海岸CDOM的光学特性

Estuarine,CoastalandShelfScience(2000)51,267–278doi:10.1006/ecss.2000.0645,availableonlineat(CDOM)inDanishCoastalWatersC.A.Stedmona,b,S.MarkagercandH.Kaasc,daDepartmentofMarineandCoastalEcology,DanishInstituteforFisheriesResearch,Kavalerga˚rden6,DK-2920Charlottenlund,DenmarkbPresentaddress:NationalEnvironmentalResearchInstitute,DepartmentofMarineEcology,P.O.Box358,Frederiksborgvej399,DK-4000Roskilde,Denmark;E-mail:[e-mail]ResearchInstitute,DepartmentofMarineEcology,P.O.Box358,Frederiksborgvej399,DK-4000Roskilde,Denmark;E-mail:[e-mail]:DHI-WaterandEnvironment,AgernAlle11,DK-2970Hørsholm,Denmark;E-mail:[e-mail]9February2000andacceptedinrevisedform5May2000Theopticalpropertiesofchromophoricdissolvedorganicmatter(CDOM)inDanishestuariesandcoastalwaterswereinvestigated.Anewmethodforestimatingthespectralslopecoefficient(S)wasexaminedandfoundtogiveacloserfitforthemeasuredabsorption(92%reductioninsumofresiduals)thanthetraditionalmethod.ThespectralpatterninresidualsproducedbythenewtechniqueenabledtheidentificationofCDOMoriginatingfromareasofdifferentlanduses/typeswhereScoefficientsweresimilar.Svalueswerefoundtobehaveconservativelywithrespecttosalinityinallwatersexceptfortheoff-shoreNorthSearegionwhereCDOMfrommarinesourceswassuspectedtohavemoreinfluence.ThespecificabsorptioncoefficientofCDOMdidnotvarysignificantlyinDanishcoastalwaters.Thesewatersareofparticularinterestinbio-opticalstudiesastheyallowthestudyofCDOMacrossagradientfromestuarine,brackishtonear-oceanicenvironments.TheresultsdemonstratethatitispossibletodevelopregionalmodelsfortheopticalpropertiesofCDOMwhichwouldthenallowhigherprecisioninremotesensingapplications.
2000AcademicPressKeywords:lightabsorption;CDOM;Scoefficient;estuaries;NorthSea;BalticSeaIntroductionChromophoricDissolvedOrganicMatter(CDOM)isfoundinallnaturalwatersanditcanmakeupasizeablefractionoftheDOMpool.Itconsistsofavariedmixtureofaliphaticandaromaticpolymersthatarederivedfromthedegradationofterrestrialandaquaticplantmatter(Kirk,1994).CDOMisoneofthemajorlightabsorbingconstituentsinnaturalwaters.Itsabsorptionisstrongestintheultravioletregionanddiminishestonearzerointheredregion.ThisbehaviourcanbemodelledbyEquation1(Jerlov,1968;Lundgren,1976;Bricaudetal.,1981),a
=a
oeS(
o
)(1)wherea
istheabsorptioncoefficientatacertainwavelength,
,anda
oistheabsorptioncoefficientatareferencewavelength,
o.Sistheexponentialslopecoefficientandisameasureofhowtheabsorptiondecreaseswithrespecttowavelength.AsaconsequenceofitsopticalbehaviourCDOMhasthepotentialtosignificantlyeffecttheproductivityofthewatercolumn.CDOMcanenhanceprimaryproductionintheupperwatersviablockingoutharm-fulUVradiationwhileatthesametimelimitproduc-tionatdepthduetoitsshadingeffect.Itscoloursignalisalsoanimportantfactorinremotesensingappli-cationswheretheeffectofCDOMmustbeaccountedforbeforereflectionmeasurementscanbeutilizedforphytoplanktonandsuspendedsedimentobservations(Tassan,1988;Karabashev,1992).Howeverasaresult,thisalsoopensthepossibilityofusingremotesensingmethodsforthemeasurementofasizeablefractionofthedissolvedcarbonpoolinthesurfacewaters(Hogeetal.,1995;Vodaceketal.,1995;Ferrarietal.,1996).InmanycoastalwatersCDOMhasbeenfoundtobehavequasi-conservativelyduringmixing,showingconsiderableremovalatlowsalinitiesbutbehavingconservativelyathighersalinities(Reuteretal.,1986;Højerslev,1988,1989;Laane&0272–7714/00/080267+12$35.00/0
2000AcademicPressKramer,1990).ThispropertymeansthatCDOMisausefultoolintracingwatermassmovementsespeciallyincoastalregionswheretemperaturedoesnotbehaveconservatively(Højerslev,1971;Laane&Kramer,1990;Karabashevetal.,1993;Højerslevetal.,1996).Asaresultofabsorbingsolarradiation,CDOMisbrokendownbyphotoreactionstoformavarietyofreactiveproductsinfluencingvariousbiogeochemicalcyclesinnaturalwatersandtheatmosphere(Moran&Zepp,1997;Pettersonetal.,1997;Uher&Andreae,1997).ThisprocesshasbeenshowntocausebothanincreaseinbioavailabilityofCDOMandbleachingoftheCDOMpoolresultinginthedeeperpenetrationofharmfulUVradiation(Moran&Zepp,1997;Gao&Zepp,1998).SomeoftheearliestresearchintoCDOMinopenwaterswascarriedoutintheBalticandNorthseas(Kalle,1937,1956;Jerlov,1953,1955).TheopticaleffectsofCDOMareofparticularinterestinthisareaduetoalargegradientfromhighconcentrationintheBalticandGermanBightwaterstonearoceanicconditionsintheSkagerrak(seeFigure1forlo-cations).TheimportanceofCDOMwithrespecttoremotesensingapplicationsintheregionhasbeenemphasisedbymanyearlierstudies(Højerslev,1980;Reuteretal.,1986;Karabashev,1992;Aarup,1994).TheaimofthisstudywastomaptheopticalpropertiesofCDOMinDanishcoastalwatersinordertoimprovethecorrectionforitseffectonwatercolourinremotesensingapplications.Asecondobjectivewastoinvesti-gatetherelationshipbetweenCDOMandtheconcen-trationoftotalorganiccarbon(TOC).TheworkwascarriedoutaspartoftheDanishEnvironmentalMoni-toringofCoastalWaters(DECO)program,whichisexploringthefeasibilityofremotesensingtechniquesformonitoringpu