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Biogeosciences,12,4421–4445,2015 www.biogeosciences.net/12/4421/2015/ doi:10.5194/bg-12-4421-2015 ©Author(s)2015.CCAttribution3.0License. Iron budgets for three distinct biogeochemical sites around the Kerguelen Archipelago (Southern Ocean) during the natural fertilisation study, KEOPS-2 A.R.Bowie1,2,3,P.vanderMerwe1,F.Quéroué1,2,3,T.Trull1,4,M.Fourquez2,5,F.Planchon3,G.Sarthou3, F.Chever3,a,A.T.Townsend6,I.Obernosterer5,J.-B.Sallée7,8,9,andS.Blain5 1AntarcticClimateandEcosystemsCooperativeResearchCentre(ACECRC),PrivateBag80,Hobart,Tasmania7001, Australia 2InstituteforMarineandAntarcticStudies(IMAS),UniversityofTasmania,PrivateBag129,Hobart,Tasmania7001, Australia 3LaboratoiredesSciencesdel’EnvironnementMarin(LEMAR),UMR6539UBO/CNRS/IRD/IFREMER,Institut UniversitaireEuropéendelaMer(IUEM),TechnopoleBrestIroise,29280Plouzané,France 4CSIROMarineandAtmosphericResearch,CastrayEsplanade,Hobart,Tasmania7000,Australia 5UniversitéPierreetMarieCurie,Laboratoired’OcéanographieMicrobienne(LOMIC),UMR7621CNRSUPMC,Avenue duFontaulé,66650Banyulssurmer,France 6CentralScienceLaboratory(CSL),UniversityofTasmania,PrivateBag74,Hobart,Tasmania7001,Australia 7SorbonneUniversités,UPMCUniv.,Paris06,UMR7159,LOCEAN-IPSL,75005Paris,France 8CNRS,UMR7159,LOCEAN-IPSL,75005Paris,France 9BritishAntarcticSurvey,HighCross,CambridgeCB30ET,UK anowat:NationalOceanographyCentre,UniversityofSouthamptonWaterfrontCampus,EuropeanWay, SouthamptonSO143ZH,UK Correspondenceto:A.R.Bowie([email protected]) Received:19November2014–PublishedinBiogeosciencesDiscuss.:19December2014 Revised:22June2015–Accepted:24June2015–Published:29July2015 Abstract. Iron availability in the Southern Ocean controls waysareresponsiblefordifferencesinthemodeandstrength phytoplankton growth, community composition and the up- ofironsupply,withverticalsupplydominantontheplateau take of atmospheric CO by the biological pump. The and lateral supply dominant in the plume. Iron supply from 2 KEOPS-2 (KErguelen Ocean and Plateau compared Study “new”sources(diffusion,upwelling,entrainment,lateralad- 2)“processstudy”,tookplacearoundtheKerguelenPlateau vection,atmosphericdust)tothesurfacewatersoftheplume in the Indian sector of the Southern Ocean. This is a re- wasdoublethatabovetheplateauand20timesgreaterthan gion naturally fertilised with iron on the scale of hundreds atthereferencesite,whilstirondemand(measuredbycellu- to thousands of square kilometres, producing a mosaic of laruptake)intheplumewassimilartothatabovetheplateau spring blooms which show distinct biological and biogeo- but 40 times greater than at the reference site. “Recycled” chemical responses to fertilisation. This paper presents bio- ironsupplybybacterialregenerationandzooplanktongraz- geochemicalironbudgets(incorporatingverticalandlateral ing was a relatively minor component at all sites (<8% of supply,internalcycling,andsinks)forthreecontrastingsites: new supply), in contrast to earlier findings from other bio- an upstream high-nutrient low-chlorophyll reference, over geochemical iron budgets in the Southern Ocean. Over the the plateau and in the offshore plume east of the Kergue- plateau, a particulate iron dissolution term of 2.5% was in- len Islands. These budgets show that distinct regional envi- vokedtobalancethebudget;thisapproximatelydoubledthe ronments driven by complex circulation and transport path- standingstockofdissolvedironinthemixedlayer.Theex- PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion. 4422 A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 change of iron between dissolved, biogenic particulate and phytoplanktonbiomassduringsummer,withchlorophylllev- lithogenicparticulatepoolswashighlydynamicintimeand els increasing to more than 1 order of magnitude above the space,resultinginadecouplingoftheironsupplyandcarbon background, as revealed by NASA MODIS satellite chloro- export and, importantly, controlling the efficiency of fertili- phyllclimatologyforJanuary(2003–2010)(Westberryetal., sation. 2013).Previousresearchonbloomsintheselocalised“natu- rallaboratories”hasprovidedinvaluableinsightsintomech- anisms linking iron fertilisation and carbon cycling in the 1 Introduction Southern Ocean, especially since studies of natural systems canaddresstheeffectsofpersistent,varyingandmultipleFe The concentration of carbon dioxide in earth’s atmosphere, sources that are not accessible through deliberate artificial and therefore earth’s climate, is highly sensitive to modifi- mesoscalefertilisationexperiments. cation in the marine carbon (C) cycle due to the growth of The KEOPS-1 (KErguelen: Ocean and Plateau compared phytoplankton in the Southern Ocean (Sarmiento and Gru- Study1)project,whichtookplaceinthelateaustralsummer ber,2006).Thesesingle-cellplantsremoveinorganiccarbon ofJanuary–February2005,demonstratedthatthisnaturalfer- from surface seawater during photosynthesis, and this inor- tilisationoftheSouthernOceanresultedindramaticchanges ganic carbon can be directly transferred into the deep sea in the functioning of the ecosystem with large impacts on whentheplantsdieandsink,orindirectlythroughthefood marine biogeochemical cycles (Blain et al., 2007, 2008a). web. The Southern Ocean is responsible for 30% of global These observations of the bloom were largely confined to oceancarbonexport(Schlitzer,2002).Asfirstdemonstrated theplateauregion,whereverticalupwelledsupplyfromthe over 20 years ago, phytoplankton growth in the Southern plateausediments(Blainetal.,2008b;Zhouetal.,2014)and Oceanislimitedbytheavailabilityofthemicronutrienttrace lateral advection of water that had been in contact with the element iron (Fe; Martin, 1990). Low dissolved iron (dFe) continentalshelfofHeardIslandtothesouth(Cheveretal., availability limits the annual uptake of atmospheric carbon 2010) were the dominant sources of dissolved and particu- dioxide (CO ) by the Southern Ocean (Boyd et al., 2000), lateFe(asconfirmedusingrareearthelement(REE)andra- 2 shapes phytoplankton species composition and physiology dium (Ra) isotope tracers; van Beek et al., 2008; Zhang et (Assmy et al., 2013), the cycling of other nutrient elements al., 2008). The interaction of waters, islands and plateau of (MooreandDoney,2007),andthusthestructureoftheentire the Kerguelen Archipelago with several circumpolar fronts marineecosystem(BoydandEllwood,2010). oftheSouthernOceanallowedustomakeafirstattemptat Artificial mesoscale ocean iron fertilisation experiments placingourregionalKEOPS-1observationswithinabroader have unequivocally demonstrated the role of Fe in setting basin-scalecontext(Blainetal.,2007). phytoplankton productivity, biomass and community struc- The KEOPS-2 project was designed to improve the spa- ture in high-nutrient low-chlorophyll (HNLC) regions (de tial and temporal coverage of the Kerguelen region. During Baar et al., 2005; Boyd et al., 2007). However, the “carbon KEOPS-2, which was approved as a GEOTRACES process sequestration efficiency” of ocean fertilisation as a means study1, we studied the region above and downstream of the to sequester atmospheric CO (calculated as the additional plateau and observed a massive natural iron fertilisation on 2 (net) C that is exported from surface waters into the deep thescaleofhundredsofthousandsofsquarekilometres.This (>1000m) ocean for a given addition of Fe) varies widely producedapatchworkofbloomswithdiversebiologicaland betweenexperimentsandisconsiderablylessthanestimates biogeochemical responses, as detailed in the multiple stud- from the early iron fertilisation experiments (see discussion iesinthisspecialissueofBiogeosciences(volumes11–12). in de Baar et al., 2008). This is due to a number of factors, KEOPS-2 was also carried out in the austral spring to doc- including rapid grazing of phytoplankton in surface waters, ument the early stages of the bloom and to complement the thelossofaddedFebyitsprecipitationandscavengingonto resultsofKEOPS-1obtainedinlatesummerduringthestart sinking particles, differences in estimated or assumed iron- of the decline of the bloom, with a principal aim to better to-carbon (Fe/C) ratios of the cells, and changes in wind constrainthemechanismofFesupplytosurfacewatersear- mixedlayerdepth. lierintheseason. The natural resupply of iron to Fe-depleted waters is a SinceFeisactivelytakenupintophytoplanktonandtrans- more efficient process (Blain et al., 2007), although in part ferredthroughoutthefoodweb,includingremovalbyparti- this depends on the mode of Fe delivery (e.g. from above, cle settling and remineralisation in deep waters, the assess- laterally or from below) and on the ability of organic lig- mentofitsavailabilityisquitecomplexandcannotbejudged ands to keep the supplied Fe in solution (Gerringa et al., from dFe levels in surface waters alone (Breitbarth et al., 2008), and for continued ocean fertilisation, it is in part re- 2010). Advances in chemical oceanographic techniques for liant on the concurrent supply of other major nutrients. In trace elements through the GEOTRACES program (SCOR the Indian sector of the subantarctic Southern Ocean, natu- ralFesupplyfromtheKerguelenPlateau(Blainetal.,2007) 1http://www.geotraces.org/cruises/cruise-summary/68-science/ andCrozetIslands(Pollardetal.,2009)resultsinincreased process-studies/206-geotraces-process-studies Biogeosciences,12,4421–4445,2015 www.biogeosciences.net/12/4421/2015/ A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 4423 Working Group, 2007) now allow the measurement of Fe subjected to natural Fe fertilisation (e.g. Frew et al., 2006 associated with different phases (dissolved and particulate), andBoydetal.,2005forFeCycle-I,andEllwoodetal.,2014 internalbiologicalrecyclingandFeexportfromsurfacewa- for FeCycle-II east of New Zealand; Bowie et al., 2009 for ters.Theresultsfromearlierironbiogeochemicalbudgetsfor SAZ-Sense south of Tasmania; Planquette et al., 2011 for FeCycle-I (Boyd et al., 2005; Frew et al., 2006), KEOPS-1 CROZEXneartheCrozetIslands;andZhouetal.,2010for (Blainetal.,2007;Cheveretal.,2010),CROZEX(CROZet BlueWaterZonenearthewesternAntarcticPeninsula).The naturalironbloomandEXportexperiment;Planquetteetal., observations of dFe (Quéroué et al., 2015) and particulate 2007, 2009) and SAZ-Sense (Sensitivity of the subantarctic tracemetals(vanderMerweetal.,2015)aredetailedincom- zonetoenvironmentalchange;Bowieetal.,2009)havehigh- panionpapersinthisspecialissuetoallowthecurrentpaper lightedthatthedominant“new”Fefluxesareassociatedwith tofocusexplicitlyontheconstructionofironbudgets;how- the particulate phase. Particles thus represent an important ever,thethreepapersshouldbeseenasacollectivewhole. transport vector for trace metals in the marine ecosystem, although their bioavailability or transfer into a bioavailable 2 Materialandmethods fraction remains uncertain. Suspended particles have also been shown to be important aspects of sedimentary, bound- 2.1 Studyarea ary layer Fe sources and export processes (Tagliabue et al. 2009; Homoky et al., 2013; Marsay et al., 2014; Wadley et The KEOPS-2 (KErguelen Ocean and Plateau in compared al.,2014),withparticlesbeingtransportedlaterallyoverhun- Study 2) expedition was carried out in the Indian sector dredsofkilometresintheocean(Lametal.,2006;Lamand of the Southern Ocean in the vicinity of the Kerguelen Bishop,2008).ThebiologicalcyclingofparticulateFemay Plateau between 7 October and 30 November 2011 on the therefore be the most important aspect of the complete Fe RV Marion Dufresne (Fig. 1a). The plateau of the Kergue- biogeochemical cycle, especially since earlier budgets have lenArchipelagoisanorthwest–southeastseafloorfeatureap- demonstratedthatbiologicalFe“demand”cannotbesatisfied proximately 500m deep and is constrained by the Kergue- bythenewFesupply(Boydetal.,2005;Blainetal.,2007; len Islands to the north and the smaller volcanic Heard and Sarthouetal.,2008;Bowieetal.,2009;deJongetal.,2012). McDonaldislandstothesouth.Ourstudywasconductedin Asimpleone-dimensionalverticalmodelthatcorrectlyrep- earlyaustralspringwhenphytoplanktonbiomasswasdevel- resentedtheinputofdFetosurfacewatersduringKEOPS-1 opingrapidlyandformingamosaicofphytoplanktonblooms did not accurately represent the supply of other geochemi- intheregion(Trulletal.,2015;Lasbleizetal.,2014).Since cal tracers or particulate Fe (Blain et al., 2007; van Beek et samplingatthedifferentstationstookplaceatdifferenttimes al., 2008; Zhang et al., 2008), and the role of dissolved and over the ∼7-week study, our observations also provide a particulateFeearlierintheseason(winterstock)intheKer- temporal sequence relative to the development of surface guelenregionhasyettobequantified. biomass. Thispaperpresentsashort-term(daystoweeks)Febudget The Kerguelen bloom has two main features: a north- fortheperiodofKEOPS-2foreachofthreeprocesssites:(i) ern branch that extends northeast of the island into waters a“plateau”bloomsite(A3)onthecentralKerguelenPlateau both south and north of the PF and a larger bloom cover- studiedduringlatesummeronKEOPS-1andreoccupieddur- ing∼45000km2 southofthePFandlargelyconstrainedto ing spring on KEOPS-2; (ii) a “plume” bloom site (E) east the shallow bathymetry of the Kerguelen Plateau (<700m) oftheKerguelenIslands,whichwaslocatedwithinaquasi- (Mongin et al., 2008; Supplement in Trull et al., 2015) stationary,bathymetricallytrappedrecirculationfeaturenear (Figs. 1b and 2). Thirty-two stations were sampled during the polar front (PF); and (iii) a “reference” site (R-2) south KEOPS-2, often with repeat visits. Here, we focus on three ofthePFandupstream(southwest)oftheKerguelenIslands study sites, namely plateau A3, plume E and reference R-2 inHNLCwaters.Wefocusonmixed-layerintegratedpools (Fig.1).TwovisitsweremadetoA3atthestart(A3-1)and of dissolved Fe and particulate Fe (which we further sepa- end(A3-2)ofthevoyage(28daysapart),andfivevisitswere rate into biogenic and lithogenic fractions using elemental madetositeE(over21days)todocumentthebloomdevel- normalisers), estimate the fluxes of Fe associated with new opment.Basedonthetrajectoriesofsurfacedrifters,stations andrecycledFesources,andcompareFesupplyanddemand E-1,E-3andE-5weretakenastrackingthemiddleofare- with implications for bloom duration and magnitude. Our circulation region (d’Ovidio et al., 2015), so that they can observations also include particulate measurements in both beconsideredaspseudo-Lagrangianandtheirsuccessionin suspended-water-column (in situ pump; ISP) and sinking- timecanbeconsideredafirst-ordertimeseries.Fulldetailsof export(free-floatingsedimenttrap;“P-trap”)particlesbelow otherstationsandsamplingdesignedtodocumentthemerid- the mixed layer, with linkage to food web processes via a ionalandzonalextensionsofthebloomsontheplateauand discussion of Fe/C ratios. Finally, we present a seasonal totheeastoftheKerguelenIslandsarecontainedincompan- comparisonofourspringtimebudgetforKEOPS-2withlate ionpapersinthisspecialissueofBiogeosciences. summerobservationsfromKEOPS-1andalsomakecompar- isonwithfindingsfromothersectorsoftheSouthernOcean www.biogeosciences.net/12/4421/2015/ Biogeosciences,12,4421–4445,2015 4424 A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2   (a) Southern Ocean Leclaire Rise ‐2 (b) Figure1.(a)ThelocationofKEOPS-2intheIndiansectoroftheSouthernOcean,showingbathymetryaroundtheKerguelenArchipelago. ◦ (cid:48) ◦ (cid:48) ◦ (cid:48) Ourbiogeochemicalironbudgetsfocusonthreeprocessstations(openblackcircles):referenceR-2(50 2 S,66 4 E),plateauA3(50 4 S, ◦ (cid:48) ◦ (cid:48) ◦ (cid:48) 72 0 E)andplumeE(48 3 S,72 1 E).Blackdotsmarkthepositionsoftheotherstationsvisited,includingN–SandE–Wsurveytransects at the start of the KEOPS-2 expedition. (b) A schematic of the mean regional circulation of surface and subsurface waters around the KerguelenArchipelago,indicatingcircumpolarSouthernOceanfronts,locationsofstationsalongN–SandE–Wtransects,andpathways and origins of different water masses flowing on the plateau and offshore into the plume. The abbreviations are Antarctic Surface Water (AASW),PolarFrontSurfaceWater(PFSW),SubantarcticSurfaceWater(SASW),SubtropicalSurfaceWater(STSW),SubantarcticFront (SAF)andthepolarfront(PF)(reproducedwithpermissionfromParketal.(2014a),courtesyofIsabelleDurandandYoung-HyangPark, LOCEAN/DMPA,MNHN,Paris). (a) (b) (c) Figure 2. MODIS ocean-colour satellite images showing the development of the plateau and plume blooms during KEOPS-2. Surface chlorophyll(µgL−1)biomassisshownforthenearestclearskydaytothefinalsamplingdayatstationsR-2(a),A3-2(b)andE-5(c).The PFisshownasablackdashedlinein(b)and(c).Trulletal.(2015)discussthetimingofthestationsrelativetobloomdevelopment. ThehydrologyandcirculationaroundandabovetheKer- Zhouetal.(2014).ThemeancirculationisshowninFig.1b. guelen Plateau have been described by Park et al. (2008a, Briefly, the Kerguelen Plateau constitutes a barrier to the b, 2014a), van Beek et al. (2008), Zhang et al. (2008) and eastwardflowingAntarcticCircumpolarCurrent(ACC),the Biogeosciences,12,4421–4445,2015 www.biogeosciences.net/12/4421/2015/ A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 4425 mainjetsofwhicharetheSubantarcticFront(SAF)andPF. were rinsed liberally with sample before final collection in MostoftheACCisdeflectednorthoftheKerguelenIslands 125mLNalgeneLDPEbottles.Seawatersampleswereacid- as Subantarctic Surface Water (SASW) but some filaments ifiedwithin24hofcollectionusing2mLofconcentratedul- passbetweentheKerguelenIslandsandHeardIsland(asthe trapure hydrochloric acid (HCl, Seastar BASELINE grade) PF) and further south between Heard Island and Antarctica per litre of sample, resulting in an approximate final pH of (Roquetetal.,2009).Abovetheplateau,theremainderofthe 1.8, double bagged and stored for at least 24h at ambient ACCcomesfromthewesternpartoftheplateau.Currentsof temperatureuntilanalysis. AASW travelling along the western flank of the plateau are deflected south and east of Heard Island as a branch of the 2.2.2 Insitupumps(ISPs) Fawn Trough Current (FTC) (Sokolov and Rintoul, 2009) before travelling in a broadly northwest direction up along Suspended particles for trace elemental analysis were col- the eastern shelf break. The water flow is then deflected to- lected using 11 large-volume in situ pumps (McLane Re- ward the east of the Kerguelen Islands, where there is an searchLaboratoriesWTS6-1-142LVandChallengerOcean- intense mixing zone consisting of mesoscale eddies which ics pumps), suspended simultaneously at prechosen depths travelmanythousandsofkilometresintheACCtowardsthe following methods reported in Bowie et al. (2009). Up to AustraliansectoroftheSouthernOcean. 2000L of seawater was filtered across a 142mm diameter stack (134mm diameter active area) consisting of a 53µm 2.2 Sampling nylonpre-filterscreen(NYTEX)followedbyaQMAquartz fibrefilter(1µmnominalporesize;Sartorius).TheQMAfil- Alltracemetalsamplingandanalyticalproceduresfollowed ter was supported by a 350µm polyester mesh, which was recommended protocols in the cookbook2 published by the placed on top of the Teflon PFA grid of the pump housing. international program GEOTRACES (Bishop et al., 2012; Prior to use, NYTEX screens were conditioned by soaking Cutter and Bruland, 2012; Planquette and Sherrell, 2012). in5%H SO ,rinsed3×withMilli-Q-gradewater,driedat 2 4 All methodshavebeen successfully usedpreviously by this ambient temperature under a laminar flow hood and stored team during the KEOPS-1 (Blain et al., 2008b) and SAZ- incleanplasticZiploc® bags.QMAfilterswereconditioned Sense projects (Bowie et al., 2009). Subtle differences in for trace metal analysis (precombustion and acid cleaning) methods employed during the earlier KEOPS-1 and SAZ- followingBowieetal.(2010).Uponrecoveryofthepumps, Sense projects are described in those papers and/or later in subsamplesweretakenfromtheQMAfiltersbyusingacir- thismanuscript. cularplasticpunch(14mmdiameter)andbycuttingtheny- lon mesh using ceramic scissors. Filters were dried under 2.2.1 Tracemetalrosette(TMR) a laminar flow bench and stored at −18◦C in acid-washed PCRtraysuntilfurtheranalysisinthehomelaboratory.The Water column samples were collected using 10L externally 1–53and>53µmsizefractionsweredigestedandanalysed closing, Teflon-lined Niskin-1010X bottles deployed on an separately,andtheparticulateiron(pFe)reportedhereisthe autonomous 1018 intelligent rosette system (TMR – trace sumofbothfractions.TheISPswereshowntobeefficientin metalrosette,speciallyadaptedfortracemetalwork;General capturinglarge(>53µm)particles(Planchonetal.,2015). OceanicsInc.).Thepolyurethane-powder-coatedaluminium rosette frame was suspended on Kevlar rope which passed 2.2.3 Free-floatingtraps(P-trap) throughacleanblockwithaplasticsheave(GeneralOcean- ics)andwasloweredtoamaximumdepthof1300m.Bottles Sinking particles for trace elemental analysis were col- weretrippedatpreprogrammeddepthsusingapressuresen- lectedusingPPS3/3free-floatingsedimenttraps(Technicap, sor as the TMR was being raised through the water column atapproximately0.5ms−1. France), specially adapted for trace metals and deployed at 200m. Traps were deployed for 5.3, 5.1, 1.9 and 1.5 days AllsampleprocessingwascarriedoutunderanISOclass- at stations E-1, E-3, A3-2 and E-5, respectively. The trap 5 trace metal clean laminar flow bench in a HEPA filtered- deployed at station R-2 was lost and not recovered. Traps air clean container, with all materials used for sample han- drifted between 10 and 43km over the course of the de- dlingthoroughlyacid-washed.Samplesweredrawnthrough ployment. Full details of the trap deployments are given in C-Flex tubing (Cole Parmer) and filtered in-line through Laurenceau-Cornecetal.(2015)andPlanchonetal.(2015). 0.2µm pore size acid-washed capsules (Pall Supor mem- Samplesfortraceelementalanalysiswerecollectedinthree brane Acropak 200 or Sartorius Sartobran 300 filters). The separateacid-washedcups(specificallyfortracemetals)con- dissolvedfractionisthuslikelytocontaincolloidsandsmall tainingalowtracemetalbrinesolution(salinity∼60),each particles <0.2µm in diameter (Bowie and Lohan, 2009). opened for either 1, 3, 8 or 12h (depending on the station). All transfer tubes, filtering devices and sample containers Uponrecovery,cupsweretakentoacleanroomandparticles 2http://www.geotraces.org/libraries/documents/Intercalibration/ filteredoff-lineontoa47mmdiameter,2µmporositypoly- Cookbook.pdf carbonatefilterundergentlevacuumusingaTeflonPFAunit www.biogeosciences.net/12/4421/2015/ Biogeosciences,12,4421–4445,2015 4426 A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 (SavillexCorp.,USA)equippedwitha350µmpre-screen(to for the pump samples were estimated from filters prepared excludezooplankton). identically but not deployed on the ISPs; for the trap sam- ples this was done by re-filtering the pre-filtered seawater. 2.3 Analysis Allblankcorrectionswerelessthan2%forallsamples.The subsampling introduces uncertainties of 5–10% from inho- 2.3.1 Dissolvediron mogeneousfiltercoveragethatexceedstheanalyticaluncer- taintyof∼1%inthePOCanalysis(Trulletal.,2015). Dissolved Fe (dFe) was determined shipboard by flow in- jection analysis with chemiluminescence detection (FI-CL) 2.4 Biologicalironcycling using in-line preconcentration on an 8-hydroxyquinoline chelating resin (adapted from Obata et al., 1993, de Jong et 2.4.1 Ironuptake al., 1998 and Sarthou et al., 2003). Dissolved Fe data were quality controlled against the SAFe (Sampling and Analy- Trace metal clean seawater was collected from the mixed sisofFe)standardreferencematerials(Johnsonetal.,2007). layer (20–40m) using the TMR, was transferred into acid- Fulldataincludingcertificationresultsandanalyticalfigures washed polycarbonate bottles and 0.2nmolL−1 (final con- ofmeritarereportedinQuérouéetal.(2015). centration)ofenriched55FeasFeCl wasadded(1.83×103 3 Cimol−1 of specific activity, Perkin Elmer). Bottles were 2.3.2 Particulateiron placed at in situ temperature in on-deck incubators contin- uouslyfedbysurfaceseawater.Incubationswereconducted Particulate Fe (pFe) was determined as follows. Sampled for 24h (sunrise to sunrise) at several light intensity levels particles were acid extracted in 1mL concentrated HNO 3 (75,45,25,16,4and1%ofphotosyntheticallyactiveradi- (Seastar Baseline) for 12h on a DigiPREP HP Teflon hot- ation; PAR). For stations R-2, A3-1, E-1 and E-3, seawater plate supplied with HEPA-filtered air (SCP Science) at 120◦C using 15mL Teflon PFA Savillex vials. Digest so- wasprefilteredona25µmmeshsizebefore55Fewasadded. After incubation, 300mL of seawater was passed through lutions were diluted with 10mL ultra high-purity water to 0.2µm pore size nitrocellulose filters (47mm diameter, Nu- 10%HNO andspikedwith10ppbindiumasinternalstan- 3 clepore). To determine intracellular Fe uptake rates, 55Fe dard prior to analysis by sector field inductively coupled notincorporatedbycellswasremovedimmediatelyafterfil- plasmamassspectrometry(FinniganELEMENT2,Thermo tration using 6mL of a Ti-citrate-EDTA washing solution Scientific),followingBowieetal.(2010).Blanksfromrepli- for 2min, then rinsed three times with 5mL of 0.2µm fil- cate analysis of filters treated identically to the sample fil- tered seawater for 1min (Hudson and Morel, 1989; Tang ters but without large volumes of seawater passed through and Morel, 2006). The filters were placed into plastic vials them,weretypically2–3%and<1%ofthepFesamplecon- and10mLofthescintillationcocktail“Filtercount”(Perkin centrationsfortheISPdeploymentsandP-trapdeployments, Elmer) added. Vials were agitated for 24h before the ra- respectively. Recoveries from the analysis of the Commu- dioactivityonfilterswascountedwiththeTricarb®scintilla- nityBureauofReferenceplanktoncertifiedreferencemate- rialBCR-414wereexcellent,witha101%recovery(n=3) tioncounter(precision<10%).Controlswereobtainedwith 300mL of microwave-sterilised seawater (750W for 5min) forpFe.FulldataarereportedinvanderMerweetal.(2015). incubatedandtreatedthesameway.Subsamplesforenumer- 2.3.3 Particulateorganiccarbonandnitrogen ationbyflowcytometrywerecollectedfromeachbottlejust beforethefiltrationstep.Cellswerefixedinglutaraldehyde For particulate organic carbon (POC) and particulate nitro- (1%) and kept frozen (−80◦C) until processing and analy- gen (PN) analyses, QMA quartz filters from the ISPs were sis.DatawerecorrectedbyblanksubtractionandFeuptake subsampledinaflowbenchusinga14mmdiameterplastic rates normalised to the concentration of Fe in each incuba- punchandtransferredtosilverfoilcups(Serconbrandp/n tion(insitudFeand55Feadded).Furtherdetailsaregivenin SC0037). Samples were also collected from the P-traps for Fourquezetal.(2015). POCandPNanalyses(seeLaurenceau-Cornecetal.,2015). Samplesweretreatedwitha40µLaliquotof2NHCltore- 2.4.2 Ironremineralisation movecarbonates(Kingetal.,1998),driedat60◦Cfor48h and stored in a desiccator until analysis using a Thermo- Sinceironregenerationwasnotmeasureddirectlybyexper- Finnigan Flash EA1112 elemental analyzer (using sulfanil- iment during KEOPS-2, we used the following approach to amidestandards)attheCentralScienceLaboratory,Univer- calculate iron regeneration fluxes. Bacterial Fe regeneration sity of Tasmania. The >53µm fraction was treated in the wasestimatedfrombacterialturnovertimesdeterminedfrom same way at the Vrije Universiteit Brussel after first trans- bacterialproductionandbiomass(Christakietal.,2014),as- ferringthematerialfromonefourthofthescreen,usingpre- suming all loss of bacterial biomass through viral lysis and filteredseawater,onto25mmdiameter,1.0µmporesizesil- flagellategrazingresultedintheregenerationofFe(Strzepek vermembranefilters(Sterlitech,Concord).Blankcorrections et al., 2005) and using a bacterial iron quota of 7.5µmol Biogeosciences,12,4421–4445,2015 www.biogeosciences.net/12/4421/2015/ A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 4427 Fe (mol C)−1 (Tortell et al. 1996). The mesozooplankton and particulate (Mn, Al; van der Merwe et al., 2015) trace grazing contribution to Fe regeneration was assumed to be elements. equaltotheexperimentallydeterminedFeregenerationdur- The dFe profile at the KEOPS-2 reference station R-2 is ing KEOPS-1 (Sarthou et al., 2008). The regeneration rates similar to the KEOPS-1 reference station C11 (with the ex- per mesozooplankton individual determined in Sarthou et ceptionoftheR-2enrichmentinthe200–700mdepthstrata; al. (2008), were then multiplied by mesozooplankton abun- Fig.4a),butitshouldbenotedthatthelocationofC11was dance,calculatedfromthenumberofcellscapturedinadaily quitedifferent–inHNLCwaterstothesoutheastoftheKer- haulover200mduringKEOPS-2(Carlottietal.,2015;val- guelenPlateau(51◦39’S,78◦00’E)–andwehadonly1dFe uesreportedinTable6inLaurenceau-Cornecetal.,2015). data point in UCDW at C11. In contrast to the similarity of thedFeprofiles,thepFeprofileatC11wasgenerallylower than at R-2, with mean values through the water column 3 Resultsanddiscussion of 0.2±0.14nmolL−1 (Andrew Bowie, unpublished data) comparedto0.53±0.35nmolL−1forstationR-2. 3.1 Biogeochemicalsettingsatourthreestudysites 3.1.2 PlateaustationA3 Full descriptions of the dFe and pFe distributions can be found in Quéroué et al. (2015) and van der Merwe et al. StationsA3-1(Fig.3b)andA3-2(Fig.3c)wereinrelatively (2015), respectively, with further presentation of the distri- shallow waters on the central plateau, and were impacted butions of other micronutrient trace elements (Mn, Co, Ni, byplateausedimentsandpossiblyfluvialandglacialrunoff Cu,Cd,Pb)fromKEOPS-2tobepresentedelsewhere.How- from the basaltic rocks of Heard Island ∼300km upstream ever, briefly our subset of stations used for the iron budgets (vanderMerweetal.,2015;M.Grenier,personalcommuni- canbedescribedasfollows. cation,2014).Apycnoclinewasobservedat∼190m,above which the salinity (33.9) and nitrate (∼29µmolL−1) were 3.1.1 ReferencestationR-2 relatively constant. The mixed layer shoaled (from 165 to 123m)andincreasedintemperature(from1.7to2.2◦C)be- Intheupper100m,weobservedasalinityminimum(33.8) tweenthetwovisitstoA3,consistentwithspringtimewarm- andtemperaturemaximum(2.2◦C)characteristicofAntarc- ing of surface waters. We believe that the water masses at tic Surface Water (AASW) overlying a layer of winter wa- A3-1 and A3-2 are comparable since surface waters move ter (WW) at 180–200m (T of 1.6◦C) (Fig. 3a). Deeper slowly in this region (Park et al., 2008, 2014a; Zhou et al., min in the water column, a T of 2.5◦C at 500m (associated 2014);thiswasconfirmedbyREEdatawhichindicatedsim- max withanoxygenminimum;notshown)wasindicativeofup- ilarwatersatbothstationsmarkedwithfreshcontinentalsup- per Circumpolar Deep Water (UCDW) overlying a salinity pliesandonlymodifiedbybiologicalprocesses(M.Grenier, maximum of 34.8 at 1830m in lower Circumpolar Deep personalcommunication,2014). Water (LCDW). Phytoplankton abundance was low (0.2µg Surface chlorophyll images revealed that during the 28 Chl aL−1; Lasbleiz et al., 2014) and dominated by di- days between the first and second visits to A3, a large di- atoms,inwaterswithrelativelyhighsurfacenitrateconcen- atom spring bloom developed mostly dominated by lightly trations(>25µmolL−1;Blainetal.,2015),typicalofSouth- silicified Chaetoceros spp. (surface Chl a increasing from ernOceanHNLCconditions(Lasbleizetal.,2014). 0.2µgL−1 at A3-1 to 1.3µgL−1 at A3-2; Lasbleiz et al., Dissolved Fe concentrations were very low at the sur- 2014),whichlikely resultedinthedrawdown ofdFe(mean face (<0.1nmol L−1) and increased with depth, averaging mixedlayervaluesdecreasingfrom0.3–0.4nmolL−1atA3- 0.3nmolL−1inLCDWandbroadlytrackingthenitratepro- 1 to 0.1–0.2nmolL−1 at A3-2). The peak of biomass had file. The pFe profile showed a similar structure to the dFe passedbythetimewesampledatA3-2,withthebloomstart- profile but with surface and deep water concentrations be- ingtofade(Trulletal.,2015).Belowthemixedlayer,simi- tween 0.3 and 1.1nmolL−1 (the deepest sample was 148m lardFeprofileswereobservedduringbothvisitstoA3,with above the seafloor). The exception was at 500m, where, expected significant increases at depth towards the plateau interestingly, we observed a dFe and pFe peak of 0.4 and floor(e.g.to1.30nmolL−1 at480matA3-2;notethat,due 1.6nmolL−1, respectively. Whilst this maximum may have tooperationalconstraints,therewasnodFedatadeeperthan arisenduetotheenrichmentofFeinUCDWdeliveredfrom 340m at A3-1). Such enrichments at depth were also ob- further south, we hypothesise that the Fe supply may have servedindissolvedMnandCoprofiles(F.Quéroué,personal originatedfromsubsurfacesedimentsofthenearbyLeclaire communication,2014;datanotshown)anddFeprofilesfrom Rise (also known as Skiff Bank; Kieffer et al., 2002), a theoccupationsofstationA3duringKEOPS-1(Fig.4b),in- large seamount which rises to 250m at 49◦50’S, 65◦00’E dicativeofplateausedimentarysupply. (approximately 140km northwest of station R-2). Similar The pFe profiles at A3 showed a similar structure to the lithogenicinputswerealsoobservedforotherdissolved(Mn; dFeprofile,withlowervaluesatthesurface(<10nmolL−1 F.Quéroué,personalcommunication,2014,datanotshown) atA3-1and<4nmolL−1atA3-2)andincreasingwithdepth www.biogeosciences.net/12/4421/2015/ Biogeosciences,12,4421–4445,2015 4428 A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 (a) R‐2 (reference) (d) E‐1 (plume) dFe(nmoll‐1) Temperature (oC) Temperature (oC) 0.0 0.2 0.4 0.6 0 1 2 3 0 1 2 3 0 0 0 0 500 500 500 500 1000 1000 1000 1000 1500 1500 1500 1500 2000 2000 Temperature 2000 2000 Temperature dFe Salinity Salinity pFe Nitrate pFe Nitrate 2500 2500 2500 2500 0.0 0.5 1.0 1.5 2.0 2.5 33.8 34.0 34.2 34.4 34.6 34.8 0.0 0.5 1.0 1.5 2.0 2.5 33.8 34.0 34.2 34.4 34.6 34.8 pFe(nmoll‐1) pFe(nmoll‐1) 0 10 20 30 40 0 10 20 30 40 Salinity; nitrate (moll‐1)  Salinity; nitrate (moll‐1)  (b) A3‐1 (plateau) (e) E‐3 (plume) dFe(nmoll‐1) Temperature (oC) dFe(nmoll‐1) Temperature (oC) 0.0 0.5 1.0 1.5 0 1 2 3 0.0 0.2 0.4 0.6 0 1 2 3 0 0 0 0 100 100 500 500 200 200 1000 1000 300 300 1500 1500 400 400 Temperature 2000 2000 Temperature 500 dFe 500 Salinity dFe Salinity pFe Nitrate pFe Nitrate 600 600 2500 2500 0 10 20 30 40 33.8 34.0 34.2 34.4 34.6 34.8 0.0 0.5 1.0 1.5 2.0 2.5 33.8 34.0 34.2 34.4 34.6 34.8 pFe(nmoll‐1) pFe(nmoll‐1) 0 10 20 30 40 0 10 20 30 40 Salinity; nitrate (moll‐1)  Salinity; nitrate (moll‐1)  (c) A3‐2 (plateau) (f) E‐5 (plume) dFe(nmoll‐1) Temperature (oC) dFe(nmoll‐1) Temperature (oC) 0.0 0.5 1.0 1.5 0 1 2 3 0.0 0.2 0.4 0.6 0 1 2 3 0 0 0 0 100 100 500 500 200 200 1000 1000 300 300 1500 1500 400 400 500 dFe 500 TSeamlinpiteyrature 2000 dFe 2000 TSeamlinpiteyrature pFe Nitrate pFe Nitrate 600 600 2500 2500 0 10 20 30 40 33.8 34.0 34.2 34.4 34.6 34.8 0.0 0.5 1.0 1.5 2.0 2.5 33.8 34.0 34.2 34.4 34.6 34.8 pFe(nmoll‐1) pFe(nmoll‐1) 0 10 20 30 40 0 10 20 30 40 Salinity; nitrate (moll‐1)  Salinity; nitrate (moll‐1)  Figure 3. (a) Vertical profiles of dissolved iron (dFe) and particulate iron (pFe), potential temperature, salinity, and nitrate at reference station R-2. The seafloor depth at 2528m is shown. (b, c) Vertical profiles of dFe and pFe, potential temperature, salinity, and nitrate at plateaustationsA3-1(b)andA3-2(c).Theseafloordepthat∼530misshown.NotedifferentscalesfordFeandpFecomparedtoR-2and Estations.(d,e,f)VerticalprofilesofdFeandpFe,potentialtemperature,salinity,andnitrateatplumestationsE1(d),E3(e)andE5(f). Theseafloordepthrangingfrom1905m(E3)to2057m(E1)isshown. due to enrichment from bottom sediments (up to 33 and centrationschangedremarkablybetweenthetwovisits,and 14nmolL−1 at 440m at A3-1 and A3-2, respectively), and the full water column integrated pool was ∼70% lower at were on average 10 times greater than dissolved concentra- A3-2thanatA3-1.Interestingly,thischangewasalsoasso- tions through the water column. The mixed layer pFe con- ciated with a shift of particles from the 1–53µm size range Biogeosciences,12,4421–4445,2015 www.biogeosciences.net/12/4421/2015/ A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 4429 (a) Reference stations trationsincreasingwithdepthandenrichmentjustabovethe dFe(nmoll‐1) pFe(nmoll‐1) plateau seafloor (Fig. 4b). Through the water column, dFe was between 2 and 5 times greater during KEOPS-2 than 0.0 0.2 0.4 0.6 0.0 0.5 1.0 1.5 2.0 2.5 0 0 KEOPS-1andpFewas∼10timesgreaterduringKEOPS-2 (with the exception of the deepest samples). The lower val- 500 500 uesduringKEOPS-1werelikelytheresultofbiologicalup- takeinsurfacewatersandtheexportofFeduringthespring 1000 1000 bloom prior to our arrival at the study site, combined with seasonal changes in the strength of the supply mechanisms 1500 1500 to deeper waters at A3 (discussed in van der Merwe et al., 2015). 2000 2000 3.1.3 PlumeEstations 2500 2500 R‐2 C11 R‐2 C11 The E stations within the bathymetrically trapped complex (b) Plateau stations recirculationsystemshowedsimilarhydrographicandnutri- dFe(nmoll‐1) pFe(nmoll‐1) ent distributions below the mixed layer (Fig. 3d, e and f), 0.0 0.5 1.0 1.5 0 10 20 30 40 which shoaled from 64m at E-1 to 32m at E-3 to 39m at 0 0 E-5,withsomeinternalvariabilityinwatercolumnstructure 100 100 atmid-depths.Surfacewaterswarmedfrom2.7to3.4◦Cbe- tweentheoccupationsofE-1andE-5,althoughnosignificant 200 200 nitrate drawdown was observed (Blain et al., 2015). Below 300 300 AASW,asubsurfacetemperatureminimum(Tmin,∼1.7◦C) wasobservedbetween180m(E1)and220m(E5),character- 400 400 isticofWW.TheT featureisassociatedwithwaterssouth min ofthePF,althoughtherecirculationfeatureprobablyalsore- 500 500 ceivedSAZwatersmixedinfromthenorth(d’Ovidioetal., 600 600 2015).T,S andO2 characteristicsindicatedthepresenceof A3‐1 A3‐2 A3(1) A3(2) A3(3) A3(4) A3‐1 A3‐2 A3(1) UCDW(∼600–700m)andLCDW(deeperthan∼1300m) deeper in the water column above the seafloor (Quéroué et Figure4.(a)ComparisonofdFeandpFeatreferencestationsfor KEOPS-1 (station C11, open blue diamonds) and KEOPS-2 (sta- al., 2015). Water parcel trajectories calculated from altime- tionR-2,closedredsquares).Thewaterdepthswere3110matC11 trybasedgeostrophiccurrentsindicatedthatittookgenerally and2530matR-2.(b)ComparisonofdFeandpFeatA3plateau >2 months for Fe-rich waters from the plateau to travel to stationsforKEOPS-1(opensymbols)andKEOPS-2(closedsym- thedownstreamplumesiteassociatedwiththerecirculation bols).DataareshownforallvisitstoA3onbothKEOPScruises. feature(Estations)(d’Ovidioetal.,2015).Howevershorter NotedifferenceinscalefordFeandpFebetween(a)and(b). transport times are also possible due to episodic transport acrossthePF(Sanialetal.,2015). Watersattheplumestationsshowedthelargestspatialhet- erogeneity in surface biomass as revealed by the evolution to the >53µm size range, with the larger class tripling in ofamosaicofcomplexbloomsseeninsatelliteimages(see size (van der Merwe et al., 2015). The development of the SupplementinTrulletal.,2015).Weobservedmoderatesur- largebloombetweenourtwovisitstoA3,whichconsistedof faceChlalevelsrangingfrom0.3–0.4µgL−1atE-1andE-3 a diatom community 50–210µm in size (Trull et al., 2015), to 0.5–0.9µgL−1 at E-5 (Lasbleiz et al., 2014), noting that waslikelyresponsibleforconvertingthepFewithinthesur- as much as 50% of the chlorophyll was below the mixed facemixedlayerfromthesmallersizeclasstothelargersize layer at the plume stations due to stratification of the up- class.Thismayhavebeenduetoeither(i)physicalaggrega- perwatercolumninthewarm,springconditions.Unlikethe tionoftheparticlesontodiatomaggregatesand/or(ii)micro- plateau bloom dominated by large cells >53µm, the com- biallydrivenconversionofsmalllithogenicFe(1–53µm)to munityintheplumeEstationswasmoremixed(Laurenceau- bioavailableformsandincorporationintothelarge(>53µm) Cornecetal.,2015),withcellspresentinboththe5–20and diatoms as biogenic Fe, with potentially some fraction of 50–200µm size classes (Trull et al., 2015). The E stations these larger particles exported to depths below the mixed showedthehighestCexportfluxesofallregionsasestimated layer,aspreviouslydiscussedbyLametal.(2006),Frewet fromThdeficits,nitratedepletionsandfree-driftingsediment al.(2006)andPlanquetteetal.(2011). trap observations (Planchon et al., 2015; Trull et al., 2015; Thespring(Oct-Nov)KEOPS-2FeprofilesatstationA3 Laurenceau-Cornecetal.,2015). showed a similar structure to those from the late summer (Jan-Feb)duringKEOPS-1,withsurfacedepletion,concen- www.biogeosciences.net/12/4421/2015/ Biogeosciences,12,4421–4445,2015 4430 A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 Due to operational constraints, no dFe data were avail- particle export), whilst studies on Fe uptake and microbial able at station E-1. The dFe vertical profiles at E-3 cycling have shown that short-term fluxes within the “fer- and E-5 were quite different, with a distinct surface en- rouswheel”aredominatedbybiologicaluptakeandreminer- richment to 0.4nmolL−1 at E-3 above a minimum of alisation (Strzepek et al., 2005). Here, we follow a similar 0.2nmolL−1 at 100m. This feature was absent at sta- approach to that used by Bowie et al. (2009) for the SAZ- tion E-5, where dFe was depleted to <0.1nmolL−1 SensestudysouthofTasmania(Australia)atourthreestudy at the surface, likely due to biological Fe uptake, sites. Since all parameters in our iron budget calculations which was highest at E-5 (1745nmolm−2d−1) com- were only measured at stations R-2, A3-2 and E-5, discus- pared to A3-2 (1120nmolm−2d−1) (Table 1) and E-4E sionwillfocusonthesestations.DataforstationsA3-1,E-1 (880nmolm−2d−1;datanotshown),despitelowerPOCand andE-3aregiventoprovideacontextforspatialandtempo- primary production (see discussion below and Fourquez et ralchangesintheFepoolsandfluxesduringKEOPS-2,and al., 2015). Deeper in the water column (>500m) at E sta- theyarecollatedinTable1. tions,dFewasbroadlyuniform(0.3–0.5nmolL−1). The pFe distributions at the three E stations were simi- 3.2.1 Ironpools larwithasurface(35–40m)enrichment(1.6–1.9nmolL−1), a minimum at ∼100–200m below the mixed layer (0.7– Ironandcarbonpoolswerecalculatedbyintegratingthedis- 0.9nmolL−1;broadlyconsistentwiththeT layer),amax- solvedandparticulateprofilesdowntothebaseofthesurface min imum at 280–600m (1.7–2.4nmolL−1), and with evidence mixedlayer,definedasthedepthwherethepotentialdensity of enrichment near the seafloor at depths >1800m (up equalledthepotentialdensityat10m+0.02kgm−3(Parket to 1.5–2.3nmolL−1). By applying biogenic (using P) and al.,2014a).Themixedlayervariedfrom165matstationA3- lithogenic (using Al) normalisers to the data (see Sect. 3.2 1to32matstationE-3,consistentwiththeseasonalshoaling below), surface pFe enrichment was roughly equally com- assurfacewaterswarmed,butitremaineddeep(>120m)on posed of biogenic and lithogenic Fe, whilst the 300–600m theplateauthroughoutthestudyduetodeepmixingasare- maximum was predominantly composed of lithogenic Fe sultofseveralpassingstorms. (>100-fold greater than biogenic Fe at these depths). This Integratedpoolsofbothdissolved(∼5×)andparticulate lithogenic Fe was most likely from waters enriched by sed- (∼10×) Fe were significantly greater on the plateau (sta- iments and transported laterally eastward off the Kerguelen tion A3) compared to in the plume (station E), with stocks Plateau which sits at ∼530m below the sea surface. There at the reference station R-2 lower still. Horizontal dFe sup- wasnoobviouschangeinpFeinsurfaceordeepwatersdur- ply from the plateau to the plume was either or both via (i) ingthebloomevolutionatthepseudo-LagrangianEstations. ageostrophicpathloopingalongthenorthernsideofthePF KEOPS-1 only occupied one station in the plume east andthenbackintotherecirculationfeature(d’Ovidioetal., of the Kerguelen Islands (A11 at 49◦09(cid:48)S 74◦00(cid:48)E). Dis- 2015)and(ii)directEkmanfluxtransportofFe-richcoastal solved Fe at A11 ranged from 0.09nmolL−1 at the surface water across the PF driven by westerly winds, as indicated to 0.17nmolL−1 at 1500m (Blain et al., 2008b), and pFe byRatracers(Sanialetal.,2015).Thelatterprocessissup- ranged from 0.07nmolL−1 at the surface to 0.81nmolL−1 ported by Lagrangian trajectories of water parcels derived at 1500m (Andrew Bowie, unpublished data); thus, it was fromaltimetry,whichshowedthePFwasnotastrongbarrier much lower than our KEOPS-2 observations at the E site towatermassmovement,withtransportofwatersacrossthe (notingthatdifferentsamplinganddigestionmethodsforpFe front taking place on timescales of days to weeks but being wereusedforthetwocruises). highlyvariableinspaceandtime(d’Ovidioetal.,2015).The pFepoolshowedthesamevariabilityasthedissolvedpoolat 3.2 Constructionofironbudgets ourthreestudysitesandexceededthedFestocksatallsites byfactorsofapproximately19–26(A3),31(E)and6(R-2), Theprimaryaimofthisworkwastouseourobservationsof althoughitisestimatedthatonly∼2–3%oftheparticulate Fepoolsandfluxestounderstandthesources,sinksandbi- poolcanbeconvertedintobioavailableformsbyphysically ologicalFecyclingandtoevaluatewhetherFesupplycould or biologically mediated dissolution (Schroth et al., 2009). meet demand in both the high-Fe and low-Fe environments IfweassumethatstationA3-1representedpre-bloomcondi- inthevicinityoftheKerguelenArchipelagoduringKEOPS- tionsandtheintegratedmixedlayerpoolof54µmoldFem−2 2.Ironbudgetshavebeenconstructedforpreviousstudiesin was a good estimate of the winter stock, observations show watersfertilisedwithFebothnaturally(Sarthouetal.,2008; that only 4 weeks later at station A3-2, almost 60% of the Bowieetal.,2009;Cheveretal.,2010;Ellwoodetal.,2014) winter stock had been drawn down to 21µmolm−2. If an- and artificially (Bowie et al., 2001) as well as low-Fe con- nual variability is low, which may not always be the case ditions (Price and Morel, 1998; Boyd et al., 2005). These (Grenier et al., 2015), by late summer >90% of the winter budgets have combined geochemical and chemical compo- stockhadbeenusedwithonly4.7µmoldFem−2 remaining nents to demonstrate that the dominant long-term fluxes of in the surface mixed layer at A3 (KEOPS-1 data; Blain et Fe are associated with the particulate pool (dust supply and al., 2007). We note that this drawdown is probably a con- Biogeosciences,12,4421–4445,2015 www.biogeosciences.net/12/4421/2015/

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A. R. Bowie1,2,3, P. van der Merwe1, F. Quéroué1,2,3, T. Trull1,4, M. Fourquez2,5, when the plants die and sink, or indirectly through the food web.
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Preview Iron budgets for three distinct biogeochemical sites around the Kerguelen Archipelago

Biogeosciences,12,4421–4445,2015 www.biogeosciences.net/12/4421/2015/ doi:10.5194/bg-12-4421-2015 ©Author(s)2015.CCAttribution3.0License. Iron budgets for three distinct biogeochemical sites around the Kerguelen Archipelago (Southern Ocean) during the natural fertilisation study, KEOPS-2 A.R.Bowie1,2,3,P.vanderMerwe1,F.Quéroué1,2,3,T.Trull1,4,M.Fourquez2,5,F.Planchon3,G.Sarthou3, F.Chever3,a,A.T.Townsend6,I.Obernosterer5,J.-B.Sallée7,8,9,andS.Blain5 1AntarcticClimateandEcosystemsCooperativeResearchCentre(ACECRC),PrivateBag80,Hobart,Tasmania7001, Australia 2InstituteforMarineandAntarcticStudies(IMAS),UniversityofTasmania,PrivateBag129,Hobart,Tasmania7001, Australia 3LaboratoiredesSciencesdel’EnvironnementMarin(LEMAR),UMR6539UBO/CNRS/IRD/IFREMER,Institut UniversitaireEuropéendelaMer(IUEM),TechnopoleBrestIroise,29280Plouzané,France 4CSIROMarineandAtmosphericResearch,CastrayEsplanade,Hobart,Tasmania7000,Australia 5UniversitéPierreetMarieCurie,Laboratoired’OcéanographieMicrobienne(LOMIC),UMR7621CNRSUPMC,Avenue duFontaulé,66650Banyulssurmer,France 6CentralScienceLaboratory(CSL),UniversityofTasmania,PrivateBag74,Hobart,Tasmania7001,Australia 7SorbonneUniversités,UPMCUniv.,Paris06,UMR7159,LOCEAN-IPSL,75005Paris,France 8CNRS,UMR7159,LOCEAN-IPSL,75005Paris,France 9BritishAntarcticSurvey,HighCross,CambridgeCB30ET,UK anowat:NationalOceanographyCentre,UniversityofSouthamptonWaterfrontCampus,EuropeanWay, SouthamptonSO143ZH,UK Correspondenceto:A.R.Bowie([email protected]) Received:19November2014–PublishedinBiogeosciencesDiscuss.:19December2014 Revised:22June2015–Accepted:24June2015–Published:29July2015 Abstract. Iron availability in the Southern Ocean controls waysareresponsiblefordifferencesinthemodeandstrength phytoplankton growth, community composition and the up- ofironsupply,withverticalsupplydominantontheplateau take of atmospheric CO by the biological pump. The and lateral supply dominant in the plume. Iron supply from 2 KEOPS-2 (KErguelen Ocean and Plateau compared Study “new”sources(diffusion,upwelling,entrainment,lateralad- 2)“processstudy”,tookplacearoundtheKerguelenPlateau vection,atmosphericdust)tothesurfacewatersoftheplume in the Indian sector of the Southern Ocean. This is a re- wasdoublethatabovetheplateauand20timesgreaterthan gion naturally fertilised with iron on the scale of hundreds atthereferencesite,whilstirondemand(measuredbycellu- to thousands of square kilometres, producing a mosaic of laruptake)intheplumewassimilartothatabovetheplateau spring blooms which show distinct biological and biogeo- but 40 times greater than at the reference site. “Recycled” chemical responses to fertilisation. This paper presents bio- ironsupplybybacterialregenerationandzooplanktongraz- geochemicalironbudgets(incorporatingverticalandlateral ing was a relatively minor component at all sites (<8% of supply,internalcycling,andsinks)forthreecontrastingsites: new supply), in contrast to earlier findings from other bio- an upstream high-nutrient low-chlorophyll reference, over geochemical iron budgets in the Southern Ocean. Over the the plateau and in the offshore plume east of the Kergue- plateau, a particulate iron dissolution term of 2.5% was in- len Islands. These budgets show that distinct regional envi- vokedtobalancethebudget;thisapproximatelydoubledthe ronments driven by complex circulation and transport path- standingstockofdissolvedironinthemixedlayer.Theex- PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion. 4422 A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 change of iron between dissolved, biogenic particulate and phytoplanktonbiomassduringsummer,withchlorophylllev- lithogenicparticulatepoolswashighlydynamicintimeand els increasing to more than 1 order of magnitude above the space,resultinginadecouplingoftheironsupplyandcarbon background, as revealed by NASA MODIS satellite chloro- export and, importantly, controlling the efficiency of fertili- phyllclimatologyforJanuary(2003–2010)(Westberryetal., sation. 2013).Previousresearchonbloomsintheselocalised“natu- rallaboratories”hasprovidedinvaluableinsightsintomech- anisms linking iron fertilisation and carbon cycling in the 1 Introduction Southern Ocean, especially since studies of natural systems canaddresstheeffectsofpersistent,varyingandmultipleFe The concentration of carbon dioxide in earth’s atmosphere, sources that are not accessible through deliberate artificial and therefore earth’s climate, is highly sensitive to modifi- mesoscalefertilisationexperiments. cation in the marine carbon (C) cycle due to the growth of The KEOPS-1 (KErguelen: Ocean and Plateau compared phytoplankton in the Southern Ocean (Sarmiento and Gru- Study1)project,whichtookplaceinthelateaustralsummer ber,2006).Thesesingle-cellplantsremoveinorganiccarbon ofJanuary–February2005,demonstratedthatthisnaturalfer- from surface seawater during photosynthesis, and this inor- tilisationoftheSouthernOceanresultedindramaticchanges ganic carbon can be directly transferred into the deep sea in the functioning of the ecosystem with large impacts on whentheplantsdieandsink,orindirectlythroughthefood marine biogeochemical cycles (Blain et al., 2007, 2008a). web. The Southern Ocean is responsible for 30% of global These observations of the bloom were largely confined to oceancarbonexport(Schlitzer,2002).Asfirstdemonstrated theplateauregion,whereverticalupwelledsupplyfromthe over 20 years ago, phytoplankton growth in the Southern plateausediments(Blainetal.,2008b;Zhouetal.,2014)and Oceanislimitedbytheavailabilityofthemicronutrienttrace lateral advection of water that had been in contact with the element iron (Fe; Martin, 1990). Low dissolved iron (dFe) continentalshelfofHeardIslandtothesouth(Cheveretal., availability limits the annual uptake of atmospheric carbon 2010) were the dominant sources of dissolved and particu- dioxide (CO ) by the Southern Ocean (Boyd et al., 2000), lateFe(asconfirmedusingrareearthelement(REE)andra- 2 shapes phytoplankton species composition and physiology dium (Ra) isotope tracers; van Beek et al., 2008; Zhang et (Assmy et al., 2013), the cycling of other nutrient elements al., 2008). The interaction of waters, islands and plateau of (MooreandDoney,2007),andthusthestructureoftheentire the Kerguelen Archipelago with several circumpolar fronts marineecosystem(BoydandEllwood,2010). oftheSouthernOceanallowedustomakeafirstattemptat Artificial mesoscale ocean iron fertilisation experiments placingourregionalKEOPS-1observationswithinabroader have unequivocally demonstrated the role of Fe in setting basin-scalecontext(Blainetal.,2007). phytoplankton productivity, biomass and community struc- The KEOPS-2 project was designed to improve the spa- ture in high-nutrient low-chlorophyll (HNLC) regions (de tial and temporal coverage of the Kerguelen region. During Baar et al., 2005; Boyd et al., 2007). However, the “carbon KEOPS-2, which was approved as a GEOTRACES process sequestration efficiency” of ocean fertilisation as a means study1, we studied the region above and downstream of the to sequester atmospheric CO (calculated as the additional plateau and observed a massive natural iron fertilisation on 2 (net) C that is exported from surface waters into the deep thescaleofhundredsofthousandsofsquarekilometres.This (>1000m) ocean for a given addition of Fe) varies widely producedapatchworkofbloomswithdiversebiologicaland betweenexperimentsandisconsiderablylessthanestimates biogeochemical responses, as detailed in the multiple stud- from the early iron fertilisation experiments (see discussion iesinthisspecialissueofBiogeosciences(volumes11–12). in de Baar et al., 2008). This is due to a number of factors, KEOPS-2 was also carried out in the austral spring to doc- including rapid grazing of phytoplankton in surface waters, ument the early stages of the bloom and to complement the thelossofaddedFebyitsprecipitationandscavengingonto resultsofKEOPS-1obtainedinlatesummerduringthestart sinking particles, differences in estimated or assumed iron- of the decline of the bloom, with a principal aim to better to-carbon (Fe/C) ratios of the cells, and changes in wind constrainthemechanismofFesupplytosurfacewatersear- mixedlayerdepth. lierintheseason. The natural resupply of iron to Fe-depleted waters is a SinceFeisactivelytakenupintophytoplanktonandtrans- more efficient process (Blain et al., 2007), although in part ferredthroughoutthefoodweb,includingremovalbyparti- this depends on the mode of Fe delivery (e.g. from above, cle settling and remineralisation in deep waters, the assess- laterally or from below) and on the ability of organic lig- mentofitsavailabilityisquitecomplexandcannotbejudged ands to keep the supplied Fe in solution (Gerringa et al., from dFe levels in surface waters alone (Breitbarth et al., 2008), and for continued ocean fertilisation, it is in part re- 2010). Advances in chemical oceanographic techniques for liant on the concurrent supply of other major nutrients. In trace elements through the GEOTRACES program (SCOR the Indian sector of the subantarctic Southern Ocean, natu- ralFesupplyfromtheKerguelenPlateau(Blainetal.,2007) 1http://www.geotraces.org/cruises/cruise-summary/68-science/ andCrozetIslands(Pollardetal.,2009)resultsinincreased process-studies/206-geotraces-process-studies Biogeosciences,12,4421–4445,2015 www.biogeosciences.net/12/4421/2015/ A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 4423 Working Group, 2007) now allow the measurement of Fe subjected to natural Fe fertilisation (e.g. Frew et al., 2006 associated with different phases (dissolved and particulate), andBoydetal.,2005forFeCycle-I,andEllwoodetal.,2014 internalbiologicalrecyclingandFeexportfromsurfacewa- for FeCycle-II east of New Zealand; Bowie et al., 2009 for ters.Theresultsfromearlierironbiogeochemicalbudgetsfor SAZ-Sense south of Tasmania; Planquette et al., 2011 for FeCycle-I (Boyd et al., 2005; Frew et al., 2006), KEOPS-1 CROZEXneartheCrozetIslands;andZhouetal.,2010for (Blainetal.,2007;Cheveretal.,2010),CROZEX(CROZet BlueWaterZonenearthewesternAntarcticPeninsula).The naturalironbloomandEXportexperiment;Planquetteetal., observations of dFe (Quéroué et al., 2015) and particulate 2007, 2009) and SAZ-Sense (Sensitivity of the subantarctic tracemetals(vanderMerweetal.,2015)aredetailedincom- zonetoenvironmentalchange;Bowieetal.,2009)havehigh- panionpapersinthisspecialissuetoallowthecurrentpaper lightedthatthedominant“new”Fefluxesareassociatedwith tofocusexplicitlyontheconstructionofironbudgets;how- the particulate phase. Particles thus represent an important ever,thethreepapersshouldbeseenasacollectivewhole. transport vector for trace metals in the marine ecosystem, although their bioavailability or transfer into a bioavailable 2 Materialandmethods fraction remains uncertain. Suspended particles have also been shown to be important aspects of sedimentary, bound- 2.1 Studyarea ary layer Fe sources and export processes (Tagliabue et al. 2009; Homoky et al., 2013; Marsay et al., 2014; Wadley et The KEOPS-2 (KErguelen Ocean and Plateau in compared al.,2014),withparticlesbeingtransportedlaterallyoverhun- Study 2) expedition was carried out in the Indian sector dredsofkilometresintheocean(Lametal.,2006;Lamand of the Southern Ocean in the vicinity of the Kerguelen Bishop,2008).ThebiologicalcyclingofparticulateFemay Plateau between 7 October and 30 November 2011 on the therefore be the most important aspect of the complete Fe RV Marion Dufresne (Fig. 1a). The plateau of the Kergue- biogeochemical cycle, especially since earlier budgets have lenArchipelagoisanorthwest–southeastseafloorfeatureap- demonstratedthatbiologicalFe“demand”cannotbesatisfied proximately 500m deep and is constrained by the Kergue- bythenewFesupply(Boydetal.,2005;Blainetal.,2007; len Islands to the north and the smaller volcanic Heard and Sarthouetal.,2008;Bowieetal.,2009;deJongetal.,2012). McDonaldislandstothesouth.Ourstudywasconductedin Asimpleone-dimensionalverticalmodelthatcorrectlyrep- earlyaustralspringwhenphytoplanktonbiomasswasdevel- resentedtheinputofdFetosurfacewatersduringKEOPS-1 opingrapidlyandformingamosaicofphytoplanktonblooms did not accurately represent the supply of other geochemi- intheregion(Trulletal.,2015;Lasbleizetal.,2014).Since cal tracers or particulate Fe (Blain et al., 2007; van Beek et samplingatthedifferentstationstookplaceatdifferenttimes al., 2008; Zhang et al., 2008), and the role of dissolved and over the ∼7-week study, our observations also provide a particulateFeearlierintheseason(winterstock)intheKer- temporal sequence relative to the development of surface guelenregionhasyettobequantified. biomass. Thispaperpresentsashort-term(daystoweeks)Febudget The Kerguelen bloom has two main features: a north- fortheperiodofKEOPS-2foreachofthreeprocesssites:(i) ern branch that extends northeast of the island into waters a“plateau”bloomsite(A3)onthecentralKerguelenPlateau both south and north of the PF and a larger bloom cover- studiedduringlatesummeronKEOPS-1andreoccupieddur- ing∼45000km2 southofthePFandlargelyconstrainedto ing spring on KEOPS-2; (ii) a “plume” bloom site (E) east the shallow bathymetry of the Kerguelen Plateau (<700m) oftheKerguelenIslands,whichwaslocatedwithinaquasi- (Mongin et al., 2008; Supplement in Trull et al., 2015) stationary,bathymetricallytrappedrecirculationfeaturenear (Figs. 1b and 2). Thirty-two stations were sampled during the polar front (PF); and (iii) a “reference” site (R-2) south KEOPS-2, often with repeat visits. Here, we focus on three ofthePFandupstream(southwest)oftheKerguelenIslands study sites, namely plateau A3, plume E and reference R-2 inHNLCwaters.Wefocusonmixed-layerintegratedpools (Fig.1).TwovisitsweremadetoA3atthestart(A3-1)and of dissolved Fe and particulate Fe (which we further sepa- end(A3-2)ofthevoyage(28daysapart),andfivevisitswere rate into biogenic and lithogenic fractions using elemental madetositeE(over21days)todocumentthebloomdevel- normalisers), estimate the fluxes of Fe associated with new opment.Basedonthetrajectoriesofsurfacedrifters,stations andrecycledFesources,andcompareFesupplyanddemand E-1,E-3andE-5weretakenastrackingthemiddleofare- with implications for bloom duration and magnitude. Our circulation region (d’Ovidio et al., 2015), so that they can observations also include particulate measurements in both beconsideredaspseudo-Lagrangianandtheirsuccessionin suspended-water-column (in situ pump; ISP) and sinking- timecanbeconsideredafirst-ordertimeseries.Fulldetailsof export(free-floatingsedimenttrap;“P-trap”)particlesbelow otherstationsandsamplingdesignedtodocumentthemerid- the mixed layer, with linkage to food web processes via a ionalandzonalextensionsofthebloomsontheplateauand discussion of Fe/C ratios. Finally, we present a seasonal totheeastoftheKerguelenIslandsarecontainedincompan- comparisonofourspringtimebudgetforKEOPS-2withlate ionpapersinthisspecialissueofBiogeosciences. summerobservationsfromKEOPS-1andalsomakecompar- isonwithfindingsfromothersectorsoftheSouthernOcean www.biogeosciences.net/12/4421/2015/ Biogeosciences,12,4421–4445,2015 4424 A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2   (a) Southern Ocean Leclaire Rise ‐2 (b) Figure1.(a)ThelocationofKEOPS-2intheIndiansectoroftheSouthernOcean,showingbathymetryaroundtheKerguelenArchipelago. ◦ (cid:48) ◦ (cid:48) ◦ (cid:48) Ourbiogeochemicalironbudgetsfocusonthreeprocessstations(openblackcircles):referenceR-2(50 2 S,66 4 E),plateauA3(50 4 S, ◦ (cid:48) ◦ (cid:48) ◦ (cid:48) 72 0 E)andplumeE(48 3 S,72 1 E).Blackdotsmarkthepositionsoftheotherstationsvisited,includingN–SandE–Wsurveytransects at the start of the KEOPS-2 expedition. (b) A schematic of the mean regional circulation of surface and subsurface waters around the KerguelenArchipelago,indicatingcircumpolarSouthernOceanfronts,locationsofstationsalongN–SandE–Wtransects,andpathways and origins of different water masses flowing on the plateau and offshore into the plume. The abbreviations are Antarctic Surface Water (AASW),PolarFrontSurfaceWater(PFSW),SubantarcticSurfaceWater(SASW),SubtropicalSurfaceWater(STSW),SubantarcticFront (SAF)andthepolarfront(PF)(reproducedwithpermissionfromParketal.(2014a),courtesyofIsabelleDurandandYoung-HyangPark, LOCEAN/DMPA,MNHN,Paris). (a) (b) (c) Figure 2. MODIS ocean-colour satellite images showing the development of the plateau and plume blooms during KEOPS-2. Surface chlorophyll(µgL−1)biomassisshownforthenearestclearskydaytothefinalsamplingdayatstationsR-2(a),A3-2(b)andE-5(c).The PFisshownasablackdashedlinein(b)and(c).Trulletal.(2015)discussthetimingofthestationsrelativetobloomdevelopment. ThehydrologyandcirculationaroundandabovetheKer- Zhouetal.(2014).ThemeancirculationisshowninFig.1b. guelen Plateau have been described by Park et al. (2008a, Briefly, the Kerguelen Plateau constitutes a barrier to the b, 2014a), van Beek et al. (2008), Zhang et al. (2008) and eastwardflowingAntarcticCircumpolarCurrent(ACC),the Biogeosciences,12,4421–4445,2015 www.biogeosciences.net/12/4421/2015/ A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 4425 mainjetsofwhicharetheSubantarcticFront(SAF)andPF. were rinsed liberally with sample before final collection in MostoftheACCisdeflectednorthoftheKerguelenIslands 125mLNalgeneLDPEbottles.Seawatersampleswereacid- as Subantarctic Surface Water (SASW) but some filaments ifiedwithin24hofcollectionusing2mLofconcentratedul- passbetweentheKerguelenIslandsandHeardIsland(asthe trapure hydrochloric acid (HCl, Seastar BASELINE grade) PF) and further south between Heard Island and Antarctica per litre of sample, resulting in an approximate final pH of (Roquetetal.,2009).Abovetheplateau,theremainderofthe 1.8, double bagged and stored for at least 24h at ambient ACCcomesfromthewesternpartoftheplateau.Currentsof temperatureuntilanalysis. AASW travelling along the western flank of the plateau are deflected south and east of Heard Island as a branch of the 2.2.2 Insitupumps(ISPs) Fawn Trough Current (FTC) (Sokolov and Rintoul, 2009) before travelling in a broadly northwest direction up along Suspended particles for trace elemental analysis were col- the eastern shelf break. The water flow is then deflected to- lected using 11 large-volume in situ pumps (McLane Re- ward the east of the Kerguelen Islands, where there is an searchLaboratoriesWTS6-1-142LVandChallengerOcean- intense mixing zone consisting of mesoscale eddies which ics pumps), suspended simultaneously at prechosen depths travelmanythousandsofkilometresintheACCtowardsthe following methods reported in Bowie et al. (2009). Up to AustraliansectoroftheSouthernOcean. 2000L of seawater was filtered across a 142mm diameter stack (134mm diameter active area) consisting of a 53µm 2.2 Sampling nylonpre-filterscreen(NYTEX)followedbyaQMAquartz fibrefilter(1µmnominalporesize;Sartorius).TheQMAfil- Alltracemetalsamplingandanalyticalproceduresfollowed ter was supported by a 350µm polyester mesh, which was recommended protocols in the cookbook2 published by the placed on top of the Teflon PFA grid of the pump housing. international program GEOTRACES (Bishop et al., 2012; Prior to use, NYTEX screens were conditioned by soaking Cutter and Bruland, 2012; Planquette and Sherrell, 2012). in5%H SO ,rinsed3×withMilli-Q-gradewater,driedat 2 4 All methodshavebeen successfully usedpreviously by this ambient temperature under a laminar flow hood and stored team during the KEOPS-1 (Blain et al., 2008b) and SAZ- incleanplasticZiploc® bags.QMAfilterswereconditioned Sense projects (Bowie et al., 2009). Subtle differences in for trace metal analysis (precombustion and acid cleaning) methods employed during the earlier KEOPS-1 and SAZ- followingBowieetal.(2010).Uponrecoveryofthepumps, Sense projects are described in those papers and/or later in subsamplesweretakenfromtheQMAfiltersbyusingacir- thismanuscript. cularplasticpunch(14mmdiameter)andbycuttingtheny- lon mesh using ceramic scissors. Filters were dried under 2.2.1 Tracemetalrosette(TMR) a laminar flow bench and stored at −18◦C in acid-washed PCRtraysuntilfurtheranalysisinthehomelaboratory.The Water column samples were collected using 10L externally 1–53and>53µmsizefractionsweredigestedandanalysed closing, Teflon-lined Niskin-1010X bottles deployed on an separately,andtheparticulateiron(pFe)reportedhereisthe autonomous 1018 intelligent rosette system (TMR – trace sumofbothfractions.TheISPswereshowntobeefficientin metalrosette,speciallyadaptedfortracemetalwork;General capturinglarge(>53µm)particles(Planchonetal.,2015). OceanicsInc.).Thepolyurethane-powder-coatedaluminium rosette frame was suspended on Kevlar rope which passed 2.2.3 Free-floatingtraps(P-trap) throughacleanblockwithaplasticsheave(GeneralOcean- ics)andwasloweredtoamaximumdepthof1300m.Bottles Sinking particles for trace elemental analysis were col- weretrippedatpreprogrammeddepthsusingapressuresen- lectedusingPPS3/3free-floatingsedimenttraps(Technicap, sor as the TMR was being raised through the water column atapproximately0.5ms−1. France), specially adapted for trace metals and deployed at 200m. Traps were deployed for 5.3, 5.1, 1.9 and 1.5 days AllsampleprocessingwascarriedoutunderanISOclass- at stations E-1, E-3, A3-2 and E-5, respectively. The trap 5 trace metal clean laminar flow bench in a HEPA filtered- deployed at station R-2 was lost and not recovered. Traps air clean container, with all materials used for sample han- drifted between 10 and 43km over the course of the de- dlingthoroughlyacid-washed.Samplesweredrawnthrough ployment. Full details of the trap deployments are given in C-Flex tubing (Cole Parmer) and filtered in-line through Laurenceau-Cornecetal.(2015)andPlanchonetal.(2015). 0.2µm pore size acid-washed capsules (Pall Supor mem- Samplesfortraceelementalanalysiswerecollectedinthree brane Acropak 200 or Sartorius Sartobran 300 filters). The separateacid-washedcups(specificallyfortracemetals)con- dissolvedfractionisthuslikelytocontaincolloidsandsmall tainingalowtracemetalbrinesolution(salinity∼60),each particles <0.2µm in diameter (Bowie and Lohan, 2009). opened for either 1, 3, 8 or 12h (depending on the station). All transfer tubes, filtering devices and sample containers Uponrecovery,cupsweretakentoacleanroomandparticles 2http://www.geotraces.org/libraries/documents/Intercalibration/ filteredoff-lineontoa47mmdiameter,2µmporositypoly- Cookbook.pdf carbonatefilterundergentlevacuumusingaTeflonPFAunit www.biogeosciences.net/12/4421/2015/ Biogeosciences,12,4421–4445,2015 4426 A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 (SavillexCorp.,USA)equippedwitha350µmpre-screen(to for the pump samples were estimated from filters prepared excludezooplankton). identically but not deployed on the ISPs; for the trap sam- ples this was done by re-filtering the pre-filtered seawater. 2.3 Analysis Allblankcorrectionswerelessthan2%forallsamples.The subsampling introduces uncertainties of 5–10% from inho- 2.3.1 Dissolvediron mogeneousfiltercoveragethatexceedstheanalyticaluncer- taintyof∼1%inthePOCanalysis(Trulletal.,2015). Dissolved Fe (dFe) was determined shipboard by flow in- jection analysis with chemiluminescence detection (FI-CL) 2.4 Biologicalironcycling using in-line preconcentration on an 8-hydroxyquinoline chelating resin (adapted from Obata et al., 1993, de Jong et 2.4.1 Ironuptake al., 1998 and Sarthou et al., 2003). Dissolved Fe data were quality controlled against the SAFe (Sampling and Analy- Trace metal clean seawater was collected from the mixed sisofFe)standardreferencematerials(Johnsonetal.,2007). layer (20–40m) using the TMR, was transferred into acid- Fulldataincludingcertificationresultsandanalyticalfigures washed polycarbonate bottles and 0.2nmolL−1 (final con- ofmeritarereportedinQuérouéetal.(2015). centration)ofenriched55FeasFeCl wasadded(1.83×103 3 Cimol−1 of specific activity, Perkin Elmer). Bottles were 2.3.2 Particulateiron placed at in situ temperature in on-deck incubators contin- uouslyfedbysurfaceseawater.Incubationswereconducted Particulate Fe (pFe) was determined as follows. Sampled for 24h (sunrise to sunrise) at several light intensity levels particles were acid extracted in 1mL concentrated HNO 3 (75,45,25,16,4and1%ofphotosyntheticallyactiveradi- (Seastar Baseline) for 12h on a DigiPREP HP Teflon hot- ation; PAR). For stations R-2, A3-1, E-1 and E-3, seawater plate supplied with HEPA-filtered air (SCP Science) at 120◦C using 15mL Teflon PFA Savillex vials. Digest so- wasprefilteredona25µmmeshsizebefore55Fewasadded. After incubation, 300mL of seawater was passed through lutions were diluted with 10mL ultra high-purity water to 0.2µm pore size nitrocellulose filters (47mm diameter, Nu- 10%HNO andspikedwith10ppbindiumasinternalstan- 3 clepore). To determine intracellular Fe uptake rates, 55Fe dard prior to analysis by sector field inductively coupled notincorporatedbycellswasremovedimmediatelyafterfil- plasmamassspectrometry(FinniganELEMENT2,Thermo tration using 6mL of a Ti-citrate-EDTA washing solution Scientific),followingBowieetal.(2010).Blanksfromrepli- for 2min, then rinsed three times with 5mL of 0.2µm fil- cate analysis of filters treated identically to the sample fil- tered seawater for 1min (Hudson and Morel, 1989; Tang ters but without large volumes of seawater passed through and Morel, 2006). The filters were placed into plastic vials them,weretypically2–3%and<1%ofthepFesamplecon- and10mLofthescintillationcocktail“Filtercount”(Perkin centrationsfortheISPdeploymentsandP-trapdeployments, Elmer) added. Vials were agitated for 24h before the ra- respectively. Recoveries from the analysis of the Commu- dioactivityonfilterswascountedwiththeTricarb®scintilla- nityBureauofReferenceplanktoncertifiedreferencemate- rialBCR-414wereexcellent,witha101%recovery(n=3) tioncounter(precision<10%).Controlswereobtainedwith 300mL of microwave-sterilised seawater (750W for 5min) forpFe.FulldataarereportedinvanderMerweetal.(2015). incubatedandtreatedthesameway.Subsamplesforenumer- 2.3.3 Particulateorganiccarbonandnitrogen ationbyflowcytometrywerecollectedfromeachbottlejust beforethefiltrationstep.Cellswerefixedinglutaraldehyde For particulate organic carbon (POC) and particulate nitro- (1%) and kept frozen (−80◦C) until processing and analy- gen (PN) analyses, QMA quartz filters from the ISPs were sis.DatawerecorrectedbyblanksubtractionandFeuptake subsampledinaflowbenchusinga14mmdiameterplastic rates normalised to the concentration of Fe in each incuba- punchandtransferredtosilverfoilcups(Serconbrandp/n tion(insitudFeand55Feadded).Furtherdetailsaregivenin SC0037). Samples were also collected from the P-traps for Fourquezetal.(2015). POCandPNanalyses(seeLaurenceau-Cornecetal.,2015). Samplesweretreatedwitha40µLaliquotof2NHCltore- 2.4.2 Ironremineralisation movecarbonates(Kingetal.,1998),driedat60◦Cfor48h and stored in a desiccator until analysis using a Thermo- Sinceironregenerationwasnotmeasureddirectlybyexper- Finnigan Flash EA1112 elemental analyzer (using sulfanil- iment during KEOPS-2, we used the following approach to amidestandards)attheCentralScienceLaboratory,Univer- calculate iron regeneration fluxes. Bacterial Fe regeneration sity of Tasmania. The >53µm fraction was treated in the wasestimatedfrombacterialturnovertimesdeterminedfrom same way at the Vrije Universiteit Brussel after first trans- bacterialproductionandbiomass(Christakietal.,2014),as- ferringthematerialfromonefourthofthescreen,usingpre- suming all loss of bacterial biomass through viral lysis and filteredseawater,onto25mmdiameter,1.0µmporesizesil- flagellategrazingresultedintheregenerationofFe(Strzepek vermembranefilters(Sterlitech,Concord).Blankcorrections et al., 2005) and using a bacterial iron quota of 7.5µmol Biogeosciences,12,4421–4445,2015 www.biogeosciences.net/12/4421/2015/ A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 4427 Fe (mol C)−1 (Tortell et al. 1996). The mesozooplankton and particulate (Mn, Al; van der Merwe et al., 2015) trace grazing contribution to Fe regeneration was assumed to be elements. equaltotheexperimentallydeterminedFeregenerationdur- The dFe profile at the KEOPS-2 reference station R-2 is ing KEOPS-1 (Sarthou et al., 2008). The regeneration rates similar to the KEOPS-1 reference station C11 (with the ex- per mesozooplankton individual determined in Sarthou et ceptionoftheR-2enrichmentinthe200–700mdepthstrata; al. (2008), were then multiplied by mesozooplankton abun- Fig.4a),butitshouldbenotedthatthelocationofC11was dance,calculatedfromthenumberofcellscapturedinadaily quitedifferent–inHNLCwaterstothesoutheastoftheKer- haulover200mduringKEOPS-2(Carlottietal.,2015;val- guelenPlateau(51◦39’S,78◦00’E)–andwehadonly1dFe uesreportedinTable6inLaurenceau-Cornecetal.,2015). data point in UCDW at C11. In contrast to the similarity of thedFeprofiles,thepFeprofileatC11wasgenerallylower than at R-2, with mean values through the water column 3 Resultsanddiscussion of 0.2±0.14nmolL−1 (Andrew Bowie, unpublished data) comparedto0.53±0.35nmolL−1forstationR-2. 3.1 Biogeochemicalsettingsatourthreestudysites 3.1.2 PlateaustationA3 Full descriptions of the dFe and pFe distributions can be found in Quéroué et al. (2015) and van der Merwe et al. StationsA3-1(Fig.3b)andA3-2(Fig.3c)wereinrelatively (2015), respectively, with further presentation of the distri- shallow waters on the central plateau, and were impacted butions of other micronutrient trace elements (Mn, Co, Ni, byplateausedimentsandpossiblyfluvialandglacialrunoff Cu,Cd,Pb)fromKEOPS-2tobepresentedelsewhere.How- from the basaltic rocks of Heard Island ∼300km upstream ever, briefly our subset of stations used for the iron budgets (vanderMerweetal.,2015;M.Grenier,personalcommuni- canbedescribedasfollows. cation,2014).Apycnoclinewasobservedat∼190m,above which the salinity (33.9) and nitrate (∼29µmolL−1) were 3.1.1 ReferencestationR-2 relatively constant. The mixed layer shoaled (from 165 to 123m)andincreasedintemperature(from1.7to2.2◦C)be- Intheupper100m,weobservedasalinityminimum(33.8) tweenthetwovisitstoA3,consistentwithspringtimewarm- andtemperaturemaximum(2.2◦C)characteristicofAntarc- ing of surface waters. We believe that the water masses at tic Surface Water (AASW) overlying a layer of winter wa- A3-1 and A3-2 are comparable since surface waters move ter (WW) at 180–200m (T of 1.6◦C) (Fig. 3a). Deeper slowly in this region (Park et al., 2008, 2014a; Zhou et al., min in the water column, a T of 2.5◦C at 500m (associated 2014);thiswasconfirmedbyREEdatawhichindicatedsim- max withanoxygenminimum;notshown)wasindicativeofup- ilarwatersatbothstationsmarkedwithfreshcontinentalsup- per Circumpolar Deep Water (UCDW) overlying a salinity pliesandonlymodifiedbybiologicalprocesses(M.Grenier, maximum of 34.8 at 1830m in lower Circumpolar Deep personalcommunication,2014). Water (LCDW). Phytoplankton abundance was low (0.2µg Surface chlorophyll images revealed that during the 28 Chl aL−1; Lasbleiz et al., 2014) and dominated by di- days between the first and second visits to A3, a large di- atoms,inwaterswithrelativelyhighsurfacenitrateconcen- atom spring bloom developed mostly dominated by lightly trations(>25µmolL−1;Blainetal.,2015),typicalofSouth- silicified Chaetoceros spp. (surface Chl a increasing from ernOceanHNLCconditions(Lasbleizetal.,2014). 0.2µgL−1 at A3-1 to 1.3µgL−1 at A3-2; Lasbleiz et al., Dissolved Fe concentrations were very low at the sur- 2014),whichlikely resultedinthedrawdown ofdFe(mean face (<0.1nmol L−1) and increased with depth, averaging mixedlayervaluesdecreasingfrom0.3–0.4nmolL−1atA3- 0.3nmolL−1inLCDWandbroadlytrackingthenitratepro- 1 to 0.1–0.2nmolL−1 at A3-2). The peak of biomass had file. The pFe profile showed a similar structure to the dFe passedbythetimewesampledatA3-2,withthebloomstart- profile but with surface and deep water concentrations be- ingtofade(Trulletal.,2015).Belowthemixedlayer,simi- tween 0.3 and 1.1nmolL−1 (the deepest sample was 148m lardFeprofileswereobservedduringbothvisitstoA3,with above the seafloor). The exception was at 500m, where, expected significant increases at depth towards the plateau interestingly, we observed a dFe and pFe peak of 0.4 and floor(e.g.to1.30nmolL−1 at480matA3-2;notethat,due 1.6nmolL−1, respectively. Whilst this maximum may have tooperationalconstraints,therewasnodFedatadeeperthan arisenduetotheenrichmentofFeinUCDWdeliveredfrom 340m at A3-1). Such enrichments at depth were also ob- further south, we hypothesise that the Fe supply may have servedindissolvedMnandCoprofiles(F.Quéroué,personal originatedfromsubsurfacesedimentsofthenearbyLeclaire communication,2014;datanotshown)anddFeprofilesfrom Rise (also known as Skiff Bank; Kieffer et al., 2002), a theoccupationsofstationA3duringKEOPS-1(Fig.4b),in- large seamount which rises to 250m at 49◦50’S, 65◦00’E dicativeofplateausedimentarysupply. (approximately 140km northwest of station R-2). Similar The pFe profiles at A3 showed a similar structure to the lithogenicinputswerealsoobservedforotherdissolved(Mn; dFeprofile,withlowervaluesatthesurface(<10nmolL−1 F.Quéroué,personalcommunication,2014,datanotshown) atA3-1and<4nmolL−1atA3-2)andincreasingwithdepth www.biogeosciences.net/12/4421/2015/ Biogeosciences,12,4421–4445,2015 4428 A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 (a) R‐2 (reference) (d) E‐1 (plume) dFe(nmoll‐1) Temperature (oC) Temperature (oC) 0.0 0.2 0.4 0.6 0 1 2 3 0 1 2 3 0 0 0 0 500 500 500 500 1000 1000 1000 1000 1500 1500 1500 1500 2000 2000 Temperature 2000 2000 Temperature dFe Salinity Salinity pFe Nitrate pFe Nitrate 2500 2500 2500 2500 0.0 0.5 1.0 1.5 2.0 2.5 33.8 34.0 34.2 34.4 34.6 34.8 0.0 0.5 1.0 1.5 2.0 2.5 33.8 34.0 34.2 34.4 34.6 34.8 pFe(nmoll‐1) pFe(nmoll‐1) 0 10 20 30 40 0 10 20 30 40 Salinity; nitrate (moll‐1)  Salinity; nitrate (moll‐1)  (b) A3‐1 (plateau) (e) E‐3 (plume) dFe(nmoll‐1) Temperature (oC) dFe(nmoll‐1) Temperature (oC) 0.0 0.5 1.0 1.5 0 1 2 3 0.0 0.2 0.4 0.6 0 1 2 3 0 0 0 0 100 100 500 500 200 200 1000 1000 300 300 1500 1500 400 400 Temperature 2000 2000 Temperature 500 dFe 500 Salinity dFe Salinity pFe Nitrate pFe Nitrate 600 600 2500 2500 0 10 20 30 40 33.8 34.0 34.2 34.4 34.6 34.8 0.0 0.5 1.0 1.5 2.0 2.5 33.8 34.0 34.2 34.4 34.6 34.8 pFe(nmoll‐1) pFe(nmoll‐1) 0 10 20 30 40 0 10 20 30 40 Salinity; nitrate (moll‐1)  Salinity; nitrate (moll‐1)  (c) A3‐2 (plateau) (f) E‐5 (plume) dFe(nmoll‐1) Temperature (oC) dFe(nmoll‐1) Temperature (oC) 0.0 0.5 1.0 1.5 0 1 2 3 0.0 0.2 0.4 0.6 0 1 2 3 0 0 0 0 100 100 500 500 200 200 1000 1000 300 300 1500 1500 400 400 500 dFe 500 TSeamlinpiteyrature 2000 dFe 2000 TSeamlinpiteyrature pFe Nitrate pFe Nitrate 600 600 2500 2500 0 10 20 30 40 33.8 34.0 34.2 34.4 34.6 34.8 0.0 0.5 1.0 1.5 2.0 2.5 33.8 34.0 34.2 34.4 34.6 34.8 pFe(nmoll‐1) pFe(nmoll‐1) 0 10 20 30 40 0 10 20 30 40 Salinity; nitrate (moll‐1)  Salinity; nitrate (moll‐1)  Figure 3. (a) Vertical profiles of dissolved iron (dFe) and particulate iron (pFe), potential temperature, salinity, and nitrate at reference station R-2. The seafloor depth at 2528m is shown. (b, c) Vertical profiles of dFe and pFe, potential temperature, salinity, and nitrate at plateaustationsA3-1(b)andA3-2(c).Theseafloordepthat∼530misshown.NotedifferentscalesfordFeandpFecomparedtoR-2and Estations.(d,e,f)VerticalprofilesofdFeandpFe,potentialtemperature,salinity,andnitrateatplumestationsE1(d),E3(e)andE5(f). Theseafloordepthrangingfrom1905m(E3)to2057m(E1)isshown. due to enrichment from bottom sediments (up to 33 and centrationschangedremarkablybetweenthetwovisits,and 14nmolL−1 at 440m at A3-1 and A3-2, respectively), and the full water column integrated pool was ∼70% lower at were on average 10 times greater than dissolved concentra- A3-2thanatA3-1.Interestingly,thischangewasalsoasso- tions through the water column. The mixed layer pFe con- ciated with a shift of particles from the 1–53µm size range Biogeosciences,12,4421–4445,2015 www.biogeosciences.net/12/4421/2015/ A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 4429 (a) Reference stations trationsincreasingwithdepthandenrichmentjustabovethe dFe(nmoll‐1) pFe(nmoll‐1) plateau seafloor (Fig. 4b). Through the water column, dFe was between 2 and 5 times greater during KEOPS-2 than 0.0 0.2 0.4 0.6 0.0 0.5 1.0 1.5 2.0 2.5 0 0 KEOPS-1andpFewas∼10timesgreaterduringKEOPS-2 (with the exception of the deepest samples). The lower val- 500 500 uesduringKEOPS-1werelikelytheresultofbiologicalup- takeinsurfacewatersandtheexportofFeduringthespring 1000 1000 bloom prior to our arrival at the study site, combined with seasonal changes in the strength of the supply mechanisms 1500 1500 to deeper waters at A3 (discussed in van der Merwe et al., 2015). 2000 2000 3.1.3 PlumeEstations 2500 2500 R‐2 C11 R‐2 C11 The E stations within the bathymetrically trapped complex (b) Plateau stations recirculationsystemshowedsimilarhydrographicandnutri- dFe(nmoll‐1) pFe(nmoll‐1) ent distributions below the mixed layer (Fig. 3d, e and f), 0.0 0.5 1.0 1.5 0 10 20 30 40 which shoaled from 64m at E-1 to 32m at E-3 to 39m at 0 0 E-5,withsomeinternalvariabilityinwatercolumnstructure 100 100 atmid-depths.Surfacewaterswarmedfrom2.7to3.4◦Cbe- tweentheoccupationsofE-1andE-5,althoughnosignificant 200 200 nitrate drawdown was observed (Blain et al., 2015). Below 300 300 AASW,asubsurfacetemperatureminimum(Tmin,∼1.7◦C) wasobservedbetween180m(E1)and220m(E5),character- 400 400 isticofWW.TheT featureisassociatedwithwaterssouth min ofthePF,althoughtherecirculationfeatureprobablyalsore- 500 500 ceivedSAZwatersmixedinfromthenorth(d’Ovidioetal., 600 600 2015).T,S andO2 characteristicsindicatedthepresenceof A3‐1 A3‐2 A3(1) A3(2) A3(3) A3(4) A3‐1 A3‐2 A3(1) UCDW(∼600–700m)andLCDW(deeperthan∼1300m) deeper in the water column above the seafloor (Quéroué et Figure4.(a)ComparisonofdFeandpFeatreferencestationsfor KEOPS-1 (station C11, open blue diamonds) and KEOPS-2 (sta- al., 2015). Water parcel trajectories calculated from altime- tionR-2,closedredsquares).Thewaterdepthswere3110matC11 trybasedgeostrophiccurrentsindicatedthatittookgenerally and2530matR-2.(b)ComparisonofdFeandpFeatA3plateau >2 months for Fe-rich waters from the plateau to travel to stationsforKEOPS-1(opensymbols)andKEOPS-2(closedsym- thedownstreamplumesiteassociatedwiththerecirculation bols).DataareshownforallvisitstoA3onbothKEOPScruises. feature(Estations)(d’Ovidioetal.,2015).Howevershorter NotedifferenceinscalefordFeandpFebetween(a)and(b). transport times are also possible due to episodic transport acrossthePF(Sanialetal.,2015). Watersattheplumestationsshowedthelargestspatialhet- erogeneity in surface biomass as revealed by the evolution to the >53µm size range, with the larger class tripling in ofamosaicofcomplexbloomsseeninsatelliteimages(see size (van der Merwe et al., 2015). The development of the SupplementinTrulletal.,2015).Weobservedmoderatesur- largebloombetweenourtwovisitstoA3,whichconsistedof faceChlalevelsrangingfrom0.3–0.4µgL−1atE-1andE-3 a diatom community 50–210µm in size (Trull et al., 2015), to 0.5–0.9µgL−1 at E-5 (Lasbleiz et al., 2014), noting that waslikelyresponsibleforconvertingthepFewithinthesur- as much as 50% of the chlorophyll was below the mixed facemixedlayerfromthesmallersizeclasstothelargersize layer at the plume stations due to stratification of the up- class.Thismayhavebeenduetoeither(i)physicalaggrega- perwatercolumninthewarm,springconditions.Unlikethe tionoftheparticlesontodiatomaggregatesand/or(ii)micro- plateau bloom dominated by large cells >53µm, the com- biallydrivenconversionofsmalllithogenicFe(1–53µm)to munityintheplumeEstationswasmoremixed(Laurenceau- bioavailableformsandincorporationintothelarge(>53µm) Cornecetal.,2015),withcellspresentinboththe5–20and diatoms as biogenic Fe, with potentially some fraction of 50–200µm size classes (Trull et al., 2015). The E stations these larger particles exported to depths below the mixed showedthehighestCexportfluxesofallregionsasestimated layer,aspreviouslydiscussedbyLametal.(2006),Frewet fromThdeficits,nitratedepletionsandfree-driftingsediment al.(2006)andPlanquetteetal.(2011). trap observations (Planchon et al., 2015; Trull et al., 2015; Thespring(Oct-Nov)KEOPS-2FeprofilesatstationA3 Laurenceau-Cornecetal.,2015). showed a similar structure to those from the late summer (Jan-Feb)duringKEOPS-1,withsurfacedepletion,concen- www.biogeosciences.net/12/4421/2015/ Biogeosciences,12,4421–4445,2015 4430 A.R.Bowieetal.:SouthernOceanironbudgetsduringKEOPS-2 Due to operational constraints, no dFe data were avail- particle export), whilst studies on Fe uptake and microbial able at station E-1. The dFe vertical profiles at E-3 cycling have shown that short-term fluxes within the “fer- and E-5 were quite different, with a distinct surface en- rouswheel”aredominatedbybiologicaluptakeandreminer- richment to 0.4nmolL−1 at E-3 above a minimum of alisation (Strzepek et al., 2005). Here, we follow a similar 0.2nmolL−1 at 100m. This feature was absent at sta- approach to that used by Bowie et al. (2009) for the SAZ- tion E-5, where dFe was depleted to <0.1nmolL−1 SensestudysouthofTasmania(Australia)atourthreestudy at the surface, likely due to biological Fe uptake, sites. Since all parameters in our iron budget calculations which was highest at E-5 (1745nmolm−2d−1) com- were only measured at stations R-2, A3-2 and E-5, discus- pared to A3-2 (1120nmolm−2d−1) (Table 1) and E-4E sionwillfocusonthesestations.DataforstationsA3-1,E-1 (880nmolm−2d−1;datanotshown),despitelowerPOCand andE-3aregiventoprovideacontextforspatialandtempo- primary production (see discussion below and Fourquez et ralchangesintheFepoolsandfluxesduringKEOPS-2,and al., 2015). Deeper in the water column (>500m) at E sta- theyarecollatedinTable1. tions,dFewasbroadlyuniform(0.3–0.5nmolL−1). The pFe distributions at the three E stations were simi- 3.2.1 Ironpools larwithasurface(35–40m)enrichment(1.6–1.9nmolL−1), a minimum at ∼100–200m below the mixed layer (0.7– Ironandcarbonpoolswerecalculatedbyintegratingthedis- 0.9nmolL−1;broadlyconsistentwiththeT layer),amax- solvedandparticulateprofilesdowntothebaseofthesurface min imum at 280–600m (1.7–2.4nmolL−1), and with evidence mixedlayer,definedasthedepthwherethepotentialdensity of enrichment near the seafloor at depths >1800m (up equalledthepotentialdensityat10m+0.02kgm−3(Parket to 1.5–2.3nmolL−1). By applying biogenic (using P) and al.,2014a).Themixedlayervariedfrom165matstationA3- lithogenic (using Al) normalisers to the data (see Sect. 3.2 1to32matstationE-3,consistentwiththeseasonalshoaling below), surface pFe enrichment was roughly equally com- assurfacewaterswarmed,butitremaineddeep(>120m)on posed of biogenic and lithogenic Fe, whilst the 300–600m theplateauthroughoutthestudyduetodeepmixingasare- maximum was predominantly composed of lithogenic Fe sultofseveralpassingstorms. (>100-fold greater than biogenic Fe at these depths). This Integratedpoolsofbothdissolved(∼5×)andparticulate lithogenic Fe was most likely from waters enriched by sed- (∼10×) Fe were significantly greater on the plateau (sta- iments and transported laterally eastward off the Kerguelen tion A3) compared to in the plume (station E), with stocks Plateau which sits at ∼530m below the sea surface. There at the reference station R-2 lower still. Horizontal dFe sup- wasnoobviouschangeinpFeinsurfaceordeepwatersdur- ply from the plateau to the plume was either or both via (i) ingthebloomevolutionatthepseudo-LagrangianEstations. ageostrophicpathloopingalongthenorthernsideofthePF KEOPS-1 only occupied one station in the plume east andthenbackintotherecirculationfeature(d’Ovidioetal., of the Kerguelen Islands (A11 at 49◦09(cid:48)S 74◦00(cid:48)E). Dis- 2015)and(ii)directEkmanfluxtransportofFe-richcoastal solved Fe at A11 ranged from 0.09nmolL−1 at the surface water across the PF driven by westerly winds, as indicated to 0.17nmolL−1 at 1500m (Blain et al., 2008b), and pFe byRatracers(Sanialetal.,2015).Thelatterprocessissup- ranged from 0.07nmolL−1 at the surface to 0.81nmolL−1 ported by Lagrangian trajectories of water parcels derived at 1500m (Andrew Bowie, unpublished data); thus, it was fromaltimetry,whichshowedthePFwasnotastrongbarrier much lower than our KEOPS-2 observations at the E site towatermassmovement,withtransportofwatersacrossthe (notingthatdifferentsamplinganddigestionmethodsforpFe front taking place on timescales of days to weeks but being wereusedforthetwocruises). highlyvariableinspaceandtime(d’Ovidioetal.,2015).The pFepoolshowedthesamevariabilityasthedissolvedpoolat 3.2 Constructionofironbudgets ourthreestudysitesandexceededthedFestocksatallsites byfactorsofapproximately19–26(A3),31(E)and6(R-2), Theprimaryaimofthisworkwastouseourobservationsof althoughitisestimatedthatonly∼2–3%oftheparticulate Fepoolsandfluxestounderstandthesources,sinksandbi- poolcanbeconvertedintobioavailableformsbyphysically ologicalFecyclingandtoevaluatewhetherFesupplycould or biologically mediated dissolution (Schroth et al., 2009). meet demand in both the high-Fe and low-Fe environments IfweassumethatstationA3-1representedpre-bloomcondi- inthevicinityoftheKerguelenArchipelagoduringKEOPS- tionsandtheintegratedmixedlayerpoolof54µmoldFem−2 2.Ironbudgetshavebeenconstructedforpreviousstudiesin was a good estimate of the winter stock, observations show watersfertilisedwithFebothnaturally(Sarthouetal.,2008; that only 4 weeks later at station A3-2, almost 60% of the Bowieetal.,2009;Cheveretal.,2010;Ellwoodetal.,2014) winter stock had been drawn down to 21µmolm−2. If an- and artificially (Bowie et al., 2001) as well as low-Fe con- nual variability is low, which may not always be the case ditions (Price and Morel, 1998; Boyd et al., 2005). These (Grenier et al., 2015), by late summer >90% of the winter budgets have combined geochemical and chemical compo- stockhadbeenusedwithonly4.7µmoldFem−2 remaining nents to demonstrate that the dominant long-term fluxes of in the surface mixed layer at A3 (KEOPS-1 data; Blain et Fe are associated with the particulate pool (dust supply and al., 2007). We note that this drawdown is probably a con- Biogeosciences,12,4421–4445,2015 www.biogeosciences.net/12/4421/2015/

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A. R. Bowie1,2,3, P. van der Merwe1, F. Quéroué1,2,3, T. Trull1,4, M. Fourquez2,5, when the plants die and sink, or indirectly through the food web.
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