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Deep-Sea Research II 49 (2002) 3327–3349 Resource limitation of phytoplankton growth in the Crozet Basin, Subantarctic Southern Ocean P.N. Sedwicka,*, S. Blainb, B. Qu!eguinerc, F.B. Grifﬁthsa,d, M. Fialae, E. Bucciarellib, M. Denisc aAntarctic CRC, GPO Box 252-80, Hobart, Tasmania 7001, Australia b Institut Universitaire Europ!een de la Mer, UMR-CNRS 6539, Place Nicolas Copernic, 29280 Plouzan!e, France cCentre d'Oceanologie de Marseille, Laboratoire d'Oc!eanographie et de Biog!eochimie, Campus de Luminy, Case 901, 13288 Marseille Cedex 09, France dCSIRO Division of Marine Research, Castray Esplanade, Hobart, Tasmania 7000, Australia eLaboratoire Arago, Universit!e Pierre et Marie Curie, UMR-CNRS 7621, 66651 Banyuls-sur-mer Cedex, France Accepted 20 July 2001 Abstract In January–February 1999, we performed shipboard iron- and macronutrient-addition experiments in the Crozet Basin, Indian sector of the Subantarctic Southern Ocean, to evaluate the sufﬁciency of ambient iron and macronutrient concentrations for algal growth. Experiments were conducted with near-surface seawater collected from three locations in a narrow latitudinal band characterized by relatively low algal biomass (o0.7 mg l1 chlorophyll a), low dissolved iron concentrations (o0.

late summer

seawater inside

using seawater

iron

zone

ocean

subantarctic southern

macronutrient-addition bottle

crozet basin during

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Deep-SeaResearchII49(2002)3327–3349ResourcelimitationofphytoplanktongrowthintheCrozetBasin,SubantarcticSouthernOceana,bca,deP.N.Sedwick*,S.Blain,B.Que!guiner,F.B.Grifﬁths,M.Fiala,cbE.Bucciarelli,M.DenisaAntarcticCRC,GPOBox252-80,Hobart,Tasmania7001,AustraliabInstitutUniversitaireEurope!endelaMer,UMR-CNRS6539,PlaceNicolasCopernic,29280Plouzane!,FrancecCentred’OceanologiedeMarseille,Laboratoired’Oce!anographieetdeBioge!ochimie,CampusdeLuminy,Case901,13288MarseilleCedex09,FrancedCSIRODivisionofMarineResearch,CastrayEsplanade,Hobart,Tasmania7000,AustraliaeLaboratoireArago,Universite!PierreetMarieCurie,UMR-CNRS7621,66651Banyuls-sur-merCedex,FranceAccepted20July2001AbstractInJanuary–February1999,weperformedshipboardiron-andmacronutrient-additionexperimentsintheCrozetBasin,IndiansectoroftheSubantarcticSouthernOcean,toevaluatethesufﬁciencyofambientironandmacronutrientconcentrationsforalgalgrowth.Experimentswereconductedwithnear-surfaceseawatercollectedfromthreelocationsinanarrowlatitudinalbandcharacterizedbyrelativelylowalgalbiomass(o0.7mgl1chlorophylla),lowdissolvedironconcentrations(o0.33nM),andstrongmeridionalgradientsintemperature,salinityandmacronutrientconcentrations:(1)thePolarFrontalZone(PFZ)near461S,651E(B19mMnitrateand1.2mMsilicicacid);(2)theconﬂuenceoftheSubantarcticandSubtropicalFronts(SAF/STF)near441120S,631230E(B5.4mMnitrateand0.5mMsilicicacid);and(3)thesouthernSubtropicalZone(STZ)near431180S,621310E(o0.1mMnitrateandB1.4mMsilicicacid).Ourexperimentalresultsrevealthreedistinctregimesofresourcelimitationofphytoplanktongrowth.InthePFZ,ironavailabilityexertedtheprimarylimitationonnitratedrawdownandbiomassaccumulation,thuscommunitygrowth,withsilicicacidavailabilityexertingasecondarylimitationondiatomgrowthandbiogenicsilicaproduction.WithintheSAF/STF,irondeﬁciencywasalsotheprimarylimitationonalgalcommunitygrowth;however,hereweobservedevidenceofsecondarylimitationofnitratedrawdownandbiomassaccumulationbysilicicaciddeﬁciency,viacontrolofalgalcommunitystructure—suchthatironadditionpreferentiallystimulatedthegrowthofnon-diatomnanoplankton—suggestingthatthealgalcommunitywaspoisedclosetoco-limitationbyironandsilicicacid.Asexpected,ourexperimentalresultsindicatethatmacronutrients(nitrate/phosphate)weretheprimarylimitationoncommunitygrowthintheSTZwaters;however,ourresultsalsosuggestthatirondeﬁciencyimposedasigniﬁcantsecondarylimitationoncommunitygrowth,particularlydiatomgrowth,suchthatthealgalcommunitywaspoisedclosetoco-limitationbymacronutrientsandiron.Weconcludethatthesesameregimesofresourcelimitationarelikelytoregulatephytoplanktongrowthandexportproductionovermuchoftheopen-oceanSubantarcticregionduringthemidtolatesummer.r2002ElsevierScienceLtd.Allrightsreserved.*Correspondingauthor.Presentaddress:BermudaBiologicalStationforResearch,St.George’sGE01,Bermuda.E-mailaddress:psedwick@bbsr.edu(P.N.Sedwick).0967-0645/02/$-seefrontmatterr2002ElsevierScienceLtd.Allrightsreserved.PII:S0967-0645(02)00086-3

2338P.N.Sedwicketal./Deep-SeaResearchII49(2002)3327–33491.IntroductionTheSubantarcticregionofSouthernOcean,locatedbetweenthePolarandSubtropicalwatermasses,accountsforasigniﬁcantfractionofmarineprimaryproduction(Fieldetal.,1998),andisoneofthelargestoceanicsinksforatmosphericCO2,dueinparttobiologicaluptakeandexportbyphytoplankton(Metzletal.,1999).Fromsouthtonorth,thesurfacewaterswithinthisvastcircumpolarbeltincludetheAntarcticPolarFront,thePolarFrontalZone,theSubantarcticFront,theSubantarcticZone,andtheSubtropicalConvergenceand/orSubtropicalFront(NowlinandKlinck,1986;ParkandGambe!roni,1997;Grifﬁthsetal.,1999;RintoulandTrull,2001).Muchofthisregionischaracterizedbyhigh-nutrientlow-chlorophyll(HNLC)conditions—where‘‘nutrient’’referstonitrateandphos-phate—althoughlownitrateconcentrations(o1mM)areoftenobservedinsurfacewatersoftheSubtropicalConvergence/SubtropicalFront(Clementsonetal.,1998;Boydetal.,1999;Grifﬁthsetal.,1999).Silicicacid,whichisrequiredforthegrowthofdiatoms,iscommonlydepletedfromlevelsofB5mMormoreinspringtoverylowconcentrations(o1mM)bymidtolatesummer(Dugdaleetal.,1995;Nelsonetal.,2001;RintoulandTrull,2001),anddissolvedironconcentrationsarelikelytobelow(o0.5nM)insurfacewatersovermuchoftheregion(Sedwicketal.,1997,1999;Coaleetal.,1999;MeasuresandVink,2001).Thus,themajorcontrolsonalgalgrowthinopen-oceanwatersofthisregionmayincludeironandsilicicacidavailability,inaddi-tiontolightavailabilityascontrolledbyverticalmixing,and,inthenorth,nitrateand/orphos-phateavailability(Martinetal.,1990a;NelsonandSmith,1991;Cullen,1991;Dugdaleetal.,1995;Banse;1996,Sedwicketal.,1997;SundaandHuntsman,1997;Boydetal.,1999;Grifﬁthsetal.,1999;Hutchinsetal.,2001).Atpresent,however,welackaclearunderstandingoftherelativeimportanceofthesefactorsinregulatingalgalgrowth,communitystructureandexportproduc-tionintheSubantarcticSouthernOcean.Numerousstudiesduringthepastdecadehavecombinedtheresultsofwater-columnmeasure-ments,shipboardincubationexperimentsand,morerecently,mesoscaleperturbationexperi-ments,toinferthedominantcontrolsonalgalgrowthintheSouthernOcean(e.g.,Martinetal.,1990a;deBaaretal.,1990,1995;Lo.scheretal.,1997;vanLeeuweetal.,1997;SedwickandDiTullio,1997;Takeda,1998;Boydetal.,2000;Smetacek,2001).However,onlyafewsuchstudieshavebeenperformedintheSubantarcticregion,northoftheAntarcticPolarFront.FieldstudiesconductedinthePolarFrontalZonenear1701W,southeastofNewZealand,indicatethatphyto-planktoncommunitygrowthandbiomassarelimitedbylowironconcentrationsovermuchofthegrowingseason(Coaleetal.,1999;MeasuresandVink,2001),andthatlowsilicicacidconcentrations(o5mM)maylimitthegrowthofdiatoms,atleastduringthesummerandautumn(Francketal.,2000;Nelsonetal.,2001).Inaddition,experimentsperformedinthePolarFrontalZonearound1401E,southwestofTasma-nia,suggestthatlightavailabilitymaylimitalgalcommunitygrowthduringlatesummerandautumn(Boydetal.,2001).Furthernorth,inwatersoftheSubantarcticZonesoutheastofNewZealand,ﬁelddatasuggestthatdiatomgrowthislimitedbyavailabilityofbothironandlightduringspring,andbyironand/orsilicicacidavailabilityduringsummer,whereaswatersinandnorthoftheSubtropicalConvergenceareironreplete(Boydetal.,1999).Similarly,theresultsofﬁeldexperimentsconductedtothewestofTasmaniaindicatethatalgalgrowthinSubantarc-ticZonewatersislimitedbyironandsilicicaciddeﬁciencyduringlatesummerandautumn,althoughtheseexperimentsalsoprovidedevidenceofironlimitationwithintheSubtropicalConver-gence(Sedwicketal.,1999;Hutchinsetal.,2001).Ageneralhypothesisthatisconsistentwiththeresultsofthesestudiesisthatphytoplanktoncommunitygrowthinopen-oceanwatersoftheSubantarcticregionisprimarilyregulatedbythreeresources—iron,silicicacid,andlight—theavail-abilityofwhichvaryseasonally.Growthlimita-tionduetoinsufﬁcientlightislikelytobemostimportantbetweenlateautumnandearlyspring,whenthesurfacemixed-layerdepthtypicallyexceeds100–200m(RintoulandTrull,2001).

P.N.Sedwicketal./Deep-SeaResearchII49(2002)3327–3349Althoughsurface-waterdissolvedironconcentra-tionsmaybehighestoverthisperiod(MeasuresandVink,2001),thelowirradiancealsomayservetoprecipitateorintensifygrowthlimitationbyirondeﬁciency,giventheantagonisticrelationshipbetweentheironandlightrequirementsofphytoplankton(Raven,1990;SundaandHunts-man,1997),sothatalgalgrowthmaybephysio-logicallyco-limitedbyironandlight(Boydetal.,1999;Maldonaldoetal.,1999).Asthegrowingseasonprogresses,averagemixed-layerdepthdecreases,andalgalbiomassincreases,thecon-centrationsofbothironandsilicicacidaredepletedtoverylowvalues,whicharelikelytolimitthegrowthrateoflargerspecies,particularlydiatoms.Atthistime,betweenlatespringandearlyautumn,eitherironorsilicicacidmayexerttheproximatecontrolonalgalcommunitygrowth,andthephytoplanktoncommunitymaybepoisedclosetoco-limitationbytheseresources;infact,theavailabilityofbothresourcesmightsimulta-2339neouslylimitthegrowthofdifferentmembersofthealgalcommunity(Boydetal.,1999;Hutchinsetal.,2001).However,itisimportanttoappreciatethattheabovehypothesisisbasedondatacollectedduringarelativelysmallnumberofﬁeldstudiescarriedoutinonlytwonarrowsectorsoftheSouthernOcean.Insummer1999,wewereprovidedwiththeopportunitytoexaminetheroleofresourceavailabilityinregulatingphytoplanktongrowthintheIndiansectoroftheSubantarcticSouthernOcean,duringaresearchcruiseintheCrozetBasin,northwestofKerguelenIsland(Fig.1).ThisoceanographicsettingisquitedifferentfromtheSubantarcticregionnearNewZealandandTas-mania,inthattheSubantarcticandSubtropicalFrontsconverge,constrainedbytheKerguelenPlateauinthesouthandtheAgulhasReturnCurrentFront(AgulhasFront)inthenorth(Parketal.,1993;ParkandGambe!roni,1997).Thetransitionfromcold,fresh,nutrient-richpolarFig.1.Antares-IVcruisetrackshowinglocationofCrozetBasin(basemapcourtesyofB.Delille).Inset:DetailofcruisetrackinCrozetBasin,showingapproximatepositionsofmajoroceanographicfeatures(Y.-H.Park,pers.comm.,1999)andStations10(PFZ),7(SAF/STF)and8(STZ).

0333P.N.Sedwicketal./Deep-SeaResearchII49(2002)3327–3349waterstowarm,salty,nutrient-poorsubtropicalwatersoccursoveranarrowlatitudinalrange(Fig.1),inaregionthatappearstobecharacter-izedbyrelativelylowdissolvedironconcentrations(o0.4nM)andlowalgalbiomass(o0.7mgl1chlorophylla)duringthesummer.Herewereporttheresultsfromaseriesofshipboardbioassayexperiments,whichwereundertakentoinvestigatethesufﬁciencyofambientiron,silicicacid,andmacronutrientconcentrationsforphytoplanktongrowthinthreedistinctlow-iron,low-chlorophyllregimes;namely,(1)thePolarFrontalZone(PFZ),withhighconcentrationsofnitrate(B19mM)andlowconcentrationsofsilicicacid(B1.2mM);(2)theconﬂuenceoftheSubantarcticandSubtropicalFronts(SAFandSTF),withmoderateconcentra-tionsofnitrate(B5.4mM)andverylowconcen-trationsofsilicicacid(B0.5mM);and(3)thesouthernedgeoftheSubtropicalZone(STZ),withverylowconcentrationsofnitrate(o0.1mM)andlowconcentrationsofsilicicacid(B1.4mM).Ourexperimentalresultsdemonstratethreedistinctregimesofresourcelimitationwhichmayregulatealgalcommunityproductionandspeciescomposi-tioninwatersoftheSubantarcticregionduringthemidtolatesummer.2.MethodsTheAntares-IVcampaignwasconductedaboardNOMarionDufresneintheCrozetBasinduringJanuaryandFebruary1999,aspartoftheFrance-JGOFSprogram.AmajoraimofthisexpeditionwasanunderstandingofthefactorscontrollingphytoplanktonproductionandorganicmatterexportinthissectoroftheSubantarcticSouthernOcean,aregionthatistypicallychar-acterizedbyrelativelylowalgalbiomass,exceptforareasimmediately‘‘downstream’’fromtheislandplatformsofCrozetandKerguelen(Mooreetal.,1999).Ourstudyfocussedonanareanorthwest(‘‘upstream’’)ofKerguelenIsland,boundedbylatitudes43–461Sandlongitudes61–651E,wheretheconﬂuenceoftheSubantarcticandSubtropicalFrontswasalignedinasouthwesttonortheastdirection(Fig.1).TheapproximatepositionsoftheSAFandSTF(Fig.1)aredeﬁnedbyaxialtemperaturesandsalinitiesof61Cand34.3(SAF),and101Cand34.8(STF),atawaterdepthof200m(Parketal.,1993;Park,pers.comm.,1999).OurcruiseincludedhydrographicandbiogeochemicalmeasurementsatstationswithinthePFZ,theSAF/STFconﬂuence,andthesouthernedgeoftheSTZ.Wemeasureddissolvedironinthewatercolumnandperformediron-andmacronutrient-additionbottle-incuba-tionexperimentsusingseawaterandresidentplanktoncollectedfromtheuppermixedlayeratStations10,7and8(Fig.1,Table1),whicharehereafterreferredtoasthePFZstation,theSAF/STFstation,andtheSTZstation,respectively.Containersandapparatususedforsamplingandanalysisofironinseawaterandforiron-andmacronutrient-additionincubationexperimentswererigorouslycleanedusingthetrace-metalcleantechniquesdescribedbySedwicketal.(1997,2000).Water-columnsamplesforironmeasurementswerecollectedincustom-builtpolycarbonatewatersamplerssuspendedfromanon-metallicline,asdescribedbySedwicketal.(1997).Uponrecovery,thewatersamplerswereimmediatelytransferredintoadedicatedcleanlaboratoryvan,whereﬁlteredandunﬁlteredsubsamplesweretakenunderClass-100ﬁlteredair(Sedwicketal.,1997).Subsampleswereﬁlteredthroughacid-cleanedPoreticspolycarbonatemembranes(0.4mm),usinganin-linesystempressurizedwith0.2mm-ﬁlteredhigh-puritynitro-gengas.Theﬁlteredandunﬁlteredsubsampleswerecollectedinacid-cleanedNalgenelow-densitypolyethylenebottlesandacidiﬁedwithSeastarultrapureconcentratedhydrochloricacidasde-scribedbySedwicketal.(2000).DissolvedFe(measuredintheﬁltered,acidiﬁedsubsamples)andtotal-dissolvableFe(TDFe,measuredintheunﬁltered,acidiﬁedsubsamples,andassumedtoprovideameasureofdissolved+particulateFe)weredeterminedattheAntarcticCRCinHobartbyﬂow-injectionanalysiswithin-linepreconcen-trationonresin-immobilized8-hydroxyquinoline(Measuresetal.,1995).ThismethodhasadetectionlimitofB0.02nMFeandoverallanalyticaluncertaintiesofB20%fordissolvedFeandB30%forTDFe,overtheconcentrationrangesreportedinthisstudy(Sedwicketal.,2000).

P.N.Sedwicketal./Deep-SeaResearchII49(2002)3327–3349SAF/STF(7)441120S,631230E6Feb.19991333Table1SummaryofambientconditionsandexperimentaltreatmentsStationPosition,startdateAmbientconditionsExperimentaltreatmentsPFZ(10)461S,651E18.7mMnitrateControl(untreated)14Jan.19991.18mMsilicicacid+Si(18.3mMSi)a0.33nMdiss.iron+Fe(2nMFe)0.21mgl1Chla+Fe+Si(2nMFe,18.3mMSi)a5.4mMnitrateControl(untreated)0.5mMsilicicacid+Si(3.9mMSi)p0.19nMdiss.ironb+Fe(2nMFe)0.62mgl1Chla+Fe+Si(2nMFe,3.9mMSi)STZ(8)431180S,621310Eo0.1mMnitrateControl(untreated)12Feb.19991.4mMsilicicacid+N+P(10mMN,0.63mMP)0.09nMdiss.Fe+Fe(2nMFe)0.36mgl1Chla+Fe+N+P(2nMFe,10mMN,0.63mMP)aApproximately0.8nMFeaddedtogetherwithcontaminatedSisolutioninStation10experiment.bDissolvedFeconcentration(0.29nM)washigherthanTDFeconcentration(0.19nM)inthestartingseawateratthisstation.However,ironconcentrationsof0.19and0.29nMareanalyticallyindistinguishable,andthedissolvedFesampleisnotconsideredtohavebeencontaminated.Unﬁlteredseawaterfortheshipboardincuba-tionexperimentswascollectedfromawaterdepthofB20mthroughacid-cleanedpolyethylenetub-ingusinganOsmonicssolidTeﬂondiaphragmpump(Hutchinsetal.,1998;Sedwicketal.,1999).Thetubinginletwasloweredfromthewindwardsideoftheshipusinganon-metallicline,andseawaterwaspumpedonboardwhileheadingintothewindatB1–2knots.Thepumpedseawaterwasdischargedinsidethecleanlaboratoryvan,wheresamplecontainersusedintheexperimentswererinsedandﬁlled.Theseawaterwasnotscreenedtoexcludelargerorganisms,althoughitshouldbenotedthatlargerheterotrophsarelikelytobeundersampledinthismethodofseawatercollection.Thebottle-incubationexperimentsweresimilarindesigntothosedescribedbySedwicketal.(1999)andHutchinsetal.(2001).Thepumpedseawaterwasgentlymixedinacid-cleaned50-lNalgenepolyethylenecarboys,thenimmedi-atelytransferredintoacid-cleaned2.4-lNalgenepolycarbonatebottles,usedfortheexperimentsdiscussedhere,andacid-cleaned24-lNalgenepolycarbonatecarboys,usedforconcurrent‘‘dose-response’’iron-additionexperimentsthatexaminedtheresponseofthealgalcommunitytovariouslevelsofaddediron(seeBlainetal.,2002).Subsamplesfortheinitial(t¼0)analysesweretakendirectlyfromthe50-lmixingcarboys,exceptforthesubsamplesforironmeasurements,whichweretakenfromapolycarbonatetrace-metalwater-samplerthathadbeendirectlyﬁlledwiththepumpedseawater,andprocessedasdescribedforthewater-columnironsamples.The2.4-lexperimentalcontainerswereimmediatelyamendedwithironand/ormacronutrientsasdescribedbelow,thencapped,sealedwithPVCtape,andsetincirculatingsurfaceseawaterinsidepolyethyleneincubatorslocatedondeck.TheseincubatorsshadedthebottlestoE50%ofincidentirradiance,whichwasintendedtosimulatetheinsituirradianceintheuppermixedlayer.Tempera-turesinthedeckincubatorsweremaintainedtowithin741Cofambientsea-surfacetemperatureduringthecourseoftheexperiments.InthePFZstation(Station10)experiment,theincubationbottleswereeither(1)enrichedwithsilicicacid(+Si,18.3mM),(2)enrichedwithiron(+Fe,2nM),(3)enrichedwithironandsilicicacid(+Fe+Si,2nMand18.3mM,respectively),or(4)leftuntreatedascontrols.Ironwasaddedasasolutionofferricchloridein0.01Nhydrochloricacid,andsilicicacidwasaddedasasodiummetasilicatesolutionwhichhadbeenpuriﬁedusing

2333P.N.Sedwicketal./Deep-SeaResearchII49(2002)3327–3349acolumnofresin-immobilized8-hydroxyquino-line.However,thispuriﬁcationschemeprovedtobeinadequate,possiblyduetocomplexationofironbysilicicacidinbasicsolution.Shipboardanalysisofthepuriﬁedsodiummetasilicatesolu-tionrevealedthatitcontainedenoughironsuchthatB0.8nMironwasinadvertentlyaddedtotheincubationbottlestogetherwiththeaddedsilicicacid,thusinvalidatingthe+Sitreatmentsinthisexperiment.Anew10mMsilicicacidstocksolutionwaspreparedandpuriﬁedatseaforuseintheSAF/STFstation(Station7)experiment:reagentgradesodiummetasilicatewasdissolvedindeionizedwater,thenthissolutionwasﬁlteredthrougha0.2-mmpolycarbonatemembrane,di-lutedten-foldwithdeionizedwater,acidiﬁedtopHB3.5withSeastarultrapurehydrochloricacid,thenpassedovercolumnsofresin-immobilized8-hydroxyquinolineandWatersSep-PakC-18packingmedia(thelattertoremove8-hydroxyqui-nolinewhichmayhavebledfromtheresin-immobilized8-hydroxyquinolinecolumnduringpassageoftheacidicsolution;W.Landing,pers.comm.,1998).Thethus-puriﬁedsilicicacidsolutionwasanalyzedatseaandfoundtocontain0.94mMSiandB1nMFe,whichwasadequateforuseintheSAF/STFstationexperiment(seebelow).IntheSAF/STFstation(Station7)experiment,theincubationbottleswereeither(1)enrichedwithsilicicacid(+Si,3.9mM),(2)enrichedwithiron(+Fe,2nM),(3)enrichedwithironandsilicicacid(+Fe+Si,2nMand3.9mM,respectively),or(4)leftuntreatedascontrols.Inthiscase,thecontaminantironaddedalongwiththesilicicacidsolutionamountedtoo0.01nM,whichisas-sumednegligiblecomparedwiththeambientironconcentrationofB0.2–0.3nMinseawatercol-lectedfortheexperiment.IntheSTZstation(Station8)experiment,theincubationbottleswereeither(1)enrichedwithnitrateandphosphate(+N+P,10mMand0.63mM,respectively),(2)enrichedwithiron(+Fe,2nM),(3)enrichedwithiron,nitrateandphosphate(+Fe+N+P,2nM,10mMand0.63mM,respectively),or(4)leftuntreatedascontrols.Nitrateandphosphatewereaddedasaqueoussolutionsof20mMsodiumnitrateand20mMsodiumdihydrogenphosphate,respectively,whichhadbeentwicepuriﬁedusingcolumnsofresin-immobilized8-hydroxyquinoline.Shipboardanalysisofthesesolutionsshowedthemtocontain0.65nMand6.0nMiron,respectively,suchthattheironaddedalongwiththenitrateandphosphatesolutionsamountedtoo0.01nM,whichisassumednegligiblerelativetotheambientironconcentrationofB0.1–0.2nMinseawatercollectedfortheexperiment.Table1summarizestheambientconditionsandexperimentaltreat-mentsforthethreestationsinvestigated.Foreachexperimentaltreatment,duplicate2.4-lbottlesweresacriﬁced(i.e.,completelysub-sampled)atselectedtimepointsduringthecourseofeachexperiment,withsubsamplestakenforthefollowingmeasurements:dissolvednitrate+nitrite(N+N)andsilicicacid(Si),determinedin0.45mm-ﬁlteredsolutionsbystandardmethodsusingaTechniconsegmented-ﬂowanalysissystem;chlorophylla(Chla),determinedin>0.2mmparticulatematerialbyspectroﬂuorometry(Fialaetal.,1998);particulateorganiccarbon(POC)andparticulateorganicnitrogen(PON),determinedinGF/F-ﬁlteredmaterialbyelementalanalyzer(Qu-e!guineretal.,1997);particulatebiogenicsilica(BSi),determinedin>0.4mmparticulatematerialbyspectrophotometryaftersodiumhydroxidedigestion(Que!guineretal.,1997);rateofprimaryproduction,measuredbythe24-hincubationtechniquedescribedbyQue!guineretal.(1997);andrateofphotosynthesisvs.irradiance(Pvs.I),measuredbythesmall-bottleshort-termincuba-tiontechniquedescribedbyGrifﬁthsetal.(1999).Theresultspresentedforthe24-hprimaryproductionmeasurements,Pnet;provideanesti-mateofnetprimaryproductionoverthe24-hperiodfollowingthetimeofsubsampling,inunitsofmgCm3d1.TheresultsshownfromPvs.Imeasurements,Pmax;provideanestimateofgrosslight-saturatedrateofphotosynthesisclosetothetimeofsubsampling,inunitsofmgCm3h1,andarethusnotdirectlycomparablewiththeestimateddailynetproductionrates.ItshouldbenotedthatwehavechosennottonormalizethevaluesofPmaxtobiomassusingChla,becauseironadditionisexpectedtosigniﬁcantlyalterthecellularChlacontentofoceanicphytoplank-toninsuchexperiments(GeiderandLaRoche,1994).SamplesformeasurementsofdissolvedFe

P.N.Sedwicketal./Deep-SeaResearchII49(2002)3327–3349andtotal-dissolvableFeweretakenonlyfromthestartingseawater(t¼0),duetothelimitedvolumeoftheincubationbottles.VisualidentiﬁcationandenumerationofcellslargerthanB2mmwereperformedat400timesmagniﬁcationusinganinvertedlightmicroscopewithphasecontrast,afterovernightsettlingofB20-mlsubsamplespreservedatseawithLugol’siodinesolution.Wherepossible,morethan1000cellswerecountedineachsample.Ateachofthethreestationswhereexperimentswereconducted,asigniﬁcantcomponentofthenanoplanktoninthestartingseawaterandexperimentaltreatmentswereroughlysphericalcellsB3–10mmindiameter,bothwithandwithoutﬂagella,whichwerenotdeﬁnitivelyidentiﬁed,andareheretermed‘‘mon-ads’’assuggestedbyHasle(1978).Thesecellsaredistinguishedfromthegenerallylarger,non-sphericalnanoﬂagellates,whichwehavetermed‘‘ﬂagellates’’.However,itshouldbenotedthatsomeofthemoredelicatephytoplanktonspecies,includingcoccolithophorids,whichmaycompriseanimportantcomponentofthealgalcommunitynorthofthePolarFront(Findlay,1998),arepoorlypreservedinLugol’siodinesolution(Throndsen,1978).Thus,thealgalcellsthatwehaveidentiﬁedbymicroscopyprovideuswithanincompletepictureofphytoplanktoncommunitycomposition,inthatpicoplanktonanddelicateorcalcareousnanoplanktonwouldnothavebeencounted,sothattheseresultsmustbeconsideredassemi-quantitativeatbest.Italsoshouldbenotedthatwithourvisualcell-countingtechnique,wedidnotattempttodistinguishbetweenautotrophsandheterotrophsinthedinoﬂagellate,ﬂagellate,andmonadgroupings.Foreachexperiment,selectedsamplespreservedwithLugol’siodinesolutionwerealsoexaminedusingshadow-casttransmis-sionelectronmicroscopy(TEM)attheAustralianAntarcticDivision,whichallowedagenus-levelidentiﬁcationofsomeofthemorenumerousnanoplanktoninthesamples.3.ResultsAsinotherﬁeldstudiesthathaveusedship-boardbioassayexperiments(e.g.,Martinetal.,33331990a;deBaaretal.,1990;DiTullioetal.,1993;Fitzwateretal.,1996;Hutchinsetal.,2001),weinferrelativealgalcommunitygrowthratesintheincubationbottlesfrom(1)accumulationofalgalbiomass,asindicatedbydecreases(drawdown)indissolvedN+NandSi,andincreasesinChla,POCandPON;and(2)relativeratesofprimaryproduction,asindicatedbythe14C-basedesti-matesofdailynetprimaryproductionrate(Pnet)andshort-termgrosslight-saturatedphotosynth-esisrate(Pmax).Theseexperimentsareprincipallyintendedtobediagnostic:wherebiomassaccumu-lationandratesofproduction/photosynthesisaresigniﬁcantlyenhancedinbottlestreatedwithiron-and/ormacronutrients,relativetocontroltreat-ments,weinferthatalgalcommunitygrowthrateislimitedbydeﬁcienciesiniron-and/ormacro-nutrients,respectively.Thestatisticalsigniﬁcanceofdifferencesbetweenaveragevaluesofmeasuredparameterswereassessedusingtheunpairedstudent’st-testatthe95%conﬁdencelevel.Ineachofourthreeexperiments,therewasrelativelylittleornodrawdowninnitrateoraccumulationofbiomass(Chla,POC,PON)inthecontroltreatments(seebelow).AsdiscussedbyFitzwateretal.(1996),thiswouldsuggestthattherewasnosigniﬁcantironcontaminationoftheseawatercollectedandusedintheseexperiments.3.1.PFZstationFig.2ashowswater-columnproﬁlesoftempera-ture,salinity,density(st),ﬂuorescence,N+N,Si,anddissolvedFeobtainedatthePFZstation(Station10).Heretheupper20mofthewatercolumnwasweaklystratiﬁed,probablyasaresultofsurfacewarming,withawell-deﬁnedpycnoclinebetweendepthsofB40and60mthatwasroughlycoincidentwithamaximuminﬂuorescence.Therelativelyhighconcentrationsofnitrate(B20mM)andlowconcentrationsofsilicicacid(1–2mM)intheupperwatercolumnaretypicalofPFZwatersduringmidtolatesummer(RintoulandTrull,2001;Nelsonetal.,2001).Withintheeuphoticzone,dissolvedironconcentrationswerehighest(B0.4nM)neartheseasurface(15mdepth)andatthebaseofthepycnocline(75mdepth),andlowest(B0.2nM)nearthebaseoftheuppermixed

() 3334P.N.Sedwicketal./Deep-SeaResearchII49(2002)3327–3349(a) Station 1' - PFZ005 001 051 002 052 003 02468103333.53434.5352626.52727.500.51010203000.20.40.60.81potential temp. (˚C)salinitytfluorescenceN+N (µM)dissolved Fe (nM)Si (µM)(b) Station 7- SAF/STF005 001 051 002 052 003 101112131433.53434.53535.525.52626.52700.510510152000.20.40.60.81potential temp. (˚C)salinitytfluorescenceN+N (µM)dissolved Fe (nM)Si (µM)(c) Station 8 - STZ005 001 051 002 052 003 10121416182034.53535.53636.525.52626.52700.51051000.20.40.60.81potential temp. (˚C)salinitytfluorescenceN+N (µM)dissolved Fe (nM)Si (µM)Fig.2.Verticalproﬁlesofpotentialtemperature,salinity,st;ﬂuorescence,N+NandSi,anddissolvedFeat(a)PFZstation,(b)SAF/STFstation,and(c)STZstation.Fluorescencedatawerecollectedbetween18:30and08:30localtime.

P.N.Sedwicketal./Deep-SeaResearchII49(2002)3327–3349layer(45mdepth).Total-dissolvableFeconcen-trations(datanotshown)werewithintheanaly-ticaluncertaintyofthedissolvedFeconcentrationsthroughouttheupperwatercol-umn.Thedissolvedironconcentrationsarehigherthanvaluesofo0.2–0.3nMFethathavebeenreportedforsurfacewatersinotheropen-oceansitesneartheAntarcticPolarFront(e.g.,Martinetal.,1990b;deBaaretal.,1999;MeasuresandVink,2001),andmaythusreﬂectironinputsfromsedimentsontheCrozetIslandplatform(locatedtothewestofStation10),asreporteddownstreamfromtheKerguelenIslandplatform(Blainetal.,2001;Bucciarellietal.,2001),aswellasrecentinputsofmineralaerosol(presumablyfromsouth-ernAfrica)attheseasurface,asobservedinthecentralnorthPaciﬁc(Brulandetal.,1994).Atthisstationwesetouttoexaminethepotentialrolesofbothironandsilicicacidinlimitingalgalgrowthrate,giventhatbothspecieswerepresentatrelativelylowconcentrationsintheuppermixedlayer(B0.3nMand1.2mM,respec-tively).Thus,theincubationbottleswereamendedwithiron(+Fe),silicicacid(+Si),andiron+si-licicacid(+Fe+Si)(seeTable1).However,asdetailedinSection2,thesilicicacidstocksolutionusedinthisexperimentwassufﬁcientlycontami-natedwithironsuchthatourexperimentaltreatmentsmustbeconsideredasonly+Feand+Fe+Si(i.e.the+Siand+Fe+Sitreatmentseachcontainedsigniﬁcantadditionsofbothironandsilicicacid).ResultsfromthePFZstationexperimentareshowninFig.3.Littleornosigniﬁcantchangesindissolvednutrients,algalbiomassandalgalproductionrateswereobservedduringtheﬁrst6daysoftheincubationexperi-ment,butafter9daystherehadbeensigniﬁcantincreasesinN+Ndrawdown,Chla,POC,PONandPnetinthe+Fe,+Siand+Fe+Sitreatmentsrelativetothecontrolbottles.Signiﬁcantdiffer-encesbetweenthe+Fe,+Siand+Fe+SitreatmentswereonlydiscernableintheBSiandPmaxdata,withthegreatestincreasesinBSiandPmaxobservedintheFe-contaminated+Sitreat-ments(althoughPmaxmeasurementswerenotreplicatedforthesetreatments),followedbythe+Fe+Siand+Fetreatments.After9daysincubation,theaveragemolarSi:Ndrawdown3353ratios(drawdowninSi/drawdowninN+N)werezerointhecontrolbottles(nomeasurableSidrawdown),o0.1inthe+Fetreatments,andnear0.2inthe+Siand+Fe+Sitreatments.Inthisexperiment,thenanoplanktonandmicroplanktoncountedbymicroscopywereiniti-allydominatedbyB3–10mmmonads(roughlysphericalcellswhichwerenotdeﬁnitivelyidenti-ﬁed),withlessernumbersofdinoﬂagellates,pennatediatoms(includingPseudo-nitzschia,Navicula,andFragilariopsisspp.),andotherunidentiﬁablecells(Fig.4a).Themicroscopeobservationsindicatelittlechangeinalgalcommunitycompositioninthecontroltreatmentsafter9daysincubation,whereasthelargeincreasesinalgalbiomassinthe+Fe,+Siand+Fe+Sitreatmentswereapparentlyduetoincreasesinthenumberofsmall,lightlysiliciﬁedpennatediatoms,mainlyofthegenusPseudo-nitzschia,andmonads(Fig.4a).The+Fetreatmentscontainedsimilarabundancesofthepennatediatomsandmonads,whereasthetreatmentscontainingbothFeandSiwerenumericallydominatedbypennatediatoms,whichweremostnumerousintheFe-contaminated+Sitreatments(Fig.4a).ObservationsmadeusingTEMsuggestthatthemonadsthatgrewinthe+Fe,+Siand+Fe+Sitreatmentsweredominatedbyhapto-phytes,includingChrysochromulinaandPhaeocys-tisspecies(A.Davidson,pers.comm.2001).Inthisexperimentonly,subsampleswerealsoanalyzedbyﬂowcytometry,withresultssuggest-ingthatpicoeukaryoteswereatmostaroundhalfasabundantasnanoeukaryotes,andthatcyano-bacterialnumbersremainedrelativelylowinallexperimentaltreatments.Theﬂowcytometrydataindicateincreasesinthenumbersofbothpicoeu-karyotesandnanoeukaroytesinthe+Fe,+Si,and+Fe+Sitreatments,relativetocontrolsamples,withnanoeukaryotesshowingthegreat-estincreasesinrelativeabundance(C.Dubreuil,pers.comm.1999).Takentogether,thechemicalandbiologicalresultsofthisexperimentindicatethatironadditionincreasedalgalcommunitygrowth,pri-marilybysmallpennatediatoms,whichprobablyaccountedformostoftheaccumulatedbiomass,andmonads.Relativetoadditionofironalone,

3363P.N.Sedwicketal./Deep-SeaResearchII49(2002)3327–3349controleF+Station 1' - PFZ+Si (+Fe)+Fe+Si52020251510101550002468100246810incubation time (d)incubation time (d)201000057510050105250002468100246810incubation time (d)incubation time (d)50024051300120510002468100246810incubation time (d)incubation time (d)052030020510200101050002468100246810incubation time (d)incubation time (d)0PFimga.x3v.s.Rienscuultbsatoifonbottitmlee-.inNcuotbeattihoantetxhpeer+imSientrtseaftomretnhtesPwFeZrestcaotnitoanm(iSntaattieodnw1it)h,isrhoonwiinngthiNs+exNp,erSii,meCnhtl.a,POC,PON,BSi,Pnetand

P.N.Sedwicketal./Deep-SeaResearchII49(2002)3327–33497333Fig.4.Abundancesof>2mmalgalcellscountedbylightmicroscopyintheinitialandﬁnalsamplesfromexperimentsat(a)thePFZstation,(b)theSAF/STFstation,and(c)theSTZstation.additionofbothironandsilicicacidhadnofurthersigniﬁcanteffectonnutrientdrawdown,accumulationofbiomassasestimatedfromChla,POCandPON,orestimatednetprimaryproduc-tionrate,suggestingthatcommunitygrowthwasnotimmediatelylimitedbysilicicaciddeﬁciency.However,theelevatedBSiandPmaxvaluesandenhancedabundanceofdiatomsobservedinthe+Siand+Fe+Sitreatmentsindicatethataddi-tionofsilicicacid(incombinationwithiron)didinﬂuencephytoplanktoncommunitycomposition,biogenicsilicaproductionandgrosslight-satu-ratedrateofphotosynthesis,bypromotingthegrowthofsmallpennatediatoms.ItiscuriousthattheFe-contaminated+Sitreatmentsachievedsigniﬁcantlyhigherconcentrationsofbiogenicsilicaandgreaternumbersofdiatomsthanthe+Fe+Sitreatments,giventhatthe+Fe+Sitreatmentscontainedthesameconcentrationofsilicicacidasthe+SitreatmentsandanadditionalB1.2nMiron.WespeculatethatthecontaminantFeaddedwiththeSistocksolutionwascomplexed

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