Iron budgets for three distinct biogeochemical sites around the Kerguelen Archipelago

26 Pages·2017·2.12 MB·English

Iron budgets for three distinct biogeochemical sites
around the Kerguelen Archipelago (Southern Ocean)
during the natural fertilisation study, KEOPS-2
Andrew R. Bowie, Pier van Der Merwe, Fabien Quéroué, Thomas W. Trull,
M. Fourquez, Frédéric Planchon, Géraldine Sarthou, Fanny Chever, A. T.
Townsend, Ingrid Obernosterer, et al.
To cite this version:
Andrew R. Bowie, Pier van Der Merwe, Fabien Quéroué, Thomas W. Trull, M. Fourquez, et al..
Iron budgets for three distinct biogeochemical sites around the Kerguelen Archipelago (Southern
Ocean) during the natural fertilisation study, KEOPS-2. Biogeosciences, 2015, 12 (14), pp.4421-4445.
￿10.5194/bg-12-4421-2015￿. ￿hal-01207315￿
HAL Id: hal-01207315
https://hal.sorbonne-universite.fr/hal-01207315
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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
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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
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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-
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