Fig1Annualcompositeyear2006ofSeaWiFSdatafortheglobaloceanUpperpanelchlorophyllconcentrationChlmgm3middlepanelchromophoricdissolvedmatterindex8dimensionlessBoththesequantitiesareprovi ID: 439217
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3140A.Moreletal.:Themostoligotrophicsubtropicalzonesoftheglobalocean Fig.1.Annualcomposite(year2006)ofSeaWiFSdatafortheglobalocean.Upperpanel:chlorophyllconcentration,[Chl](mgm�3/;middlepanel:chromophoricdissolvedmatterindex,8(dimensionless).Boththesequantitiesareprovidedanddis-tributedbyNASA.Lowerpanel:mapoftheabsorptioncoefcientat443nm,ay(m�1/ofthechromophoricdissolvedorganicmatter,CDOM,computedfrom[Chl]and8viaEq.(1).Eachpanelhasitsowncolorscale.The(yellow)boxessuperimposedonthemapsshowtheoligotrophiczonesselectedforthepresentstudy(seealsoTable1fortheexactlocationsandmeaningoftheidentiers).TheblackboxinamesotrophiczoneoffPortugal(P)isselectedforacomparison(seetext). material,CDOM(or“yellowsubstance”,or“Gelbstoff”),asattestedbytheparticularlylowvaluesoftheattenuationco-efcientfordownwardirradianceintheultravioletspectraldomain(Moreletal.,2007a;Swanetal.,2009),andalsoat-testedbyspectroscopicdeterminations(Bricaudetal.,2010).Thespecialclarityofthesewatersresultsinadeepbluecolorduetothereectanceenhancementintheblue,violet,andUVpartsofthespectrum.SpaceborneOceanColorsensorsareabletodetectthisenhancement.Therecurrentdetec-tionfromspaceofahuge“bluehole”denotingahyperolig-otrophicsystemwithintheSouthPacicgyrewaslargelyattheoriginoftheBIOSOPEproject(ClaustreandMaritorena,2003).Ofcourse,thequestionarisesaboutthepossibleoc-currenceofsimilarsituationsinotherpartsoftheworldocean,notonlyintermsofalgalconcentration,butalsointermsofyellowsubstancecontent.Othersubtropicalgyresalsocharacterizedbyanticycloniccirculation,down-welling,andthickerthermoclines,areobviouscandidates,sincethedepressednutriclinelimitsthealgaldevelopmentandsubsequentbiological/biochemicalprocesses.Theseex-tensiveoligotrophicgyresarewellknowninbothhemi-sphereswithinthesubtropicalAtlanticandPacicoceans(seee.g.,Sverdrupetal.,1963;TomczakandGodfrey,1994).ThesouthIndianoceanisalsotheseatofasubtropicalanti-cyclonicsystem,whileinthenorthernIndianoceanthemon-soonregimeimpedestheregulardevelopmentofsuchasim-ilarfeature.Thesedesertoceanicareashaveunambiguouslybeendetectedbyoceancolorsensors;theirhugeextension,andtheirchlorophyllconcentrationhavebeendocumentedandmonitored(seee.g.,McClainetal.,2004;Polovinaetal.,2008).Indeed,whilethebiologicalactivityiscomparativelysmallintheseareas,theirvastsizemakestheircontributiontotheglobalproductivityandbiogeochemistrydenitelysig-nicant(seee.g.,Antoineetal.,1996).Theseultramarineoceaniczoneswerealsoselectedasof-feringwideandhomogeneoustargetsallowingin-ightcal-ibrationofspaceoceancolorsensorstobeperformedviatheRayleighscatteringcalibrationmethod(Fougnieetal.,2002).Inthisstudy,basedonSea-viewingWideField-of-viewSensor(SeaWiFS)data,thelowchlorophylllevel(thereafterdenoted[Chl])withinseveralgyreswascon-rmedanddocumented.Noattempt,however,wasmadetocomparativelyassesstheCDOMcontent.Yet,someyearslater,actuallywhenthe“GSM”algorithm(Siegeletal.,2002,2005b)wasappliedtoremotelysensedoceancolordata,relativeminimainnear-surfaceCDOMabundancewerealsodetectedinthesubtropicalgyres.Therefore,asystem-aticstudyofthesezones,withboththeirlow[Chl]andlowCDOMlevels,canprovideananswertotheinitialques-tion:fromanopticalviewpoint,dowatersidenticalto,orapproachingthoseencounterednearEasterIslandexistelse-whereintheworldocean?Moregenerally,andfromabio-geochemicalviewpoint,thetrophicstatusofthesezones,theirseasonality,theirsystematicdifferenceorresemblance,andtheinterannualstabilityoftheircharacteristicsarealsothetopicsofthepresentstudy.Anewlyproposedtechniqueallowsthechromophoricor-ganicmaterialcontenttobequantiedfromremotelysenseddataofoceancolor(MorelandGentili,2009a).Thistech-niquewillbeappliedinparallelwiththe[Chl]retrievaltocarryoutacomparativestudyofthevariousoligotrophicsub-tropicalgyres.Thelocationandextensionoftheselectedzonesinsideeachofthegyres(Table1andFig.1)werede-limitedinordertocontainthemostoligotrophicinnercore.Notethatthesezonesarefarlessextendedthantheecologicaldomainsasdenedinthesub-tropicalregionsbyLonghurst(1995),orthantheentiregyresasstudiedbyMcClainetal.(2004). Biogeosciences,7,3139– 3151 ,2010www.biogeosciences.net/7/3139/2010/ A.Moreletal.:Themostoligotrophicsubtropicalzonesoftheglobalocean3143 donotimplyaconstancyintheconcentration.Actually,aseasonal[Chl]signalclearlyoccursinfourofthesixselectedzones(asdiscussedlateron),whileinthetwoothers(bothintheNorthPacic),thissignaluctuatesratherirregularlyandisspatiallylesshomogeneous.Atrstsight(Fig.1),theCDOMdistributionintheup-perlayerbearsaroughresemblancetothephytoplanktonicchlorophylldistribution.Themodulationintroducedbythevariationofthe8factor(Eq.1)impedesatightcorrelationbetweenthetwoquantities.Anyhow,theoligotrophicareasselectedonthebasisoftheirlow[Chl]content,appearalsotobecharacterizedbylowayvalues.TheSouthPacicgyreisthelargestsubtropicalanticy-clonicgyreoftheworldocean;itshyperoligotrophiccharac-terwasalreadyacknowledged(Claustreetal.,2008;Moreletal.,2007a).Itscentralzone,aroundEasterIsland,ispresentlyselected.ItscounterpartintheNorthernHemi-sphereisahugeanticyclonicsystem,extendingfromHawai-ianIslandstoMarianaIslands.Theexaminationofoceancolorimageryshowedthatthereallyoligotrophicwatersarelyinginthewesternmostpartofthegyre,i.e.,eastofMarianaIslands;thiszone,actuallywithintheso-calledwarmpool,wasthusselected.IntheAtlanticocean,thegenerallylowlevelofphyto-planktonintheSargassoSeahasbeenknownforalongtime(MenzelandRyther,1960,1961),andregularlydocumented(BermudaAtlanticTime-SeriesStudyor“BATS”program;seee.g.,MichaelandKnap,1996).Theareapresentlyse-lectedinthesouthernSargassoSea,between22and27N,liessouthoftheBATSdeploymentarea(3140N),wherethe[Chl]levelmaybemoderatelyhighinwinter(GarverandSiegel,1997).Theselectedareaisthusclosetothesitestud-iedbyHuetal.(2006)andpresumablymorestable.Indeed,themesoscaleeddyactivity,whichmayincludeintermittentdriftofcoldcoreringsatthelatitudeofBermuda,isweaken-ingsouthward,attheselected(27N)latitudes(Steinbergetal.,2001).TheSouthAtlanticgyreoffBrazilissimilarlycharacter-izedbyalowalgalcontentwithapparentlyalimitedE-Wextensionprobablyduetotheremoteinuenceoftheeu-trophicBenguelasystem(Fig.1).DataobtainedalongtheAtlanticMeridionalTransect(Aikenetal.,2009)haveregu-larlyconrmedthelow[Chl]valuesinthisarea.Midwaybe-tweenMadagascarandAustralia,intheSouthIndianocean,thescarcelydocumentedanticyclonicgyreissystematicallyseenbyoceancolorsensorasanoligotrophiczone,andthuswasretainedforthepresentstudy.Finally,anotherareasituatedsouthoftheHawaiianIs-landshasalsobeenconsidered.Actually,thisareawhichbelongsalsototheNorthPacicsubtropicalgyresystem,isnolongerlocatedinitscenter,butatitseasternperiphery.Itdoesnotexhibitthelowest[Chl]levelintheNorthPacic,which,assaidbefore,arefoundwestward,neartheMari-anaislands,inthequietercenterofthegyre.Thisareawasneverthelessconsideredforapossiblecomparativestudy,inparticularbecauseinsitudataareregularlygatheredattheALOHAreferencestation,andattheMOBYinstrumentedsite,devotedtosatellitecalibrationandvalidationactivitiesandopticalmeasurements.Theregionnallykept,how-ever,ismoreinthesouth(between18and10N),about500–1300kmsouthoftheHawaiianarchipelago.Ifitisout-sideoftheeddyactivityeldintheleeoftheislands(CalilandRichards,2010),itliesyetinsidetheeasternbranchoftheNorthEquatorialcurrent,drivenwestwardbythetradewinds(WyrtkiandKilonsky,1984).Lateraladvectionandoccurrenceofeddy-relatedrandomuctuationscannotbeex-cludedinthiszone.4ResultsThemonthly[Chl]and8SeaWiFSvalueshavebeenspa-tiallyaveragedovereachofthesixzones.Withthesevalues,thecorrespondingCDOMabsorptionvalues,ay(443),havebeenstraightforwardlyderivedviaEq.(1).ThesemonthlyquantitiesaredisplayedinFig.2fromJanuary1998toDe-cember2007.Then,meanannualcycles(Fig.3)werepro-ducedforeachzonebycumulatingandaveragingthecyclesobservedduringthetenyears(thoseinFig.2);thestandarddeviationwhichiscomputedanddisplayedasverticalbarscharacterizes,foreachmonth,theyear-to-yearvariabilityofthequantityinvolved,[Chl],ay,and8.Finally,ageneralannualmeanvalueforeachzone,andthepooledstandarddeviations(theaverageofthemonthlystandarddeviations)werealsoproducedfromthecyclesshowninFig.3;there-sultsaregiveninTable2,wherethe6zoneshavebeenrankedaccordingtoincreasing[Chl]values.Whenneeded,theeight-day[Chl],8,andPARcompos-iteshavealsobeenexamined(“PAR”means“photosyntheti-callyavailableradiation”,from400to700nm).4.1ThechlorophyllconcentrationThepermanentoligotrophiccharacterisconrmedwithinthesixselectedareas.Indeed,theannualmean[Chl]values(Table2),whicharebelow0.05mgm�3inallzonesexceptnearHawaii(0.06mgm�3/,arelowcomparedwiththemeanvalueforthedeepglobaloceanamountingto0.193mgm�3(Wangetal.,2005).TheSouthPacicgyre(EasterIsland,E)denitelyap-pearstobehyperoligotrophic(mean[Chl]0.026mgm�3/.ThewesternpartofNorthPacicgyre(nearMarianaIslands,M)issimilarlyandsteadilyhyperoligotrophic.Incontrast,thehighestmean[Chl]valueisobservednearHawaii(H),attheeasternperipheryofthegyre.TheSouthAtlantic(B)andIndian(I)gyres,aswellastheSargassoSea(S)areinin-termediateposition,withannualmean[Chl]valuesrangingfrom0.038to0.049mgm�3.Comparedtotheseasonal[Chl]valuesintheBATSandHOTsites(Figs.1and2inWerdelletal.,2007),thepresentvaluesinHandSarenotablylower, www.biogeosciences.net/7/3139/2010/Biogeosciences,7,3139– 3151 ,2010