CDOM紫外可见光谱与DOC的联系

LinkingCDOMspectralabsorptiontodissolvedorganiccarbonconcentrationsandloadingsinborealestuariesEeroAsmalaa,*,ColinA.Stedmonb,DavidN.Thomasa,caMarineResearchCentre,FinnishEnvironmentInstitute(SYKE),Helsinki,FinlandbNationalInstituteforAquaticResources,SectionforOceanographyandClimate,TechnicalUniversityofDenmark,Charlottenlund,DenmarkcSchoolofOceanSciences,BangorUniversity,MenaiBridge,Anglesey,UKarticleinfoArticlehistory:Received22February2012Accepted28June2012Availableonline11July2012Keywords:dissolvedorganiccarbonchromophoricdissolvedorganicmatterriverineorganicloadingabsorptioncoefficientestuarinegradientabstractThequantityofchromophoricdissolvedorganicmatter(CDOM)anddissolvedorganiccarbon(DOC)inthreeFinnishestuaries(Karjaanjoki,KyrönjokiandKiiminkijoki)wasinvestigated,withrespecttopredictingDOCconcentrationsandloadingsfromspectralCDOMabsorptionmeasurements.Altogether87sampleswerecollectedfromthreeestuarinetransectswhichwerestudiedinthreeseasons,coveringasalinityrangebetween0and6.8,andDOCconcentrationsfrom1572mmoll
1infreshwaterto222mmoll
1incoastalwaters.CDOMabsorptioncoefficient,aCDOM(375)valuesfollowedthetrendinDOCconcentrationsacrossthesalinitygradientandrangedfrom1.67to33.4m
1.ThelinkbetweenDOCandCDOMwasstudiedusingarangeofwavelengthsandalgorithms.Wavelengthsbetween250and270nmgavethebestpredictionswithsinglelinearregression.Totaldissolvedironwasfoundtoinfluencethepredictioninwavelengthsabove520nm.DespitesignificantseasonalandspatialdifferencesinDOCeCDOMmodels,auniversalrelationshipwastestedwithanindependentdatasetandfoundtoberobust.DOCandCDOMyields(loading/catchmentarea)fromthecatchmentsrangedfrom1.98to5.44gCm
2yr
1,and1.67to11.5aCDOM(375)yr
1,respectively.
2012ElsevierLtd.Allrightsreserved.1.IntroductionDissolvedorganicmatter(DOM)ismadeupfromaheteroge-neousarrayofcompoundsandthesubsetofthesethatabsorbultraviolet(UV)andvisiblelightandarereferredtoaschromophoricorcoloreddissolvedorganicmatter,CDOM(Bricaudetal.,1981;GreenandBlough,1994).CDOMabsorptionreducesthepenetrationofUVlightthroughthewatercolumnandthusprotectsmarineorgan-ismsfromexposuretoharmfulUVradiation,whilstalsodecreasingtheamountofvisiblelightavailableforpelagicandbenthicphotosynthesis(Bidigareetal.,1993).ThroughouttheUVandvisiblewavelengthrange,thelightabsorptionbyCDOMdecreasesapproximatelyexponentially(Bricaudetal.,1981).Theslopecoef-ficient(spectralslopeorSvalue),fittedtotheexponentialcurveofCDOMspectrum,isasameasureofhowrapidlytheabsorptiondecreaseswithincreasingwavelength(GreenandBlough,1994;StedmonandMarkager,2003).AvarietyofpartiallyoxidizedligninsderivedfromvascularplantsourcesarethoughttoberesponsibleforthemajorityofCDOMabsorptioninhumic-richwaters(Boyleetal.,2009).ReliablemeasurementofDOCiscrucialincontemporaryaquaticresearchsinceDOCconcentrationsareexpectedtoincreasewithelevatedterrestrialrunoff,thusaffectingtheaquaticecosystems(Freemanetal.,2001;Evansetal.,2006;Larsenetal.,2011).InFinnishrivers,94%oftotalorganiccarbonisindissolvedform(Mattssonetal.,2005),whichhighlightstheimportanceofDOCmeasurementsforestimatingenvironmentalcarbonbudgets.Methodstopredictdissolvedorganiccarbon(DOC)concentra-tionsfromabsorbancecharacteristicshavebeenpursuedforseveraldecades(Banoub,1973;LewisandTyburczy,1974;StewartandWetzel,1981;Carteretal.,2012),andsuchanapproachshouldenableaquickandinexpensivealternativetodirectanalyticalmethodsformeasuringDOCconcentrations(Spenceretal.,2007).However,aprerequisiteforasuccessfulpredictionofDOCconcentrationisthatthenon-absorbingDOCisataconstant,oratalowlevel(FerrariandDowell,1998).Despiteseverallimi-tations,CDOMmeasurementshavebeenusedasindicatorsofDOMfluxes,especiallywhenstudyingasystemwithasinglehumic-richend-member(BakerandSpencer,2004).AmoreuniversalapproachwassuggestedbyFichotandBenner(2011)whopre-sentedanapproachforestimatingdissolvedorganiccarbon(DOC)concentrationsfromCDOMcoefficientsat275and295nm.This*Correspondingauthor.E-mailaddresses:eero.asmala@ymparisto.fi,eero.asmala@helsinki.fi(E.Asmala).ContentslistsavailableatSciVerseScienceDirectEstuarine,CoastalandShelfSciencejournalhomepage:
2012ElsevierLtd.Allrightsreserved.(2012)107e117regionoftheCDOMspectrum,moreaccuratelythespectralslopebetween275and295nm(S275
295),haspreviouslybeenintro-ducedasaproxyformolecularweightofdissolvedorganicmatter(Helmsetal.,2008).S275
295waschosenasitshowsthewidestvariabilityacrosscontrastingenvironmentsandcanbeaccuratelydeterminedevenforCDOMinhighlyphotobleachedoceanwaters.SpectralslopeshavealsobeenusedasproxiesforcompositionofCDOM,includingtheratioofhumicacidstofulvicacids(Carderetal.,1989)andphoto-bleachingeffects(Vodaceketal.,1997).BesidesCDOM,thereareothercompoundspresentinnaturalwatersthatabsorblight.Mostsignificantly,nitrate,nitriteandbisulfideaffectabsorptionpropertiesupto240nm,andbromideupto260nm(Collosetal.,1999;JohnsonandColetti,2002).Highdissolvedironconcentrationsmayresultasaprocesswherephoto-Fe