Export fluxes in a naturally fertilized area of the Southern - BGD

31 Pages·2014·1.07 MB·English

BiogeosciencesDiscuss.,11,17089–17150,2014 D
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doi:10.5194/bgd-11-17089-2014 us
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©Author(s)2014.CCAttribution3.0License. io
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Thisdiscussionpaperis/hasbeenunderreviewforthejournalBiogeosciences(BG). p
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PleaserefertothecorrespondingfinalpaperinBGifavailable. r
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Export fluxes in a naturally fertilized area D
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of the Southern Ocean, the Kerguelen s
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Plateau: ecological vectors of carbon and P
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biogenic silica to depth (Part 2) r
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M.Rembauville1,2,S.Blain1,2,L.Armand3,B.Quéguiner4,andI.Salter1,2,5 Dis
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1SorbonneUniversités,UPMCUnivParis06,UMR7621,LOMIC,Observatoire ss
Océanologique,Banyuls-sur-Mer,France ion
2CNRS,UMR7621,LOMIC,ObservatoireOcéanologique,Banyuls-sur-Mer,France P
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3DepartmentofBiologicalSciencesandClimateFutures,MacquarieUniversity,NewSouth pe
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Wales,Australia
4Aix-MarseilleUniversitéUniversitédeToulon,CNRS/INSU,IRD,MOI,UM110,Marseille, |
France D
5Alfred-Wegener-InstituteforPolarandMarineresearch,Bremerhaven,Germany is
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Received:7November2014–Accepted:13November2014–Published:10December2014 s
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Correspondenceto:M.Rembauville([email protected])
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PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion. p
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17089 |
Abstract D
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The chemical (particulate organic carbon and nitrogen, biogenic silica) and biological s
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(diatomsandfaecalpellets)compositionofthematerialexportedtoamooredsediment io
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trap located under the winter mixed layer of the naturally-fertilized Kerguelen Plateau P
a
5 in the Southern Ocean was studied over an annual cycle. Despite iron availability in pe
spring, the annualparticulate organic carbon (POC)export (98.2mmolm−2) at 289m r
waslowbutannualbiogenicsilicaexportwassignificant(114mmolm−2).Thisfeature |
was related to the abundance of empty diatom frustules and the ratio of full:empty D
is
cell exerted a first order control in BSi:POC export stoichiometry of biological pump. c
u
10 ChaetocerosHyalochaetespp.andThalassiosiraantarcticarestingsporeswerefound ssio
toberesponsibleformorethan60%oftheannualPOCthatoccurredduringtwovery n
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shortexportevents(<14daysinspring-summer)representingthemajorityofcaptured a
p
export.Lowdiatomfluxeswereobservedovertheremainderoftheyear.Faecalpellet e
r
contributiontoannualcarbonfluxwaslow(34%)andreachedit’sseasonalmaximum
|
in autumn and winter (>80%). The seasonal progression of faecal pellet types re-
15
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vealedacleartransitionfromsmallsphericalshapes(smallcopepods)inspring,larger is
cylindrical and ellipsoid shapes in summer (euphausiids and large copepods) and fi- cu
s
nally large tabular shapes (salps) in autumn and winter. We propose that in this High sio
Biomass, Low Export (HBLE) environment, small, highly silicified, fast-sinking resting n
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spores are able to bypass the high grazing pressure and efficient carbon transfer to a
20 p
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higher trophic levels that are responsible for the low fluxes observed the during the r
remainderoftheyear.Ourstudyalsoprovidesastatisticalframeworklinkingtheeco-
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logicalsuccessionofdiatomandzooplanktoncommunitiestotheseasonalityofcarbon
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andsiliconexportwithinaniron-fertilizedbloomregionintheSouthernOcean. is
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17090 |
1 Introduction D
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TheSouthernOceanistheplaceofexposureofoldupwelledwaterstotheatmosphere s
s
andtheformationofmodalwaters,therebyventilatinganimportantpartoftheglobal io
n
Oceanandplayingacentralroleindistributingheat,carbonandnutrientsintheglobal P
a
5 Ocean(Sarmientoetal.,2004;Takahashietal.,2012;Salléeetal.,2012).Silicontrap- pe
r
pingoccursintheSouthernOceanbecausesiliconisstrippedoutoftheeuphoticzone
moreefficientlythanphosphorousandnitrogen(Holzeretal.,2013).Itisgenerallyac- |
knowledged that regional variations in plankton community structure are responsible D
is
for variations in nutrient stoichiometry in the Southern Ocean (Jin et al., 2006; We- c
u
10 ber and Deutsch, 2010) and that the biological pump is a central process regulating ssio
this stoichiometry (Ragueneau et al., 2006; Salter et al., 2012; Primeau et al., 2013). n
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ThesecharacteristicsemphasizetheimportanceofbiologicalprocessesintheSouth- a
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ernOceanwatersfortheavailabilityofsilicicacidandnitrate(Sarmientoetal.,2004; e
r
Dutkiewiczetal.,2005)aswellasphosphate(Primeauetal.,2013)atlowerlatitudes,
|
thereby regulating part of the productivity of the global Ocean. It has been proposed
15
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that change in the uptake ratio of silicate and nitrate by Southern Ocean phytoplank- is
ton in response to increased iron availability during the Last Glacial Maximum could cu
s
have played a substantial role in varying atmospheric CO2 (Brzezinski et al., 2002; sio
Matsumotoetal.,2002). n
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PrimaryproductionintheSouthernOceanisregulatedbymacro-andmicronutrients a
20 p
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availability(Martinetal.,1990;Mooreetal.,2001,2013;Nelsonetal.,2001)andlight- r
mixingregime(VenablesandMoore,2010;Blainetal.,2013).Thecomplexinteraction
|
of these factors introduces strong spatial heterogeneity in the distribution of primary
D
producer biomass (Arrigo et al., 1998; Thomalla et al., 2011). In particular, High Nu- is
c
trient, Low Chlorophyll (HNLC) areas in the open ocean contrast strongly with highly u
25 s
s
productive,naturallyfertilized,bloomslocateddownstreamofislandsystemssuchas io
n
the Keguelen Plateau (Blain et al., 2001, 2007), Crozet Islands (Pollard et al., 2002) P
a
andSouthGeorgia(Parketal.,2010;Tarlingetal.,2012).Thediatom-dominatedphy- p
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17091 |
toplankton blooms characteristic of these island systems are the product of multiple D
is
environmentalconditionsfavorablefortheirrapidgrowth(Quéguiner,2013),whichap- c
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pear to promote POC export from the mixed layer (Nelson et al., 1995; Buesseler, ss
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1998). However the ecological traits of certain species can impact the BSi:POC ex- n
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5 portstoichiometry(Crawford,1995;Salteretal.,2012),andmaythereforecontrolthe a
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biogeochemicalfunctionofaparticularregionoftheSouthernOcean(Smetaceketal., e
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2004;Assmyetal.,2013).
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Among the numerous ecological characteristics of plankton communities, algal ag-
D
gregation(Jacksonetal.,2005;BurdandJackson,2009),mesozooplanktonfaecalpel- is
10 let(Lampittetal.,1990;Wilsonetal.,2008,2013),verticalmigrationsofzooplankton cus
andmesopelagicfish(JacksonandBurd,2001;Steinbergetal.,2002;Davisonetal., s
io
2013),radiolarianfaecalpellet(Lampittetal.,2009),anddiatomrestingsporeforma- n
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tion(Salteretal.,2012;Rynearsonetal.,2013)haveallbeenhighlightedasefficient a
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vectorsofcarbonexportoutofthesurfacemixedlayer.Thechallengeindescribingthe er
principalecologicalprocessesregulatingPOCexportfluxesistherequirementtohave
15 |
directaccesstosinkingparticles.Manyoftheprocessesdescribedoccurintheupper
D
layersoftheocean,wherecirculationcanstronglyinfluencethereliabilityofsediment is
c
trapcollections(Bakeretal.,1988;Buesseleretal.,2007).Shorttermdeploymentsof u
s
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freedriftingsedimenttrapscanbe anefficientsolutiontominimizethehydrodynamic io
n
20 bias(Buesseleretal.,2000;Lampittetal.,2008)butspatialandtemporaldecoupling P
a
ofproductionandexportneedstobeconsidered(Salteretal.,2007;Rynearsonetal., p
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2013). In regions characterized by relatively weak circulation, moored sediment trap r
observations in areas of naturally fertilized production can track temporal succession |
of exported material from long-term (several month) blooms (Westberry et al., 2013). D
Suchanapproachcanpartiallyresolvehowecologicalprocessesinplanktoncommu- is
25 c
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nities regulate POC and biomineral export out of the mixed layer, although selective s
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processesduringexportmaymodifyoriginalsurfacefeatures. io
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The central Kerguelen Plateau is a good environment to study the ecological vec- P
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torsofexportwithsedimenttrapsduetothenaturallyfertilizedrecurrentbloom(Blain p
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17092 |
etal.,2007)andshallowbathymetrythatbreaksthestrongAntarcticCircumpolarCur- D
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rentflow(Park etal.,2008,2014).Asreportedin thecompanionpaper(Rembauville c
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etal.,2014),annualPOCexportmeasuredbythesedimenttrapdeploymentat289m ss
beneaththesoutheasterniron-fertilizedKerguelenbloomis98 4mmolm−2yr−1.This ion
± P
5 downwardfluxofcarbonmayaccountforaslittleas 1.5%ofseasonalnetcommunity a
carbon production (6.6±2.2molm−2, Jouandet et a∼l., 2008) and <2% of seasonally per
integrated POC export estimated at 200m from a dissolved inorganic carbon budget
(5.1molCm−2; Blain et al., 2007). A comparison of POC fluxes over short-term inter- |
D
vals(<1month)fromawidevarietyofapproachesrevealedsimilarreductionsinPOC is
10 flux between 200 and 300m during spring and summer (Rembauville et al., 2014). cus
Such a rapid attenuation of flux appears to be inconsistent with microbial remineral- s
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ization of settling particles (Rembauville et al., 2014). Previously, we have suggested n
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theroleofhighertrophiclevels(mesozooplanktonandmesopelagicfish)feedingatthe a
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baseofthemixedlayerasanexplanationforthe“HighBiomass,LowExport”(HBLE, er
LamandBishop,2007)environmentcharacterizingtheproductiveKerguelenPlateau.
15 |
HBLEstatusappearstobeacommonfeatureofotherproductivesitesoftheSouthern
D
Ocean (Lam and Bishop, 2007; Ebersbach et al., 2011; Lam et al., 2011; Maiti et al., is
c
2013).DescribingthetemporalsuccessionofPOCandBSifluxvectorsfromtheKer- u
s
s
guelenPlateauisofinteresttoincreaseourunderstandingoftheecologicalprocesses io
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20 characterizing HBLE environments. In particular, phytoplankton community composi- P
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tionandfaecalpelletfluxesmaybeasignificantcomponentofparticlesexportedfrom p
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theKerguelenPlateau. r
Several studies have described diatom fluxes from sediment trap records in the |
Southern Ocean (Leventer and Dunbar, 1987; Fischer et al., 1988, 2002; Abelmann D
and Gersonde, 1991; Leventer, 1991; Gersonde and Zielinski, 2000; Pilskaln et al., is
25 c
u
2004; Ichinomiya et al., 2008; Salter et al., 2012; Grigorov et al., 2014; Rigual- s
s
Hernandez et al., 2014). Highest diatom fluxes (>109cellsm−2d−1) are observed in ion
the Seasonal Ice Zone (SIZ) near Prydz Bay and Adélie Land and are dominated by P
a
FragilariopsiskerguelensisandsmallspeciesofFragilariopsiscurtaandFragilariopsis p
e
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17093 |
cylindrus(Suzukietal.,2001;Pilskalnetal.,2004).Thesehighfluxesoccurinspring D
is
andareassociatedwiththemeltingofseaice.Changesinlightavailabilityandmeltwa- c
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terinputappeartoestablishfavorableconditionsfortheproductionandexportofphy- ss
io
toplankton cells (Romero and Armand, 2010). In the Permanently Open Ocean Zone n
5 (POOZ),diatomfluxesaretwoordersofmagnitudelower 107cellm−2d−1(Abelmann Pa
∼ p
and Gersonde, 1991; Salter et al., 2012; Grigorov et al., 2014) and typically repre- e
r
sented by F. kerguelensis and Thalassionema nitzschioides, except in the naturally
|
fertilizedwatersdownstreamoftheCrozetPlateauwhererestingsporesofEucampia
D
antarcticavar.antarcticadominatethediatomexportassemblage(Salteretal.,2012). is
10 OtherstudieshavereportedthefaecalpelletcontributiontoPOCfluxesintheSouth- cus
ern Ocean (Dunbar, 1984; Wefer et al., 1988, 1990; Wefer and Fisher, 1991; Dubis- s
io
char and Bathmann, 2002; Suzuki et al., 2001, 2003; Accornero and Gowing, 2003; n
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Schnack-SchielandIsla,2005;Gleiberetal.,2012)withaparticularemphasisonshelf a
p
processeswherefaecalpelletcontributiontoPOCfluxishigherthanintheoceanicre- er
gions (Wefer et al., 1990; Wefer and Fischer, 1991; Schnack-Schiel and Isla, 2005).
15 |
In the Ross Sea there is a northward decreasing contribution of faecal pellets to car-
D
bon flux of 59, 38 and 15% for southern, central and northern areas reported from is
c
235msedimenttrapsdeployments(Schnack-SchielandIsla,2005).Faecalpelletsin u
s
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the Ross Sea, are generally represented by larger shapes with only 2 to 3% of them io
n
20 present as small spherical or ellipsoid shapes and total faecal pellet flux is slightly P
higherthan103pelletm−2d−1.Highfaecalpelletcontributiontocarbonfluxes(>90%) ap
e
has been observed in the Bransfield Strait and the Marginal Ice Zone of the Scotia r
Sea,andlinkedtotheabundanceoftheAntarctickrillEuphausiasuperba,resultingin |
maximumrecordedfluxesof>5 105pelletsm−2d−1(Bodungen,1986;vonBodungen D
etal.,1987;Weferetal.,1988).×Thestrongcontributionofkrillfaecalpelletstocarbon is
25 c
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flux in the western Antarctic Peninsula was confirmed over several years of observa- s
s
tions,withthehighestcontributionstocarbonfluxsucceedingthephytoplanktonbloom io
n
inJanuaryandFebruary(Gleiberetal.,2012). P
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In the present study, particulate material exported from the mixed layer in the nat- D
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urally fertilized Permanently Open Ocean Zone (POOZ) of the Kerguelen Plateau is c
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describedfromanannualsedimenttrapmooring.Todevelopourunderstandingofsea- ss
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sonal variability in the ecological flux vectors and particle biogeochemistry we inves- n
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5 tigatethelinkbetweenthechemical(POC,PON,BSi)andbiological(diatomspecies a
p
and faecal pellet types) components of exported particles. Furthermore, we advance e
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thelimitationsofpreviousstudiesbyexplicitlydistinguishingfullandemptydiatomcells
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in the exported material and thereby determine species-specific roles for carbon and
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silicaexport. is
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10 2 Materialsandmethods ion
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As part of the multidisciplinary research program KEOPS2 a moored sediment trap p
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(Technicap PPS3) was deployed at 289m at the representative bloom station A3 r
(50◦38.30S–72◦02.60E) for a period of 321 days (21 October 2011 to 7 Septem- |
ber 2012). The sediment trap mooring was located within an iron-fertilized bloom site D
15 onthesouthernpartoftheKerguelenPlateau(Blainetal.,2007).Detailsofsediment iscu
trapdesign,hydrologicalconditionsanddeploymentconditionsareprovidedinacom- s
s
panion paper (Rembauville et al., 2014). The cup rotation dates of the sediment trap ion
arelistedinTable1. P
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2.1 Sedimenttrapsampleprocessing
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20 Detailsofsedimenttrapsampleretrievalandprocessingmethodshavebeenpresented D
in Rembauville et al. (2014). In brief, swimmers were removed and classified from is
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samplesunderadissectingmicroscope.Thesampleswerequantitativelydividedinto s
s
8 aliquots using a Jencons peristaltic splitter with a precision of 2.9%. Particulate io
∼ n
material was separated from the overlying preservative fluid by a centrifugation and P
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freeze-dryingprocedure. p
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2.2 Chemicalmeasurements D
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POCandPONanalyseshavebeenpreviouslydescribedinRembauvilleetal.(2014). s
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In summary, 3 to 5mg of freeze-dried material were weighed directly into pre- io
n
combusted (450◦C, 24h) silver cups and decarbonated through the addition of 2N Pa
5 HCl. Samples were dried overnight at 50◦C and POC and PON were measured with pe
r
aCHNanalyzer(PerkinElmer2400SeriesIICHNS/OElementalAnalyzer)calibrated
with glycine. Samples were analyzed in triplicate with an analytical precision of less |
than0.3%. D
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Fortheanalysisofbiogenicsilica(BSi)andlithogenicsilica(LSi),2to8mgoffreeze- c
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10 driedmaterialwereweighed(Sartoriusprecisionbalance,precision10−4g)andplaced ssio
intofalcontubes.Theextractionofsiliconfrombiogenicandlithogenicparticlephases n
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wasperformedfollowingtheRagueneauetal.(2005)tripleNaOH/HFextractionproce- a
p
dure. Silicic acid (Si(OH) ) resulting from NaOH extractions was measured automati- e
4 r
callyonaSkalar5100autoanalyzerwhereasSi(OH) resultingfromHFextractionwas
4 |
measuredmanuallyonaMiltonRoySpectronic401spectrophotometer.Si(OH) acid
15 4
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analyses were performed colorimetrically following Aminot and Kerouel (2007). Stan- is
dardsfortheanalysisofsamplesfromtheHFextractionwerepreparedinanHF/H BO cu
3 4 s
matrix,ensuringtheuseofanappropriatecalibrationfactorthatdiffersfromMilli-Qwa- sio
ter. The contribution of LSi to the first leaching was determined by using Si:Al ratios n
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fromasecondleachingstep(Ragueneauetal.,2005).Aluminumconcentrationswere a
20 p
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measuredbyspectrophotometry(Howardetal.,1986).Thetripleextractionprocedure r
is designed for samples with a BSi content <10µmol. For some samples (cup #3,
|
#4, #6, #7, #8, #9 and #10) the Si:Al molar ratio in the second leachate was high
D
(>10)indicatingtheincompletedissolutionofBSi.Forthesesamplesitwasnotpos- is
c
sibletouseSi:AlratiostocorrectforLSileaching.AcrustalSi:Almassratioof3.74 u
25 s
s
(Taylor and McClennan, 1986) was used instead and applied to all the samples for io
n
consistency.Precision(estimatedfrommeasurementof25independentsamples)was P
13nmolmg−1,whichrepresents<1%oftheBSicontentinallsamplesand14%ofthe ap
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meanLSicontent.Blanktriplicatesfromeachextractionwerelowerthanthedetection D
is
limit.BSiresultsfromthismethodwerecomparedtothekineticmethodfromDeMaster c
u
(1981).Therewasanexcellentagreementbetweenthetwomethods(Spearmanrank ss
io
correlation,n=12,p<0.001,BSi =1.03BSi 0.08,datanotshown) n
kinetic tripleextraction− P
5 InordertocorrectforthedissolutionofBSiduringdeploymentandstorage,Si(OH)4 ap
excess was analyzed in the overlying preservative solution. Particulate BSi fluxes e
r
were corrected for dissolution assuming that excess silicic acid originated only from
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the dissolution of BSi phases. Si(OH) excess was always <10% of total (dis-
4
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solved+particulate) Si concentrations. Error propagation for POC, PON, BSi fluxes is
10 andmolarratioswerecalculatedasthe quadratic sumoftherelativeerrorfromtripli- cus
catemeasurementsofeachvariable. s
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2.3 Diatomidentification,fluxesandbiomass a
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Many sediment trap studies reporting diatom fluxes in the Southern Ocean use a mi-
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cropaleontologicalprotocolthatoxidizesorganicmaterial(KMnO ,HCl,H O )thereby
4 2 2
facilitatingtheobservationofdiatomfrustules(seeRomeroetal.,1999,2000forade- D
15 is
scription). In the present manuscript, our specific aim was to separately enumerate c
u
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full and empty diatom cells captured by the sediment trap to identify key carbon or s
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siliconexportersamongstthediatomspecies.Wethereforeusedabiologicalmethod n
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followingasimilarprotocoltothatofSalteretal.(2007,2012).Topreparesamplesfor a
p
counting,2mLofagentlyhomogenized1/8wetaliquotweredilutedinatotalvolume e
20 r
of20mLofartificialseawater(S=34).Inordertominimizetheexclusionand/orbreak-
|
ingoflargeorelongateddiatomfrustules(e.g.Thalassiothrixantarctica),thepipettetip
D
usedforsub-samplingwasmodifiedtoincreasethetipapertureto>2mm.Thediluted is
c
andhomogenizedsamplewasplacedinaSedgewick–Raftercountingchamber(Pyser u
s
25 SGE S52, 1mL chamber volume). Each sample was observed under an inverted mi- sio
croscope(OlympusIX71)withphasecontrastat200xand400xmagnification.Diatom n
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enumerationandidentificationwasmadefromonequartertoonehalfofthecounting a
p
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17097 |
chamber(dependingoncellabundance)followingthetaxonomicdescriptioninHasle D
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andSyvertsen(1997). c
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Due to the lower magnification used and preserved cell contents sometimes ob- ss
io
scuring taxonomic features on the valve face, taxonomic identification to the species n
P
5 levelwasoccasionallydifficultandnecessitatedthecategorizingofdiatomspeciesto a
p
genusortaxagroupingsinthefollowingmanner:Chaetocerosspeciesofthesubgenus e
r
Hyalochaeterestingspores(CRS)werenotdifferentiatedintospeciesormorphotypes
|
but were counted separately from the vegetative cells; Fragilariopsis separanda and
D
Fragilarsiopsis rhombica were grouped as Fragilariopsis separanda/rhombica; Mem- is
10 braneis imposter and Membraneis challengeri and species of the genera Banquisia cus
and Manguinea were denominated as Membraneis spp. (Armand et al., 2008a); di- s
io
atoms of the genus Haslea and Pleurosigma were grouped as Pleurosigma spp.; all n
P
Pseudo-nitzschia species encountered were grouped as Pseudo-nitzschia spp.; Rhi- a
p
zosolenia antennata and Rhizosolenia styliformis were grouped as Rhizosolenia an- er
tenanta/styliformis; large and rare Thalassiosira oliverana and Thalassiosira tumida
15 |
were grouped as Thalassiosira spp.; Thalassiosira antarctica resting spores (TRS)
D
were identified separately from the vegetative cells; small centric diatoms (<20µm) is
c
representedbyThalassiosiragracilisandotherThalassiosiraspeciesweredesignated u
s
s
asSmallcentrics(<20µm);andfinallylargeandrarecentricsincludingAzpeitiatabu- io
n
20 laris,Coscinodiscusspp.andActinocycluscurvatulusweregroupedasLargecentrics P
a
(>20µm).Fullandemptyfrustulesofeachspeciesortaxagroupingweredistinguished p
e
andenumeratedseparately.Thecellfluxforeachdiatomspeciesortaxagroupingwas r
calculatedaccordingtoEq.(1): |
D
Cellflux=Ndiat×d×8×Vcup×0.1125×da1ys×chamberfraction (1) iscu
s
s
io
25 Where Cell flux is in cells m−2d−1, Ndiat is the number of cells enumerated for each nP
diatomclassification,d isthedilutionfactorfromtheoriginalwetaliquot,8isthetotal ap
e
number of wet aliquots comprising one sample cup, V is the volume of each wet r
cup
17098 |
aliquot, 0.125 is the Technicap PPS/3 sediment trap collecting area (m2), days is the Dis
collectingperiod,chamberfractionisthesurfacefractionofthecountingchamberthat cu
s
wasobserved(onequarteroronehalf). s
io
We directly compared the micropaleontological and biological counting techniques n
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inoursedimenttrapsamplesandnotedthelossofseveralspecies(Chaetocerosde- a
5 p
cipiens/dichaeta, Corethron pennatum/inerme, Guinardia cylindrus and Rhizosolenia er
chunii) with the micropaleontological technique. We attribute this to the aggressive
|
chemical oxidation techniques used to “clean” the samples which appears to selec-
D
tivelydestroy/dissolvecertainfrustules.Forthespeciesthatwerecommonlyobserved is
c
by both techniques, total valve flux was in good agreement (Spearman rank correla- u
10 s
s
tion,n=12,ρ=0.91,p<0.001,datanotshown)althoughconsistentlylowerwiththe io
n
micropaleontolgical technique, probably due to the loss of certain frustules described P
a
above.Fulldetailsofthismethodcomparisonareinpreparationforaseparatesubmis- p
e
sion. r
15 Biomass calculations for both Chaetoceros Hyalochaete resting spores (CRS) and |
Thalasiosiraantarcticarestingspores(TRS)weredeterminedfrom>50randomlyse- D
lectedcompleterestingsporesobservedinsplitsfromcups#4to#11.Morphometric is
c
u
measurements(pervalvarandapicalaxis)weremadeusingtheFijiimageprocessing s
s
package (available at http://fiji.sc/Fiji) on images taken with an Olympus DP71 cam- io
n
era.CellvolumesfollowedappropriateshapedesignatedcalculationsfromHillebrand P
20 a
et al. (1999) (Table 2). The cell volume coefficient of variation was 46 and 54% for p
e
r
CRSandTRS,respectively.CRScarboncontentwasestimatedfromthederivedcell
volumeusingthevolumetocarbonrelationshipof0.039pmolCµm−3establishedfrom |
the resting spore of Chaetoceros pseudocurvisetus (Kuwata et al., 1993), leading to D
25 ameanvalueof227pgCcell−1 (Table2).Thereiscurrentlynovolumetocarbonrela- iscu
tionshipforThalassiosiraantarcticarestingsporesdescribedintheliterature,therefore, ss
the allometric relationship for vegetative diatoms (Menden-Deuer and Lessard, 2000) ion
wasusedtocalculateourTRScarboncontent,givingameanvalueof1428pgCcell−1 Pa
p
e
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17099 |
(Table2).Restingsporesfluxeswereconvertedtocarbonfluxesasfollows: D
is
c
CRSflux 227pgCcell−1 us
Cflux = × (2) s
(CRS) M12C×109 ion
P
Cflux(TRS)= TRSfluxM×14281p0g9Ccell−1 (3) aper
12C×
|
where C flux(RS) is the carbon flux carried by each resting spore (mmol C m−2d−1), D
5 RS flux is the full spore numerical flux (cellm−2d−1), M12C is the molecular weight of isc
12C (12gmol−1) and 109 is a conversion factor from pmol to mmol. Other diatoms us
s
species that contributed to more than 1% of total cell flux (E. antarctica var. antarc- io
n
tica,Fragilariopsiskerguelensis,Fragilariopsisseparanda/rhombica,Pseudo-nitzschia P
a
spp.andThalassionemanitzschioidesspp.)wereconvertedtocarbonfluxusingcell- p
e
specificcarboncontentfordiatomscommunitiesoftheKerguelenPlateaufromCornet- r
10
Barthauxetal.(2007). |
D
2.4 Faecalpelletcompositionandfluxes is
c
u
s
To enumerate faecal pellets an entire 1/8 aliquot of each sample cup was placed in sio
n
a gridded petri dish and observed under a stereomicroscope (Zeiss Discovery V20)
P
15 coupled to a camera (Zeiss Axiocam ERc5s) at 10x magnification. Photographic im- ap
ages(2560 1920pixels,3.49µmpixel−1)coveringtheentiresurfaceofthepetridish er
×
were acquired. Following Wilson et al. (2013), faecal pellets were classified into five |
types accordingto theirshape: spherical,ovoid, cylindrical,ellipsoid and tabular.The
D
fluxofeachfaecalpelletclass(nbm−2d−1)wascalculatedasfollows: isc
u
1 1 s
s
20 Faecalpelletflux=NFP×8×0.125×days (4) ion
P
whereNFPisthenumberofpelletswithineachclassobservedinthe1/8thaliquot.The ape
otherconstantsareasdescribedinEq.(1).Individualmeasurementsofthemajorand r
17100 |
minoraxisforeachfaecalpelletwereperformedwiththeFijisoftware.Thetotalnumber D
is
ofspherical,ovoid,cylindrical,ellipsoidandtabularfaecalpelletsmeasuredwas4041, c
u
2047, 1338, 54 and 29, respectively. Using these dimensions, faecal pellet volume ss
io
was determined using the appropriate shape equation (e.g. sphere, ellipse, cylinder, n
5 ovoid/ellipse) and converted to carbon using a factor of 0.036mgCmm−3 (Gonzalez Pa
p
andSmetacek,1994).Duetotheirregularityofthetabularshapespreventingtheuse e
r
ofsingleequationtocalculatetheirvolume,aconstantvalueof119µgCpellet−1 rep-
|
resenting a midrange value for tabular shapes (Madin, 1982), was applied to tabular
D
faecalpellets(Wilsonetal.,2013).Thisvaluewasrelevantbecausetheobservedtab- is
10 ularfaecalpelletswerecomprisedinthesizerangereportedinMadin(1982).Ranges cus
andmeanvaluesoffaecalpelletvolumesandcarboncontentarereportedinTable3. s
io
Faecalfluffanddisaggregatedfaecalpelletswerenotconsideredinthesecalculations n
P
because quantitative determination of their volume is difficult. We acknowledge that a
p
fragmentationoflargerpelletsmayrepresentanartifactofthesamplesplittingproce- er
dure. Alternatively, their presence may also result from natural processes within the
15 |
watercolumn,althoughdedicatedsamplingtechniques(e.g.polyacrylamidegeltraps)
D
are required to make this distinction (Ebersbach and Trull, 2008; Ebersbach et al., is
c
2011,2014;Laurenceauetal.,2014).Consequentlyourpresentquantificationoffae- u
s
s
calpelletcarbonfluxshouldbeconsideredaslower-endestimates. io
n
20 Theprecisionofourcalculationsdependsonthereliabilityofcarbon-volumeconver- P
a
sionfactorsoffeacalpellets,whichvarywidelyintheliterature,aswellasvariabilityin p
e
diatomrestingsporevolumes(Table2).Toconstraintheimportanceofthisvariability r
onourquantitativeestimationofCflux,wecalculatedupperandlowererrorboundsby |
aconstantscalingoftheconversionfactors( 50%). D
± is
c
2.5 Statisticalanalyses u
25 s
s
io
Correspondence analysis was performed to summarize the seasonality of diatom ex- n
P
port assemblages. This approach projects the original variables (here full and empty a
p
e
cells) onto a few principal axes that concentrate the information of the Chi-squared r
17101 |
(Chi2) distance between both observations and variables (Legendre and Legendre, Dis
1998). Chi2 distance is very sensitive to rare events. Consequently, only full- and cu
s
empty-cells fluxes >10% of the total mean flux of all sample cups were retained in sio
the correspondence analysis. This step avoided the inclusion of rare species, which n
P
potentiallycarryalotofweightintheanalysisdespiteprovidingweakinformation. a
5 p
Partial least square regression (PLSR) analysis was used to examine the relation- er
ships between ecological flux vectors (full and empty diatom cells and faecal pellet
|
fluxesascolumnsoftheXmatrix,cupsbeingtherows)andbulkgeochemicalproper-
D
ties(POCflux,PONflux,BSiflux,POC:PONandBSi:POCmolarratioandcolumns is
c
intheYmatrix)oftheexportedmaterial.TheprincipleofPLSRistodecomposeboth u
10 s
s
theXandYmatrixintotheirprincipalcomponentsusingprincipalcomponentanalysis io
n
andthenusetheseprincipalcomponentstoregressYinX(Abdi,2010).PLSRisca- P
a
pableofmodelingresponsevariablesfromalargesetofpredictors.Thesamefilteras p
e
for the correspondence analysis (full- and empty-cell fluxes >10% of the total mean r
15 flux)wasapplied. |
D
is
c
3 Results u
s
s
io
3.1 Chemicalcompositionofthesettlingmaterial n
P
a
p
Time series of the chemical signature of the settling material are presented in Fig. 1. e
r
SurfacechlorophyllaconcentrationabovethetraplocationandPOCfluxes(Fig.1aand
|
b)havebeendescribedanddiscussedinRembauvilleetal.(2014).Elementalfluxes
20
acereptredpuorirntegdtwinoTsahbolert1a.nPdOinCteflnusxeesexwpeorretelovwenmtsosintoefatrhlyeDtimeceem(≤b0e.r5(m1.m6molmmo−l2md−−21)de−x1-) Discu
andlateJanuary–earlyFebruary(1.47mmolm−2d−1).Assuminganegligiblefluxdur- ssio
ing the unsampled period (the month in September), the annual POC export was n
25 9se8a.2s±on4a.3l pmamtteorlnmt−h2a(ntoPtaOlC±sfluumxesof(tFimige.-1inct)ewgritahtelodwSDflu).xBesSi(<flu1xemsmeoxlhmib−it2edd−t1h)eesxacempet Paper
17102 |
LdSuriinflguxtehsewtweoreinhteignhseesetvinenintss(p2r.i6n0g±(>01.003µamnodlm2.−129d±−10.i1n0cmupmso#l1m−to2d#−41,,Tmabelaen1±).STDh)e. Discu
s
contributionofLSitototalparticulateSiwas5and10%respectivelyincups#1and#12 s
io
andlowerthan3%theremainderoftheyear.ThePOC:PONmolarratioshowedlow n
P
variability,rangingbetween6and8.1,withamaximumvalueobservedinautumn(cup a
5 p
#11).TheBSi:POCmolarratiowashighestatthebeginningoftheseason(between er
2.18 0.19 and 3.46 0.16 in the first three cups, blue line in Fig. 1c) and dropped
± ± |
to 0.64 0.06 in cup #5, following the first export event. BSi:POC ratios were close
± D
inthetwoexportevents(1.62 0.05and1.49 0.08).ThelowestBSi:POCratiowas is
± ± c
observedinautumnincup#11(0.29 0.01). u
10 ± ss
io
3.2 Diatomfluxes n
P
a
p
Diatomsfrom33taxawereidentifiedandtheirfluxesdeterminedacrossthe11months e
r
timeseries(Table4).Fullandemptycellfluxesforthetotalcommunityandforthetaxa
|
thatarethemajorcontributorstototaldiatomflux(eighttaxathataccountfor>1%of
D
15 totalcellsannualexport)arepresentedinFig.2.Thefluxoffull-andempty-cellfluxes is
c
foreachdiatomspeciesortaxaisreportedinTable4. u
s
During spring (cups #1 to #3) and winter (cups #11 and #12) the total flux of sio
tfoull5c.5elalsndwa9s.5<510×71c0e6llscemll−s2md−−21d(−c1up(sFi#g4. 2aan)d. T#9h,eretostpaelcfltiuvxelyo)fdfuulrlincgeltlswoinecpreisaosdeidc nPap
× e
20 (<14 days) sedimentation events. The two largest flux events (cups #4 and #9) were r
also associated with significant export of empty cells with respectively 6.1 107 and
|
2(C.9R×S1),0f7uclleclleslmls−fl2udx−e1s(oFfig4. 2a1)0.7FcoerllsCmh−a2edto−c1earonsdH7.y8aloc1h0a7ecteellssmpp−.2dre−s1tian×cgcosupnotreeds Dis
× × c
for 76 and 83% of the total full cell flux during these two events, respectively us
s
25 (Fig. 2b), whereas a smaller contribution of Thalassiosira antarctica resting spores io
(TRS)(2.7 106cellsm−2d−1,5%oftotalfullcells)wasobservedduringthefirstevent nP
× a
(Fig.2h).CRSalsodominated(79–94%)thecompositionoffullcellsintheinterven- p
e
r
17103 |
i2n.g5pe1r0io7dce(cllsupms−2#d5−-#18))b,yalcthoomupgahritshoenm(Faiggn.it2ubd)e. Ionfccuepll fl#u4x, twhaesemmopdtyercaetell(fl9u×x1c0o6n–- Discu
× s
tained 61% of C. Hyalochaete spp. vegetative empty cells and 27% of unidentified s
io
Small centrics (<20µm) empty cells. In cup #9, the total empty cells flux contained n
P
60%ofC.Hyalochaetespp.vegetativestageandonly2%ofSmallcentrics(<20µm) a
5 p
emptycells. er
Fragilariopsis kerguelensis, and Fragilariopsis separanda/rhombica (Fig. 2d and e)
|
weremostlyexportedfromspringthroughtheendofsummer(cups#1to#10)withtotal
D
(full+empty)fluxes<3 106cellsm−2d−1,avalue 20timeslowerthanthehighest is
× ∼ c
CRS fluxes recorded. During this time, these species were represented by >50% of u
10 s
s
emptycells.Inautumnandwinter,(cups#10and#11),thesespecieswereonlyrepre- io
sented by low fluxes (<0.5 10−6cellsm−2d−1) of empty cells. Thalassionema nitzs- nP
× a
chioidesspp.fluxeswerehighestinspringandearlysummer(cups#1to#4)withtotal p
fluxescomprisedbetween3.5 106 and6.7 106cellsm−2d−1 (Fig.2g).Theremain- er
15 der of the year, total flux was <×2 106cells×m−2d−1 and was essentially represented |
×
by full cells. Pseudo-nitzschia spp. were mostly represented by full cells (Fig. 2f) with D
thehighestfluxof1.2×107cellsm−2d−1 observedinthesecondintenseexportevent iscu
(cup#9).Eucampiaantarcticavar.antarcticatotalfluxeswerealwaysrepresentedby s
s
>50%offullcells(Fig.2c).TotalcellfluxesofEucampiaantarcticavar.antarcticagrad- io
n
20 ually increased from <1×105 to 1.3×106cellsm−2d−1 from spring to summer (cups Pa
#1to9#)andthendecreasedtoanegligiblefluxinwinter(cup#12).Thisspecieswas p
e
r
observed as both the lightly silicified, chain-forming, vegetative form and the highly
silicifiedwintergrowthstageform.Bothformswereobservedthroughouttheyearwith- |
outspecificseasonalpattern.Smallcentricspecies(<20µm)wereessentiallyrepre- D
25 sented by empty cells (Fig. 2i). Their total fluxes were <4×106cellsm−2d−1, except iscu
inthefirstexportevent(cup#4)wheretheirfluxrepresentedaconsiderableexportof ss
1.7 107cellsm−2d−1. ion
D×iatomsandsamplingcupprojectiononthefirsttwoaxesfromthecorrespondence Pa
analysisispresentedinFig.3.Chi2 distanceinthecorrespondenceanalysisisbased pe
r
17104 |
on frequency distribution, therefore the results of the analysis must be considered as D
is
representative of the community composition as opposed to cell flux. The first two c
u
factorsaccountedforthemajority(75.6%)oftotalexplainedvariance.Earlyinthesea- ss
io
son(cups#1–#3),duringtheperiodofbiomassaccumulationinthesurface(Fig.1a), n
P
5 diatomfluxeswerecharacterizedbyemptycellsofT.nitzschioidesspp.andF.kergue- a
p
lensis.FullTRScellswereobservedincup#3followingtheinitialbloomdecline.The e
r
first major flux event (cup #4) contained mostly TRS, empty Small centrics (<20µm)
|
cellsandemptyC.Hyalochaetespp.cells.Thesummerfluxperiod(cups#5to#8)pri-
D
marilyconsistedofCRS,althoughE.antarcticavar.antarctica,Pseudo-nitzschiaspp, is
10 and Thalassiothrix antarctica were present as full cells and Plagiotropis spp., Mem- cus
braneisspp.,Pseudo-nitzschiaspp.asemptycells.Thesecondmajorfluxevent(cup s
io
#9)wastightlyassociatedwithCRSandfullPseudo-nitzschiaspp.cells.Subsequent n
P
cups (#10 and #11) were characterized by full cells of E. antarctica var. antarctica a
p
and Thalassiotrix antarctica and empty cells of Corethron inerme, P. alata, F. sepa- er
randa/rhombicaandF.kerguelensis.Winterfluxes(cup#12)weresimilartotheinitial
15 |
three cups characterized primarily by empty cells of small diatom taxa. The central-
D
izedprojectioninFig.3offullF.kerguelensisandT.nitzschioidesspp.highlightstheir is
c
constantpresencethroughouttheannualrecord. u
s
s
Thetotalempty:fullcellratioispresentedinFig.2a(blueline).Thisratiowashighest io
n
20 inspringandearlysummer(cups#1to#4),rangingbetween1.1and2.4,suggesting P
a
moreemptycellstofullcells.Theratiowaslowest,representingconsiderablymorefull p
e
cellstoemptycellsincups#5to#10withvaluesbetween0.1and0.4.Inautumn(cup r
#11),theempty:fullratioincreasedto0.7.Inthewintercup#12,thetotalamountoffull |
diatomcellswasverylowandthereforewecouldnotcalculatearobustempty:fullratio. D
Across thetime-series certaindiatom taxawere observedexclusively as emptycells, is
25 c
u
notablyChaetocerosatlanticusf.bulbosum,andCorethronpennatum.Fordiatomtaxa s
s
present as full and empty cells we calculated an annually integrated empty:full ratio io
n
(Fig. 4) and arbitrarily defined threshold values of 2 (representing species mainly ob- P
a
served as empty cells) and 0.5 (representing species mainly observed as full cells), p
e
r
17105 |
respectively.Indecreasingorder,thediatomtaxaexhibitingempty:fullratios>2were D
is
Thalassiosiralentiginosa,Smallcentrics(<20µm),Probosciaalata,Rhizosoleniaan- c
u
tennata/styliformis, Chaetoceros decipiens, Corethron inerme, Dactyliosolen antarcti- ss
io
cus, Large centrics (>20µm), and Asteromphalus spp. The diatom taxa displaying n
P
5 an empty:full ratio <0.5 were Thalassiothrix antarctica, Rhizosolenia simplex, CRS, a
p
Eucampia antarctica var. antarctica, Thalassiosira spp. and Navicula spp. Species or e
r
groupedtaxawithratiovaluesfallingbetweenthethresholds(<2and>0.5;R.chunii,
|
through to C. dichaeta in Fig. 4) wereperceived as being almost equally represented
D
byfullandemptycellswhenintegratedannuallyacrossthetimeseries. is
c
u
s
10 3.3 Faecalpelletfluxes sio
n
The seasonal flux of faecal pellet type, volume and their estimated carbon flux are P
a
summarizedinFig.5andTable5.Totalfaecalpelletfluxwas<2 103pelletsm−2d−1 pe
× r
in spring (cups #1 to #3). Cups #4 and #5 were characterized by the highest fluxes
of 21.8 103 and 5.1 103pelletsm−2d−1 (Fig. 5a, Table 5). Faecal pellet numerical |
× ×
flux decreased gradually from mid-summer (cup# 5) to reach a minimal value in win- D
15 ter(140pelletsm−2d−1 incup#12).Inspring(cups#1to#3),sphericalandcylindrical iscu
s
shapesdominatedthenumericalfaecalpelletfluxes.Ellipsoidandtabularshapeswere s
io
absentfromthesespringcups.Thefirstexportevent(cup#4),wasnumericallydomi- n
P
natedbythesphericalshapedpellets,howevertheremainderofthesummer(cups#5 a
p
to #10) contained spherical, ovoid and cylindrical shapes in comparable proportions. e
20 r
Ellipsoidshapeswereobservedfrommid-summertoautumn(cups#7to#11)buttheir
|
overall contribution to pellet flux was low (<6%, Table 5). Rare tabular shapes were
D
observed in summer (cups #6 and #8) and their contribution to numerical fluxes was is
c
highestinautumnandwinter(cups#11and#12). u
s
25 Themedianfaecalpelletvolumeshowedaseasonalsignalwithamaximumpeak> sio
5.5 106µm3inmid-summer(cups#6to#8)andvalues<4 106µm3theremainderof n
× × P
theyear(Fig.5b).Concomitantlywiththehighestmedianvolume,thelargestvariance a
p
e
r
17106 |
in faecal pellet size was also observed in the summer (highest interquartile values in D
is
Fig.5b). c
u
Total faecal pellet carbon flux was lowest in spring (<0.05mmolCm−2d−1 in cups ss
io
#1 to #3, Fig. 5c, Table 5). The highest total faecal pellet carbon flux of nearly n
5 0.5mmolCm−2d−1 was observed during the first export event in cup #4 and was Pa
p
essentially composed of spherical shapes (83%, Table 5). For the remainder of the e
r
summer (cups #5 to #10), total faecal pellet carbon flux was between 0.03 and
0.15mmolCm−2d−1 with a dominant contribution of cylindrical, ellipsoid and tabular |
D
shapes.Inautumnandwinter(cups#11and#12),faecalpelletcarbonfluxesof0.13 is
10 and 0.06mmolCm−2d−1 were strictly dominated by tabular shapes (>90% to total cus
faecalpelletcarbonfluxes,Table5). s
io
n
P
3.4 Statisticalanalysisofbiologicalandbiogeochemicalsignatures a
p
e
r
Theβ correlationcoefficientsofstandardizedvariablesobtainedfromthePLSRanal-
|
ysis are presented as a heatmap in Fig. 6. The full cell fluxes of all diatom taxa, in
additiontosphericalandovoidandellipsoidfaecalpelletfluxeswerepositivelycorre- D
15 is
latedtoPOCandPONfluxes.Bycontrast,emptycellfluxesofF.kerguelensis,P.alata, c
u
s
T. nitzschioides spp., T. lentiginosa and cyclindrical, ellipsoid and tabular pellet fluxes s
io
wereeitheruncorrelatedornegativelycorrelatedwithPOCandPONfluxes.Full-and n
P
empty-cellfluxesofalldiatomtaxawerepositivelycorrelatedwithBSifluxes,although a
p
this correlation was notably weak for empty cells of C. inerme, P. alata and T. lentigi- e
20 r
nosa.OnlysphericalandovoidfaecalpelletswerepositivelycorrelatedwithBSifluxes.
|
FullcellsfluxesofCRSandE.antarcticavar.antarcticawerethemostnegativelycor-
D
relatedwithBSi:POCmolarratio,whereasTRS,F.kerguelensis,T.nitzschioidesspp. is
c
andT.lentiginosafullcellsfluxeswerepositivelycorrelated.Sphericalandovoidfaecal u
s
25 pelletswereweaklyandnegativelycorrelatedwiththeBSi:POCmolarratiowhereas sio
the cylindrical, ellipsoid and tabular shapes were more strongly negatively correlated n
P
totheBSi:POCmolarratio.Allthebiologicalcomponentsexhibitedweakornocorre- a
p
e
lationstothePOC:PONmolarratio. r
17107 |
The first two latent vectors of the PLSR accounted for 61.3 and 74.1% of cumu- D
is
lative variance in X (full and empty diatom and pellet fluxes) and Y (biogeochemical c
u
properties). In order to visualize how the seasonal succession of flux vectors was re- ss
io
latedtothebulkgeochemicalpropertiesofparticles,thesamplingcups,biologicaland n
P
5 chemical factors were projected on the first two latent factors of the PLSR analysis a
p
(Fig.8).PositivelyprojectedonthefirstaxisarethePOC,PONandBSifluxes,close e
r
to the export events sampled in cups #4 and #9. The closest biological components
|
comprise a complex assemblage of full and empty cells and spherical and ovoid fae-
D
cal pellet shapes. All the other cups are projected far from these two export events. is
10 The second axis opposes the spring cups (#1 to #3) to the autumn (#11) and winter cus
(#12)cups.EmptyfrustulesofF.kerguelensis,T.lentiginosaandT.nitzschioidesspp. s
io
are projected close to the spring cups (#1 to #3) together with the BSi:POC molar n
P
ratiowhereasautumn(#11)andwintercups(#12)areprojectedfarfromtheBSi:POC a
p
molarratioandclosetothetabularandcylindricalfaecalpelletshapes. er
|
3.5 Partitioningcarbonfluxesamongecologicalvectors
15
D
is
Weestimatedthecontributionofrestingsporesandfaecalpelletstocarbonflux,calcu- c
u
s
latedtheircumulativevaluesandcomparedthemtomeasuredvalues(Fig.8aandb). s
io
Ahighlysignificantcorrelation(Spearmanrankcorrelation,n=36,ρ=0.84,p<0.001) n
P
was evident between calculated and measured carbon flux suggesting that the main a
p
ecologicalfluxvectorsobservedinthesamplewerecapableofexplainingtheseasonal e
20 r
variationintotalPOCflux.Table6liststhecontributionofeachvectortothecalculated
|
flux. In cup #1, CRS and other diatoms dominated the calculated POC fluxes, with
D
respectively 25.3 and 38.6%. Diatoms other than spores dominated the calculated is
c
carbon flux (35.4%) together with cylindrical faecal pellets (36.4%) in cup #2. TRS u
s
25 dominated the POC fluxes in cup #3 (85.1%). CRS strictly dominated the calculated sio
POCfluxesinsummer(cups#4to#10)withacontributionrangingfrom46.8to88.1%. n
P
Duringtheautumnandwinter(cups#11and#12),POCfluxeswerealmostexclusively a
p
e
associated to tabular faecal pellets, 81 and 93.3%, respectively. At annual scale di- r
17108 |