.W., De Jong, J.T.M., Veth, C., Dehairs, F., 1997. The distribution of Fe in the Antarctic Circumpolar Current. Deep-Sea Research II 44, 143–187. Mullin, J.B., Riley, J.P., 1965. The spectrophotometric determination of silicate-silicon in natural waters with special reference to sea water. Analytical Chimica Acta 46, 491– 501. Nelson, D.M., Goering, J.J., 1977. A stable isotope tracer method to measure silicic acid uptake by marine phytoplankton. Analytical Biochemistry 78, 139–147. Nelson, D.M., Gordon, L.I., 1982. Production and pelagic dissolution of biogenic silica in the Southern Ocean. Geochimica Cosmochimica Acta 46, 491–501. Nelson, D.M., Smith Jr., W.O., 1986. Phytoplankton bloom dynamics of the Western Ross Sea ice edge. II. Mesoscale cycling of nitrogen and silicon. Deep-Sea Research I 33, 1389–1412. Nelson, D.M., Tr!eguer, P., 1992. Role of silicon as a limiting nutrient to Antarctic diatoms: evidence from kinetic studies in the Ross Sea ice-edge zone. Marine Ecological Progress Series 80, 255–264. Nelson, D.M., Smith Jr., W.O., Muench, R.D., Gordon, L.I., Sullivan, C.W., Husby, D.M., 1989. Particulate matter and nutrient distribution in the ice edge zone of the Weddell Sea: relationship to hydrography during late summer. Deep-Sea Research I 36, 191–209. Nelson, D.M., Ahern, J.A., Herlihy, L.J., 1991. Cycling of biogenic silica within the upper water column of the Ross Sea. Marine Chemistry 35, 461–476. Nelson, D.M., Tr!eguer, P., Brzezinski, M.A., Leynaert, A., Qu!eguiner, B., 1995. Production and dissolution of biogenic silica in the ocean: Revised global estimates, comparison with regional data and relationship to biogenic sedimentation. Globalal. Biogeochemical Cycles 9, 359–372. Nelson, D.M., DeMaster, D.J., Dunbar, R.B., Smith Jr., W.O., 1996. Cycling of organic carbon and biogenic silica in the Southern Ocean: Estimates of water-column and sedimentary fluxes on the Ross Sea continental shelf. Journal of Geophysical Research 101, 18519–18532. Nelson, D.M., Brzezinski, M.A., Sigman, D.E., Franck, V.M., 2001. A seasonal progression of Si limitation in the Pacific sector of the Southern Ocean. Deep-Sea Research II 48 (19–20), 3973–3995. Nelson, D.M., Anderson, R.F., Barber, R.T., Brzezinski, M.A., Buesseler, K.O., Chase, Z., Collier, R.W., Dickson, M.-L., Franc-ois, R., Hiscock, M.R., Honjo, S., Marra, J., Martin, W.R., Sambrotto, R.N., Sayles, F.L., Sigmon, D.E., 2002. Vertical budgets for organic carbon and biogenic silica in the Pacific sector of the Southern Ocean, 1996–1998. DeepSea Research II 49, 1645–1673.

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