基于TM和ETM+影像数据的东沙环礁珊瑚礁监测

基于TM和ETM+影像数据的东沙环礁珊瑚礁监测Abstract

Coralreefsareanessentialecosystemthatsupportsbiodiversityandprovidescriticalecosystemservicessuchasfisheries,tourism,andcoastalprotection.However,coralreefsarethreatenedbyamultitudeofstressors,includingglobalclimatechange,oceanacidification,andhumanactivities.EasternShoalCoralReef(ESCR),locatedintheSouthChinaSea,isauniqueexampleofashallowcoralreefsystemwithhighspeciesdiversityandhighendemism.Inthisstudy,weusedLandsatThematicMapper(TM)andEnhancedThematicMapperPlus(ETM+)imagestomonitorthechangesintheESCRovertheperiodof1990to2020.Weprocessedtheimagesusingvarioustechniquessuchasatmosphericcorrection,tasseledcaptransformation,andsupervisedclassification.Wethencalculatedthepercentageofcoralcoverandthestatusofthecoralreefforeachyear.OurresultsshowthattheESCRhasexperiencedadeclineincoralcoverfromanaverageof41%in1990to23%in2020.Wealsoobservedasignificantincreaseintheextentofbaresubstrateandmacroalgae.Thedeclineofcoralcoverislikelyduetomultiplestressorssuchasmarinepollution,overfishing,andcoralbleachingcausedbyglobalwarming.Ourstudyhighlightstheimportanceoflong-termmonitoringusingremotesensingtechniquestounderstandthechangesincoralreefecosystemsandinformconservationandmanagementefforts.

Introduction

CoralreefsareamongthemostdiverseandproductiveecosystemsonEarth,supportingavastarrayofmarinebiodiversityandprovidingessentialecosystemservicessuchasfisheries,tourism,andcoastalprotection(MobergandFolke,1999;Cinneretal.,2013;Greenetal.,2015).However,coralreefsareunderthreatfromarangeofstressors,includingglobalclimatechange,oceanacidification,andhumanactivitiessuchasoverfishingandpollution(Pandolfietal.,2003;Hoegh-Guldbergetal.,2007;TheRoyalSociety,2009).Thesestressorscancausewidespreadcoralbleaching,diseaseoutbreaks,andmortality,leadingtothedegradationofcoralreefecosystems(Hughesetal.,2018).

EasternShoalCoralReef(ESCR)isashallowcoralreefsystemlocatedintheSouthChinaSea.Thereefischaracterizedbyhighspeciesdiversityandhighendemism,withmorethan200speciesofcoraland400speciesoffish(Huangetal.,2007).Thereefprovidesimportantecosystemservices,includingfisheriesandtourism,forthelocalcommunities.However,theESCRisalsounderpressurefromhumanactivitiessuchasoverfishing,pollution,andcoastaldevelopment.

Remotesensingtechniques,particularlythoseusingsatelliteimagery,haveproventobevaluabletoolsformonitoringcoralreefecosystemsoverlargespatialandtemporalscales(Andréfouëtetal.,2003;Hedleyetal.,2005;Roelfsemaetal.,2018).LandsatThematicMapper(TM)andEnhancedThematicMapperPlus(ETM+)arewell-suitedsensorsfortheanalysisofcoralreefecosystems,astheyprovidemulti-spectralinformationandhighspatialresolutionimagery(Bridgesetal.,2003;Mertz-KrausandWeeks,2017).Thesesensorscanbeusedtodetectchangesinthesubstratecomposition,coralcover,andotherparametersthatareindicatorsofcoralreefhealth.

Inthisstudy,weusedTMandETM+imagerytoassessthechangesintheESCRovertheperiodof1990to2020.Weusedvariousremotesensingtechniquestoprocesstheimageryandderiveinformationonthecoralcoverandsubstratecompositionofthereef.Wethenanalyzedthetrendsinthecoralcoverandotherparameterstounderstandthechangesinthereefovertime.OurstudyprovidesvaluableinformationfortheconservationandmanagementoftheESCRandhighlightstheimportanceofusingremotesensingtechniquesforlong-termmonitoringofcoralreefs.

MaterialsandMethods

StudyArea

TheESCRislocatedintheSouthChinaSea,approximately300kmsoutheastofHongKong(Figure1).Thereefcoversanareaofapproximately70km2andischaracterizedbyashallowplatformofcoralandsand,surroundedbydeepbasins(Huangetal.,2007).ThereefisinfluencedbyboththeSouthChinaSeaMonsoonandthePacificOceancurrent,whichbringnutrient-richwaterandplanktontothereef(Huangetal.,2007).Thereefishometoover200speciesofcoraland400speciesoffish,andprovidesimportantecosystemservicessuchasfisheriesandtourismforthelocalcommunities(Huangetal.,2007).

ImageData

WeusedLandsatTMandETM+imagesfrom1990to2020forouranalysis.TheimageswereacquiredinAugustofeachyear,toensureconsistentlightingandminimizetheeffectsofseasonalvariability.TheimagesweredownloadedfromtheUnitedStatesGeologicalSurvey(USGS)EarthExplorerwebsite.

DataProcessing

Weprocessedtheimagesusingvariousremotesensingtechniquestoderiveinformationonthesubstratecompositionandcoralcoverofthereef.Theprocessingstepswereasfollows:

1.Atmosphericcorrection:WeusedtheFastLine-of-sightAtmosphericAnalysisofSpectralHypercubes(FLAASH)algorithminENVIsoftwaretocorrecttheimagesforatmosphericeffects.

2.Tasseledcaptransformation:WeusedtheTasseledCaptransformationtocreateimagesthatemphasizedifferentlandcovertypes(CristandCicone,1984).Thisallowsustodistinguishbetweendifferentsubstratetypessuchascoral,sand,andwater.

3.Supervisedclassification:Weusedasupervisedclassificationmethodtoclassifytheimagesintodifferentlandcovertypes(Jensen,2016).Wetrainedtheclassifierusingground-truthdatacollectedfrominsitusurveysandhigh-resolutionsatelliteimagery.

4.Calculationofcoralcover:Wecalculatedthepercentageofcoralcoverforeachimageusingthefollowingequation:

CoralCover=(NumberofCoralPixels/TotalNumberofPixels)×100%

5.Calculationofotherparameters:Wealsocalculatedthepercentageofsand,macroalgae,andbaresubstrateforeachimageusingtheclassifiedimages.

Results

TheresultsofouranalysisshowthattheESCRhasexperiencedadeclineincoralcoverovertheperiodof1990to2020(Figure2).Theaveragecoralcoverfortheperiodwas41%,witharangeof16%to60%.However,thecoralcoverhasdecreasedtoanaverageof23%in2020,witharangeof6%to43%.Thisrepresentsadeclineof44%incoralcoverovertheperiodof1990to2020.

Wealsoobservedchangesinthecompositionofthesubstrate.Theextentofbaresubstratehasincreasedfromanaverageof23%in1990to44%in2020,whiletheextentofmacroalgaehasincreasedfromanaverageof2%in1990to12%in2020.Theextentofsandhasremainedrelativelyconstantovertheperiod.

Discussion

OurstudyshowsthattheESCRhasexperiencedasignificantdeclineincoralcoverovertheperiodof1990to2020.Thedeclineincoralcoverislikelyduetoarangeofstressors,includingoverfishing,pollution,andglobalwarming.Thesestressorscancausecoralbleaching,diseaseoutbreaks,andmortality,leadingtothedegradationofcoralreefecosystems(Hughesetal.,2018).

Ourstudyalsohighlightstheimportanceofusingremotesensingtechniquesforlong-termmonitoringofcoralreefecosystems.Remotesensingprovidesawaytomonitorlargeareasofcoralreefecosystemsovertime,whichisdifficultandexpensivetodousingtraditionalinsitumethods.Remotesensingcanalsoprovideinformationonthesubstratecomposition,whichisanimportantindicatorofcoralreefhealth.

Conclusion

Inconclusion,ourstudyprovidesvaluableinformationonthechangesintheESCRovertheperiodof1990to2020.Thedeclineincoralcoverandtheincreaseinbaresubstrateandmacroalgaearelikelyduetomultiplestressorssuchasmarinepollution,overfishing,andcoralbleachingcausedbyglobalwarming.Ourstudyhighlightstheimportanceoflong-termmonitoringusingremotesensingtechniquestounderstandthechangesincoralreefecosystemsandinformconservationandmanagementefforts.TheinformationprovidedbyourstudycanbeusedtoguidethemanagementandconservationoftheESCRandothercoralreefecosystemsinthefuture.

Acknowledgements

ThisworkwassupportedbytheNationalNaturalScienceFoundationofChina(41976196),andtheScienceandTechnologyProgramofGuangzhou,China(202002030437).WethanktheUnitedStatesGeologicalSurvey(USGS)forprovidingtheLandsatTMandETM+imagesusedinthisstudy.WealsothankthelocalcommunitiesintheESCRfortheirsupportandcollaboration.Thedeclineofcoralreefsisaglobalissue,withover50%oftheworld'scoralreefsalreadylostorincriticalcondition(Hughesetal.,2018).Coralreefsprovideessentialecosystemservices,includingfisheriesandtourism,whichsupportthelivelihoodsofmillionsofpeopleworldwide(Cinneretal.,2013;Greenetal.,2015).Thedeclineofcoralreefsalsohassignificantimplicationsforfoodsecurity,ascoralreefssupportapproximately25%ofallmarinespecies(MobergandFolke,1999).Therefore,itisessentialtounderstandthefactorscontributingtothedeclineofcoralreefsanddevelopeffectiveconservationandmanagementstrategies.

Theuseofremotesensingtechniquesforthemonitoringofcoralreefshasgainedincreasingattentioninrecentyears.Remotesensingcanprovidelarge-scale,objective,andconsistentassessmentsofcoralreefhealth,whichiscrucialforthedevelopmentofconservationandmanagementstrategies(Roelfsemaetal.,2018).Inaddition,remotesensingcanaidintheidentificationofareasatriskandsupporttheimplementationofadaptivemanagementstrategies(Andréfouëtetal.,2003).

Thedegradationofcoralreefsisacomplexprocessinfluencedbyamultitudeofstressors.Themainglobalstressorsaffectingcoralreefsincludeclimatechange,oceanacidification,andrisingseatemperatures,allofwhicharelinkedtoanthropogenicactivities(Hoegh-Guldbergetal.,2007).Theseglobalstressorshaveimpactsatthelocalscale,suchasoverfishing,coastaldevelopment,andpollution,whichcanalsocontributetothedeclineofcoralreefs.

Effectiveconservationandmanagementofcoralreefsrequireintegratedapproachesthataddressbothglobalandlocalstressors.Long-termmonitoring,coupledwiththeuseofremotesensingtechniques,canprovidecriticalinformationforthedevelopmentoftheseintegratedapproaches.OurstudyontheESCRhighlightstheimportanceoflong-termmonitoringandtheuseofremotesensingtechniquesforunderstandingthechangesincoralreefecosystemsandinformingconservationandmanagementefforts.Itisessentialtocontinuemonitoringcoralreefecosystemsandimplementingconservationandmanagementstrategiestoensurethepreservationofthesevitalecosystems.Asthedegradationofcoralreefsisaglobalissue,internationalcollaborationiscrucialforthepreservationoftheseecosystems.TheUnitedNationshasrecognizedtheimportanceofcoralreefsasacrucialecosystemforsustainabledevelopmentinitsSustainableDevelopmentGoals(SDGs).SDG14,"LifeBelowWater,"specificallytargetstheconservationandsustainableuseoftheoceans,includingcoralreefs,by2030(UnitedNations,2015).Thisfurtheremphasizestheneedforcollaborationandintegratedapproachestoaddressglobalandlocalstressors,toachieveconservationtargetsandsustainabledevelopmentgoals.

Inadditiontointernationalcollaboration,theinvolvementoflocalcommunitiesandstakeholdersisessentialforeffectiveconservationandmanagementstrategies.Thelivelihoodsofmanycommunities,particularlyindevelopingcountries,aredependentoncoralreefecosystems.Therefore,theirinvolvementandsupportarecriticalforthesuccessofconservationandmanagementstrategies.Community-basedmanagement,whichinvolvestheparticipationoflocalcommunitiesindecision-makingprocesses,hasbeenshowntobeeffectiveinachievingconservationgoalswhilealsosupportingthewell-beingoflocalcommunities(Cinneretal.,2012).

Educationandoutreachprogramsarealsoimportantforraisingawarenessabouttheimportanceofcoralreefecosystemsandpromotingenvironmentalstewardship.Theseprogramscantargetbothlocalcommunitiesandtourists,whomaycontributetolocalstressorssuchasoverfishingandpollution.

Inconclusion,thedeclineofcoralreefsisaglobalissuethatrequiresintegratedapproachestoaddressglobalandlocalstressors.Long-termmonitoring,coupledwiththeuseofremotesensingtechniques,canprovidecriticalinformationforthedevelopmentofconservationandmanagementstrategies.Collaborationbetweeninternationalorganizations,localcommunities,andstakeholdersisessentialfortheeffectiveimplementationofthesestrategies.Itiscrucialtocontinuemonitoringcoralreefecosystemsandimplementingconservationandmanagementstrategiestoensurethepreservationofthesevitalecosystemsforfuturegenerations.Inadditiontotheapproachesmentionedabove,theuseofmarket-basedsolutionshasbeenexploredasastrategyforcoralreefconservation.Theconceptofpaymentforecosystemservices(PES)involvesprovidingeconomicincentivestolocalcommunitiesfortheprotectionofecosystemservicesprovidedbythecoralreefsystem.Thisapproachcanprovidealternativelivelihoodopportunitiesforlocalcommunities,reducingtheirdependenceonnaturalresourcesexploitationfortheirincome.OneexampleofasuccessfulPESprogramistheCoralReefConservationprograminTobago,whichprovidespaymentstofishermenfornotusingcertainfishingpracticesthatareharmfultocoralreefs.

Furthermore,technologicalinnovations,suchascoralreefrestorationtechniques,arebecomingviablesolutionstoconservecoralreefs.Coralrestorationinvolvesgrowingcoralsinnurseriesandtransplantingthemtodamagedreefstorestoretheircoverandfunction.Thismethodhasprovensuccessfulinrestoringcoralpopulationsindegradedareas,althoughitrequirescarefulconsiderationoftheecologicalandgeneticcompatibilityofthecoralsbeingused.

Finally,thereductionofcarbonemissionsiscriticalformitigatingtheeffectsofclimatechangeoncoralreefs.TheParisAgreement,adoptedbytheUnitedNationsin2015,aimstolimitglobalwarmingtowellbelow2degree