Deep Sea Research II

Marine Biology

Description

Niveau: Supérieur, Licence, Bac+1
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. Qu!eguinera,*, M.A. Brzezinskib aCentre 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 bDepartment 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 sub- systems 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 l1) were recorded in late November at the southern border of the PFZ.

• area during


• zone de l'oc


• ees


• zone


• mmol poc


• production rate


• bsi


• southern ocean


• jgofs cruise

Sujets

• Spring
• Marseille
• Biology
• Science
• Barbara
• Outline of industrial organization
• Southern Ocean

Informations

• Publié par: profil-ontoe-2012
• Nombre de lectures: 22
• Langue: English
• Poids de l'ouvrage: 1 Mo

Extrait

Deep-SeaResearchII49(2002)1765–1786 Biogenicsilicaproductionratesandparticulateorganicmatter distributionintheAtlanticsectoroftheSouthernOcean duringaustralspring1992 B.Que ! guiner a, *,M.A.Brzezinski b a Centred’Oce ! anologiedeMarseille,Laboratoired’Oce ! anographieetdeBioge ! ochimie,ParcScientifiqueetTechnologiquedeLuminy, UMRCNRS6535,Case901,F-13288MarseilleCedex9,France b DepartmentofEcology,EvolutionandMarineBiology,andtheMarineScienceInstitute,UniversityofCalifornia,SantaBarbara, CA93106,USA Abstract Severalofthecomponentsofthesiliconcycle F orthosilicicacid(Si(OH) 4 ),biogenicsilica(BSi),andbiogenicsilica productionrates(rSi) F havebeeninvestigated,togetherwiththedistributionofparticulateorganiccarbon(POC), particulateorganicnitrogen(PON)andcarbonprimaryproduction(rC),onaseriesoftransectsacrossthreesub- systemsintheAtlanticsectoroftheSouthernOcean(6 1 W):theseasonalicezone(SIZ),thepermanentlyopenocean zone(POOZ),andthesouthernboundaryofthepolarfrontalzone(PFZ).ThestudywasconductedinSpring1992as partoftheEuropeanSO-JGOFScruiseaboardtheR.V.Polarstern.HighBSiconcentrations(maximum:11.7mmol Sil
1 )wererecordedinlateNovemberatthesouthernborderofthePFZ.Incontrast,nolargeBSibiomasswasfound intheothersubsystemsstudied.IntheSIZ,nodiatombloomwasobserved,despiteasea-iceretreatof200kmduring thestudyperiod,andBSibiomassneverexceeded0.6mmolSil
1 .ThePOOZalsoshowedverylowBSibiomass ( o 0.5mmolSil
1 ),andlowBSi/POCmolarratiosfromthesurfaceto200m(0.04–0.06at53 1 S)suggestthatthiswasan areawherephytoplanktonwerenotdominatedbysiliceousorganisms.AtthesouthernborderofthePFZ,BSi/POC molarratioswereamongthehighesteverrecordedinthesurfacewatersoftheSouthernOcean(maximum:1.33).This couldbearesultofthepresenceofheavilysilicifieddiatomsoralsocouldreflectamorerapidrecyclingofPOCas comparedtoBSi.HighconcentrationsofBSi(>1.5mmolSil
1 )extendedwellbelowtheeuphoticzoneto200mdepth between49 1 Sand51 1 S,suggestingsignificantsedimentationofsiliceousparticlesinthatarea.HighvaluesofrSialso wereobservedinthePFZ(29.6–60.7mmolSim
2 d
1 ,duringtheproductionmaximum)indicatingthatthissubsystem isimportantinthebiogeochemicalbudgetoftheSouthernOcean.Highdepth-integratedrSi/rC(0.25–0.46)andBSi/ POC(0.53–0.85)inthePFZimplytheproductionofdiatomsrichinsilicacomparedtoorganicmatter.Thehighrates ofsilicaproductionobservedinthePFZsupporttherecenthypothesisthattheformationoftheabyssalsiliceousoozes thatencirclemuchofAntarticaformprimarilyastheresultofhighlevelsofsilicaproductioninsurfacewatersrather thanasaresultofhighratesofopalpreservationashasbeensuggestedinthepast.r2002ElsevierScienceLtd.All rightsreserved. R ´ esum ´ e Lesrecherchespre ! sente ! esconcernentladistributiondeplusieursparame " tresducyclebioge ! ochimiquedu silicium F l’acideorthosilicique(Si(OH) 4 ),lasilicebioge ! nique(BSi)etlestauxdeproductiondesilicebioge ! nique *Correspondingauthor.Tel.:[phone];fax:[phone]. E-mailaddress:[e-mail](B.Que ! guiner). 0967-0645/02/$-seefrontmatterr2002ElsevierScienceLtd.Allrightsreserved. PII:S0967-0645(02)00011-5

6671B.Que ! guiner,M.A.Brzezinski/Deep-SeaResearchII49(2002)1765–1786 (rSi) F ainsiqueducarboneorganiqueparticulaire(POC),del’azoteorganiqueparticulaire(PON)etdelaproduction primaire(rC),surplusieursradialessuccessivestraversantlestroissous-syste " mesdusecteurAtlantiquedel’Oce ! an Australa " lalongitude6 1 W:lazonesaisonnie " redesglaces(SIZ),lazonedel’oce ! anouverteenpermanence(POOZ)etla borduresuddelazonepolairefrontale(PFZ).L’e ! tudeae ! te ! mene ! eauprintemps1992danslecadredel’ope ! ration EuropeanSO-JGOFSa " borddunavireoce ! anographiquePolarstern.Alafindumoisdenovembredefortes concentrationsdeBSi(maximum:11.7mmolSil
1 )ontainsie ! te ! observe ! esausuddelaPFZ.Aucontraire,lesautres sous-syste " mesn’ontpasmontre ! defortesteneursdeBSi.DanslaSIZ,enparticulier,aucunde ! veloppementde diatome ! esn’apue # tremisene ! videncemalgre ! unretraitdelabanquisesur200kmpendantlape ! rioded’e ! tudeetles 1
concentrationsdeBSin’ontjamaisde ! passe ! 0.6mmolSil.LesteneursdeBSisontaussireste ! estre " sfaiblesdansla POOZ( o 0.5mmolSil
1 )etlesfaiblesvaleursdurapportmolaireBSi/POCentrelasurfaceet200mdefond(0.04–0.06 a " 53 1 S)ame " nenta " penserquelephytoplanctondecettezonen’e ! taitpasdomine ! pardesorganismessiliceux.Parcontre, ausuddelaPFZ,lesrapportsmolairesBSi/POCquionte ! te ! e ! value ! ssontparmilespluse ! leve ! sjamaisenregistre ! sdans l’Oce ! anAustral(maximum:1.33).Cetteobservationpeute # treinterpre ! te ! ecommerefle ! tantlapre ! sencedediatome ! es fortementsilicifie ! esoubiencommere ! sultantd’unrecyclageplusrapideduPOCparrapporta " laBSi.Defortes concentrationsenBSi(>1.5mmolSil
1 )ontaussie ! te ! misesene ! videnceenprofondeur,largementau-dessousdelazone euphotique,jusqu’a " 200mdefondentre49 1 Set51 1 S,cequisugge " reunphe ! nome " nedese ! dimentationmassivedes particulessiliceusesdecettezone.LesfortesvaleursderSiobserve ! esausuddelaPFZ(29.6–60.7mmolSim
2 J
1 au maximumdeproduction)indiquentquecesous-syste " meestparticulie " rementimportantpourlebilandusiliciumdans l’Oce ! anAustral.Lesrapportsinte ! gre ! ssurlaverticalerSi/rC(0.25a " 0.46)etBSi/POC(0.53a " 0.85)delaPFZ impliquentquelesdiatome ! essonticiparticulie " rementsilicifie ! es.Enfin,lesvaleurstre " se ! leve ! esdeproductiondesilice bioge ! niquequenousavonsobserve ! confirmentl’hypothe " sere ! centedelaformationdesde ! po # tssiliceuxquientourentle continentAntarctique,formationlie ! ea " desvaleursparticulie " remente ! leve ! esdeproductiondansleseauxdesurfaceet nonpasa " unemeilleurpre ! servationdel’opaleabyssaledanscettere ! gion. 1.Introduction TheSouthernOceanisthemajorareaof seaflooropaldepositsintheWorldOcean. However,themechanismsleadingtotheforma- tionofthosesedimentsarestillamatterofdebate. Nelsonetal.(1995)arguedthatespeciallyhigh biogenicsilicaproductionrates(rSi)insurface watersandanunusuallyhighburialefficiencyfor biogenicsilica(BSi)arethetwolikelyexplana- tions.Nelsonetal.(1995)favoredthesecond hypothesis,withthecaveatthatdirectmeasure- mentsofBSidissolutionrateswerescarceinthe literature.Recently,Pondavenetal.(2000)have questionedthehighefficiencyofBSipreservation intheIndiansectoroftheSouthernOcean;using BSiproductiondata,siliconandnitrogenseasonal depletion,sediment-trapfluxmeasurements,and 230 Th-normalizedburialrateinsedimentsthey haveconcludedthattheoverallburialefficiencyof BSiisinfactsimilartotheglobalmeanof2–5% (Calvert,1983).Nelsonetal.(2002)alsohave concludedthattheopalpreservationefficiencyin thePacificsectorisindistinguishablefromthe globalaverageof3%.TheconclusionsofPonda- venetal.(2000)andNelsonetal.(2002)areof particularimportancebecausetheyofferthe possibilityofusingsedimentaryopalaccumulation ratestoreconstructpastpalaeoceanographicBSi productionratesinthesurfacewaters.Iron availabilityalsohasbeenshowntocontrolthe Si:Cratioofdiatoms(Hutchins&Bruland,1998; Takeda,1998).Recently,Que ! guineretal.(in preparation)andFrancketal.(2000)haveadded furthercomplexitytothequestionofthecontrolof particulateSi:Cratiosbyshowingthatiron availabilitycaninducemodificationsinthesilicic aciduptakeparametersinsomeareasofthe SouthernOcean. Onlyafewstudieshavedirectlyexaminedthe productionofBSiinthedifferentsubsystemsof theSouthernOcean,andmostofthesedidnot resolveseasonalpatternsorvariability(e.g., NelsonandGordon,1982;Que ! guineretal., 1991;Leynaertetal.,1993).Onemajorexception istheworkthathasbeenconductedintheRoss

B.Que ! guiner,M.A.Brzezinski/Deep-SeaResearchII49(2002)1765–17861767 Sea(NelsonandSmith,1986;Nelsonetal.,1991), wheresuccessivecruiseshaveledtoareasonable understandingoftheseasonalevolutionofsilic- eousbiomasswithinthisareaofthecoastaland continentalshelfzone(CCSZ)(Nelsonetal., 1996).Also,therecentAESOPScruisesinthe PacificsectoroftheSouthernOceanprovidenew insightsintotheseasonalpatternofbiogenicsilica productionwithinthedifferentsub-systemsofthe Antarcticcircumpolarcurrent(ACC)(Brzezinski etal.,2001). In1992,wehadtheopportunitytodocument thespringtemporalvariationinbiogenicsilica productioninthreedifferentsub-systemsofthe AtlanticsectoroftheSouthernOcean:the seasonalicezone(SIZ),thepermanentlyopen Oceanzone(POOZ),andthesouthernedgeofthe polarfrontalzone(PFZ),referredtoelsewhereas thepolarfrontregion(PFr)(Que ! guineretal., 1997).Thestudywasconductedduringthecourse ofaustralspring,andpreliminarydataconcerned withtheBSidistributionattheendofspringhave alreadybeenpublished(Que ! guineretal.,1997). Herewepresentthecompletetemporalevolution ofsiliceousphytoplanktonproductionduring spring,includingdirectmeasurementsofsilicic aciduptakebythenaturalphytoplanktoncom- munities. Inthispaperweaddressthefollowingquestions: (1)HowdoestheBSicontentofsurfacewaters varyduringspring?(2)Howdothedifferentsub- systemscompareintermsofbiogenicsilica production?(3)Isitpossibletoderiveanestimate ofthecontributionofthedifferentsub-systemsto thebiogeochemicalbudgetofsiliconintheSouth- ernOcean? 2.Materialsandmethods Thedistributionsofdissolvedorthosilicicacid, particulateBSi,particulateorganiccarbon(POC), particulateorganicnitrogen(PON),andrSiwere measuredbetweenOctoberandNovember1992, duringtheAntarktisX/6-S.O.JGOFScruise onboardR.V.PolarsternintheAtlanticsectorof theSouthernOcean(fordetailsofthecruise,see Smetaceketal.,1997).Carbonprimaryproduc- tionalsowasmeasuredduringthecruise,and resultshavebeenpresentedelsewhere(Jochem etal.,1995;Que ! guineretal.,1997).TheAntarktis X/6-S.O.JGOFScruisewasprincipallydevotedto thestudyofalatitudinalsectiononthe6 1 W, betweenthepack-iceandthesouthernborderof thePolarFront;i.e.between59 1 30 0 Sand47 1 S (Fig.1),duringtheperiodofsea-iceretreat. Resultspresentedherecomefromtransects2and 3(11stations,between48and57 1 S,from12to22 October1992),transect5(18stationsbetween47 and56 1 Sfrom24to31October1992),and transect11(16stationsbetween10and21 November1992).Thissurveyencompassedthe threesub-systemsmentionedabove:theSIZsouth of54 1 30 0 S,thePOOZbetween54 1 30 0 Sand 50 1 30 0 S,andthePFZfrom50 1 30 0 Stothenorth- ernmostboundary(Que ! guineretal.,1997).Dur- ingthecruiseseaiceretreatedfrom55 1 Stoabout .S 1 85 Watersamplesweretakenatsixdepthsderived fromPARmeasurements(100%,25%,10%,3%, 1%,and0.1%ofsurfacePAR)foreverypara- meter;PARprofiles(400–700nm)wereobtained Fig.1.LocationofthestudyareaduringAntarktisX/6-S.O. JGOFScruise(29September–29November1992).Themajor limitsofthesub-systemsareindicated(PFZ:PolarFrontal Zone;POOZ:PermanentlyOpenOceanZone;SIZ:Seasonal IceZone).

8671B.Que ! guiner,M.A.Brzezinski/Deep-SeaResearchII49(2002)1765–1786 usingaLI-CORquantummeter(LI-193SA sphericalquantumsensorconnectedtoaLI-100 DataLogger).Threeadditionaldeeperlevelswere sampledforparticulatematterandorthosilicic acidconcentrations,at100,150,and200m. Orthosilicicacidconcentrationsweremeasured onboardship.Sampleswereanalyzedonan AutoanalyzerTechniconIIusingthemethodof MullinandRiley(1965);precision: 7 0.05mM. Forchlorophyllaanalysis,0.5-lseawater sampleswerefilteredontoWhatmanGF/Ffilters (nominalcut-offsize:0.7mm),transferredintodry Pyrextubesandimmediatelyfrozenonboard (
20 1 C).Sampleswerelateranalyzedbythe fluorimetricmethodofYentschandMenzel (1963),usingacalibratedTurner112fluorometer; precision: 7 1%. Forparticulatesilicaanalysis,1-lseawater sampleswerefilteredonto0.4-mmNuclepore polycarbonatefilters.Filterswerethenovendried (60 1 C)onboard,storedinplasticPetridishes,and returnedtothelaboratoryforfurtheranalysis.BSi wasmeasuredbythehotNaOHdigestionmethod ofPaasche(1973)modified
b 1 yNelsonetal.(1989); blanks:0.006 7 0.005mmoll,precision: 7 10%in therange0–20mmoll
1 . ForPOCandPONanalysis,2.5-lseawater sampleswerefilteredontoWhatmanGF/Ffilters pre-combustedat450 1 C.Filterswerestoredfrozen (
20 1 C)inglassboxes.Aftereliminationof inorganiccarbonbyfumingwithconcentrated HCl,POCandPONconcentrationsweremea- suredbyacombustionmethod(Stricklandand Parsons,1972)usingaCarloErbamodelN1500 analyzer;blanks:0.1 7 0.01mmolPOCl
1 and 0.02 7 0.002mmolPONl
1 ,precision: 7 10%in therange0–14mmolPOCl
1 and0–2mmol PONl
1 . ForrSimeasurements,samplesweredrawnin 1-lcleanpolycarbonatebottlescoveredwith neutraldensityscreenstosimulatethelight intensityofthesamplingdepths(seeabove). Sampleswerespikedwith11.5mlof1.7mM Na 3 2 0 SiO 3 solutionor13mlof1.6mMNa 3 2 0 SiO 3 solution,resultinginanincreaseofthefinal ambientorthosilicicacidconcentrationofabout 20mMSi(OH) 4 .DuetothelowSi(OH) 4 concen- trationsmeasuredatthenorthernstations( o 4mM Si(OH) 4 ),thetraceradditioncouldhavesignifi- cantlyincreasedthenaturaluptakerates,andthis pointwillbediscussedlater.Sampleswerethen placedinadeckincubatorcooledbyrunningsea surfacewater.After24hincubation,sampleswere filteredthrough0.4-mmNucleporepolycarbonate filtersandstoredliketheparticulatesilicasamples. SampleswereanalyzedusingeithertheMAAS6– 60massspectrometeroftheMarineScience Institute(UniversityofCaliforniaSantaBarbara) usingthemethodofNelsonandGoering(1977), orthemassspectrometeroftheInstitutUniversi- taireEurope ! endelaMer(Universite ! deBretagne Occidentale,Brest,France)usingthemethod describedbyCaubert(1998). Particulatesilicasamplesfromthesurfaceand the0.1%lightdepthwerealsosize-fractionatedby filteringsamplesthrough10mmfollowedby 0.4mmNucleporepolycarbonatefilters. 3.Results 3.1.Silicicacid ThedistributionofSi(OH) 4 showsageneral south–northdecreaseinconcentration(Fig.2). ThisgeneraldecreaseislikelyrelatedtoEkman northerntransportofAntarcticsurfacewater (AASW)originatingfromcontinuousmixingof uppercircumpolardeepwater(UCDW)with surroundingwatermassesoftheACC(Veth etal.,1997),althoughthereisevidencethatthe positionofthesilicicacidgradientisinfluenced significantlybyinsitubiologicalconsumptionof silicicacid(DafnerandMordasova,1994;Brze- zinskietal.,2001).Indicationsofaseasonal decreaseinsilicicacidconcentrationsrelatedto biologicalactivitycanbeseeninthePFZ,andalso inthePOOZfrom52 1 Sto54 1 S,betweentransect5 andtransect11(Fig.2bandc),aperiodcoinciding withanincreaseinproductionandbiomassas discussedlater.Atthesouthernedgeofthestudy area,thesteepsilicicgradientobservedinsurface waterscorrespondedtotheACC-WeddellGyre boundaryfrontwhichVethetal.(1997)positioned at57 1 S–58 1 30 0 Sduringourcruise.Thatfrontal areaischaracterizedbytheoccurrenceofeddies

B.Que ! guiner,M.A.Brzezinski/Deep-SeaResearchII49(2002)1765–17861967 Fig.2.Temporalevolutionoforthosilicicaciddistribution(mM):(a)transect2+3(12–22October),(b)transect5(24–31October),(c) transect11(10–21November).Thesea-iceextentissymbolizedbythequadrangleontoprightoftheframe. andmeanders.TheobservationsofVethetal.PolarFront,whichismanifestedasabi-modal (1997)duringtheAntarktisX/6cruisealsominimumintheSi(OH) 4 distributioninthesurface revealedthepresenceofanactivemeanderinthewaters(Fig.2).

0771B.Que ! guiner,M.A.Brzezinski/Deep-SeaResearchII49(2002)1765–1786 3.2.Chlorophylla Thetemporalevolutionofchlorophyllawas characterizedbystronglydifferentpatternsinthe southernandthenorthernpartsofthestudy region(Fig.3).InthePFZ,thephytoplankton biomasswasalreadyelevatedatthebeginningof thestudyperiod,withvaluesquitehomogeneously distributedovertheupper0–80m(0.4–0.5to >0.6mgl
1 ).Chlorophyllaconcentrationsin- creasedinthePFZduringthecruise,andadeep chlorophyllmaximum(DCM)becameestablished bytheendofthestudyperiod(maximum >1.4mgl
1 at30–60mdepthdependingonthe location).Althoughwewerenotabletosamplethe POOZadequatelyduringtransect2+3duetoa stormevent,thechlorophylladistributionob- servedduringtransect5suggestedamoderate phytoplanktondevelopmentwithhighestvaluesof >0.3 m gl
1 .Afterwards,chlorophyllalevels declinedandvalues o 0.2mgl
1 characterizedthe centralPOOZduringtransect11(Fig.3c); althoughhighervalueswereobservedatthe northernmostPOOZstationnearthePolarFront. TheSIZalsocontainedlowphytoplanktonbio- massduringtransect11(Fig.3c),withchlorophyll aconcentrationscloseto0.2mgl
1 presentinthis zonedownto100m.Averysmallincreaseof chlorophyllaconcentrations( o 0.25mgl
1 )was noticedinthevicinityoftherecedingiceedgeat theendofthecruise. 3.3.Particulateorganicmatter ThedistributionofPOC(Fig.4)andPON (Fig.5)stronglyparalleledthatofchlorophylla, withthemainseasonalfeaturebeingthedevelop- mentofthebloominthePFZ.Themaindifference wasobservedattheendofthestudyperiodwhen highconcentrationsofbothPOC(>8mmolCl
1 ) andPON(>1mmolNl
1 )wereobservedbelow theeuphoticzoneatdepthsexceeding100mat 50 1 S,suggestingbloomsedimentationatthattime. TherewasamoderateincreaseofPOCandPON withintheSIZ,especiallyattheiceedge,but permanentdeepwind-inducedmixingresultedina deepentrainmentandquitehomogenousvertical distributionofparticulateorganicmatterastheice edgemovedawaysouthward. 3.4.Biogenicsilica BSiconcentrationswerehighinthePFZatthe beginningofthestudyperiod,withvalueswell above1.0mmolSil
1 intheupper100m(Fig.6a). AsmentionedaboveforPOCandPON,thehigh concentrationlayerofBSiextendedtoconsider- abledepthnear50 1 Sattheendofthelasttransect (Fig.6c),butcontrarytoPOCandPON,this patternwasalreadyestablishedforBSiatthe beginningofthefirsttransect,suggestingearly sedimentationofSi-richmaterialanddecoupling ofBSiversusPOCandPONcyclinginthesurface layer.InthecourseofaustralSpringBSivalues increasedattainingmaximalvaluesinthePFZ(at 49–50 1 S)bytheendofthestudyperiodwhereBSi concentrations>11mmolSil
1 wereobserved between40and60mdepth(maximumvalue: 11.7mmolSil
1 at49 1 S).Highvaluesextended downto200mwithconcentrationswellabove 1mmolSil
1 . Incontrast,BSiconcentrationswererelatively lowinthePOOZandtheSIZduringallthree transects.Initialvaluesweremoderateatthe beginningofthefirsttransectandquitehomo- geneouslydistributedoverthesurfacelayer(0.2– 0.3mmolSil
1 intheopenwatersofthePOOZas wellasintheice-coveredwatersoftheSIZ).Assea iceretreatedfrom55 1 Sto59 1 S,theBSiincrease wasmoderateintheSIZ,andmaximumvalues wereobservedat57 1 Sinanareaofincreased verticalstabilitycorrespondingtotheACC- WeddellGyreBoundaryFront(seeVethetal., 1997).However,themaximumBSivalue (0.66mmolSil
1 )wasfarlowerthanthemaximum valuereachedinthePFZ.InthePOOZ,aslight increaseinBSioccurredbetweenthefirstandthe secondtransect,withconcentrationsreaching >0.4mmolSil
1 .Thedominantchangewithin thePOOZwastheseasonalevolutionofanorth– southgradientofBSistartingat52 1 Sassociated withthedevelopmentofthebloominthePFZ. Thesize-fractionationofBSi(Fig.7)clearlyshows themajorroleoflarge(>10mm)oversmall ( o 10mm)siliceousphytoplanktoninthebloom

B.Que ! guiner,M.A.Brzezinski/Deep-SeaResearchII49(2002)1765–1786 withinthePFZ.Therelativecontributionoflarge andsmallfractionstoBSididnotshowanyclear seasonaltrend,andthemajorvariationswere morerelatedtospatialheterogeneitybetweenthe threesub-systems.The>10mmfractionac- countedfor,respectively,64.7% 7 11.4%, 75.7% 7 5.5%,89.9% 7 3.9%ofthesurfaceBSi and55.5% 7 15.3%,73.3% 7 9.1%, 82.2% 7 11.5%ofthedeep(0.1%lightlevel) BSi,intheSIZ,thePOOZandthePFZ.The increasedcontributionofsmallphytoplankton towardstheSIZparalleledashiftfromphyto- planktonassemblagesdominatedbyCorethron criophilum/inermeandFragilariopsiskerguelensis inthePFZtosmallpennatecommunities(Pseudo- nitzschiaspp.)attheiceedge(seeBathmanetal., 1997). 3.5.Elementalratiosoftheparticulatematter Theelementalcompositionoftheparticulate matteralsoreflectedthedevelopmentoflarge diatomsinthePFZ.Si:Cratiosincreasedfrom quitehighvalues(0.09–0.37)atthebeginningof thestudytoextremelyhighvaluesbythelast transect(Fig.8),especiallywithinthesubsurface BSimaximumwherethemaximumSi:Cratioof 1.33wasobserved.Incontrast,Si:Cratiosinthe POOZandtheSIZweresimilartotypicalvalues fornutrient-repletediatoms(Brzezinski,1985), andvaluesdidnotshowmuchchangefrom transect2+3(Fig.8a),whenvaluesrangedfrom 0.05to0.18,totransect11(Fig.8c)whenvalues between0.03and0.22wereobserved.C:Nratios (datanotshown)followedtheclassicalvertical increase(seeHonjoandManganini,1993),and therewasnoclearseasonalpattern.TheC:Nratio rangedfrom6.1–22.0to6.1–15.1inthePOOZand SIZ,andfrom6.6–15.4to6.1–16.0inthePFZ duringthestudyperiod. 3.6.Biogenicsilicaproductionrates rSiincreasedinthePFZduringthestudy, whereasconsistentlylowratescharacterizedthe othersubsystems(Fig.9).Duetothestormevent duringthetwofirsttransects,wearenotableto documentpreciselythedistributionofrSiatthe 1771 beginningofthecruise,buttherearesome indicationsofaslightlyhigheractivityinthe PFZreachingupto0.29mmolSil
1 d
1 insub- surfacewaters(30m)at49 1 Swhereasvalues p 0.05mmolSil
1 d
1 wererecordedinice-covered watersoftheSIZ.rSiremainedquitelowinthe POOZandtheSIZduringalltransects,barely exceeding0.05mmolSil
1 d
1 ,withtheexception ofthesurfacewatersattheiceedgeduringtransect 11whererSivaluesslightlyexceeded0.10mmol Sil
1 d
1 ,indicativeofmoderatesiliceousphyto- planktongrowth(Fig.9c). Themainfeatureofthechangeinsilica productionwithtimewastheespeciallyhigh valuesreached
1 in
t 1 hePFZ,whererSiratesupto 0.80mmolSildwereobservedontransect5 (Fig.9b)withvaluesreaching1.08mmolSil
1 d
1 ontransect11at49 1 S(Fig.9c).Thesevaluesarein thelowerendoftherangeoftheveryhighvalues measuredbyNelsonandSmith(1986)inabloom followingtherecedingiceedgeoftheRossSeaand arenearlyequivalenttothosemeasuredwithinan intensediatombloomintheSIZoverlappingPFZ inthePacificsectoralong170 1 WbyBrzezinski etal.(2001).Itisinterestingtonotethat,contrary tothedistributionofBSistocks,whichexhibited sub-surfacemaxima,silicaproductionrateswere elevatedatthesurfaceandwerequitehomoge- neouslydistributedinthe0–50mlayer.This furthersupportstheideaofsignificantexportof siliceousbiomasswhilethebloomwasstill growing.Asmentionedintheprevioussection, theisotopeadditionresultedinalargeincreaseof orthosilicicacid(+20mM)relativetotheambient concentrationinthePFZ,sothattherSivalueswe measuredneedtobeconsideredaspotential maximumrates.However,webelievethatthe rateswemeasurearequiteclosetothetruevalues fortworeasons.Firstly,onlyafewlocations locatedatthenorthernboundaryofthestudyarea exhibitedloworthosilicicacidconcentrations(i.e. o 5mM),andthesestationswerenottheones wherethemaximumratewererecorded.HighK m valuesfororthosilicicacid(>10mM)havebeen reportedintheliteratureforSouthernOcean open-waterdiatoms(Jacques,1983;Sommer, 1986,1991;Caubert,1998;Nelsonetal.,2001) butsomestudiessuggestK m values o 5mM

2771B.Que ! guiner,M.A.Brzezinski/Deep-SeaResearchII49(2002)1765–1786 Fig.3.Temporalevolutionofchlorophylladistribution(mgl
1 ):(a)transect2+3(12–22October),(b)transect5(24–31October),(c) transect11(10–21November).Thesea-iceextentissymbolizedbythequadrangleontoprightoftheframe.

B.Que ! guiner,M.A.Brzezinski/Deep-SeaResearchII49(2002)1765–17863771 Fig.4.Temporalevolutionofparticulateorganiccarbondistribution(mmolCl
1 ):(a)transect2+3(12–22October),(b)transect5 (24–31October),(c)transect11(10–21November).Thesea-iceextentissymbolizedbythequadrangleontoprightoftheframe.

4771B.Que ! guiner,M.A.Brzezinski/Deep-SeaResearchII49(2002)1765–1786 1
Fig.5.Temporalevolutionofparticulateorganicnitrogendistribution(mmolNl):(a)transect2+3(12–22October),(b)transect5 (24–31October),(c)transect11(10–21November).Thesea-iceextentissymbolizedbythequadrangleontoprightoftheframe.

B.Que ! guiner,M.A.Brzezinski/Deep-SeaResearchII49(2002)1765–17867715 Fig.6.Temporalevolutionofbiogenicsilicadistribution(mmolSil
1 ):(a)transect2+3(12–22October),(b)transect5(24–31 October),(c)transect11(10–21November).Thesea-iceextentissymbolizedbythequadrangleontoprightoftheframe.