蜂蜜系列四(终)：然后,牵起季风-骸云 30页

ProgressinOceanography59(2003)130www.elsevier.com/locate/poceanDistributionsofCalanusspp.andothermesozooplanktonintheLabradorSeainrelationtohydrographyinspringandsummer(19952000)∗E.J.H.Head,L.R.Harris,I.YashayaevDepartmentofFisheriesandOceans,OceanSciencesDivision,BedfordInstituteofOceanography,P.O.Box,1006Dartmouth,N.S.,B2Y4A2,CanadaReceived10April2003;accepted28June2003AbstractWecollectedmesozooplanktonsamplesintheupper100minspringorearlysummereachyearbetween1995and2000alongasectionfromHamiltonBank(Labrador)toCapeDesolation(Greenland),andalongadditionalsectionsinspring1997andearlysummer1995.TheNorthAtlanticwatersofthecentralbasinwerecharacterisedbythepresenceofthecopepodsCalanusfinmarchicus,EuchaetanorvegicaandScolecithrocellaminorandeuphausiids.Cal-anusglacialis,CalanushyperboreusandPseudocalanusspp.wereassociatedwiththeArcticwatersovertheshelves.Amongsttheotherenumeratedgroupslarvaceanswereconcentratedovertheshelvesandaroundthemargins.Amphi-pods,pteropodsandthecopepodsOithonaspp.andOncaeaspp.showednodefinablerelationshipswithwatermassesorbathymetry,whilethedielmigrantostracodsandchaetognathswereconfinedtodeepwater.Metridalonga,alsoastrongdielmigrant,andMicrocalanusspp.,amainlydeepwaterspeciesandpossibledielmigrant,werebothsometimesquiteabundantontheshelvesaswellasinthecentralbasin,consistentwiththeirlikelyArcticorigins.AnalysisofcommunitystructurealongthesectionacrosstheLabradorSeaindicatedthatstationscouldbegroupedintofivedifferentzonescorrespondingto:theLabradorShelf;theLabradorSlope;thewesternandcentralLabradorSea;theeasternLabradorSeaandGreenlandSlope;and,theGreenlandShelf.Theboundariesbetweenzonesvariedspatiallybetweenyears,butcommunitycompositionwasrelativelyconsistentwithinagivenzoneandagivenseason(springversusearlysummer).Therelationshipbetweencommunitycompositionandwatermasseswasnotentirelystraightforward.Forexample,LabradorShelfwaterwasgenerallyconfinedtotheshelf,butinspring2000whenitalsodominatedtheadjacentslopezone,thecommunityintheLabradorSlopezonewassimilartothosefoundinotheryears.Conversely,inspring1997,whenArcticorganismswereunusuallyabundantintheLabradorSlopezone,therewasnoincreasedcontributionofshelfwater.Inaddition,NorthAtlanticorganismswereoftenfoundontheshelveswhennoslopeorcentralbasinwaterwaspresent.Althoughotherorganismsweresometimesveryabundant,themesozooplanktonpreserveddryweightbiomasswasdominatedeverywherebythethreespeciesofCalanus,whichtogetheralwaysaccountedfor
70%.Onespecies,C.finmarchicus,comprised60%ofthetotalmesozooplanktonbiomassand80%oftheabundanceoflargecopepodsinspringandsummerthroughoutthecentralLabradorSea.Inwesternandcentralregionsofthecentralbasinaverage22C.finmarchicusbiomasswasca4gdryweightmandaverageabundance,ca17000moverbothseasons.Highest∗Correspondingauthor.E-mailaddress:heade@mar.dfo-mpo.gc.ca(E.J.H.Head).0079-6611/$-seefrontmatterÓ2003ElsevierLtd.Allrightsreserved.doi:10.1016/S0079-6611(03)00111-3 2E.J.H.Headetal./ProgressinOceanography59(2003)1–30levels(ca7gdryweightm2,100000m2)occurredinthenorthernLabradorSeainspringandineasternandsouthwestregionsinearlysummer.C.hyperboreuscontributedca20%ofthetotalmesozooplanktonbiomassinthecentralbasininspringand5%inearlysummer,whileC.glacialisaccountedfor1%.Overtheshelves,C.hyper-boreuscontributedamaximumof54%and3.6gdryweightm2,andC.glacialis,amaximumof29%and1gdryweightm2,tothetotalmesozooplanktonbiomass.Ó2003ElsevierLtd.Allrightsreserved.Keywords:NWAtlantic;LabradorSea;Annualsurveys(19952000);Mesozooplanktondistribution;Calanusspp.Contents1.Introduction..........................................................32.Materialsandmethods....................................................42.1.Zooplanktonsampling..................................................52.2.Analysisofzooplanktoncommunitystructure....................................52.3.Hydrographicmeasurements...............................................53.Results.............................................................53.1.Broad-scalespatialdistributionsofmesozooplankton................................63.1.1.Spring,MayJune1997...............................................63.1.2.Earlysummer,July1995..............................................93.2.InterannualpatternsofmesozooplanktonabundanceandbiomassdistributionalongtheL3section...113.3.HydrographicconditionsintheLabradorSeaduringspringandsummer...................154.Discussion..........................................................184.1.Distributionsofmesozoooplanktoninspringandearlysummer–broadscalesurveysandtimeseriesontheL3section......................................................184.1.1.Calanusfinmarchicus................................................194.1.2.Calanushyperboreus................................................214.1.3.Calanusglacialis..................................................224.1.4.Euchaetanorvegica.................................................224.1.5.Pseudocalanusspp..................................................234.1.6.Metridialonga....................................................234.1.7.Oithonaspp......................................................244.1.8.Oncaeaspp......................................................244.1.9.Microcalanusspp...................................................244.1.10.Scolecithrocellaminor...............................................244.1.11.Chaetognaths.....................................................254.1.12.Euphausiids......................................................254.1.13.Amphipods......................................................254.1.14.Larvaceans......................................................264.1.15.Pteropods.......................................................264.1.16.Ostracods.......................................................264.2.RelationshipbetweenzooplanktoncommunitystructureandthedistributionofwatermassesalongtheL3section...........................................................275.Summary...........................................................28 E.J.H.Headetal./ProgressinOceanography59(2003)1–3031.IntroductionTheLabradorSeaisanareawheretheeffectsofglobalwarmingarelikelytobefeltinfuturedecadesandcenturies.Modelpredictionsincludefresheningofthesurfacelayers,withincreaseddischargeofArcticice-melt,andwarmingduetoincreasedairtemperatures(e.g.Rahmstorf&Ganopolski,1999;ICPP,2001).Modelsalsosuggestthatoveralongertimescaletheremaybeaslow-down,orshut-downofthedeepconvectivemixingthatisanimportantelementofthecurrentglobalthermohalinecirculation.Howsuchchangesinhydrographymighteffectthemarineecosystemisasubjectforspeculation.Itisknown,how-ever,thatdecadalchangesinthedistributionsofseveralkeymesozooplanktonspecieshaveoccurredinvariousregionsoftheNorthAtlanticthatareapparentlyrelatedtoclimaticvariations(Fromentin&Planque,1996;Greene&Pershing,2000;Johns,2001).InordertofollowtheeffectsofglobalwarmingintheLabradorSeainfuture,itisclearthatweneedadetaileddescriptionofpresentconditions.ScientistsattheBedfordInstituteofOceanographyarecurrentlyinvolvedinaprogrammeaimedatthelong-termmonitoringofhydrographicandbiologicalconditionsintheLabradorSea.Here,wewillpresentresultscollectedduringthefirst6yearsofsystematicbiologicalobservations,andwewillcompareourfindingsonthedistributionsofthemesozooplanktonwiththefewreportsalreadypresentinthescientificliterature.TheLabradorSeaformspartoftheNorthwestAtlanticsub-polargyreandcontainsmainlyrelativelywarm,saltywaterofNorthAtlanticorigininitscentraldeepbasin.Itsshelvesandshelfmargins,however,arecoveredmainlybycoolerfresherArcticwater.Inthewest,theBaffinIslandCurrentandtheoutflowfromHudsonStraitprovidethesourceofwatertotheLabradorShelfandcontributetotheshelfandshelf-breakportionsoftheLabradorCurrent(Loder,Petrie,&Gawarkiewicz,1998).Intheeast,theEastandWestGreenlandCurrentsprovidetheArcticwatersourcestotheGreenlandShelfandoffshoreoftheshelf-breakthereisalsoinflowofwarmersaltierIrmingerSeawaterviatheIrmingerCurrent.ItistobeexpectedthattheoccurrenceofdifferentwatermassesinthedifferentregionsoftheLabradorSea,itsmarginsandfringingcontinentalshelveswillaffectthedistributionandcompositionofthemeso-zooplankton.Forexample,thecopepodCalanusfinmarchicusisassociatedwithAtlanticwatersnorthoftheGulfStream(Planque,Hays,Ibanez,&Gamble,1997),whiletwocongenericspecies,C.glacialisandC.hyperboreus,aremorecharacteristicofArcticwaters(Conover,1988).ContinuousPlanktonRecorder(CPR)datahaveshownthatconcentrationsofC.finmarchicusalongthesouthernfringeoftheLabradorSeaareamongstthehighestintheNorthAtlantic.ThesamerecordsalsoshowthatconcentrationsofC.hyperboreusandC.glacialisaregenerallyhigherinthewest(overtheNewfoundlandShelf)thanintheeast(Johns,2001).Thesedatawere,however,mostlygatheredalongasectionbetweenReykjavik(Iceland)andSt.Johns(Newfoundland)andinformationfromfarthernorthismorelimited.Therearetheobser-vationsofzooplanktonabundanceoveranannualcycleatOceanStationBravoin1950(Kielhorn,1952)andalongsectionsacrosstheGreenlandShelfandintotheIrmingerandLabradorSeasin1963(ICNAF,1968).Also,therearetheobservationsofCalanusspp.distributionsbyGrainger(1963)inBaffinBay,HudsonBayandthenorthernLabradorSea,andofthemajorzooplanktonspeciesbyHuntley,Strong,andDengler(1983)inDavisStraitandinnorthwestLabradorSea.Morerecently,wehavedescribedthestagedistributionsofC.finmarchicusatstationscoveringarelativelylargeareaoftheLabradorSeainMay/June1997(Head,Harris,&Campbell,2000).There,wealsodiscussedhowdifferencesinthestateofdevelopmentofpopulationsindifferentareaswererelatedtothedynamicsoflocallyoccurringspringblooms.Here,wewillpresentinformationwehavecollectedonthedistributionsofthedominantmeso-zooplanktonspeciesandgroupsduringcruisestotheLabradorSeafora6yearperiodbetween1995and2000.Thisisfromstationsthatcoveredrelativelylargespatialareasinspring1997andearlysummer1995,andfromstationsalongasection(L3section,Fig.1)betweenLabradorandGreenlandfor3springand3earlysummerperiods.FortheL3sectionwewillalsocomparethezooplanktondistributionswithhydrography.WewillreportonthestagedistributionsofCalanusspp.forthesamestationsduringthe 4E.J.H.Headetal./ProgressinOceanography59(2003)1–30Fig.1.Mesozooplanktonsamplingstationsoccupiedduring6yearsofsampling(19952000)intheLabradorSea,superimposedonacompositeimageofseasurfacetemperatureovertheperiodMay1531,1997,collectedbytheNOAA14satellite.Whitepatchescorrespondeithertoice(alongtheGreenlandandLabradorcoasts)orsustainedcloudcover.AllstationsoftheL3section(greensquares)areshown,butnotallweresampledeveryyear.Additionalstationssampledinspring1997(orangecircles)andinearlysummer1995(yellowtriangles)arealsoshown.Thecontourcorrespondstothe200misobath.same6yearsofsamplingelsewhere,anddiscussseasonal,interannualandspatialvariationsandhowtheyrelatetohydrographicconditionsandspringbloomdynamics(Head&Harris,2003).2.MaterialsandmethodsBiologicalsampleswerecollectedandhydrographicmeasurementsmadealongtheL3section(=WOCEAR7Wsection,Fig.1),betweenHamiltonBank(LabradorShelf)andCapeDesolation(Greenland)every E.J.H.Headetal./ProgressinOceanography59(2003)1–305year.SamplingalongtheL3sectionoccurred:July1013(1995);May1827(1996);May2128(1997);June26July2(1998);July19(1999);May24June5(2000).Therewasmoreextensivespatialcoveragealongadditionalsectionsin1995(Samplingperiod,July6July18)and1997(SamplingperiodMay13June9).2.1.ZooplanktonsamplingZooplanktonwerecollectedinverticalnethaulsintheupper100musinga0.75mdiameterringnetfittedwitha200mmmesh.Thecod-endwasattachedviaaclamptoaweightedhydro-wireandthetowingbridlewasattachedtoacrossbowmountedonthewireataheightabovethecod-endsuchthatthenetwasheldvertically.Inthisconfigurationzooplanktonwereonlycollectedasthenetwastowedupwards.Thetowingspeedwasca0.5ms1andthevolumeofwatersampledwasassumedtobethevolumeofthecylindersampledbythenet.Sampleswerepreservedin2%formalin.ForC.finmarchicus,C.hyper-boreusandC.glacialis,sizefrequencydistributionsofsizes-at-stagewereconstructedforallstagesateachstation.Thesewereusedtodistinguishtheearlycopepoditestages(CICII)ofC.finmarchicusfromthoseofC.glacialis.Therewasnosizeoverlapinthelaterstages(CIIICVI).Othertaxawereidentifiedtothelevelofspecies(sometimestostage),genusorgroup,dependingontheirabundanceinthesamples.DryweightsweredeterminedforgroupsofindividualsbelongingtoeachstageoftheCalanusspp.andforvariousotherindividualspeciesorgroupsofspecies.Sub-samplesweredriedat60°Cfor13daysbeforeweighing.Dryweightbiomassdistributionsarepresentedwithoutcorrectionforformalinpreser-vation.2.2.AnalysisofzooplanktoncommunitystructurePatternsofabundancedistributionwereanalysedforindividualyearsusingthePRIMER5(PRIMER-ELtd,2000)softwareusingallcategoriesofmesozooplanktonidentifiedaboveforsamplescollectedalongtheL3section.Thedataweredouble-squareroottransformedtoreducetheimportanceofdominantgroupsandplottedasmultidimensionalscaling(MDS)plots,whichdisplaytherelativesimilaritybetweensamplesasadistancebetweenpointsinatwo-dimensionalrepresentation.Samplesappearedinanumberofclus-teredgroups,withinwhichsampleswererelativelysimilar,andbetweenwhichtheywererelativelydiffer-ent.Thestresslevelisameasureofthequalityoftherepresentation:lowstress(0.1)correspondstoagoodrepresentation;highstress(0.2),apoorone.Inourdata,inonlyonecase(1998)wasthestresslevel0.1andinthatcaseitwas0.11.2.3.HydrographicmeasurementsTemperature(T)andsalinity(S)profileswerecollectedusingaSeabird(ModelSBE25)sensormountedonaCTDrosette.Analysiswasperformedforeachyearbydividingthesectionintowesternandeasternportionsforeachcruise,andbyexaminingthedistributionofTversusSat1mintervalsbetween0and100m.WaterclasseswereidentifiedasclustersofpointssharingsimilarTScharacteristics.Pointsdistrib-utedbetweentheclusterswereassumedtobemixturesofwaterclasses,whoseproportionscouldbedeterminedbytheirrelativedistancesfromthecentresoftheclusters.3.ResultsInthefollowinganalysiswerecognisedtwosamplingseasons:spring,whichcorrespondstoMay/earlyJuneandearlysummer,whichcorrespondstolateJune/July.Thesedesignationsaremadeonthebasisof 6E.J.H.Headetal./ProgressinOceanography59(2003)1–30temperatureconditionsandnutrientconcentrationsinthenearsurfacelayersofthecentralLabradorSearegionoftheL3section.Temperatureswerelowerandsimilarinspringandhigherandsimilarinearlysummer(seebelow),whilenutrientconcentrationswerehighinspringandreducedbyaboutonehalfinearlysummer(datainHead&Harris,2003)3.1.Broad-scalespatialdistributionsofmesozooplankton3.1.1.Spring,May–June1997CalanusfinmarchicuswasfoundateverystationsampledduringMayJune1997(Fig.2).AbundanceswerehighestinthenorthandattheshelfmarginsofthemostnortherlysectionacrosstheLabradorSea.Thesehighabundanceswereassociatedwiththepresenceoflargenumbersofyoungcopepoditesoftheyearsnewgeneration(Headetal.,2000).Atotherstations,particularlyinthecentralLabradorbasinandsouthofGreenland,overwinteredadultfemalesweredominant.Thus,thedistributionofC.finmarchicusbiomasswasmoreeventhanthatofabundance(Fig.3).C.finmarchicusbiomassgenerallyexceededthatofeveryotherzooplanktoncategoryatdeepwaterstations(cf.Figs.3and4).Calanushyperboreusabun-danceandbiomasswerehighestinnorthernandwesternregions,loweradjacenttoorontheGreenlandFig.2.Abundanceofcopepods(0100m)intheLabradorSeainspring,MayJune1997.Abundancesareproportionaltotheareasofthefilledcirclesthatareatthesamplingpositions.Eachpanelisscaledtothemaximumvalue(N)shownaboveit.ThesizesofthesymbolsrepresentingN,N/4,N/10andN/40areshownatthebottomofthefigure.Contourscorrespondtothe200,1000and3000misobaths. E.J.H.Headetal./ProgressinOceanography59(2003)1–307Fig.3.Preserveddryweightbiomassofcopepods(0100m)intheLabradorSeainspring,MayJune1997.Biomassconcentrationsareproportionaltotheareasofthefilledcirclesthatareatthesamplingpositions.Symbolscalingandcontourlines,asinFig.2.ShelfandlowestintheeasterncentralLabradorSeaandsouthofGreenland.CalanusglacialiswaslargelyabsentfrommostofthecentralLabradorSeaandfromthewaterssouthofGreenland.Itsabundanceandbiomasswerehighestinshoreofthe1000mcontouroffLabradorandNewfoundland,withlowvaluesintheextremenorthandalongtheGreenlandShelf.C.hyperboreusandC.glacialisbiomasslevelswereoftenclosetoandsometimesexceededthoseofC.finmarchicusatstationsontheLabradorShelf,buttheyweregenerallymuchlowerinthecentralLabradorBasin.Twoothercopepodgenera,theveryabundantOithonaspp.andthelessabundantMicrocalanusspp.,werewidelydistributedthroughoutmostofthesamplingareabothinshoreandoffshoreofthe1000mcontour.Bothtendedtobemoreconcentratedinthesouthandeast,however,andthelatterwasabsentfromthenortherncentralLabradorSea.EuchaetanorvegicaandScolecithrocellaminorwerefoundevery-whereoffshoreofthe1000mcontour,whilePseudocalanusspp.andMetridialongaabundances(andM.longabiomass)showedhighestvaluesoffLabradorandNewfoundlandinshoreofthe1000mcountour,withthelatterhavingamoreseawarddistribution.Thelastenumeratedcopepodgenus,Oncaeaspp.wasfoundindeepwaterregions,insubstantiallylowerconcentrationsthananyothercopepodgroup. 8E.J.H.Headetal./ProgressinOceanography59(2003)1–30Fig.4.Preserveddryweightbiomassofchaeognaths,euphausiidsandamphipods(0100m)intheLabradorSeainspringandearlysummer.Symboldesignationsandscaling,andcontourlines,asinFig.2.Amongstthenon-copepodmesozooplankton,chaetognaths(mainlyEukrohniaspp.andSagittaspp.)andamphipods(mainlyParathemistospp.)werewidelydistributedthroughoutthesamplingarea(Fig.5).Euphausiids(mainlyThysanoessaspp.)werefoundinmostareas,buttendedtobemoreconcentratedaroundthemarginsoftheLabradorSea,especiallyinthenorthandwest,andintheoffshoreareasouthofGreenland.Euphausiidpopulations(juvenileandadultstagescombined)showedlargevariationsinaveragesize,sothatabundanceandbiomassdistributionswerenotsimilar(cf.Figs.4and5).LarvaceansweremostabundantinthenortherncentralLabradorSea,alongthewesternmarginandnorthofFlemishCap,whereOikopleuraspp.wasdominantandon,orcloseto,theGreenlandShelfwhereFritillariaspp.wasdominant.Pteropods(mainlyLimacina)weremostabundantovertheLabradorShelfandoffshoreinsouthernregions,whileostracodswerecommoninwesternandsouthernareasoffshoreofthe1000mcontour. E.J.H.Headetal./ProgressinOceanography59(2003)1–309Fig.5.Abundanceofselectedmesozooplanktongroups(0100m)intheLabradorSeainspring,MayJune1997.Symboldesig-nationsandscaling,andcontourlines,asinFig.2.3.1.2.Earlysummer,July1995C.finmarchicusoccurredateverystationsampledduringJuly1995(Fig.6).AbundanceswerehighestattheeasternendoftheL3sectionandinthesouthwest,includingtheslopewatersoftheGrandBank,whereyoungstagecopepoditesweredominant(Head&Harris,2003).AbundanceswerelowontheNew-foundlandShelfandattheonestationontheGrandBank.Thedistributionofbiomassdidnotcloselymatchthatofabundance(cf.Figs.6and7)duetovariationsintheproportionsofthedifferentdevelopmen-talstagespresentinthepopulations.MaximalabundancesinJulyintheeasterncentralLabradorSeaandthesouthwestwerecomparablewiththosemeasuredinspringinthenortherncentralLabradorSea.Maxi-malbiomassvalueswereslightlylowerthaninspring,duetohigherproportionsofyoungerstages.C.hyperboreusshowedhighestabundanceandbiomasslevelsovertheLabradorandNewfoundlandShelves,withlowerlevelsoverthecentralandeasternportionsoftheL3section,andlowestlevelsalongtheeastern 10E.J.H.Headetal./ProgressinOceanography59(2003)1–30Fig.6.Abundanceofcopepods(0100m)intheLabradorSeainearlysummer,July1995.Symboldesignationsandscaling,andcontourlines,asinFig.2.longitudinalsection.C.glacialiswasgenerallyrestrictedtotheLabrador,NewfoundlandandGreenlandShelves,withverylowlevelsalsopresentintheslopewaterssoutheastoftheGrandBank.MaximalbiomasslevelsofC.hyperboreuswerelowerinearlysummerthaninspring,whereasthoseofC.glacialisweresimilar.Oithonaspp.,Oncaeaspp.andMicrocalanusspp.occurredinrelativelyhighabundancebothinshoreandoffshoreofthe1000mcontour.Thelattertwoweremore-or-lessabsentfromtheeasternlongitudinalsection,however.ThemaximalabundanceofOithonaspp.inearlysummerwassimilartoitsmaximalabundanceinspring,whileOncaeaspp.hadahigher,andMicrocalanusspp.,alower,maximalabundance.E.norvegica,M.longaandS.minorweregenerallyfoundindeepwater.Allweregenerallylessabundantthantheyhadbeeninspring,withE.norvegicahavingasimilar,andM.longaalower,maximalbiomass.Amongstthenon-copepodmesozooplankton,thesamegeneraofchaeognaths,euphausiids,amphipods,larvaceansandpteropodswerefoundinearlysummerashadbeenpresentinspring.Thefirstthreegroupsandostracodsweremoreabundantoverdeepwaterthaninnearshoreareas,andchaetognathsandeuphausi-idsgenerallyshowedlowerlevelsofabundanceandbiomassthaninspring(cf.Figs.4and8).Amphipodconcentrationsinearlysummerweresimilartothoseatmoststationsinspring,whilebiomasslevelsweregenerallyalittlelower.LarvaceansandpteropodsweremostabundantinnearshoreareasoffLabrador,NewfoundlandandGreenland,exceptatonestationinthesouthwestLabradorSea,wherepteropodsshowedtheirmaximalabundance.Maximalabundancesoflarvaceansandpteropodsinearlysummerweregenerallysimilartothoseinspring. E.J.H.Headetal./ProgressinOceanography59(2003)1–3011Fig.7.Preserveddryweightbiomassofcopepods(0100m)intheLabradorSeainearlysummer,July1995.Symboldesignationsandscaling,andcontourlines,asinFig.2.3.2.InterannualpatternsofmesozooplanktonabundanceandbiomassdistributionalongtheL3sectionDifferentnumbersofstationsweresampledalongtheL3sectionduringeachoftheyears(Table1).Sometimesthiswasduetotimeconstraintsbutoftensamplingonorneartheshelveswaspreventedbyice.MDSplotsoftheabundanceofthezooplanktongroupsforeachofthe6yearsofsamplingshowedthatstationsfellinto5(orfewer)groups.MDSplotsforspring(1996,May/June)andearlysummer(1999,June/July),forwhichdatasetsweremostcomplete,areshowninFig.9.Geographicallythegroups(zones)more-or-lesscorrespondedto:theLabradorShelf(Zone1);theLabradorSlope(Zone2);thecentralandwesternLabradorSea(Zone3);theeasternLabradorSeaandGreenlandSlope(Zone4);andGreenlandShelf(Zone5).EveryyeartheboundarybetweenZones1and2wasthesame,whereastheboundariesbetweentheotherzonesvaried(Table1andFig.10).Zone3wasthemostextensiveandincludedthelargestnumberofstations.Insomeyearssomezoneswererepresentedbyonly1stationorwerenotsampledatall.Inthedescriptionofthezooplanktondistributionpatternspresentedbelow,abundanceandbiomassdataonthemesozooplanktongroupsfromallofthestationswithinagivenzonewereaveraged(Tables24).Forzoneshavingmorethantwostations,averagecoefficientsofvariationinabundancefor2eachofthecopepodgenera/specieswithinazonewereca50%forvalues1000mandca95%for2valuesbetween100and1000m.Averagecoefficientsofvariationinbiomasslevelsforeachofthe2copepodgenera/specieswithinazonewereca40%forlevels1000mgmandca80%forlevels 12E.J.H.Headetal./ProgressinOceanography59(2003)1–30Fig.8.Abundanceofselectedmesozooplanktongroups(0100m)intheLabradorSeainearlysummer,July1995.Symboldesig-nationsandscaling,andcontourlines,asinFig.2.2between10and1000mgm.Averagecoefficientsofvariationforabundancesforeachoftheother2mesozooplanktongroupswithinazonewereca68%forabundances1000mandca80%forabun-dancesbetween10and1000m2.Nobiomassvaluesforthesegroupswere1000mgm2,andaverage2coefficientsofvariationforbiomassvaluesbetween10and1000mgmwereca140%.C.finmarchicusoccurredineveryzoneineveryyearofsampling,withzonalconcentrationsranging22betweenonelowvalueofca2000minZone1inspring1996andahighof165000minZone24insummer1995(Table2).Concentrationswere10000minallzonesinsummerandwere50002minallzonesinspring,exceptfortheonelowvalue.C.hyperboreusalsooccurredinallzonesinall2years,butconcentrationswerealways1200minZone3andwerealwayshigherovertheshelves2(Zones1and5)thanelsewhere.Ontheshelves,concentrationswerehighest(10000m)ontheLabra- E.J.H.Headetal./ProgressinOceanography59(2003)1–3013Table1StationssampledontheL-3line(19952000)andzonaldesignationsStationno.LatitudeLongitudeSpringEarlysummer(decimal°)(decimal°)NW199619972000199519981999153.6855.551ndnd111253.8055.44ndndndnd11353.9955.25ndndndnd11454.2255.021ndnd111554.4954.76nd1ndndnd1654.7654.4911ndnd11754.9654.291nd1111855.1154.14222nd228.555.1654.04ndnd3ndnd2955.2653.98223nd221055.4253.83223nd221155.6153.63ndnd33231255.8453.39333nd231356.1253.123nd3ndnd31456.5452.683333331556.9652.24ndnd3nd3nd1657.3851.7933333316.557.5951.55ndnd3ndnd31757.8051.34333ndnd31858.2250.883333331958.6450.4233ndnd332059.0649.953344332159.4849.47334nd332259.7549.164344342359.9848.90ndnd4ndnd42460.1848.68434ndnd424.560.2448.63ndnd4ndndnd2560.2948.5444ndndnd42660.3748.4554nd4nd426.560.4048.41ndndndndnd52760.4548.365ndnd5nd52860.5648.235ndndndndndnd=nodata.dorShelfinspring1997andsummer1998andlowest(ca2000m2)ontheGreenlandShelfinsummer21995.C.glacialisabundanceswerealwaysrelativelyhigh(4000m)overtheshelves(Zones1and5)2insummer,sometimeshighovertheshelvesinspring(range16344165m),andlowerinthesloperegions(Zones2and4,range171092m2)inbothspringandsummer.C.glacialiswasabsentfrom2Zone3insummerandpresentinonlyverylownumbers(100m)inspring.2Oithonaspp.occurredeverywhereinallyears.Thelowestconcentrationwasca.1600monthe2LabradorShelf(Zone1)inspring1996andthehighest,168000minZone3insummer1995.Pseudocalanusspp.wasmostabundantontheLabradorShelf(Zone1)andmoreabundantinsummerthaninspring.ConcentrationsinZones3and4weregenerallyverylow.Microcalanusspp.occurredinallzonesinallyears,withhighestabundanceseitherovertheshelves(spring,1996;summer,1999)orinZone2(spring2000)(Table2).S.minorwasfoundonlyoverdeepwater(Zones24),aswasgenerally 14E.J.H.Headetal./ProgressinOceanography59(2003)1–30Fig.9.MDSplotsshowingsimilaritiesinmesozooplankton(0100m)communitystructureatstationsoftheL3sectionforspring1996(upperpanel)andearlysummer1999(lowerpanel).Distancesbetweenindividualstationsarerelatedtothesimilarityofthecommunitystructure.Fiveclusters(zones)aredistinguishedineachcase.thecaseforE.norvegica.Bothspeciesweremore-or-lessequallyabundantinspringandsummer.Distri-butionpatternsofM.longaabundancewerenotveryconsistent.Inspringconcentrationswerehighest22(range3952034m)inZones1or2.Inearlysummerconcentrationswerelow(170m)everywhere2in1995and1998,buthigh(
678m)inZones1,2and5in1999.Lastly,Oncaeaspp.wasmoreabundantinsummerthaninspring,andmostabundantoverinZone5in1999.Amongstthenon-copepodtaxa,chaetognathsandostracodsweregenerallyabundantonlyindeepwaterzonesandshowednosystematicdifferencesinconcentrationbetweenspringandsummer(Table3).Larva-ceansandpteropods,bycontrast,showedtheirhighestabundancesovertheshelvesand(theformeronly) E.J.H.Headetal./ProgressinOceanography59(2003)1–3015Fig.10.GraphicalrepresentationoftheL3samplingsectionshowingthepositionsofthesamplingstations(invertedtriangles)andthebathymetry(solidline).Thehorizontalbarsshowtheportionsofthelineoccupiedbythedifferent(0100m)mesozooplanktoncommunityzonesduringeachofthesamplingyears.inthesloperegionsandbothshowedhigherabundancesinsummerthaninspring.HighestconcentrationsofeuphausiidswereinZone4andnumbersweregenerallyverylowinZone1.Amphipodsweregenerallypresentinlownumbersandtendedtoshowhighestvaluesindeepwater(Zones24).Neithereuphausiidsnoramphipodsshowedanysystematicdifferencebetweenspringandsummerconcentrations.ThethreespeciesofCalanusgenerallyaccountedfor80%ofthebiomassofcopepodsand70%ofthetotalmesozooplanktonbiomass(Table4).Whentheircontributionwaslower,E.norvegica,M.longaand/orchaetognathsmadeincreasedcontributions(Zones2and4,2000;Zone3,1998;Zone2,1999).3.3.HydrographicconditionsintheLabradorSeaduringspringandsummerContouredplotsofprofilesoftemperature,salinity,fluorescenceandnitratehavealreadybeenpresentedforsectionsofthebroad-scalesurveytakeninMayJune1997(Headetal.,2000).Similarplotsofthesamevariableswillbepresentedelsewhereforsectionsofthebroad-scalesurveytakeninJuly1995(Head&Harris,2003).Ingeneral,theyshowcolderfresherwatersovertheshelves,warmersaltierwaterwithinthedeepareasandwelldefinedfrontalregionsatthebasinmargins.FortheL3sections,detailedTSanalysisidentifiedatotalof15waterclasses(Fig.11),althoughnotalltypeswereencounteredeveryyear,andtheactualTSpropertiesofeachclassvariedfromyeartoyear.Thecentralbasinalwayshadthewarmestsaltiestwater.Itwasdividedintowesternandeasternsections,eachhaving1(WLSUP)or2(ELSUP1,ESLUP2)typesofwaterintheupperlayersthatweremoreinfluencedbyadvectionofcool,fresherwaterfromtheshelvesthantheunderlyingwatertypes(WLS,ELS1,ELS2).WesternLabradorSeawaterclassesweregenerallycolderthaneasternLabradorSeawaterclasses.Threewatertypeswereidentifiedovereachoftheshelves:allwererelativelycoldandfresh.Foreachshelftherewasaclassthathadnotbeenaffectedbysurfacewarming(LSH2,GSH2),andfortheLabradorShelftherewasaclassthathad(LSH1).Thesewatersoverlaidthethirdtypewhichcorrespondedtoacoldercoldintermediatelayer(LCIL,GCIL)formedbyverticalmixingduringwinter.Twoclasses 16E.J.H.Headetal./ProgressinOceanography59(2003)1–30Table2AverageabundancesofmesozooplanktoninthefivezonesalongtheL3section(nos.m2)YearZoneCalanusCalanusCalanusOithonaPseudocalanusMicrocalanusScolecithrocellaEuchaetaMetridiaOncaeafinmarchicushyperboreusglacialissp.sp.sp.minornorvegicalongasp.Spring199612046560921401455775573064942339516921352226302772015678995986767224703143259388284952841155915490434873184410921631403335190140590513787961741651898118862300720199718699113901634271202056613560902710222159635410958769209428331974152990457331024484217110337128331249123512541423648850147118659017293053502305ndndndndndndndndndnd200018609447418242497311933452011320302560024327231536856547461808158220340313756118160793178419108364527174198461788178362631126729920910710155ndndndndndndndndndndSummer19951143642681191011569725066176030155272ndndndndndndndndndnd3329708360130005168105111974001291557416515713021961681891474121348837316514975599532222414576986217118400011841998111443120408138164983261952708903012148426322777001361455421058104683197122780586758885152011651675234ndndndndndndndndndnd5ndndndndndndndndndnd199915578877344233368062671750016296781942141781975791029490305180813848251131397545003254412714898122067524155057353663612911474633334380194201655248558655968305104520542402313492486nd=nodata.ofslopewaterwererecognisedateachmarginwhichwerecomprisedofcentralbasinwatercontainingeithermore(LSL1,GSL1)orless(LSL2,GSL2)shelfwater.Whileitwasnecessarytoidentifyallthesewatertypesinordertocalculatethecontributionofeachtotheupper100mateachstation,thenumberofwatertypeswascondensedtosix,whenexaminingthehydrographicmake-upineachofthezooplanktoncommunityzones(Fig.12).Thesewatertypesare:LabradorShelf(LSH=LSH1+LSH2+LCIL);LabradorSlope(LSL=LSL1+LSL2);westernLabradorSea(WLS=WLSUP+WLS);easternLabradorSea(ELS=ELSUP+ELS1+ELS2),GreenlandSlope(GSL=GSL1+GSL2);and,GreenlandShelf(GSH=GCIL+GSH2). E.J.H.Headetal./ProgressinOceanography59(2003)1–3017Table3Averageabundancesofthenon-copepodmesozooplanktoninthefivezonesalongtheL3section(nos.m2)YearZoneChaetognathsEuphausiidsAmphipodsLarvaceansPteropodsOstracodsSpring1996114772110238645112182885336628852158313776727212729325874215492656026281721454132205305001997123271136949226452128930431886031661205311156423411310610404147819342045158235ndndndndndnd2000190113316113013562322945651130113271355548943983258157845541069510755372037465ndndndndndndSummer199514750482339118602ndndndndndnd3216016421011216223115247747228153880011105149003849145070199812448083232782712029701538108283220163231086139446640938608624ndndndndndnd5ndndndndndnd19991611556883604489027187712685512501647310898402992539499108943896861176277538141311501413110205664520nd=nodata.Zone1containedonlyLSHwater,exceptin1999.Zone2containedonlyLSHin2000,butotherwisemixturesofLSLandWLS,withorwithoutsmallamountsofLSHwater.Zone3containedmostlyvaryingamountsofWLSandELS,exceptin2000,whentherewasnoELSanywhere.LabradororGreenlandslopewaterssometimesmademinorcontributionstoZone3.Zone4alwayscontainedsomeESLwater,butELSandevenWLS(1999,2000)watersometimesmadesubstantialcontributions,andGSHwaterwaspresentin2000.Zone5containedeitheramixtureofELSandGSHwater,orGSHwateralone. 18E.J.H.Headetal./ProgressinOceanography59(2003)1–30Table4AveragebiomassofmesozooplanktoninthefivezonesalongtheL3section(mgm2)YearZoneCalanusCalanusCalanusEuchaetaMetridiaOtherChaetognathsEuphausiidsAmphipodsOtherfinmarchicushyperboreusglacialisnorvegicalongacopepods(non-copepods)Spring19961644.04783.88627.724.07116.22237.5045.43104.7537.91265.6624464.712667.5596.58160.6196.9576.24893.0886.1812.58464.2034570.171573.866.20245.6112.9635.53299.3737.9241.38102.5844534.46542.4044.7572.558.2918.23327.7013.94334.5631.115327.85389.55192.100.230.5321.625.4212.887.0178.6519971658.681641.10706.8226.5587.12394.828.1436.617.57191.4221516.161180.77220.12349.62257.3479.70185.6218.2318.53137.5632769.74812.705.33227.6029.5451.21174.2553.638.2062.384533.59153.6820.5728.935.4229.4686.225.422.8393.345ndndndndndndndndndnd200011991.743653.40520.2521.8164.52206.4549.4945.5412.32143.9621282.101395.32327.25786.03442.9667.3563.51171.531.3634.1334249.601486.8233.62332.9284.4734.60214.5473.0222.50117.1941461.73631.1114.43177.2624.1443.54619.58143.0283.73316.065ndndndndndndndndndndSummer19951630.311184.62992.4413.563.11239.680.000.0021.85285.972ndndndndndndndndndnd34517.51348.610.00413.5854.75209.67417.8739.49295.50261.2648092.46470.980.60222.3860.04194.5988.8934.5093.49258.5451877.38698.57404.540.000.0088.144.970.000.00401.3819981939.142483.551381.052.3741.02397.469.7997.9722.30923.9624562.44316.2620.841050.2743.1755.64229.7149.7783.57211.8535398.52143.140.001062.4365.12160.01609.04178.14108.93460.194ndndndndndndndndndnd5ndndndndndndndndndnd199913373.861720.09830.5824.60226.10222.8437.5557.0860.92449.0024643.90906.9437.061047.34277.6468.82331.77267.3059.32700.1533616.09183.760.00725.5125.9034.96412.5648.8256.25321.6447224.2373.6357.49202.862.1737.2086.51420.4537.341051.3552388.59985.36738.576.5513.1164.180.009.6130.74970.78nd=nodata.4.Discussion4.1.Distributionsofmesozoooplanktoninspringandearlysummer–broadscalesurveysandtimeseriesontheL3sectionThemainobjectiveofoursamplingintheLabradorSeahasbeenastudyofthedistributionandecologyofCalanusspp.Forthisreasonwedonotidentifyallofthemesozooplanktongroupstospeciesorsome-timeseventogenuslevel.Itisforthesamereasonthatweusea200mmmeshnet,whichwehavefound E.J.H.Headetal./ProgressinOceanography59(2003)1–3019Fig.11.Plotsoftemperatureversussalinityforthe15waterclassesdistinguishedintheupper100malongtheL3sectionforeachofthesamplingyears.Seetextforanexplanation.capturesStage1C.finmarchicuscopepoditesfromtheLabradorSeawithca100%efficiency(Head&Harris,unpubl.data).Werecognisethatusingthismeshleadstounderestimationoftheabundanceofthesmallerspecies,includingOithonaspp.,Oncaeaspp.,Microcalanusspp.,Pseudocalanusspp.andS.minor.Wealsonote,however,thatevenifoursamplingunderestimatedthebiomassofsmallcopepodsbyasmuchasafactorof5,theirsummedbiomasswouldstillalwayshavebeen15%ofthetotalmesozooplank-tonbiomassinthecentralLabradorSeaand40%overtheshelves(Table4).Also,sinceweusedverticalhaulsweexpectthatsomeofthelargermoreactiveswimmers,suchaschaeotognaths,euphausiidsandamphipodsmayhavebeenunderestimated,duetonetavoidance.Evenifourestimatesofabundanceofsomeorganismswerenotalwaysaccurate,however,ourobservationsofthepatternsofdistributionofthespeciesorgroupsshouldstillbevalid.4.1.1.CalanusfinmarchicusC.finmarchicusisfoundthroughouttheNorthAtlanticnorthoftheGulfStreamandwhereverNorthAtlanticwaterpenetratesfarthernorth(e.g.theBarentsSea,ArcticOcean,BaffinBay)(Planqueetal.,1997;Hirche&Mumm,1992;Grainger,1963;Longhurst,Sameoto,&Herman,1984).Consistentwiththisview,thecentralLabradorSeacontainsmainlyNorthAtlanticwaterandhighlevelsofC.finmarchicus.Indeed,theLabradorSeamaybeacentreofdistributionforthespeciesinthewesternNorthAtlantic(Wiebe,2001).FromthecentralLabradorSeaC.finmarchicuscanbetransportedontoitsfringingshelves 20E.J.H.Headetal./ProgressinOceanography59(2003)1–30Fig.12.Percentagecontributionsofsixwaterstypestotheupper100mofthefivemesozooplanktoncommunityzonesoftheL3section:Zones15,lefttoright.Average0100mtemperaturesareshownbelowthex-axisforeachzone.Gapscorrespondtozonesthatwerenotsampledinaparticularyear(LSH=LabradorShelf,LSL=LabradorSlope,WLS=WesternLabradorSea,ELS=EasternLabradorSea,GSL=GreenlandSlope,GSH=GreenlandShelf). E.J.H.Headetal./ProgressinOceanography59(2003)1–3021and,viatheLabradorCurrent,totheslopewatersoftheScotianShelfandtheGulfofMaineandtheiradjacentshelves(Loderetal.,1998;Pershingetal.,2001).Inourbroadscalesurveys,inthecentralLabradorSeanorthoftheL3sectioninspringandonthelongitudinaleasternsectionsinspringandearlysummer,C.finmarchicusmadeup64%ofmesozooplanktonbiomassinspringandca80%inearlysummer(datanotshown).OntheL3sectionC.finmarchicusconstitutedca6469%ofthetotalmeso-zooplanktonbiomassinZone3everyyearandinZone4inearlysummeritmadeup78%(Table4).ClearlyC.finmarchicusisthemostimportantcomponentofthemesozooplanktonbiomassinthecentralLabradorSeaatthistimeofyear.Intermsofnumbers,forthebroadscalesurveys,inthecentralLabradorSeanorthoftheL3sectioninspring,andonthelongitudinaleasternsectionsinspringandearlysummer,C.finmarchicusalwaysaccountedfor89%ofthelargecopepods.AlongtheL3section,C.finmarchicusmadeup80%ofthelargecopepodseveryyearinZone3,and99%inearlysummerinZone4(Table2).Stationshavingthehighestconcentrationswerestationsatwhichyoungcopepoditesoftheyearsnewgenerationweredominant(Headetal.,2000;Head&Harris,2003).Overall,however,theseasonalpro-gression(fromspringtoearlysummer)wasnotobviouslyaccompaniedbyageneralincreaseinabundanceofC.finmarchicusthroughouttheregion.Nevertheless,forthebroadscalesurveysitcouldbearguedthatagreaterproportionofstationshadhighconcentrationsinearlysummerthaninspring,andalongtheL3sectionthehighestconcentrationsoccurredinearlysummer(Zone4).ThedistributionofC.finmarchicusinspring1997wassimilartothatreportedintheMayJuneNorwestlantIIsurvey(ICNAF,1968),whichshowedhighabundancesinthenortherncentralLabradorSeaandnearthenortheastmarginandlowerabundancesinthecentralLabradorSeafarthersouth.OnenotabledifferencewasthatalonglongitudinalsectionssouthofGreenlandconcentrationswereaboutsixtimeshigherinspring1997thaninMayJune1963.Similarly,CPRdatashowincreasedabundanceofC.finmarchicusintheIrmingerSeainthe1990s,relativetothe1960s(Johns,2001).Huntleyetal.(1983)foundlevelsinthenorthcentralLabradorSeainMayandJune197778thatwerecomparabletothoseofotherreports.FortheJulyAugust1963NorwestlantIIIsurvey,youngstagesofC.finmarchicuswerenotdistinguishedfromthoseofC.glacialis.Duringthatsurvey,however,averageconcentrationsof‘C.finmarchicus’alongasectionnearthecentralregionofourL3section,whereC.glacialiswouldlikelyhavebeenabsent,weresimilartoaveragevaluesobservedinZone3in1999,andlowerthanthosefoundin1995and1998.FurtherdetailsoftheC.finmarchicusdistributionpatterns(e.g.stagecomposition)willbepresentedelsewhere(Head&Harris,2003).4.1.2.CalanushyperboreusC.hyperboreusisfoundinoceanicArcticwaters,includingtheArcticOcean,BaffinBayandthroughouttheeasternCanadianArcticarchipelago(Grainger,1963;Conover,1988;Thibault,Head,&Wheeler,1999).InourstudyregionC.hyperboreusoccurredinthecentralLabradorSeabasin,alongitsmarginsandespeciallyonitsshelves.ThespeciesisrelativelyabundantintheGreenlandSea(Hirche,1991),whichcouldbeasourcefortheGreenlandShelf,viatheeastandwestGreenlandCurrents,andinBaffinBay(Buchanan&Sekerak,1982;Longhurstetal.,1984),whichcouldbeasourcefortheLabradorShelfandSlope.Inaddition,however,lownumbersofC.hyperboreusmayoverwintersuccessfullyinthecentralLabradorSea,sinceitisfoundthroughouttheregioninspring.C.hyperboreuscontributedlittletothemesozooplanktonbiomassatstationsalongthelongitudinaleasternsectionsinspringorearlysummer(ca4%).OntheL3section,however,italwayscontributedasignificantproportionofthetotalmesozooplanktonbiomassovertheLabradorandGreenlandshelves(Zones1and5,Table4),withamaximumof54%(Zone1,spring2000)andaminimumof19%(Zone5,summer1999).LevelsinZone2wereconsistentlyca30%oftotalmesozooplanktonbiomassinspringandmuchlowerinearlysummer(512%).InZones3and4C.hyperboreusmadeupca20%ofthemesozooplanktonbiomassinspring,but6%inearlysummer.Intermsofnumbers,forthebroadscalesurveys,C.hyperboreusabundancesweregenerallylowestatstationsalongthelongitudinaleasternsectionsinspringandearlysummerandhighestinthenorth 22E.J.H.Headetal./ProgressinOceanography59(2003)1–30andalongthewesternLabradorSeamargin,includingtheLabradorShelf(Figs.2and6).ConcentrationsinthenortherncentralLabradorSeainspring1997weregenerallysubstantiallyhigher(´10)thanthoserecordedfortheMayJune1963NorwestlantIIsurvey(ICNAF,1968)andsomewhathigher(´5)thanthoseinMayJuneof197778(Huntleyetal.,1983).EarlysummeraveragevaluesinZone3weresimilartovaluesalongasectionnearthecentralregionofourL3sectionsampledduringtheNorwestlantIIIsurveyinJulyAugust1963andvaluesontheLabradorShelfwerealsosimilar(ICNAF,1968).OvertheshelvesStages1and2weredominantinspring,whereasStages3and4weremostabundantinearlysummer,suggestingaseasonalprogression.InthecentralLabradorSeaStage5sandadultfemalesweremostabundantinspring,whereasStage4sweremostabundantinearlysummer.ThissuggeststhattheStage4swereprobablyadvectedintothecentralLabradorSeafromelsewhereandthatindividualC.hyperboreushadstartedtoleavethesurfacelayersfortheiroverwinteringdepthsbyJune/July.4.1.3.CalanusglacialisC.glacialisisanArcticspeciesanditisoftenassociatedwithshelf/shallowregionsthatareseasonallycoveredwithice(Grainger,1963;Conover,1988).ItisabundantinHudsonBay(Runge&Ingram,1991)andtheeasternCanadianArcticarchipelago(Sameoto,Herman,&Longhurst,1986).ItalsooccursindeeperwatersnorthoftheLabradorSeasuchasthewesternandnortheastBaffinBay(Buchanan&Sekerak,1982;Sameoto,1984);theGreenlandSea(Hirche,1991)andtheArcticOcean(Grainger,1965;Hirche&Mumm,1992;Thibaultetal.,1999),butitwasnotfoundinthecentralLabradorSea.Instead,itwasassociatedwiththeshelves.ThedistributionofC.glacialiscontrastswiththatofC.hyperboreus,whichisalsoanArcticspecies,butwhichdidoccurinthecentralLabradorSea(albeitinlownumbers).WewouldexpectC.glacialistobeadvectedintothecentralLabradorSeafromitsmargins,inthesamewaythatC.hyperboreusseemstobe,butitwasneverfoundthereinappreciablenumbers.WesuggestthatthismaybebecauseC.glacialisisnotastolerantoftherelativelywarmtemperaturesofthecentralbasineitherinthesurfacelayersoratdepth.InthecentralLabradorSeatemperaturesare3°Cthroughoutthewatercolumninspringand3.5°Cyear-roundatprobableoverwinteringdepths(500m).Inthenearsurfacelayers,populationsovertheshelvesseldomexperiencetemperatures1°C(Fig.11)andwhentheydotheycanretreattothecoldsub-surfacelayers.Longhurstetal.(1984)reportedthatthepreferredtemperatureofC.glacialisinnorthernBaffinBayandKaneBasinis0.5°C.SourcepopulationstotheLabradorShelfmayoverwinterinHudsonBayorthechannelsoftheeasternCanadianArcticarchipelago,wherenearbottomtemperaturesare0°C(Ingram&Prinsenberg,1998;Sameotoetal.,1986)andBaffinBay,wheretemperaturesatdepthare1°C(Harrison,Li,Smith,Head,&Longhurst,1987).Elsewhere,whereC.glacialisareknowntooverwinter,nearbottomtemperaturesare0°C(e.g.WhiteSea,Kosobokova,1999).ConcentrationsofC.glacialisinthenorthcentralLabradorSeaweresimilartothosereportedfortheMayJune1963NorwestlantIIsurvey(ICNAF,1968)andarguablyhigherthanthoseobservedinMay-June1977-78(Huntleyetal.,1983).ThesourcetothisregionmightbeDavisStraitortheGreenlandShelfviatheWestGreenlandcurrent.4.1.4.EuchaetanorvegicaE.norvegicawerewidelydistributedoffshoreofthe1000mcontour.Concentrationspersquaremetreweresimilarto,orgreaterthan,thosereportedrecentlyforParaeuchaeta(=Euchaeta)norvegicaintheNorwegianSea(Fleddum,Kaartvedt,&Ellertsen,2001).Thoseestimateswereforthe0700mdepthrange,however,andincludedsubstantialconcentrationsbelow100m.Althoughweareonlypresentingdatafrom0to100mtowsinthispaper,inearlysummer1999wemadeverticallystratifiedtowstodepthsof2000matfourstationsalongtheL3section(L310,16,20and23,seeFig.10).AtthreeofthesestationsconcentrationsofE.norvegicaintheupper100mwere3070%ofthetotalwatercolumnconcen-trations,whileatthestationneartheGreenlandShelftherewereveryfewE.norvegicaatanydepth.Thus,fortheentirewatercolumn,concentrationswereprobablysomewhathigherintheLabradorSeathanthey E.J.H.Headetal./ProgressinOceanography59(2003)1–3023wereintheNorwegianSea.IntheNorwegianSeaadultswereseldomseenintheupper50100mandtherewaslittlesignofverticalmigrationamongjuveniles.AlongtheL3sectionintheLabradorSeazonal2averagefemaleconcentrationswereca200mintheupper100minZone2inspring(2000)andearlysummer(1998).FemalesandStage5swerecollectedonlyatstationsthatweresampledatnight,but,asintheNorwegianSea,theyoungerstagesshowednoday-nightdifferencesinabundance.Springandearlysummer0100mconcentrationsreportedherearecomparablewiththosefoundincorrespondingareasduringtheNorwestlantIIandIIIsurveys(ICNAF,1968).Therewasaseasonalprogressioninthedevelop-mentalstagesintheE.norvegicapopulations.InspringatstationsoftheL3sectionStage3wasgenerallythemostabundantcopepoditestage,withStage2snextmostabundantinZone2andStage4s,inZone3.Inearlysummer,Stage4sand5sweregenerallythemostabundantstages.E.norvegicaprobablyfeedonthecopepoditesofCalanusspp.,especiallyC.finmarchicus,withtheolderstagestakingprogressivelylargerprey.SmallercopepodsandCalanuseggsandnaupliimaybetakenbytheyoungestcopepoditestages.ThebiomassofE.norvegicawassometimes20%ofthatoftheCalanuspopulation(Table4),sothatitmayhaveasignificantpredatoryimpact.E.norvegicawasmuchmoreabundantintheLabradorSeathaninBaffinBay(Buchanan&Sekerak,1982;Sameoto,1984)supportingtheviewthatitisaNorthAtlanticspecies.4.1.5.Pseudocalanusspp.PseudocalanusminutusiscommonintheshallowwatersofHudsonBay(Runge&Ingram,1991)andtheeasternCanadianArcticarchipelagoinbothice-covered(Conover,Bedo,Herman,Head,Harris,&Horne,1988)andopenwater(Sameotoetal.,1986)seasons.Itisalsofoundindeepwaters,atsomewhatlowerconcentrations,inwesternandnortheastBaffinBay(Buchanan&Sekerek,1982;Sameoto,1984)andinArcticregions(Grainger,1965).Overall,thespecieshasadistributionsimilartothatofC.glacialisbothfarthernorthandintheLabradorSearegionand,asforC.glacialis,probablesourcesofPseudocalanusspp.totheLabradorShelfareHudsonBay,viaHudsonStraitand/ortheBaffinIslandCurrent.Also,asforC.glacialis,thelowconcentrationsinthenorthcentralLabradorSeainspringmayhavebeentrans-portedtherefromfarthernorthand/orfromtheGreenlandShelf.ThesourcetotheGreenlandShelfitselfisprobablytheeastGreenlandCurrent.4.1.6.MetridialongaM.longaisfoundintheArcticOcean,whereitoftencontributessignificantlytothetotalzooplanktonbiomass(Grainger,1965;Hirche&Mumm,1992;Thibaultetal.,1999).Itiscommoninwestern(Buchanan&Sekerak,1982)andnortheast(Sameoto,1984)BaffinBayandasfarsouthastheslopewatersoftheScotianShelf,althoughthereitislessabundantthanitscongenerMetridialucens(Head&Harris,unpubl.data).Inourstudyoftheupper100m,M.longawasconcentratedintheLabradorShelfandsloperegionsinspring,anditwasgenerallylessabundantinsummerthanspring,exceptin1999(Table2,Figs.2and6).IndeepwateralongtheL3section(Zones24),concentrationsofM.longawerehigheratstationssampledatnightthanatthosesampledduringtheday,suggestiveofdielmigration.Elsewhere,M.longawasfoundmostlyatdepths100m(Buchanan&Sekerak,1982;Sameoto,1984)andintheverticallystratifiedtowswemadein1999,theabundanceofM.longaabove100mwas6%ofthetotalwatercolumnabundanceatthreestationsandca20%atthefourth.Thus,watercolumnconcentrationsofM.longawereunderestimatedbymeasurementsintheupper100mandthespatialdistributionpatternforthetotalwatercolumnmighthavebeendifferentfromthe0100mpattern.Forthefourstationsthatweresampledto2000m,however,concentrationswerehighernearerthemarginsthaninthecentralLabradorSeabasin,aswasseengenerallyseeninthe0100mtows.Overall,thedistributionofM.longawasconsistentwithithavinganArcticsource,andwithitspreferringdeepratherthanshallowwater.Intheverticallystratifiedtowsmadein1999,M.lucenswassometimesobservedatdepths100m,consistentwithitsNorthAtlanticorigin. 24E.J.H.Headetal./ProgressinOceanography59(2003)1–304.1.7.Oithonaspp.Oithonaspp.arewidelydistributedthroughouttheAtlanticOcean(Sabatini&Kiorboe,1994andrefs.therein)andwerefoundatallstationsintheLabradorSearegion.OithonasimilisoccursasfarnorthastheArcticOcean(Grainger,1965;Thibaultetal.,1999).Inwestern(Buchanan&Sekerak,1982)andnortheast(Sameoto,1984)BaffinBay,Oithonaspp.concentrationsinsummer/fallweresomewhatlower2thansummerconcentrationsintheLabradorSea(7000mversusvaluesinTable2).Ithasbeenarguedthatsamplingbymeansofa200mmmeshnetgreatlyunderestimatesthecontributionofOithonaspp.tothetotalabundanceandbiomassofcopepodcommunities(Gallienne&Robins,2001).Inourstudy,abun-dancesofOithonaspp.mightoftenhaveexceededthoseoftheCalanusspp.,hadtheybeenadequatelyassessed,butourestimatesofOithonaspp.biomasswerealways3.5%andoften1%ofthetotalcopepodbiomassinallzonesoftheL3section.SincewewouldhavebeencatchingthelargeststagesthatwouldhavecontributedmosttotheOithonaspp.biomass,itseemsunlikelythatthecontributionofOithonaspp.tothezooplanktonbiomassintheLabradorSeaiseververysignificant.4.1.8.Oncaeaspp.2AbundancesofOncaeaspp.wererelativelylow(8000m)inourstudyregioninbothspringandsummer.Kielhorn(1952)reportedfindingonlyonespecimenofthegenusduringayearofsamplingatStationBravointheLabradorSeaandBuchananandSekerak(1982)andSameoto(1984)foundlow2abundances(100m)inwesternandnortheastBaffinBay.Grainger(1965)foundittobethesecondmostabundantspeciesinfinenettowstakenatT-3(ArcticOcean)andThibaultetal.(1999)reporteditsfrequentoccurrenceonasectionacrosstheArcticOcean,butconsidereditscontributiontothezooplank-tonbiomassinsignificant.Althoughitsabundancewasundoubtedlyunderestimatedinourstudyduetoitssmallsize,itscontributiontothezooplanktonbiomasswasstillprobablyinsignificant.4.1.9.Microcalanusspp.MicrocalauspygmaeuswasreportedtobethemostabundantcopepodcollectedinfinenetstowsatT-3intheArcticOcean,wheretheyoccurredmainlyinthe0300mdepthrange(Grainger,1965).InwesternBaffinBay,however,mostanimalswereatdepthsof250minsummer/fall(Buchanan&Sekerak,1982)andtheseauthorsalsonotethatotherforms(species)mayhavebeenpresent.IntheLabradorSeaconcen-trationsofMicrocalanusspp.intheupper100mweregenerallyhigherinspringthaninearlysummer(Figs.2and6,Table2),andalongtheL3sectiontheyweregenerallylowerinthecentralbasinthanontheshelves.Inthedepth-stratifiedtowstakeninearlysummer1999,Microcalanusspp.abundancesinthe0100mdepthrangewere2%ofthosethroughoutthe02000mdepthrange,withmostofthepopulationfoundbetween100and1000m.OurobservationsareconsistentwiththeviewthatMicrocalanusspp.isprobablyofArcticorigin,andwiththesuggestion(Kielhorn,1952)thatthespeciesundergoesaseasonalmigrationtodeeperwaterinearlysummer,perhapsasaresponsetowarming.Withoutspringverticaldistributiondata,however,thissuggestionremainsspeculative.OursamplingofMicrocalanusspp.intheupper100mprobablydoesnotreflectitsoverallwatercolumnabundanceverywell,whileitscontributiontothezooplanktonbiomasswithinthislayerwasinsignificant.AlongtheL3section,inspringeveryyearandinearlysummerin1999,Microcalanusspp.concentrationswerehigheratstationsthatweresampledatnightthanatthosesampledduringtheday,suggestiveofdiel,aswellasseasonal,migration.4.1.10.ScolecithrocellaminorS.minorwasmore-or-lessubiquitousatdeepwaterstationsinourLabradorSeastudyareaandlargelyabsentfromtheshelves.ItsdistributionseemstoindicateanassociationwithAtlanticwater,sinceitwaspresentinverylowconcentrationsinBaffinBay(Buchanan&Sekerak,1982;Sameoto,1984)andwasapparentlyabsentfromtheArcticOceanandadjacentwaters(Grainger,1965)andLancasterSound(Sameotoetal.,1986).Althoughitwaspresentinthesurfacelayersatnearlyeverydeepwaterstation, E.J.H.Headetal./ProgressinOceanography59(2003)1–3025forthestationswhereverticallystratifiedtowsweremadein1999,only1535%ofthepopulationswereintheupper100m.S.minorshowednoevidenceofdielverticalmigrationandcontributedonlyasmallportionofthezooplanktonbiomass.4.1.11.ChaetognathsChaetognathswerepredominantlyfoundoverdeepwater.Intheverticallystratifiedtowscarriedoutinearlysummer1999,between12and75%ofthechaetognathswerefoundintheupper100m,butactualcountswereratherlow,sothattheseproportionsshouldbeviewedwithscepticism.Itiscommonlythoughtthatchaetognathscarryoutdielverticalmigrations,butday/nightdifferencesinconcentrationsinZones2,3and4oftheL3sectionwereonlyseeninspring1997andinearlysummer1998.Itislikelythatsomechaetognathswouldhaveavoidedcaptureinourverticaltows,duetotheirrelativelystrongswimmingability,sothatourquantitativeestimatesofchaetognathbiomassandabundanceareprobablytoolow.Ourspringestimateswere,however,generallyhigherthanthoserecordedduringtheNorwestlantIIMayJune1963survey(ICNAF,1968)forcomparableareas.OurabundanceestimatesforZone3(Table3)weresubstantiallyhigherthanthosereportedforwesternBaffinBay,usingthesametowingstrategy(Buchanan&Sekerak,1982).Ontheotherhand,theyweresimilartothoseofSameoto(1984)innortheastBaffinBayandlowerthanthoseofLonghurstetal.(1984)incentralBaffinBay.Bothoftheselatterinvestigationsweremadeusingobliquelytowednets,forwhichnetavoidancewouldhavebeenlessimportant.ChaetognathsarereportedlyvoraciouspredatorsonCalanusspp.copepoditesandadultsandinourstudy,asinthatofKielhorn(1952),manyindividualswereobservedtocontaincopepods,especiallyCalanusspp.,invariousstagesofdigestion.WithinthesurfacelayersinZone2in1996,thebiomassofchaetognathswas29%ofthatoftheCalanusspp.populationimplyingasignificantpredatoryimpact.InmostoftheLabradorSea(Zone3),however,chaetognathbiomassrangedbetween4%(spring)and11%(earlysummer)ofthatoftheCalanusspp.population(Table4).Thesenumbersimplyalowpredatoryimpact,butgiventhelikelihoodofdielmigrationandourprobableunderestimationofchaetognathabun-dance,predationratesontheCalanusspp.populationsinthesurfacelayersmaygenerallybefairlysignifi-cant.4.1.12.EuphausiidsInouranalysisallcategoriesofjuvenileandadulteuphausiidswerecombined.Sincetheseencompassawiderangeofsizes,therewerelargedifferencesinweightsperindividual,whichleadtodisparitiesinthepatternsofabundanceandbiomassdistribution(cf.Figs.4,5and8).Euphausiidsarestrongswimmersandareknowntoavoidplanktonnets,evenwhentowsarehorizontalorobliqueandatrelativelyrapidspeeds(Sameoto,1983).Thus,wedonotregardourabundanceandbiomassestimatesofeuphausiidstobeveryreliable.Nevertheless,wedidfindeuphausiidswidelydistributedthroughoutourstudyregion,whereastheywereapparentlyabsentfromverticalnettows(Buchanan&Sekerak,1982)andnearlyabsentfromhorizontalnettows(Sameoto,1984)inBaffinBay.Thus,euphausiidsareassociatedwithAtlantic,andnotArctic,waters.OurestimatesofeuphausiidabundanceinthecentralLabradorSeaandsouthofGreenlandweregenerallymuch(10times)higherthanthosereportedfortheNorwestlantIIandIIIsurveys(ICNAF,1968).4.1.13.AmphipodsConcentrationsofamphipodsintheLabradorSeainspringweregenerallyhigher(ca510times)thantheaveragesreportedforthecentralLabradorSeaandtothesouthofGreenlandfortheNorwestlantIIsurvey(ICNAF,1968).Inaddition,theywerehigherthanthosereportedforwesternBaffinBay(Buchanan&Sekerak,1982),butalittlelowerthanthosereportedforcentralandnortheastBaffinBay(Longhurstetal.,1984;Sameoto,1984).Thelatterhighervaluesmighthavebeenpartlytheresultofreducednetavoidance,duetothehighertowingspeedsemployed.Inpreviousstudies,asinours,themost 26E.J.H.Headetal./ProgressinOceanography59(2003)1–30abundantgenuswasParathemisto.Atsomestationsabundancesgreatlyexceededaveragevalues(e.g.Figs.5and8).Kielhorn(1952)reportedthatamphipodssometimesoccurredinswarmsatStationBravo,ononeoccasionaccountingfor66%ofthemesozooplanktonabundance.Ourobservationsaresimilar,butthehighestproportionthatamphipodseverreachedinourcatcheswasca21%,atonestationinthecentralLabradorSeain1996(datanotshown).Amphipodsreachedamaximumof13.4%ofthetotalmesozooplanktonbiomassatonestationinZone4in1995,andinthatzonetheywereimportantcontribu-torstothenon-copepodzooplanktonbiomassin1995and1996.ItisprobablethatamphipodswouldsometimeshavehadasignificantpredatoryimpactontheCalanusspp.populations.4.1.14.LarvaceansKielhorn(1952)reportedOikopleuralabradoriensistobeoneofthemoreimportantmembersoftheplanktonoftheLabradorSea,althoughduringtheNorwestlantIIsurveyFritillariaspp.reachednumericallyhigherconcentrations(ICNAF,1968).Grainger(1965)foundOikopleuravanhoeffeniandFritillariabore-alisintheArcticOceanandadjacentwatersandSameoto(1984)foundunidentifiedlarvaceansatlowaverageconcentrationsinnortheastBaffinBay.NeitherBuchananandSekerak(1982)norHuntleyetal.(1983)reportedlarvaceansintheirspecieslistsforwesternBaffinBayandthenorthernLabradorSea,butwesuspectthattheywerepresent,butnotenumerated.InourstudylarvaceanswereconcentratedontheshelvesandalongtheeasternandnorthernregionsofthecentralLabradorSeainspring1997andintheseareastheyoftendominatedthenon-copepodmesozooplanktonnumerically.AsimilarpatternwasobservedduringthespringNorwestlantIIsurvey,andconcentrationsweresimilarinequivalentareas.Becauselarvaceansarefragileandoftendamagedduringcapturetheirbiomasswasnotassessedseparately,althoughtheydidsometimescontributesubstantiallytotheOthernon-copepodbiomass.4.1.15.PteropodsGrainger(1965)foundbothLimacinahelicinaandClionelimacinaintheArcticOceanandadjacentwatersandLonghurstetal.(1984)foundboththroughouttheeasternCanadianArcticarchipelagoandinBaffinBay.Spiratellaspp.wastheonlygenusthatwasreportedfortheNorwestlantIIandNorwestlantIIIsurveys(ICNAF,1968),butSpiratellaspp.arenowclassifiedasbelongingtotheLimacinagenus.Inourstudyareapteropodswerequiteabundantatahandfulofstations,butgenerallynotveryimportant.TheytendedtobemoreabundantovertheshelvesandinthesouthandtheywereconsistentlylowinabundanceinZones24oftheL3section.Atstationswhereabundanceswerehigh,theywerehigherthantheaveragevaluesreportedbyLonghurstetal.(1984),whileelsewheretheyweresimilar.AbundancesweregenerallyhigherthanthosereportedfortheNorwestlantIIandIIIsurveys(ICNAF,1968).FollowingthediscussionofKielhorn(1952)itseemspossiblethatovertheshelvesLimacinahelicina,theArcticform,wouldhavebeendominant,whereasfarthersouthoverthedeepwaterLimacinaretroversamighthavereplacedit.Pteropodbiomasswasnotassessedseparately,butwasincludedintheNon-copepodbiomass.4.1.16.OstracodsOstracodswererestrictedtothedeepwateranditisprobablethattheproportionofthepopulationintheupper100mwasgenerallylow.Inourseriesofverticallystratifiedtowsinearlysummer1999,6498%oftheostracodsoccurredatdepthsbetween100and1000mintowsmadeto2500m,with029%intheupper100m.IntheLHPR(LonghurstHardyPlanktonRecorder)towsofLonghurstetal.(1984)maximumconcentrationswerealsobelow200matthefourstationsforwhichprofilesareshownandBuchananandSekerak(1982)reportthatostracodswereabundantbelow250minwesternBaffinBay.Atdeepwaterstationswholewatercolumnostracodabundancesincentral(Longhurstetal.,1984)andnortheastBaffinBay(Sameoto,1984)werehigherthanandorsimilartothoseintheupper100matsomestationsintheLabradorSea.ZonalaverageabundancesalongtheL3sectionwere,however,generally E.J.H.Headetal./ProgressinOceanography59(2003)1–3027somewhatlowerthanthosereportedinthestudiesofLonghurstetal.(1984)andSameoto(1984).Ostracodsarethoughttoexhibitdielverticalmigration,butthiswasonlyseenourdatainspring1997.Theircontri-butiontothemesozooplanktonbiomasswasneverimportant.4.2.RelationshipbetweenzooplanktoncommunitystructureandthedistributionofwatermassesalongtheL3sectionThehydrographicmeasurementsreportedhereandelsewhere(Headetal.,2000;Head&Harris,2003)areconsistentwithourunderstandingofcirculationandwatermassesthatoccurinthestudyregionasoutlinedintheIntroduction.AlongtheL3section,thecentralLabradorSearegion(Zone3)containedmainlyrelativelywarmrelativelysaltywaterofNorthAtlanticorigin.Westernregionsofthecentralbasintendedtobecoolerthaneasternregion.Thisisbecausetheyaremoreaffectedbytheconvectivedeepwatermixingthatoccurshereinwinter.Interannualchangesintheintensityofconvection,however,(strongerin1997and2000,weaker1995,1996,1998and1999,Yashayaev,unpublisheddata)hadnoveryobviouseffectswithinthesurfacelayers.EasternregionsofthecentralLabradorSeaweregenerallywarmerthanwesternregionspartlybecausetheydonotundergosuchdeepmixingandpartlybecauseoftheinfluxofwarmerwaterviatheIrmingerCurrent.In2000,however,therewasnowarmeasternLabradorSeawateranywhere.OnthisoccasiontherewereanunusuallylargenumberoficebergsonandneartheGreenlandShelfandcoldshelf-typewaterwasfoundbeyondtheshelf-break.ZooplanktoncommunitystructureinZone3wassimilarinwesternandeasternregions(e.g.Fig.9)and,foragivenseason,concentrationsofthecharacteristiczooplankton(C.finmarchicus,E.norvegica,S.minorandeuphausiids)weresimilarbetweenyears(Table2),despitedifferencesintheproportionsofwesternandeasternwaters.Averagetemperaturesintheupper100minZone3weresimilarinspringandinearlysummer,butca1.2°Chigherinearlysummerthaninspring(Fig.12).AveragetemperaturesinZones2and4werevariable,dependingontheproportionsofcoldshelfwater,coolslopewaterandwarmcentralbasinwaterpresent.AmongstthezooplanktongroupsinZone2,noneshowedanyobviousassociationwithanyofthewatertypes,althoughin2000whenLabradorShelfwaterwasdominant,chaetognathsandostracodswerelessabundantthanduringtheotherspringcruises,andM.longawasmoreabundant(Table2).Ontheotherhand,in1995,whentheanalysisofcommunitystructuresuggestedthatZone2hadnotbeensampled,thehydrographicanalysisconfirmedthatnoneofthestationssampledcontainedanyslope-typewater.AsinZone2inthewest,inZone4intheeast,nozooplanktongroupsshowedanyparticularassociationwithanyofthewatertypesandtheconcentrationsofmostgroupswerequiteconsistentinbothspringandearlysummer.Zone4wasremarkable,however,inthatconcentrationsofC.finmarchicuswereconsistentlyhigherherethanintheotherzones,especiallyinearlysummer.Thisappearstoberelatedtothespringbloomdynamicsintheregionandtheiraffectonreproductionandthedevelopmentoftheyearsnewgeneration(Head&Harris,2003).Averagetemperaturesovertheshelvesvariedamongstyearsandseasonally(Fig.12)andweregenerallylowerthanthoseinadjacentslopewaters.C.glacialisandPseudocalanusspp.werethegroupsmostassociatedwiththeshelves,althoughC.hyperboreuswerealsoconcentratedthereandlarvaceansshowedtheirhighestabundancesontheshelvesandinGreenlandSlopewaters(Zone4).Inearlysummer1999,whenslopewatersmadeimportantcontributionstoZones1and5,concentrationsoftheseshelforganismswerenolowerthaninotherearlysummers.Ontheotherhand,M.longawasmoreabundantonbothshelvesin1999thaninotheryears,andeuphausiidsweremoreabundantontheGreenlandShelf,perhapsbecauseofthepresenceofslopewater.Inspring1997,allfourshelf-associatedorganismswereunusuallyabundantintheLabradorslopewaters(Zone2),buttheseincreaseswerenotapparentlyaccompaniedbyanincreasedcontributionofshelfwater. 28E.J.H.Headetal./ProgressinOceanography59(2003)1–305.SummaryOurobservationsofthedistributionsofthemesozooplanktonintheLabradorSeaareconsistentwithpreviousreportsfromtheareaandwithwhatweknowoftheirdistributionselsewhere.TheArcticspecies(C.glacialis,C.hyperboreusandPseudocalanusspp.)weremostabundantintheArcticwatersovertheshelves.TheNorthAtlanticorganisms(C.finmarchicus,E.norvegica,S.minor,euphausiids)weremostabundantinthecentralbasin.Thegroupswerenotcompletelysegregated,however,sothatallArcticorganismswerealsofoundinslopewatersandC.hyperboreus,inthecentralbasin.Similarly,NorthAtlanticorganismswerefoundnotonlyinthecentralLabradorSea,butalsoinslopewatersandsometimesontheshelves.Thesespilloversarelikelytheresultofmixingofshelfandcentralbasinwateracrossthefrontsassociatedwiththebasinmargins,asourhydrographicanalysisshowscanhappen.AlongtheL3section,however,wegenerallysawlittleevidenceoflocalslopeorcentralbasinwatersontheLabradorShelf:intrusionsmayoccur,however,farthernorthupstreamofoursamplingstations,wherethebathymetryislessabrupt.Organismsthatundertakedielverticalmigrations,orthatgenerallylivebelow100m(E.norvegica,M.longa,Microcalanusspp.,S.minor,chaetognaths,euphausiids,ostracods)arelikelytoberestrictedtothedeeperwaters,aswasgenerallyobservedexceptforM.longaandMicrocalanusspp..BothoftheseareprobablyArcticspecies,withsourcesinArcticwaters.TheyareprobablytransportedtotheLabradorSeaovertheshelvesandalongthebasinmargins.BecausetheyspendmuchoftheirtimeatdepththeywilltendtoaccumulateinthedeepcentralbasinoftheLabradorSea,althoughnotintheupper100m.Theirobserveddistributionswithintheupper100marethustheresultofboththeirsourcesandtheirdielmigratorybehaviourorpreferenceforlivingatdepths100m.Thedistributionpatternsofmesozooplanktonthatweobservedweregenerallyrathersimilaramongstyearsforagivenseason,whereasCPRobservationshaveshownhigherconcentrationsofC.hyperboreusandC.glacialisonandneartheNewfoundlandShelfinthe1990srelativetothe1960s(Johns,2001).ThismayberelatedtoincreaseddischargefromtheArcticviaDavisand/orHudsonStrait,sincesalinitiesovertheLabradorShelfinthe1990swereconsistentlylowerthanthoseinthe1960s(Yashayaev,unpub-lisheddata).CPRobservationsalsoshowedlowerconcentrationsofC.finmarchicusintheIrmingerSeainthe1960srelativetothe1990s.Inagreementwiththisobservation,wesawsubstantiallyhigherconcen-trationsofC.finmarchiciussouthofGreenlandinspring1997thanthosereportedforspring1963(ICNAF,1968).DifferencesintheabundanceanddistributionofC.finmarchicusarethoughttobelinkedchangesinclimaticconditions,manifestedinthephaseoftheNorthAtlanticOscillation(NAO)(Fromentin&Planque,1996;Greene&Pershing,2000),whichwaspredominantlynegativeinthe1960sandgenerallypositiveduringoursamplingyears.In1996,however,theonlyyearofourstudywhentheNAOwasnegative,theabundanceanddistributionofC.finmarchicusatstationsalongtheL3linewerenotaffectedinanyobviousway.Inabroadercontext,ithasbeenarguedthattheLabradorSeaisacriticalareaintheearthsclimatesystem.ModelsimulationsforagloballywarmingclimatesuggestincreaseddischargeoffresherwaterfromtheArctic(Vinnikovetal.,1999)andawarmingoftheLabradorSea,leadingtoaweakening,orshut-down,ofdeepwaterconvectivemixing(e.g.Rahmstorf&Ganopolski,1999;ICPP,2001).Theeffectsofthesechangesontheplanktonicecosystemarenoteasytopredict,butitishardtobelievethattheywillbenegligible.Theobservationsreportedherefora6yearperiodinthelate1990swillprovideabaselinefortheassessmentofthelong-termeffectsofachangingclimateonthemesozooplanktonoftheLabradorSeaecosystem. 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