Comparing Springtime Ice-Algal Chlorophyll a and Physical Properties of Multi-Year and First-Year

26 Pages·2015·1.14 MB·English

RESEARCHARTICLE
Comparing Springtime Ice-Algal Chlorophyll
a
and Physical Properties of Multi-Year and
First-Year Sea Ice from the Lincoln Sea
BenjaminA.Lange1,2,3*,ChristineMichel4,JustinF.Beckers3,J.AlecCasey3,
HaukeFlores1,2,IdoHatam5,GuillaumeMeisterhans4,AndreaNiemi4,ChristianHaas3,6
1 PolarBiologicalOceanography,Alfred-Wegener-InstitutHelmholtz-ZentrumfürPolar-und
Meeresforschung,Bremerhaven,Germany,2 UniversityofHamburg,CentreforNaturalHistory(CeNak),
ZoologicalMuseum,BiocenterGrindel,Hamburg,Germany,3 DepartmentofEarthandAtmospheric
Sciences,UniversityofAlberta,Edmonton,Alberta,Canada,4 FreshwaterInstitute,FisheriesandOceans
Canada,Winnipeg,Manitoba,Canada,5 DepartmentofBiologicalSciences,UniversityofAlberta,
Edmonton,Alberta,Canada,6 DepartmentofEarthandSpaceSciencesandEngineering,YorkUniversity,
Toronto,Ontario,Canada
* [email protected]
OPENACCESS
Abstract
Citation:LangeBA,MichelC,BeckersJF,CaseyJA,
FloresH,HatamI,etal.(2015)Comparing
Withnear-completereplacementofArcticmulti-yearice(MYI)byfirst-yearice(FYI)pre-
SpringtimeIce-AlgalChlorophyllaandPhysical
dictedtooccurwithinthiscentury,itremainsuncertainhowthelossofMYIwillimpactthe
PropertiesofMulti-YearandFirst-YearSeaIcefrom
theLincolnSea.PLoSONE10(4):e0122418. abundanceanddistributionofseaiceassociatedalgae.Inthisstudywecomparethe
doi:10.1371/journal.pone.0122418 chlorophylla(chla)concentrationsandphysicalpropertiesofMYIandFYIfromtheLin-
AcademicEditor:ConnieLovejoy,LavalUniversity, colnSeaduring3springseasons(2010-2012).Coreswereanalysedfortexture,salinity,
CANADA andchla.Weidentifiedannualgrowthlayersfor7of11MYIcoresandfoundnosignifi-
Received:November14,2014 cantdifferencesinchlaconcentrationbetweenthebottomfirst-year-iceportionsofMYI,
upperold-iceportionsofMYI,andFYIcores.Overall,themaximumchlaconcentrations
Accepted:February20,2015
wereobservedatthebottomofyoungFYI.However,therewerenosignificantdifferences
Published:April22,2015
inchlaconcentrationsbetweenMYIandFYI.Thissuggestslittleornochangeinalgal
Copyright:©2015Langeetal.Thisisanopen
biomasswithashiftfromMYItoFYIandthatthespatialextentandregionalvariability
accessarticledistributedunderthetermsofthe
ofrefrozenleadsandyoungerFYIwilllikelybekeyfactorsgoverningfuturechangesin
CreativeCommonsAttributionLicense,whichpermits
unrestricteduse,distribution,andreproductioninany Arcticseaicealgalbiomass.Bottom-integratedchlaconcentrationsshowednegative
medium,providedtheoriginalauthorandsourceare logisticrelationshipswithsnowdepthandbulk(snowplusice)integratedextinctionco-
credited.
efficients;indicatingastronginfluenceofsnowcoverincontrollingbottomicealgalbio-
DataAvailabilityStatement:Theauthorsconfirm mass.ThemaximumbottomMYIchlaconcentrationwasobservedinahummock,
thatalldataunderlyingthefindingsarefullyavailable
representingthethickesticewithlowestsnowdepthofthisstudy.Hence,inthisand
withoutrestriction.Alldataareavailableinthepublic
otherstudiesMYIchlabiomassmaybeunder-estimatedduetoanunder-representation
repositoryPANGAEA.http://doi.pangaea.de/10.1594/
PANGAEA.842377. ofthickMYI(e.g.,hummocks),whichtypicallyhavearelativelythinsnowpackallowing
forincreasedlighttransmission.Therefore,wesuggesttheon-goinglossofMYIinthe
Funding:ThisstudywassupportedbytheAlberta
IngenuityScholarshipprogram(RN:200700172)to ArcticOceanmayhavealargerimpactonice–associatedproductionthangenerally
CH,NSERCDiscoveryGrantstoCH(356589-08) assumed.
andCM(http://www.nserc-crsng.gc.ca/index_eng.
asp),andtheCanadianFederalProgrammeon
EnergyResearchandDevelopmenttoCM(http://
www.nrcan.gc.ca/energy/funding/current-funding-
PLOSONE|DOI:10.1371/journal.pone.0122418 April22,2015 1/26
MYIvs.FYI:ChlorophyllaandPhysicalProperties
programs/perd/4993).Somein-kindsupportwas Introduction
providedbythePolarContinentalShelfProject
Arcticfirst-yearseaice(FYI),fromlowerlatitudeandshelfregions,isgenerallymoreproductive
(PCSP;http://www.nrcan.gc.ca/the-north/polar-
continental-shelf-program).BAL,JB,JAC,andIH thanmulti-yearice(MYI),whichleadstotheassumptionthatareplacementofMYIbyFYIwill
weresupportedbytheNorthernScientificTraining resultinanoverallincreaseofseaicealgalbiomass.Arcticseaicehasalreadyundergoneadra-
Program(NSTP)andCircumpolar-BorealArctic maticreductionofMYIwithpronouncedlossesoftheoldestandthickestMYI[1–3].InSeptem-
Research(C-BAR)program.Additionalfundingwas
ber2012,anewrecordArcticseaiceextentminimumwasset,farexceedingthepreviousrecord
providedtoHFandBALaspartoftheHelmholtz
minimumof2007,whichwasitselfaremarkabledeclinefrompreviousyears[4,5].Thedecline
YoungInvestigatorsGroup"Iceflux"(VH-NG-800).
Thefundershadnoroleinstudydesign,data ofsummerseaicehasoccurredconcurrentlywithanincreaseindurationofthemeltseasonand
collectionandanalysis,decisiontopublish,or changesinthetimingofmeltonsetandfreeze-up[5–8].Thesefindingsinconjunctionwithcli-
preparationofthemanuscript. mate-modelsimulations[9–12]demonstratethatcontinuedArcticwarminganddecliningArc-
CompetingInterests:Theauthorshavedeclared ticseaice,withthereplacementofMYIbyFYI,islikelytocontinueunabatedintothefuture,
thatnocompetinginterestsexist. havingprofoundconsequencesforclimatefeed-backs,physicaloceanprocesses,ecosystemlink-
ages,andArcticbiodiversity[5,13].
Therapidlossofseaicerepresentsanequallyrapidchangeinhabitatforseaicealgae,pro-
tists,andice-associatedfauna.Seaicealgaerepresentanimportantandhighqualityfood
source,directlyorindirectly,formanykeyorganismsfoundinpolarregions(e.g.copepods,
amphipods,seabirds,polarcod,seals,polarbears;[14–17]).IntheArctic,thetimingofice
algalgrowthisimportantforthereproductionandgrowthofkeygrazingzooplanktonspecies,
suchascopepods[18,19].Icealgaeprovidefoodforpelagicgrazersormaysinkatthetimeof
icemelttothebenthoswheretheyareconsumedbybenthiccommunitiesorsequesteredinto
thesediments(e.g.,[20]).Therefore,changesinicealgalbiomassanddistributionareexpected
tostronglyimpactArcticfoodwebsandtheArcticcarboncycle,whichcanhavecascadingim-
pactsonglobal-scaleecologicalinteractionsandtheglobalcarbonbudget.
Seaicedecline,thinningofArcticseaice,andthelossofMYIhaveresultedinreducedArctic-
wideseaicealbedo[21]andmorelightreachingtheunder-iceenvironmentinsummer[22].
Suchconditionshavebeensuggestedtobeconducivetothedevelopmentofundericephyto-
planktonblooms,whichmaybecomemoreprominentinthefuture[23].Reductionsinseaice
thicknessandextenthavealsobeenlinkedtoincreasesinprimaryproductionincoastalshelfre-
gions[24,25].However,currentandfutureestimatesforprimaryproduction,includingicealgal
andphytoplanktongrowth,inthecentralArcticOceanremainuncertain.Evenwithincreased
lightavailability,primaryproductionmaybelimitedbynutrientsupply,resultinginpartfrom
increasedsurfacewaterstratification[26].
Thedevelopmentofseaicealgalcommunitiesisinfluencedbyseaicemicrostructure(e.g.,sa-
linityandtemperaturewhichinfluencepermeability),nutrientsupply,andtransmittedirradi-
ance(seerecentreviewin[27]).Duringspring,themaininfluencesonunder-iceirradianceare
thesnowdepthdistribution,withsnowextinctioncoefficientsbetween4to80m-1[28–31],and
icethickness,toalesserextent,withextinctioncoefficientsbetween0.8to1.55m-1[28,32–34].
Initialgrowthofseaicealgae,duringearlyspring,isprimarilycontrolledbythesnowdistribu-
tion,whichistypicallyevidentbyanegativerelationshipbetweenchlorophylla(chla)andsnow
depth(e.g.,[35,36]).Duringtheprogressionofmelt,lighttransmissionincreasesduetochanges
intheopticalpropertiesofsnowandice[34,37].Consequently,icealgalgrowthincreasesand
shiftstoamorenutrient-limitedsystem,whichcanbeaccompaniedbyacombinationofother
limitingfactorssuchas:self-shading,diurnallightpatterns,oriceablation[38–40].Insomein-
stanceswhenlighttransmissionincreasesfasterthanalgalcommunitiescanadapt,theincreased
lightfieldcanreduceactivityandbiomassofalgalcommunitiesduetophoto-inhibition[41,42].
Icealgalgrowthandthebloomperiodareterminatedduringadvancedandrapidmelt[40].
Manystudieshavecharacterizedtherelationshipbetweensnowdepth,transmittedirradi-
ance,andchlaforFYI(e.g.,[35,36]),however,littleisknownabouttheserelationshipsfor
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MYIvs.FYI:ChlorophyllaandPhysicalProperties
Table1. SummaryofrelevantstudiesonArcticMYIchlorophyllabiomass.
Region Season Year(s) Study
Beaufort-ChukchiSeas Year-round 1997–1998 Melnikovetal.(2002)[77]*
FramStrait Winter 1993 Thomasetal.(1995)[76]
FramStrait Winter-Spring&Summer 2002&2003 SchünemannandWerner(2005)[49]*
GreenlandSea&FramStrait Spring-Summer 1997 WernerandGradinger(2002)[79]*
BeringSea Spring <1974 McRoyandGoering(1974)[78]
CentralArcticOcean Summer 1991&1994 Gradinger(1999)[81];Gosselinetal.(1997)[92]
Beaufort-ChukchiSeas Summer 2002&20032005 Gradingeretal.(2005)[51]Gradingeretal.(2010)[50]
GreenlandSea Summer 1994&1995 Gradingeretal.(1999)[95];WernerandZhang(2002)[79]*
BarentsSea Summer 1993 GradingerandZhang(1997)[96]
*studiesconductedduringmultipleseasons.
doi:10.1371/journal.pone.0122418.t001
MYI.Ingeneral,thelargemajorityofstudiesdealingwithicealgaeorchlabiomassfocuson
landfastFYI(e.g.,[43–47],seealsosummaryin[48]).Theselimitationsstemfromthelogistical
constraintsofsamplingwithintheArcticOcean,particularlywithinregionsdominatedbyMYI.
AsummaryofstudiesconcerningArcticMYIchlabiomass(Table1)revealstheneedfor
morerecentobservationswithinMYI-dominatedregionsduringtheonsetofalgalgrowth
(e.g.,ApriltoMay).TheavailableMYIstudiesareallcurrentlyovernineyearsoldwiththema-
joritycoveringthesummerseason(Table1).Thefourstudiesconductedduringthewinter-
springtransitionwereconductedwithintheBeringSea,GreenlandSea,FramStraitandBeau-
fortSea(Table1)leavingalargeportionoftheMYIcoveredArcticwithnoobservationsdur-
ingthistransitionalperiod.OfalltheseMYIstudies(Table1),nonecharacterizethechla-
snowdepthrelationshiporprovideadetailedcomparisonbetweenFYIandMYIchlabiomass
forthesameregion,whichcouldprovideinsightintoafutureArcticOceanwithlittleorno
MYI.MostoftheMYIstudieslistedinTable1,exceptforthethreemostrecentstudies(e.g.,
[49–51]),wereconductedinadifferentArcticsystemwhenthemeltseasonwasshorter[7],
temperatureswerecolder[52],seaicewasthicker[3],andMYIdominated[1].Thus,itmaybe
statedthatourcurrentunderstandingofArcticseaicealgaeandchlabiomassisbasedonob-
servationswithlimitedspatialandtemporalcoveragefromregionsthathaveexperiencedpro-
nouncedchanges.Asaresult,thereisaneedforadditionalMYIchlorophyllobservationstofill
importantspatial,temporalandseasonal(i.e.,springperiod)gaps.
Thenorth-easterncoastofCanada,includingtheLincolnSea,representsanimportantre-
gionasitishometosomeoftheoldestandthickesticeintheArcticandwilllikelybeoneof
thelastremainingrefugesforMYIinthefuture[1,5,53,54].Despitetheimportanceofthisre-
gion,weareawareofonlytwoseaicebiogeochemicalstudiesintheLincolnSea,characterizing
microbialcommunities[55]anddenitrification[56].TheLincolnSeaisoneofthelastremain-
ingplaceswherebaselineobservationsofolder(>3years)MYIbiogeochemicalpropertiesare
possibleandacomparisonbetweenMYIandFYIwouldprovidemuchneededinsightintothe
futureofArcticmarineecosystems.BasedonthelimitedstudiesofMYIandthespatialbiasof
FYIstudies,itisdifficulttoestimatehowArcticseaicealgalbiomasswillchangewithashiftto
aFYIdominatedsystem.
ThemaingoalofourstudywastodetermineifFYIhas,orhasthepotentialfor,higherchla
biomassthanMYIintheLincolnSeaanddiscusstheimplicationsofourresultsinthecontext
ofafutureArcticwithlittleornoMYI.Weaddressourscientificquestionfirstbyproviding
detailedanalysesofthephysicalpropertiesandchlaconcentrationsofseaice(bothMYIand
FYI)inthreeconsecutivespringseasons,fromaregionwherenosimilarstudieshavebeen
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MYIvs.FYI:ChlorophyllaandPhysicalProperties
Fig1.OverviewMapsofthestudyregionandicecoringsites.a)MapoftheArcticOceanwithanoutlineofthestudyregion.b)MapoftheLincolnSea
andneighboringregions.Driftingicesites(packice),oceanbathymetry,andanoutlineoflandfasticesitesareindicated.c)Mapoflandfasticecoringsites,
immediatelyoffshorefromCFSAlert.
doi:10.1371/journal.pone.0122418.g001
reported.Thenweevaluatepotentialdifferencesofice-algalchlaconcentrationsandbiophysi-
calseaicepropertiesbetweenicetypes,iceages,andtextureclasses.Lastly,weinvestigatethe
relationshipbetweenseaicechlaconcentrationsandenvironmentalproperties,suchassnow
depth,seaicestructure,andlightavailability.
MaterialsandMethods
Studyregion
Inordertoconductthisresearchprojectinaccordancewithregulationssetforthbythegoverning
agenciesresponsibleforthestudyregion,allrelevantresearchlicensesandpermissionswereac-
quiredfromtheNunavutResearchInstitute(Licensenumbers:02–07510R-M;02–10811R-M;
0201212R-M)andNunavutImpactReviewBoard(ScreeningDecisionReport08YN057).
TheLincolnSea(Fig1)hasbeencalledthe“LastIceArea”bytheWorldWildlifeFund
basedonrecommendationsfromArcticCouncilAssessments,indicatingthattheLincolnSea
requiresspecificattentionandresearch[57,58].Moststudiesinthisregionfocusonphysical
properties(oftheMYI)andhavedocumentedaslightdeclineinmodalicethicknesssince
2004frombetween4.0and4.5m(pre-2008observations)to3.5m(post-2008observations),
whichislikelytheresultoflessoldicealongthenortherncoastofCanada[5,54].
PLOSONE|DOI:10.1371/journal.pone.0122418 April22,2015 4/26
MYIvs.FYI:ChlorophyllaandPhysicalProperties
TheLincolnSeaisadynamicareaduetointeractionwith,andexchangeof,seaicewiththe
ArcticOcean.TheLincolnSeaicecoveriscomprisedofimmobilelandfastcoastalseaiceatthe
southernedgesandmobilepackiceatitsnorthernextent.Thelandfasticeconsistsprimarilyof
consolidatedpackicewithsmalleramountsofFYIformingintheinterstitialspaceduring
freeze-up.Thedivisionbetweenlandfasticeandpackiceisnotadistinctlinebutratheratransi-
tionalregionthatcanbecharacterizedbyicewithlimitedmobilityduetogeographicbarriers
andtheintermittentnatureoficeexporttothesouththroughNaresStrait.SeaiceintheLincoln
SeatypicallycomesfromtheCentralArcticOceantransportedbytheBeaufortGyreandTrans-
polarDriftcirculationpatterns[59,60].However,theoriginofseaiceintheLincolnSeaisun-
certainbecauseiceagesaretypicallybetween2to5years,withadecreasingproportionof>5
yearoldice[1].Thismeansthaticeinthisregioncouldhaveoriginatedfromanywhereinthe
ArcticOcean.
Sampling
SamplingwasconductedduringspringinthefirsttwoweeksofMay2010,2011and2012in
theLincolnSea.OnesitewaslocatednorthoftheLincolnSea(Fig1).Seaicecoresweretaken
atatotalof18sites:11MYIsites(4in2010,4in2011and3in2012),and7FYIsites(1in
2010,2in2011and4in2012),includinglandfasticeandmobilepackice(Fig1).Landfastice
siteswerevisitedbysnowmobile,andpackicesiteswerevisitedbyhelicopterorTwin
Otteraircrafts.
Duringthistimeofyear,whentemperaturesaretypicallybelow-10°Candthelowsalinity
MYIisveryhard,coringthickerthan~3.5mbecomesexponentiallymoredifficult.Wethere-
forechoserelativelylevelsitesthatweknewwerebelow~3.5mbasedonpre-drilled2inch
augerthicknessholes.
Ateachsitethreeicecoreswereextractedwithin1mofeachotherusinga9cminnerdiam-
etericecorer(KovacsEnterpriseMarkII)andstoredinsterileU-Linebags.Onecorewassam-
pledfortextureandbulksalinity(“Texturecore”),onecoreforchlorophylla(“chlacore”)and
onetotwocoresformicrobialgenetics(geneticmethodology/protocolandresultsfromone
MYIsite“1–11”presentedelsewhere,see[55]).Duetosmalldiscrepanciesbetweencore
lengthsatthesamesite,texturecorelengthswereadjustedtocorrespondtothechlacore
lengthbylinearlyinterpolatingeachdepthvalue(e.g.textureclass,bulksalinity,temperature
andbrinevolume)ofthetexturecoresproportionally.Allcoresweretransportedfromthe
fieldbacktotheCanadianForcesStation(CFS)Alert,Nunavut,Canada(82.5°N,62.5°W)and
storedat-15to-20°Cinthedark.
On-sitemeasurements
Ateachcorelocation,snowdepth(hereafterreferredtoascore-location-snow-depth),free-
boardandcorelengthweremeasured.Thesemeasurementsrepresentthelocalconditions(i.e.,
onesinglepoint)atsamplinglocations,whichshouldnotbeconfusedwiththelarger-scale
snowdepthandicethicknesssurveymeasurements.Internalicetemperaturesweremeasured
ontexturecoresbydrillingholesandinsertingathermometer(Testo720)immediatelyafter
coreextraction.Temperaturesweremeasuredfromsurfacetobottomatintervalsof0.1m
(cores:1–10,3–10and4–10)and0.5m(core5–10).In2012,onlyicesurfacetemperature
(depth0.1m)wasmeasured.Forthesecorestheinternalicetemperatureswerelinearlyinterpo-
latedbetweenthesurfaceandassumed(theoretical)bottomtemperatureof-1.78°Cwithtypical
surfacewatersalinitiesintheLincolnSeaof~32[61].Duringthestudyperioddailytemperature
variationwithintheicewasminimalandbasedonthemeasuredtemperatureprofilesalinear
relationshipwithicedepthdemonstratedagoodfit(R2=0.94).Brinevolumeestimateswere
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Table2. Descriptionofeachseaicetextureclass.
IceClass Description
Snow-Ice Lookslikegranularbutisclearinun-polarizedimages.Formsduringfloodingorwith
presenceliquidwaterandsnownearfreezingandformssmallgranularcrystalsduring
rapidfreezing.
MeltPond Freshwater,clearinappearance,atorverynearthesurfaceoftheice.Sometimes
overlaidbysnow-ice.
Retextured Clearicewithunusualcrystalsorverylargecrystals,formsnearsurfacebelowwater
level
Deteriorated Transformedcolumnarormixedicewithlargebrineorairpockets,nearthesurface
usuallyabovewaterlevel.
Granular Consolidationoffraziliceusuallynearthesurface(typicallywithmixedlayerunderneath).
ThiscanoccurwithinMYIandisevidenceofsuper-cooling,turbulentwaterand/or
presenceofadjacentre-freezingleadwhichcreatesconditionsforrapidfreezingand
formationoffrazilice.
Mixedcol./ Mixtureofcongelationandgranularice.Thisclassalsoincludesintermediate
gran. congelation/granularicebecausetheyaredifficulttodistinguish.
Columnar Elongatedcrystals
doi:10.1371/journal.pone.0122418.t002
calculatedforcoreswithtemperaturemeasurementsusingequationsin[62].Brinevolumeval-
uesarereportedinpartsperthousand(ppt).
Snowdepthandicethicknesssurveyswereconductedalongtransectsadjacenttoeachcoring
site.Thesemeasurementsrepresentthelarger-scalecharacteristicsofthesampledicefloes,which
shouldnotbeconfusedwiththepointmeasurements:core-location-snow-depthandcorelength.
Snowdepthwasmeasuredusingametalprobeat1or10mintervalsandicethicknesswasmea-
suredin2inchaugersholesdrilledintheiceat10mintervals.Thelengthofeachtransectand
numberofmeasurementsweredependentonicetypeandtimeconstraints(range=0to400m,
mean=100m).Snowdensitymeasurementswerecalculatedfor5snowsamplescollected,atsite
2–10,usinganAdirondacksnowsampler.Additionaldensityvaluesfromthesamestudyregion
wereacquiredduringtheCryoSatValidationExperiment(CryoVEx:11to18April,2011[63]).
AirtemperaturedatawereprovidedbytheEnvironmentCanadaweatherstationlocated
onshoreatCFSAlert,Nunavut.Meandailyairtemperaturesduringthestudywereonaver-
age-12.5°C(2010to2012combined),witharangebetween-20.3to-7.4°C,andamaximum
temperatureof-3.1°C.Downwellingtotalsolarirradiancemeasurementsrepresentativeofthe
samplingareawerealsomeasuredatthenearbyCFSAlertweatherstation.Meanandrange
ofvalueswerecalculatedfortheperiodMay1–11,2010to2012(mean=984,range=213
to2313μmolphotonsm-2s-1;dataacquiredfromNOAA/ESRL/GMD/GRAD,the
GMD-RadiationGroup,ftp://aftp.cmdl.noaa.gov/data/radiation/baseline/alt/).
Texturecores
Textureanalysiswasconductedat~-15°C.Texturecoreswerecutinto0.10to0.15mvertical
sectionsthatwerefurthercutintoverticalthicksections~<5mmthin,usinganelectricband
saw.Foreachsection,theiceremainingaftercuttingwasputintoplasticcontainers,melted
andanalyzedforbulksalinityusingasalinometer(WTW3300i).Bulksalinitiesarereportedin
partsperthousand(ppt).Thicksectionswereimagedundercrossedpolarizers.Analysisofthe
imagesprovidedastratigraphicdescriptionofeachicecorebyidentifyingdifferenticetexture
classesandtheboundariesbetweenclasses.Herewedividedicetypesinto7textureclasses
basedongrainstructureandappearance,followingclassificationsystemsoutlinedin[64–67]
(Table2andFig2).Foreachsectionofthechlacores,thedominanttextureclass(i.e.the
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MYIvs.FYI:ChlorophyllaandPhysicalProperties
Fig2.Crosspolarizedimageryoficecorethinsectionsshowingdifferenticetypes.a)core7–12;b)
core7–12;c)core2–10;d)core3–11;e)core1–12;andf)core7–12.
doi:10.1371/journal.pone.0122418.g002
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MYIvs.FYI:ChlorophyllaandPhysicalProperties
textureclasswiththehighestarealcoverage)fromthecorrespondingtexturecorewasassigned.
Forsurfacepiecestheuppermosttextureclasswasalwaysassignedtothesectionbecausedete-
rioratediceandsnow-icearedistinctlayersandonlylocatedatthesurface.
InMYIcores,weidentifiedthepreviousyear’sannuallayerin7coresbyidentifyingpeaks
inbulksalinityprofilesthatcorrespondedwithchangesincrystalstructureatthesamedepth,
asdescribedin[68,69].Accordingly,MYIcoresweredividedintotwogroups:thesections
abovetheannuallayer(olderice)wereclassifiedasmulti-year(MY)andsectionsbelowthean-
nuallayer(newice)wereclassifiedasfirst-year(FY).Therefore,inlateranalysesicetypecate-
goriescompriseMYIcoresandFYIcores,whereasiceagecategoriescompriseMY(multi-year
sectionsofMYIcores),FY(first-yearicesectionsofMYIcores)andFYI(first-yearicecores).
MYportionsrepresenticethathassurvivedatleastonesummer.TheFYportionsrepresent
icethathasgrownundertheMYportionsandtypicallybeginstoformduringfreeze-upinSep-
tember-October[7].IngeneralFYIandFYrepresenticeofsimilarages(e.g.<1year),howev-
er,FYIcanrepresenticethatstartedtoformeitherduringfreeze-uporatalaterstageasopen
waterleadsformandrefreeze.FYalsogrowsslowerthanFYIduetoincreasedthermalinsula-
tionofthethickerice(MY)andsnowlayeraboveit.
Chlorophyllacores
Inordertominimizeanypotentialinfluencesonthechlameasurements,thesecoreswereal-
waysstoredbelow-15°Cinthedark,foramaximumofninedays.Thecoreswerethen
shipped,viaairatamaximumtemperatureof-10°Cinthedark,toResoluteBay,Nunavut,
wheretheywerestoredbelow-20°Cinthedarkfor1to2days.Coreswerecutusinganelectric
bandsaw(sterilizedwith95%ethanol)ina-20°Cwalk-infreezer.Coreswerecutinto10cm
sectionsexceptforendpieces(range:0.09to0.17m),placedinsterileWhirl-Pack(NASCO)
bagsandmeltedinthedark.Endpiecesfor2010and2012samplesrefertotheice-airinterface.
The2011coreswerecutintheoppositedirection,thereforetheseendpiecesrefertotheice-
waterinterface.Coresectionsweremeltedwithouttheadditionoffilteredseawater(FSW)be-
causewewerealsomeasuringdissolvedconstituents(i.e.DOC,nutrients;datanotpresented
here)inthecoresections.Ithasbeenshownthatforcommonbiologicalanalyses(e.g.,chloro-
phyllaconcentrations)meltingwithouttheadditionofFSWisanacceptableprocedure[70].
Chlaconcentrationsweredeterminedonsub-samplesfilteredontoWhatmanGF/Ffilters,
after24hextractionin90%acetoneat4°Cinthedark,usinga10AUTunerDesignfluorome-
tercalibratedwithpurechlorophyllextractfromAnacystisnidulans(Sigma;[71]).Chlacon-
centrationsweredeterminedusingequationsfrom[71]andcorrespondinginstrument
calibrationcoefficients.Chlaconcentrationsareexpressedvolumetrically(mgm-3)orarever-
ticallyintegrated(mgm-2)forthebottom0.2mcoresections(hereafterreferredtoasbottom-
integrated),ageclasscoresections,ortotalcorelength.
Statisticalanalyses
Initialdataexplorationdemonstratedthatthedistributionsofchladatawerehighlyskewed.To
achievethenormaldistributionpatternsrequiredforparametricstatisticalanalyses,log-trans-
formationswereappliedtothechladata.Two-samplet-testswereconductedtodeterminethe
effectoficetype(MYI&FYI)onthechlaconcentrationsoftheseaice.Varianceanalyses
(ANOVAs)wereconductedinordertodeterminetheeffectoficeageclass(MY,FY&FYI);
yearandtextureclassonthechlaconcentrationsoftheice.Post-hoctests(Tukey’sHSD)were
conductedwhenbothparametricANOVA(transformeddata)andnon-parametricKruskal-
Wallis(non-transformeddata)analysesshowedsignificantdifferences.
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MYIvs.FYI:ChlorophyllaandPhysicalProperties
Toinvestigatethepotentialinfluenceofsnowdepthandseaiceopticalpropertieson
bottom-integratedchlaconcentrationsalogisticregressionmodelwasapplied.Alogisticre-
gressionwasusedinordertoidentifypotentialcriticalvalues(inflectionpoint)oftheindepen-
dentvariables(e.g.,snowdepthandbulkintegratedextinctioncoefficients)thatcouldindicate
athresholdvalueforoptimalalgalgrowth.Logisticregressionanalysisrequiredvaluesofthe
dependentvariableintherange0to1.Therefore,bottom-integratedchlavalueswerenormal-
izedtotherange0to1bydividingeachchlavalue(y)bythemaximumvalueforallcores
i
(y ).Therelationshipbetweenbottom-integratedchlaconcentrationsandbulkintegrated
max
extinctioncoefficients,forvisibleradiation,wasanalyzedinasimilarmanner.Forallcalcula-
tionsweusedextinctioncoefficientsforsnow:k =20.0m-1[28];MYI:k =1.55m-1;andFYI:
s m
k =1.45m-1[32].Thevalueofk,usedhere,waschosenfromatableofk values[28]basedon
f s s
acorrespondingsnowdensitycomparabletomeasuredvaluesforourstudyregionbetween
260to281kgm-3(seesectionPhysicalproperties).Thevaluesofk,k ,andk wereintegrated
s m f
overthedepthofthecorrespondingiceandsnowlayersforeachcoresiteresultingin“integrat-
edextinctioncoefficients”(dimensionless),i.e.,avalueforeachsnowandicelayerateachsite.
The“bulk(snowplusice)integratedextinctioncoefficient”issimplythesumoftheintegrated
extinctioncoefficientsforsnowandice(alsodimensionless),i.e.,onevalueforeachcoresite.
Intheresultingbulkintegratedextinctioncoefficients,largervaluesmeanshallowerpenetra-
tionoflight.
Resultsarereportedasarithmeticmean±onestandarddeviation(μ^±1σ).
AllstatisticalanalyseswereconductedwiththeRsoftwarepackagev-2.15.2[72].
Results
Physicalproperties
Basedonsite-averageddrillholethicknessmeasurements,i.e.,characterizingthelargerscale
icepropertiesofthesampledfloes,MYIsitesweremorethantwiceasthick(3.28±0.56m)as
FYIsites(1.42±0.42m).MeanMYIcorelength(2.62±0.24m),i.e.,onlycharacterizinglocal
icepropertiesofcoresamplinglocations,wasalsonearlytwiceasthickasFYI(1.39±0.52m),
withhighervariabilityinFYIcorelengths.FYIsitesrepresentedtwodifferentkindsofice:ice
thatformedduringthefallwhenthelandfasticeconsolidated(i.e.,olderandthickerFYIwith
moresnow);andicethatformedlaterinmobileicewhenopenleadsformedandthenrefroze
(i.e.,youngerandthinnericewithtypicallylesssnow).
Site-averagedsnowdepthatMYIsites(0.39±0.10m),ingeneral,wasthickerthanatFYI
sites(0.26±0.15m).AlthoughMYIhadlowervariabilitybetweensite-averagedsnowdepth
values,FYIhadlowervariabilitywhenconsideringeachsiteindividually.Thiswasillustrated
bythemeanofsitestandarddeviationsforFYIsnowdepth:0.08m,comparedtoMYI:0.17m.
Meansnowdensityatsite2–10(Fig1)was260±0.03kgm-3(n=5)andduringCryoVex2011
was281±0.7kgm-3(n=11[63]).
Therewasnosignificantinter-annualdifferenceinthemeanphysicalproperties(e.g.snow
depthandicethickness)ofFYIorMYI(ANOVA,p>0.05).ForFYI,thereweresignificantpos-
itiverelationshipsbetweencore-location-snow-depthandicecorelength(R2=0.56,p=0.05,
n=7),andbetweensnowdepthandmeanicethicknesssurveymeasurements(R2=0.66,
p<0.05,n=7).ForMYI,therewasaninverserelationshipbetweensnowdepthandicecore
length(R2=0.51,p=0.01,n=11),andnosignificantrelationshipbetweensnowdepthand
meanicethicknesssurveymeasurements(R2=0.12,p=0.3,n=11).
TwoFYIcoreswereexceptional(3–10and6–12;Table3),exhibitingconsiderablylowerice
thickness,snowdepthandcorelengththantheotherFYIcoresanalyzedinthisstudy.These2
sitesweredeterminedtoberefrozenleadsthatcorrespondtoyoungerFYIthatformedlaterin
PLOSONE|DOI:10.1371/journal.pone.0122418 April22,2015 9/26
MYIvs.FYI:ChlorophyllaandPhysicalProperties
0 58 7 14 6 6
det - - - 0.1 0.000. - -- - 0.1 - 0.101. - - - -- - 0.0 -00. ulk
b
6 6 9 6 8 2 75 = ar,
TextureClassLengths xgranretexmps-i 030.02--- 40--0.16- 190.27-0.220.1 000.72--- 072.27---66109-01901... 061.32--- 110.01---08066---. 24-0.180.390.0 520.81--- -1.010.260.0 13----3008105903300.... 070.28--- 040.05--- 34--- 050.01---13011---. 81--0.040.0 19---- 29-0.300.110.043-03000700... core-location-snow-depth,sal meanfreeboard,col=column
m 0. 1. 0. 1. 0.0. 0. 0.0. 0. 0. - 0.0. 0. 0. 0. 0.0. 0. 0. 0.0. = =
w b
o f
col 0.88 0.67 1.94 0.73 0.71101. 0.23 1.40082. 1.51 1.45 1.20 2.35163. 1.42 1.25 1.42 0.77121. 1.79 2.31 1.69931. h,sn mber,
gt u
s 1 1 0 14 n n
vey N 10 6 42 1 4123 1 53 10 12 9 1010 10 21 1 4141 40 14 2225 ele ple
owDepthSur σd 80.045 70.1100 30.2200 0-0 20.2200602125. 8-0 1-400-20 60.2100 20.2100 90.2100 50.2100602100. 80.1100 30.120 7-0 70.04090140. 90.2400 00.1140 00.2200602247. etry,core=cor ance,N=sam
Sn μ^ 0.0 0.3 0.3 0.5 0.203. 0.3 0.203. 0.3 0.5 0.3 0.504. 0.2 0.3 0.4 0.002. 0.3 0.3 0.403. ym dist
07 13 23 09 3219 04 0706 29 17 33 1724 11 05 06 0005 24 29 2024 bath d=
IceThicknessSurveys μ^σdNfb 0.93-2540. 3.070.910060. 3.490.420090. 2.45-010. 3.100.720090.3030712560... 1.67-030. 1.310.24050.149022040... 3.851.1100100. 2.800.6100120. 4.481.410090. 3.020.8100100.35410100100... 2.00-10040. 1.510.12020. 1.68-010. 0.830.14030.151014030... 3.690.9400110. 3.231.312070. 2.900.5200100.3270924090... ude,lon=longitude,bath.=σ=onestandarddeviation, w-ice,det=deteriorated.
hcoreandcoresite. CoreLocation CoreSnowSalBV 0.930.096.1189.4 2.230.423.188.1 2.800.323.4- 2.540.502.0133.0 3.110.052.173.326703226982.... 1.600.384.5- 1.520.175.5-15602850-... 2.410.553.5- 2.960.322.1- 2.530.323.9- 2.590.302.4-26203730-... 1.770.204.490.4 1.340.333.7- 1.770.473.187.7 0.830.045.9141.4143026431065.... 2.670.473.3116.5 2.570.312.0115.9 2.460.603.6184.0256046301388.... ationsandsymbols:lat=latitμ^me(ppt),=arithmeticmean, ed,mp=melt-pond,s-i=sno
foreac bath. 257 91 95 199 90119 189 242215 100 199 159 260180 57 99 2156 125609 86 125 99103 Abbrevi nevolu retextur
data n 2.0 2.6 2.3 2.0 2.6 2.3 2.2 2.4 2.0 2.2 6.0 2.7 2.4 8.1 8.6 2.7 8.6 2.4 (m). dbri ex=
Table3.Summaryofphysical SiteOverview YearTypeidlatlo –2010FYI31082.6-6–2010MYI11082.5-6–MYI21082.8-6–MYI41082.6-6–MYI51082.5-6μ^--- –2011FYI21182.6-6–FYI41182.6-6μ^--- –2011MYI11182.5-6–MYI31182.6-6–MYI51182.6-6–MYI61183.5-6^---μ –2012FYI21282.5-6–FYI31282.5-6–FYI41286.1-7–FYI61282.9-5μ^--- –2012MYI11282.5-6–MYI51282.9-5–MYI71282.5-6μ^--- Allmeasurementsareinmeters salinity(ppt),BV=coreaverage mx=mixed,gran=granular,ret“”-=Notavailable. doi:10.1371/journal.pone.0122418.t003
PLOSONE|DOI:10.1371/journal.pone.0122418 April22,2015 10/26