Deep-Sea Research II 49 (2002) 3189–3206

Particulate biogenic silica and carbon production rates andparticulate matter distribution in the Indian sector of the

Subantarctic Ocean

K. Leblanca,*, B. Qu!eguinera, M. Fialab, S. Blainc, J. Morvand, R. Corvaisierc

aCentre d’Oc!eanologie de Marseille, Laboratoire d’Oc!eanographie et de Biog!eochimie, Campus de Luminy, Case 901,

F-13288 Marseille Cedex 09, FrancebObservatoire Oc!eanologique de Banyuls, Universit !e P. et M. Curie, UMR-CNRS 7621, F-66651 Banyuls-sur-mer Cedex, France

cUMR CNRS 6539, Institut Universitaire Europ!een de la mer, Place Nicolas Copernic, F-29280 Plouzan!e, FrancedEcole Nationale de chimie de Rennes, Avenue du G!en!eral Leclerc, F-35700 Rennes, France

Received 9 October 2000; accepted 18 October 2001

Abstract

The silicon and carbon cycles in the Subantarctic region have been investigated in the summer 1999 during the

Antares 4 cruise in the Indian sector of the Southern Ocean. Both biogenic silica (BSi) biomass and production values

were low and indicative of the end of the productive period. Biogeochemical parameters revealed that the siliceous

components of the phytoplankton were scarce. The study area was characterized by nanoplanktonic communities.

Depth-integrated values of BSi showed a north–south positive gradient: average values were 45.65mmolm�2 in the

Polar Frontal Zone (PFZ) and 19.82mmolm�2 in the Subtropical Zone (STZ). Diatom frustules appeared slightly

deteriorated, and BSi distribution suggested particulate matter sedimentation especially at the southern boundary of the

study area, where the highest concentrations were found (750 nmol l�1). BSi tended to accumulate along the Subtropical

Front, with values reaching up to 420 nmol l�1. Depth-integrated BSi production rates showed an inverse pattern

compared to BSi: the maximum and the minimum values were found, respectively, in the STZ (0.72mmol Sim�2d�1)

and in the PFZ (0.25mmol Sim�2 d�1). The Subantarctic region, located between the PFZ and the STZ, showed

intermediate values for both BSi standing stock (31.58mmolm�2) and production rates (0.50mmol Sim�2 d�1). A

production regime based on orthosilicic acid regeneration is hypothesized in the STZ where surface temperatures

appear high enough to sustain higher BSi dissolution rates. During summer, BSi production in the frontal zone of the

Indian sector is comparable to the lowest production rates of oligotrophic waters. r 2002 Elsevier Science Ltd. All

rights reserved.

1. Introduction

The Southern Ocean has long been considered aparticularly rich marine ecosystem, this idea

resting on diverse observations such as the highlevel of nutrients, the importance of krill biomass,and intensive phytoplankton blooms. Anotherreason supporting the concept of a rich environ-ment is the major role played by the SouthernOcean in the silicon cycle. Recent studies haveshown that the high opal accumulation rates are

*Corresponding author.

E-mail address: leblanc@com.univ-mrs.fr (K. Leblanc).

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 7 8 - 4

related to a higher biogenic opal rain rate in theSouthern Ocean, directly related to unusually highBSi production in surface waters (Pondaven et al.,2000; Nelson et al., 2002). The perception of theSouthern Ocean as an eutrophic ecosystem per-sisted until the beginning of the first multidisci-plinary campaigns in the 1970s when productionestimates were reconsidered and consequentlydown graded. At first considered as a single vastecosystem, the conception of the Southern Oceanthen evolved into several complex sub-systems,some of which being highly productive and othersremaining biologically poor all year long (Tr!eguerand Qu!eguiner, 1991).The main factors controlling the development

and accumulation of phytoplankton biomass inthe Southern Ocean have been related to thediminution of the mixed-layer depth (MLD),increasing light intensity (Sverdrup, 1953) asreformulated in the Sullivan’s model (Sullivanet al., 1988) for the Seasonal sea-Ice Zone andmore recently for the different sub-systems byNelson and Smith (1991), the availability of traceelements (Martin and Fitzwater, 1990), as well asgrazing pressure (Sakshaug et al., 1991). Recentfindings have demonstrated that in the beginningof the productive period, biomass maximums areconcentrated along two major circumpolar frontalfeatures: the Polar Front (Qu!eguiner et al., 1997)and the confluence zone of the Subantarctic Front(SAF) and the Subtropical Front (STF) (Banse,1996). The latter benefits from an enhanced inputof nutrients such as iron, which could explainphytoplankton development (Laubscher et al.,1993). This frontal zone is also considered to bea major carbon dioxide sink for the SouthernOcean, which could absorb up to 1GtC yr�1

(Metzl et al., 1999). Covering 20% of the worldocean, the Southern Ocean represents a crucialstudy area for a better comprehension of theglobal biogeochemical cycle of both silicon andcarbon. Sedimentary opal can be used as a tool forreconstructing paleoproductivity, using biogenicsilica (BSi) as a proxy for carbon production(Bareille et al., 1991). Understanding the varia-bility of the Si/C ratio of Antarctic and Sub-antarctic phytoplankton is necessary to establishmore accurate estimations of paleoproduction

rates and consequently direct production ratesand biomass measurements are needed. Unfortu-nately, only a few BSi production measurementsare available in this area. The aim of this study wasto acquire data in order to characterize theassociated silicon and carbon cycles in the differentsub-systems of the frontal zone.The present article deals with the results of the

Antares 4 (ANTarctic RESearch) campaign, aFrench contribution to SO-JGOFS. The generalobjectives of the Antares 4 cruise were thedescription and modelling of the carbon cycleand its associated biogenic elements (Si, C, N) inthe area encompassed by the STF and the SAF inthe Crozet-Kerguelen basin. This article will focuson particulate matter distribution and on produc-tion rates data obtained in the three sub-systems ofthe frontal zone. Differences between these sub-systems as well as factors controlling biomassdistribution will be further discussed.

2. Materials and methods

2.1. Sampling strategy

The sampling period extended from 16 Januaryto 21 February 1999 and water collection wasrealized on board the N.O. Marion-Dufresne. Thestudy area was located in the frontal zone of theAntarctic circumpolar current (ACC) in the Indiansector of the Southern Ocean, between 42–471Sand 61–651E (Fig. 1). The sampling strategydifferentiated two types of observations:

1. Physical and biological parameters were inves-tigated at 14 short-term stations on twodifferent transects. Water was sampled using12-l Niskin bottles at 11 depths from surface to300m. The first transect, orientated NE–SW,was sampled from 24 to 26 January; the secondtransect, orientated NW–SE, was sampled from25 January to 3 February.

2. Process studied were carried out at three long-term stations (lasting around 5 days) in eachsub-system and a total of five productionprofiles were obtained: one at station 3 in thePFZ, two at station 7 in the SA region (SAr),

K. Leblanc et al. / Deep-Sea Research II 49 (2002) 3189–32063190

and two at station 8 in the STZ. At each so-called ‘‘production station’’, the ship wasallowed to drift along with a free-floatingsediment trap deployed at 200m depth, in orderto follow the same water mass. For theproduction experiments sea water was collectedat depths corresponding to photometric levels:100%, 50%, 25%, 10%, 3%, 1% and 0.1% ofsurface light. Incubations were carried outunder simulated in situ conditions (using acombination of nickel and neutral photographicscreens) in a deck incubator cooled by runningsea-surface water.

2.2. Nutrients

The nutrient analyzes (nitrate+nitrite and silicicacid) were performed on board by standardautomated colorimetric methods (Tr!eguer and LeCorre, 1975) on a Technicon autoanalyzer. Theprecision of the method is 750 nM, and the limitof detection is 20 nM.

2.3. Chlorophyll a (Chl a)

Chl a was determined after filtration of 1 l ofseawater on to 47-nm Whatman GF/F glass-fibrefilter at a vacuum differential of 520 cm Hg. ForChl a extraction, the filters were ground manually

in 100% pure acetone (water retention in the filterbringing the final concentration to 90%). Extrac-tion was carried on during 24 h at 51C. Fluores-cence of the acetone extract was measured on aPerkin Elmer MPF 66 spectrofluorometer (Neveuxand Panouse, 1987).

2.4. Biogenic silica (BSi) and lithogenic silica

(LSi)

BSi and LSi were measured using the method ofPaasche (1973) modified by Nelson et al. (1989).For each sample, 1 l of sea water was filteredimmediately after collection on a polycarbonateNucleopore membrane (pore size 0.6 mm, diameter47mm). The filters were placed in a plastic Petridish and oven-dried at 601C for 24 h. The filterswere then stored at room temperature until furtheranalysis at the laboratory. Blank values were577 nmol l�1 for BSi and 573 nmol l�1 for LSi.

2.5. Biogenic silica production (rSi)

275-ml samples were spiked with 0.04 mCi(1.48 kBq) of the radioactive isotope silicon-32(32Si), and incubated in the deck incubator for 24 h(dawn-to-dawn). Two 32Si stock solutions whichspecific activities were, respectively, 27.3 and17.3 kBq mg Si�1 were used during the cruise. For

Fig. 1. Left: geographical settings of the study area (filled square). Right: detailed location of sampling stations and approximate

position of hydrological fronts (Park, pers. comm. 1999). (K) short stations, ($) long stations. Vertical profiles from stations 4a to P5

will be referred to as transect 1, and vertical profiles from station 4a–P5 as transect 2.

K. Leblanc et al. / Deep-Sea Research II 49 (2002) 3189–3206 3191

70% of the samples, Si addition increased initialSi(OH)4 concentration by o5%. The highestaddition in the other samples represented 11.3%of initial Si(OH)4 concentration. After incubation,samples were filtered on polycarbonate Nucleo-pore membrane (0.6 mm, 47mm). Filters wererinsed with filtered (0.2 mm) seawater, and placedin scintillation vials. The 32Si uptake was measuredin a Packard 1600-TR scintillation counter byCerenkov effect, following the method describedby Tr!eguer et al. (1991) and Leynaert (1993).Precision of the method averages 10%.

2.6. Carbon primary production (rC)

275-ml samples were spiked with 4 mCi(148 kBq) of NaH14CO3 and incubated on deckfor 24 h (dawn-to-dawn) in the same incubator asthat used for 32Si. After incubation, samples werefiltered on Whatman GF/F membranes (25mmdiameter) and rinsed with filtered (0.2 mm) sea-water. The determination of 14C uptake was doneon board in a liquid scintillation Packard 1600-TRcounter. Precision of the method averages 5%.

2.7. Particulate organic carbon (POC) and

nitrogen (PON)

Two liters of seawater were filtered on 25-mmWhatman GF/F filters (precombusted at 4501C)using Pyrex filter towers (Millipore). Filters werethen stored in closed glass boxes at �201C. Beforeanalysis, inorganic carbon was removed from thefilters with fuming HCl. POC and PON weredetermined using the combustion method ofStrickland and Parsons (1972) on a Carlo Erbamodel N 1500 Analyzer. Blanks were 0.1 mmol l�1

for POC and 0.02 mmol l�1 for PON.

2.8. Phytoplankton determination and counting

For each depth two samples were collected andtreated with two different fixing agents: formalinand acidified lugol, which allowed, respectively,good coccolithophorid and diatom preservation.Identifying and counting of micro- and nanophy-toplankton were made by direct observation using

an inversed microscope according to Uterm .ohl(1931).

3. Results

3.1. Hydrological environment

The characteristic feature of the sampling areawas the confluence of three hydrological fronts(Fig. 1): the Agulhas Front (AF), the STF, and theSAF (from north to south). These fronts, else-where well differentiated in the Southern Oceanand spanning over 1000 km, converged in oursurvey area into a narrow band 100–200 kmwide (Park et al., 2002). The STF constitutes thesouthern boundary of the Subtropical Zone (STZ),while the Polar Front Zone (PFZ) extends south ofthe SAF to the Polar Front itself. The SubantarcticZone (SAZ) is the area extending between the SAFand the STF (Park and Gamb!eroni, 1997). On thetransects, the different fronts are identified bytemperature and salinity values at 200m depth(Figs. 2 and 3): 101C, 34.8 for the STF and 61C,34.3 for the SAF. The AF, positioned north ofboth transects, is identified by 141C and 35.5 (Parket al., 1993). However, the mixed layer is subject tostrong northward advection, which somewhatmodifies the position of the hydrological regionsin the surface waters by comparison to the welldefined location of fronts at 200m. In this study,the stations representing the warm oligotrophicwaters of the STZ are found north of the AF,whereas the stations positioned between the AFand SAF are typical Subantarctic waters charac-terized by a strong temperature and salinitygradient. The latter area (AF–SAF) here will bereferred to as the Subantarctic region (SAr).Cold and nutrient-replete PFZ waters are foundin agreement with the usual definition, south ofthe SAF.The distribution of nutrients in the convergence

zone was characterized by a decreasing south–north gradient (Fig. 4). The cold deep waterscoming from the PFZ were enriched with nutrients([NO3]=26 mm, [Si(OH)4]=13 mM) and becamerapidly depleted from the SAF to the STZ wheresurface waters were almost oligotrophic

K. Leblanc et al. / Deep-Sea Research II 49 (2002) 3189–32063192

([NO3]o1 mM). The orthosilicic acid gradient isquite marked below 100m; however, the surfacelayer showed little variation on a north–south axis,with homogeneous concentrations under 1 mM,typical of a High Nutrient Low Silicate LowChlorophyll (HNLSiLC) system.

3.2. Particulate matter distribution

BSi concentrations were very low(o460 nmol l�1) on both transects (Fig. 5). At

the southern edge of the study area, station 4aexhibited three distinct peaks of BSi: at the surface(300 nmol l�1), at 50m (440 nmol l�1) and a deepmaximum at 200m (460 nmol l�1). Both transectsshowed a sub-surface maximum (40–50m) centredon the STF at stations 4e (420 nmol l�1) andG2 (310 nmol l�1). The lowest BSi concentrations(50–150 nmol l�1) were found at the northern endof the transects in the STZ near the AF, and in thePFZ just south of the SAF (station 4d–4b fortransect 1 and station G1 for intersect 2). LSi

Fig. 2. Physical parameters of transect 1. Top: potential

temperature (1C), middle: salinity (psu), bottom: potential

density anomaly (kg dm�3). Dark lines show the position of

fronts according to Park et al. (2002); STF: Subtropical Front,

SAF: Subantarctic Front.

Fig. 3. Physical parameters of transect 2. Top: potential

temperature (1C), middle: salinity (psu), bottom: potential

density anomaly (Kgdm�3). Dark lines show the position of

fronts according to Park et al. (2002); STF: Subtropical Front,

SAF: Subantarctic Front.

K. Leblanc et al. / Deep-Sea Research II 49 (2002) 3189–3206 3193

concentrations were low and typical of regions farfrom continental influence. LSi was less abundantthan BSi at all stations and values ranged fromundetectable values (o5 nmol l�1) to 140 nmol l�1

(Fig. 5). The maximum values were found at thesouthern boundary of the study area for bothtransects (stations 4a, 4b and G1), and the LSidistribution at these stations was similar to that ofBSi, showing three distinct maxima (surface,100m, 200m). Elsewhere, concentrations did notexceed 40 nmol l�1, except at station 4f where LSireached 80 nmol l�1 at 125m. On the secondtransect, LSi was undetectable from station P3 toP5 throughout most of the water column.During the time-course experiments up to 4

vertical profiles were sampled for particulatematter analysis at each production station, en-abling to detail daily variations. Particulate silicawas most variable at station 3 in the PFZ (Fig. 6).

Maximum BSi concentration was found at 50m,with 440 nmol l�1 on day 2, and nearly doubledthe next day with 750 nmol�1 at 60m, which wasthe highest value encountered during the cruise.Station 7, located on the northern boundary of theSTF, presented the same vertical structure asstations 4e and G2, also positioned on this front,with maximum concentrations (up to 300 nmol l�1)almost uniformly distributed from the surface to50m. Profiles on days 3 and 4 were completelysimilar, although slightly lower than day 2. Station8, located in the STZ, showed identical profiles forday 2 and 4, and was characterized by very lowand uniform concentrations (o110 nmol l�1)throughout the entire water column.As was shown for BSi, LSi also exhibited the

highest daily variability in the PFZ (Fig. 6), whilestations located in the SAr and STZ presented nosignificant differences from one day to the other.

Fig. 4. Nutrient distribution along transects 1 and 2. Top left: nitrate concentration ([NO3]) (mM) on transect 1, top right: nitrate

concentration [NO3] (mM) on transect 2, bottom left: orthosilicic acid concentration ([Si(OH)4]) (mM) on transect 1, bottom right:

[Si(OH)4] (mM) on transect 2.

K. Leblanc et al. / Deep-Sea Research II 49 (2002) 3189–32063194

The highest concentrations were again found atstation 3, with a peak at 50m on day 2(450 nmol l�1).Chl a was restricted within the first 50m north

of both transects and progressively extended todepth (125m) southward (Fig. 7). This distribu-tion paralleled the north to south deepening ofthe mixed layer (Figs. 2 and 3). For both transects,the Chl a distribution was characterized by a sub-surface maximum (up to 0.8 mg l�1) extending allover the SAr from 50m depth along the SAF to25m at the northern boundary (stations 4e–5 ontransect 1, stations G2–P5 on transect 2). At thesouthern boundary of the study area higher values(up to 0.5 mg l�1) also were recorded at 60–70m atstation 4a. By contrast, a band of very lowconcentrations was located just south of the SAFon transect 1 (stations 4b–4d) and on the SAFitself on transect 2 (station G1).

POC concentrations ranged from 2 to20 mmol l�1 on both transects (Fig. 7). The dis-tribution was quite similar to that of Chl a, withmaximum values found north of the STF fromsurface to 50m. At station 5, the highest POCvalue (20 mmol l�1) coincided with the Chl a

maximum. As was mentioned above for the otherparameters characterizing the particulate matter,concentrations decreased south of the SAF ontransect 1 (station 4d–4b) and on the SAF ontransect 2 (station G1). PON concentrationsranged from 0.1 to 2.9 mmol l�1 (data not shown)and showed a distribution very similar to that ofPOC. Maximum value was found at station 5 at20m (2.9 mmol l�1).At production stations, POC concentrations

ranged from 1.5 to 17.8 mmol l�1 (Fig. 8). Thelowest values were found in the PFZ (station 3),while the highest concentrations were reached at

Fig. 5. Particulate silica distribution on transect 1 and 2. Top left: biogenic silica ([BSi]) (mmol l�1) on transect 1, top right: biogenic

silica [BSi] (mmol l�1) on transect 2, bottom left: lithogenic silica ([LSi]) (mmol l�1) on transect 1, bottom right: [LSi] (mmol l�1) ontransect 2.

K. Leblanc et al. / Deep-Sea Research II 49 (2002) 3189–3206 3195

station 7 in the surface layer (17.8 mmol l�1). Smallvariations in POC concentrations were observed inthe PFZ and in the SAr, whereas the STZpresented nearly identical profiles during the 4days. Nevertheless, POC variability at station 3and 7 was much lower than that of BSi and LSi.PON profiles showed the same trend (not shown),with distributions similar to POC and lowvariability from the beginning to the end of thesurvey. Concentrations ranged from 0.12 to2.44 mmol l�1, with the maximum value found at20m at station 7 (day 1) corresponding to POCmaximum.

3.3. Silicon and carbon production rates

In the SAr and STZ, the silicon and carbonproduction profiles realized on the second andfourth day showed the same trends (Fig. 9).Surface Si production rate (rSi) was extremelylow (o20 nmol Si l�1 d�1) at every station. Thelowest surface production rate was recorded atstation 3 (5 nmol Si l�1 d�1). rSi increased towardsthe STZ, where the maximum surface valuereached 19 nmol Si l�1 d�1. Despite rather similarvalues in the surface layer at stations 7 and 8 (12–19 nmol Si l�1 d�1), production decreased rapidly

Fig. 6. [BSi] and [LSi] distribution (mmol l�1) at production stations 3, 7 and 8 located, respectively, in the Polar Front Zone (PFZ),

Subantarctic region (SAr), and Subtropical Zone (STZ) during the 4 days time-course experiment.

K. Leblanc et al. / Deep-Sea Research II 49 (2002) 3189–32063196

Fig. 7. Top left: Chl a (mg l�1) on transect 1, top right: (Chl a) (mg l�1) on transect 2, bottom left: particulate organic carbon ([POC])

concentration (mmol l�1) on transect 1, bottom right: [POC] (mmol l�1) on transect 2.

Fig. 8. POC concentrations (mmol l�1) at production stations 3, 7 and 8 during the 4 days time-course experiment.

K. Leblanc et al. / Deep-Sea Research II 49 (2002) 3189–3206 3197

at station 7 below 25m, while the decrease atstation 8 was pronounced only below 50m. Onlysmall daily variability was observed at stations 7and 8 considering the extremely low Si productionrates. Specific Si uptake rate V ; given by (rSi/BSi)/ln 2 (assuming logarithmic growth), showed agradient similar to that of production (data notshown). V was very low and quite homogeneous atstation 3 (o0.03 d�1), increased at station 7, with asurface maximum of 0.12 d�1, and was highest inthe STZ with 0.26 d�1 at 5m.

rC values (Fig. 9) showed the same generaltrend as rSi, with lowest values characterizing thePFZ (o0.49 mmol C l�1 d�1), but the highestsurface rC values were found at station 7 (up to

1.78 mmol C l�1 d�1 at the surface). In the STZ, theproduction profile exhibited an intermediate max-imum value of 0.99 mmol C l�1 d�1 at a sub-surfacelevel (20m) on day 4. Carbon production profilesobtained at station 7 were almost identical,however, carbon production exhibited a neartwo-fold increase at 20m in the STZ, station 8,with 0.6 mmol l�1 d�1 on day 2 and 1 mmol l�1 d�1

on the fourth day.

3.4. Phytoplanktonic communities

Water samples were chosen according tothe position of production and biomass maximaand analyzed for phytoplankton determination.

Fig. 9. Orthosilicic acid uptake rates (rSi) (nmol Si l�1 d�1) and carbon primary production (rC) (mmol C l�1 d�1) at production

stations 3, 7 and 8 during the 4 days time-course experiment.

K. Leblanc et al. / Deep-Sea Research II 49 (2002) 3189–32063198

Nanophytoplankton was dominant at stations 3,4e and 4b, while microphytoplankton was domi-nant and more diverse at stations 7 and 8. Diatomabundance was lowest at station 3 and increasedprogressively towards station 7. Pennate diatomswere less diversified than centric diatoms but weredominant in cell number. Pseudonitzschia spp. wasthe major contributor to diatom abundance at allstations, with a maximum relative contribution upto 90% at station 4e. Upon microscopical exam-ination, several frustules showed evidence ofpartial dissolution at the three study sites. Silico-flagellates were present in all samples but in verylow abundance as compared to diatoms. Silico-flagellates proportion was the highest at station 4e,mainly due to the occurrence of Dictyocha fibula.Dinoflagellates were the dominant group in

terms of cell abundance in the >10 mm size-class.Dinoflagellates abundance at stations 7 and 8 wasone order of magnitude greater than at station 4eand two orders of magnitude higher than atstations 3 and 4b. Gymnodinium spp. were themost abundant dinoflagellates at station 3, 7 and8, while Oxytoxum spp. dominated at station 4e.Coccolithophorids, as well as many single

coccoliths, were found at all stations, with thehighest abundance recorded at station 4e and 8,followed by stations 3 and 7. Station 3 was alsocharacterized by an important accumulation ofcoccoliths of Michaelsarsia spp.

4. Discussion

As a general feature the study area wascharacterized by low BSi and Chl a concentrationsand low silica and carbon production rates. Thegeneral occurrence of rSi/rC ratios and BSi/POCratios (Fig. 10) much lower than the usual value of0.09 (diatoms o20 mm) to 0.13 (diatoms >20 mm)found for living diatoms (Brzezinski, 1985), as wellas direct microscopical observations, indicates thatthe plankton community was dominated by apopulation of microplanktonic flagellates (dino-flagellates, ciliatesy) and nanoflagellates at thattime of the year. rSi/rC ratios ranged between0.006 and 0.016 at station 3, between 0.007 and0.043 at station 7, and between 0.008 and 0.036 at

station 8. The highly variable BSi/POC ratiosobserved at station 3 (Fig. 10) were related tochanges in BSi concentrations (compare Figs. 6and 8). Integrated values of biogeochemicalparameters within each hydrological region arereported in Table 1. SBSi and SPOC were verypoorly correlated, revealing a decoupling of Si andC in the study region at the end of summer, whichis further evidenced by the low SBSi/SPOC ratios.The poor preservation state (empty, partly dis-solved frustules) of the dominant diatoms Pseudo-

nitzschia spp. indicates that the productive periodoccurred earlier in the season. This suggests thatthe community was at the end stage of the bloom,shifting towards a nanoplanktonic community. Asimilar transition has been observed by Laubscheret al. (1993) in the Atlantic frontal zone: diatom-s>to 20 mm (Chaetoceros spp. and Pseudonitzschia

spp.) dominated the early summer bloom inDecember at the SAF and Polar Front (PF),whereas a shift towards dominance of nanophy-toplankton (1–20 mm) occurred later in the season,by February.Our overall mean POC/PON ratio was 6.3

(n ¼ 365), which is very close to Redfield’s ratio(Redfield et al., 1963) as compared to the values of7.3 found during January–March 1984 by Tr!egueret al. (1988) and 5.5 found in March–April 1974 byCopin-Mont!egut and Copin-Mont!egut (1978) inthe Indian sector of the Southern Ocean.It is interesting to note that SBSi and SrSi

follow opposite gradients: SBSi increased towardsthe PFZ, while SrSi was the highest in the STZ(Table 1). SChl a, SPOC, SPON and integratedcarbon primary production all exhibited highervalues in the SAr. SLSi, like SBSi, increased fromthe STZ to the PFZ.

4.1. Characteristics of particulate matter

distribution in the different subsystems

4.1.1. The Polar Front Zone (PFZ)

In the present study, the PFZ exhibited en-hanced BSi concentrations compared to POC.This is well evidenced at production station 3 aswell as stations 4a and 4b on the first transect(Fig. 5). At those stations, the subsurface max-imum (50m), located just above the pycnocline,

K. Leblanc et al. / Deep-Sea Research II 49 (2002) 3189–3206 3199

could have resulted from in situ growth, which is aclassical feature allowing phytoplankton to benefitfrom a higher nutrient availability while remainingat a sufficient light level (Takahashi et al., 1985).However, the specific uptake rates measured atstation 3 tended to invalidate that hypothesis, aslow rSi (o5 nmol l�1 d�1) resulting in high BSidoubling time (58 days) could not explain such abiomass accumulation. BSi accumulation in thePFZ then might either be due to an in situ growthprevious to the sampling or to allochtonous inputof BSi by lateral advection. BSi may have beenproduced in the PFZ and advected northward withthe general movement of the water masses. A

second deep BSi accumulation, around 200m atstations 4a and 4b, was undoubtedly detrital sinceit did not coincide with any Chl a increase. SrSi/SrC production ratios were lowest in the PFZ(0.009) and indicative of the weak contribution ofdiatom to primary production. POC/Chl a ratios(weight-to-weight) were considerably higher thanvalues considered to be representative of livingautotroph communities (e.g., Redfield et al., 1963).In the first 100m the mean POC/Chl a ratio of thePFZ exceeded 300 (68 being the lowest valuefound throughout the cruise), which stronglysupported the presence of detrital matter and/orthe dominance of the community by heterotrophs.

Fig. 10. rSi/rC ratios (mol:mol) and BSi/POC ratios (mol:mol) at production stations 3, 7 and 8 during the 4 days time-course

experiment.

K. Leblanc et al. / Deep-Sea Research II 49 (2002) 3189–32063200

Table 1

Integrated values of the different biogeochemical parameters measured at all stations, bold figures represent mean values and standard deviation (SD) for each

hydrological region. Biomass parameters are integrated over 300m, whereas Si and C production are integrated over 150m

Stations Longitude

(1E)

Latitude

(1S)

SBSi(mmolm�2)

SLSi(mmolm�2)

SChl a(mgm�2)

SPOC(mmolm�2)

SPON(mmolm�2)

SrSi(mmol Sim�2 d�1)

SrC(mmol Cm�2 d�1)

SrSi/rC SBSi/POC

SPOC/PON

PFZ

3 day 1 62156 46101 15.34 27.37 27.9 1228 138 0.012 8.9

3 day 2 62147 46102 45.27 17.04 33.4 1751 293 0.25 27.6 0.009 0.026 5.9

3 day 3 62145 45158 53.29 11.97 36.5 1261 192 0.042 6.6

4a 63111 45120 77.16 18.85 56.2 1545 275 0.050 5.6

4b 63125 45105 60.61 24.15 32.9 1618 235 0.037 6.9

4c 63140 44149 35.06 15.40 33.4 1531 190 0.023 10.9

4d 63152 44134 45.33 13.41 40.4 1513 140 0.030 10.8

G1 63105 45111 33.16 8.15 32.7 1459 193 0.023 7.6

Mean 45.65 17.04 36.7 1488 207 0.25 27.6 0.009 0.030 7.9

SD 718.75 76.34 78.7 7174 757 70.012 72.1

SA region

7 day 2 63146 44104 32.47 1.71 53.9 1343 173 0.58 46.6 0.012 0.024 7.8

7 day 3 64111 44101 20.81 1.67 45.3 1651 194 0.013 8.5

7 day 4 64121 44103 21.27 2.27 36.6 1501 157 0.41 44.2 0.009 0.014 9.6

4e 64105 44120 49.96 9.53 51.4 1996 259 0.025 7.7

4f 64118 44105 31.13 9.70 36.8 1607 205 0.019 7.8

5 64130 43150 25.72 8.87 33.5 1469 186 0.017 7.9

G2 62155 44150 40.54 5.97 42.6 1890 282 0.021 6.7

P3 62148 44138 39.38 5.81 46.0 1929 269 0.020 7.2

G3 62144 44129 — — 35.7 1549 267 5.8

G4 62133 44120 22.90 2.21 42.1 1930 295 0.012 6.5

Mean 31.58 5.30 42.4 1534 229 0.50 45.4 0.011 0.018 7.5

SD 710.07 73.46 76.9 7551 751 70.12 71.7 70.004 71.1

STZ

8 day 2 62132 43109 20.59 1.83 37.3 1248 201 0.93 35.8 0.025 0.016 6.2

8 day 4 63101 42158 22.19 2.64 35.3 1460 175 0.51 43.9 0.011 0.015 8.3

P5 62128 44110 16.68 3.17 32.3 1421 153 0.012 9.3

Mean 19.82 2.55 35.0 1376 176 0.72 39.9 0.018 0.014 7.9

SD 72.83 70.67 72.5 7113 724 70.29 75.7 70.002 71.6

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BSi/POC ratios close to the usual value (0.09–0.13)in the first 50m (Fig. 10) then did not necessarilyreflect an increase of the contribution of livingdiatoms to biomass, but rather indicated theincrease of detrital BSi vs. POC by means ofdifferential regeneration. Indeed, the abundance ofdiatoms was much lower at station 3 than atstations 7 and 8 where BSi/POC ratios were lower.Direct microscope observations confirmed theweak contribution of diatoms to phytoplanktonbiomass, with a dominance of nanoplankton atstation 3.

4.1.2. The Subantarctic region (SAr)

The most striking feature of the SAr was theaccumulation of BSi on the STF itself. Severalhypothesis may be invoked: either in situ growth,or passive accumulation in the frontal jet due toconvergence currents (both hypothesis are notmutually exclusive). Both SrC and SrSi werehigher in the SAr (45.4mmol Cm�2 d�1 and0.50mmol Sim�2 d�1) than in the PFZ (Table 1).Specific Si uptake rates (V ) were also higher,resulting in a mean BSi doubling time of 13 days.Orthosilicic acid uptake rates fell within the samerange as that was found in March 1998 inthe Australian sector of the Indian Ocean duringthe SAZ cruise (Qu!eguiner, 2002), i.e. o15 nmolSi l�1 d�1. Growth rates remained extremely low,which supports the hypothesis of passive accumu-lation in the convergence zone along the STF.The extremely low rSi/rC and BSi/POC ratios inthe surface layer at station 7 (Fig. 10) also wereindicative of the very weak contribution ofdiatoms to both production and biomass. MeanSBSi/POC ratio (0.017) and mean SrSi/SrC ratio(0.010) at station 7 were up to 10 times lower thanBrzezinski’s ratio, confirming the strong decou-pling of Si and C in this sector of the SouthernOcean. POC/Chl a were also very high, with amean value of 446, again reflecting the detritalnature of the particulate matter and/or thedominance by heterotrophs.The SAr also was characterized by a secondary

structure, located along the SAF, whereevery particulate matter parameter (BSi, LSi,POC and Chl a) showed a strong decrease. Thisdepletion was hardly explained by a control by

macronutrient availability (see Fig. 4). Wehypothesize that this particulate matter minimumwas a consequence of hydrological forcingrelated to the physics of the SAF andpossibly involving downward vertical water move-ments.

4.1.3. The Subtropical Zone (STZ)

Interestingly and surprisingly, the STZ pre-sented the highest SrSi values (mean: 0.72mmolSim�2 d�1) together with the lowest SBSi(19.82mmol Sim�2). This may be explained by aregeneration production regime based on a recy-cling of BSi in surface waters. Unfortunately, wewere not able to run dissolution experiments in thecourse of the cruise, but high dissolution rates inthe ACC are reported elsewhere (Nelson andGordon, 1982). There is evidence from theliterature that higher temperatures (>121C) foundin the surface waters of this area, as well asbacterial lysis of the organic coating protectingdiatom frustules, may enhance BSi dissolution(Kamatani, 1982; Bidle and Azam, 1999), whichwould provide sufficient orthosilicic acid concen-trations to sustain observed production rates.POC/Chl a and POC/PON ratios were the

highest in the STZ. SrSi/SrC ratios, and SBSi/POC ratios (Table 1) remained very low (meanvalue of 0.018 and 0.014, respectively), well underthe Brzezinski ratio, confirming the very weakabundance of diatoms.

4.2. Growth limiting factors

The most commonly reported growth limitingfactors for the Southern Ocean are light (Nelsonand Smith, 1991), nutrients (Nelson and Tr!eguer,1992), and the deficiency of trace elements (Boydet al., 2002).

4.2.1. Temperature

Using Eppley’s equation (1972), the role oftemperature as a limiting factor can be discarded.In the course of the cruise, temperature variedbetween 71C and 161C in the mixed layer, whichcould not explain the very low growth ratesobserved in every sub-system.

K. Leblanc et al. / Deep-Sea Research II 49 (2002) 3189–32063202

4.2.2. Light

Production in the Subantarctic waters at thistime of year was similar to that found in the mostoligotrophic waters (Qu!eguiner, 2002). Watertransparency emphasized this feature: at station7, where the highest pigment content was noted,the 0.1% light level was located at 76m, at station8 it was found at 95m, and it was as deep as 138mat station 3. We were not able to calculate thecritical depth (Zc) because surface incident lightwas not recorded during the cruise, but using thevalue of 151m calculated by Jacques and Minas(1981) in the same sector (621E, 42–461S) at thesame time of the year (end of summer), we mayassume that Zc was deeper than the MLD, whichvaried from 50 to 80m from north to south. Wemay then reasonably discard light as a limitinggrowth factor at the end of summer.

4.2.3. Nutrients

Nitrate did not appear to be limiting in the PFZnor in the SAr where mean values for surfacewaters were, respectively, 15.6 and 5.6 mM. In theSTZ, however, concentrations fell below 1 mM andmight have been limiting to phytoplanktongrowth. The enrichment experiments, which wereconducted during the cruise to evaluate the relativeinfluence of micro- and macronutrients on phyto-plankton production (Sedwick et al., 2002),indicated a co-limitation by Fe and macronutri-ents (N and P) in the STZ. This is consistent withthe observation that nitrate was undetectable inthe first 60m at station 8 during the 5 days of theexperiment. By comparison with the mean nitrateKm values of o0.2 mM for oligotrophic ando1 mM for eutrophic waters reported by MacIsaac and Dugdale (1969), it is quite obvious thatSTZ waters were NO3-limited. In the PFZ and theSAr, iron was found to be the primary limitingfactor, with a co-limitation by orthosilicic acid(Sedwick et al., 2002). Indeed, orthosilic acidconcentrations were quite low in the mixed layer,with the lowest values found in the SAr and theSTZ (o1 mM). Km values for orthosilicic acid varybetween 0.4 at 5 mM (Nelson and Tr!eguer, 1992),and [Si(OH)4] therefore may have severely limiteddiatom growth at all stations in the surface layer.Enrichment experiments showed that phytoplank-

tonic production was limited by iron in the threesub-systems. Iron concentrations of subantarcticand subtropical waters were typically o0.5 nM(Sedwick et al., 2002) and consequently potentiallylimiting (Martin and Fitzwater, 1990). LSi can beused as a proxy for iron inputs: both elementsare constituents of terrigenous material and arebrought together to oceanic waters through thesame processes (eolian transport, river discharge,land effect). Thus, the similarity between BSi andLSi profiles in the PFZ may be explained by a pastiron enrichment, which had stimulated diatomgrowth, resulting in an accumulation of BSi. Onthe other hand, BSi maxima observed at the STFlevel (4e and G2) do not appear to be correlatedwith LSi. This would tend to confirm the hypoth-esis of passive accumulation in the convergencecurrent rather than an in situ stimulated growth.

4.3. Comparison with other oceanic provinces

We have compared our integrated values ofdaily silica production rates with results obtainedin other studies conducted in the same sub-systemsof the Southern Ocean, yet in different oceanicsectors (Table 2). As a general feature, one cannote that the production rates measured in thisstudy are the lowest ever found in the SouthernOcean, and even close to the minimum range valuefor the World Ocean. This is not surprisingconsidering that sampling occurred at the end ofsummer, after the productive period, in an areaknown to encounter multiple limiting factors. Inorder to make a realistic estimate of the relativecontribution of the frontal region to the SouthernOcean silica production, spring production mea-surements are strongly needed. Indeed previousstudies have demonstrated that this area couldsustain very high carbon production rates duringthe spring–summer transition (up to 67–151mgCm�3 d�1, Laubscher et al., 1993). This is alsoconfirmed by the high silica production ratesrecently observed in the PFZ during spring(Brzezinski et al., 2002), which are one to twoorders of magnitude higher than the productionrates found in our study (Table 2). This situation(enhanced BSi production in the PFZ or at the PF)seems to be a recurrent feature throughout the

K. Leblanc et al. / Deep-Sea Research II 49 (2002) 3189–3206 3203

latest studies (Qu!eguiner and Brzezinski, 2002;Brzezinski et al., 2002) and this was also observedfor the summer season. For instance, the studyconducted in the Australian SAZ in summer(Qu!eguiner, 2002), which presents productionrates comparable to ours, also points out moreelevated production in the PFZ (2.58mmolm�2 d�1) than in the other sub-systems. The verylow values we observed in the PFZ then probablyreflects the ultimate seasonal stage of diatomgrowth in that sub-system fixing a temporal limitfor summer production rate integration in futureannual budgets of silicon.

5. Conclusion

In summer, the frontal zone of the Indian sectorwas characterized by BSi production rates similarto those found in the most of oligotrophic waters,as well as a very low accumulation of siliceousbiomass. Diatom assemblages were at the very endof their productive stage. A BSi production regime

based on the regeneration of Si(OH)4 might beconsidered in the STZ where surface temperaturesappeared high enough to allow fast recycling ofBSi. The concept of a seasonal transition from adiatom new production to a diatom regeneratedproduction already has been invoked (Brzezinskiand Nelson, 1989; Brzezinski et al., 2002) but ispoorly documented. Dissolution rates of BSi in thesurface layer are still a matter of investigation andneed to be refined. The known role played by thefrontal zone as a major CO2 sink (Metzl et al.,1999), as well as the importance of the opalsedimentary belt underlying this area, calls forfuture cruises to be conducted during the produc-tive period (October–January), since only occa-sional data for Si production are now available.Moreover, accumulating evidence for enhanced Siproduction near the PFZ calls for a bettercomprehension of diatom life cycle in this area,and more particularly for a better grasp on the linkbetween diatom bloom occurrence and limitingfactors such as iron, silicic acid or light avail-ability.

Table 2

Comparative values of integrated biogenic silica production (SrSi) for the different sub-systems of the frontal zone in the Southern

Ocean

Oceanic region BSi production (in

mmolm�2 d�1)

References

Atlantic sector of the Southern Ocean

Spring (PFZ) 13.3�43.2 Qu!eguiner and Brzezinski (2002)

Pacific sector of the Southern Ocean

Spring (October–November) (PFZ) 12.273.7

Spring (December) (PFZ) 27.576.9 Brzezinski et al. (2002)

Summer (January) (PFZ) 22.6720

Summer (February–March) (PFZ) 5.572.8

Australian sector of the Southern Ocean

(PFZ) 2.5871.58

Summer (SAZ) 1.0771.20 Qu!eguiner (2002)

(STZ) 0.6070.30

Indian sector of the Southern Ocean

(PFZ) 0.25 This study

Summer (SAZ) 0.28�0.41(STZ) 0.51�0.93

World Ocean 0.1�1100 Nelson et al. (1995)

K. Leblanc et al. / Deep-Sea Research II 49 (2002) 3189–32063204

Acknowledgements

The Antares 4 cruise was made possible throughthe support of the Institut Fran-cais pour la

Recherche et la Technologie Polaire and theauthors would like to thank Bernard Ollivier andClaudie Bournot (INSU) for technical assistanceat sea. We are also indebted to the Captain andcrew of N.O. Marion Dufresne II for their support.This work is part of the PROOF/JGOFS-Franceprogram and received financial support from theInstitut National des Sciences de l’Univers of theCentre National de la Recherche Scientifique. Weare very grateful to Louise Oriol for help in thepigment measurements on board. Thanks are alsodue to Annick Masson who made the analyses ofPOC/PON at the laboratory.

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