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TSUNAMI IMPACTS IN ACEH PROVINCE AND NORTH SUMATRA, INDONESIA

BY

ANNELISEB.HAGAN,1ROBERTFOSTER,2NISHANPERERA,3CIPTOAJIGUNAWAN,4IVANSILABAN,4YUNALDIYAHA,4YANMANUPUTTY,4IBNU

HAZAM,4andGREGORHODGSON2

ABSTRACT

ThehugeearthquakeandresultingtsunamiwhichoccurredonDecember26,2004offthewestcoastofSumatraresultedinregionallyvariablepatternsofimpactinandaroundtheIndianOceanbasin.ThecoastofSumatrawasclosetotheearthquakeepicenterandwasthefirsttobestruck,withinonehouroftheevent.AcollaborativeexpeditionbetweentheKhaledbinSultanLivingOceansFoundation,ReefCheckInternationalandIUCN(WorldConservationUnion)tothenorthwestcoastofSumatraandAcehProvince,Indonesia,wasconductedinOctober2005.

Reefsurveyswereconductedusingtwomethods:MantaTowandtheReefCheckPlusprotocol.Atotalof9sites(8offshoreislandsitesand1mainlandAcehsite)weresurveyedoveradistanceof650km.Typicallytsunamidamagewasobservedasoverturnedcoralcoloniesandtreedebrisonthereef.Overhalfofthereefssurveyedindicatedthattherehadbeennotsunamidamageandonly15%ofthesitessurveyedindicatedahighlevelofdamage.However,eveninareaswhereseveretsunamidamagewasrecordedandcoralswerekilledasaresultoftheevent,therewerestilllargeareasofintactreefpresent,whichwillbeabletorepopulatethedamagedreefinthefuture.Similarpost-tsunamisurveysinThailandsuggestthatfullrecoveryofthesereefsshouldoccurwithinthenext5-10years.

Therewasevidencethattheearthquakecausedbothupliftandsubsidenceofsomeislands.Theseprocesseshaveresultedinthreeimpactsonreefs:1)extensivemortalityofupliftedreef-flatcorals,2)thebringingofreef-frontcoralsintothereef-flatzoneand3)therelocationofreef-flatcommunitiestothereef-front.Bothupliftandsubsidencethereforehaveimplicationsfornear-futurereefecosystemdynamicsintheregion.

1KhaledbinSultanLivingOceansFoundation,8181ProfessionalPlace,Suite215,Landover,MD20785,USAandCambridgeCoastalResearchUnit,DepartmentofGeography,UniversityofCambridge,Cambridge,CB23EN,UK.2ReefCheckFoundation,17575PacificCoastHighway,PacificPalisades,CA90272,USA.3IUCN(WorldConservationUnion),SriLankaCountryOffice,53HortonPlace,Colombo-7,SriLanka.4ReefCheckFoundationIndonesia,Jln.PengembakNo.1Sanur,Denpasar,Bali,Indonesia.

No. 544. Ed. Stoddart, D.R. July 2007. Tsunamis and coral reefs

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INTRODUCTION

OnDecember26,2004anearthquakemeasuring9.3inmagnitude(Bilham,2005)occurredatlatitude3oN,offthewestcoastofSumatrawherethenorthwardmovingIndo-AustralianplateissubductedbelowthecontinentalEurasianplate.ThisearthquakewasthemostsevereeventsincetheAlaskanearthquakeof1964andwasthesecondlargestsincemodernseismographicrecordingbeganoverahundredyearsago.Theenergyitreleasedwasasmuchasalltheglobalearthquakescombinedbetween1976and1990.Thishugeearthquaketriggeredtsunamiwaves,whichcauseddevastationthroughouttheIndianOceanbasin.ThecoastofSumatrawasthefirsttobestruck,withinonehouroftheevent.ThetsunamiwavesreachedSriLankaandIndiain2-3hours,SeychellesandMauritiusin7hours,EastAfricain9hoursandSouthAfricain11-14hours.Thistsunamieventwasthemostcatastrophicsucheventinrecenthistoryresultinginthedeathsofover300,000people(Spencer,2007).

Theeffectsofhurricanesandcyclonesoncoralreefshavebeenwelldocumentedformorethan20years(e.g.Woodleyetal.,1981;Bythelletal.,2000)buttherearenosuchreportsontheeffectsoftsunamiwavesoncoralreefs.AttheInternationalCoralReefInitiative’s(ICRI)10thAnniversarymeetingintheSeychellesinApril2005,areviewofpost-tsunamireefdamageassessmentswasmade.ThereviewrevealedthatnumerousreefsurveyshadbeenconductedthroughouttheIndianOcean(e.g.Thailand,Seychelles,Maldives,SriLanka)toobservecoral-reefdamagefollowingtheDecember2004tsunami,buttherewasanevidentlackofsurveysalongthewestcoastofSumatra,thecoastlineclosesttotheepicenteroftheearthquake.NorthwestSumatraexperiencedverysevereterrestrialtsunamidamage;waterinundationreached3-4kminlandandwavescourandcoastalsubsidencesetbacktheshorelineby1.5km(Borrero,2005).Theaimofthisexpeditionwastosurveya650kmstretchofthewestcoastlineandoffshoreislandsofSumatra,Indonesia,fromSibolgatoBandaAceh(inAcehProvince)(Fig.1)inordertodocumentthestateofthereefsinthisareafollowingtheDecember2004tsunamiandtofillagapintheknowledgeoftheimpactsofthetsunamiaroundtheIndianOceanbasin.

REEFS OF NORTH SUMATRA

Sumatra,withacoastlineofapproximately4,500km(excludingoffshoreislands)isoneoftheleastknownIndonesianislandswithregardtocoralreefdistribution(Tomasciketal.,1997).Extensivefringingreefs,approximately200minwidth,occurinthenorth,aroundAceh,alongthewestcoast,andaroundthenorthernislands,especiallyPulauWeh(Tomasciketal.,1997).An85kmlongbarrierreefisreported20kmoffthecoastofAceh,butthisisasubmergedordrownedsystem13-20mbelowthesurface,andthedegreeofactivecoralgrowthhereisunknown(Spaldingetal.,2001).SeasurfacetemperaturealongSumatra’scoastlinerangesfrom26o-30oCandsalinityrangesfrom33-34ppt(Tomasciketal.,1997).Indonesia,specificallyeasternIndonesia,isknowntobetheworld’scentreofcoralbiodiversity,exhibiting581specieswithin82genera(Veron,2000).CoraldiversityinSumatrahasnotbeendocumented.


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Figure 1. MapofAcehProvince,Sumatra,Indonesiaillustratingexpeditionitinerary17-31October2005.NumbersrepresentsitenumbersasdefinedinTable1.ReddotindicatesapproximateDecember26,2004earthquakeepicenter.

Region Site Name Site Number on Fig. 1 Banyak Pulau Bangkaru 1

Pulau Baleh 2 Pulau Bagu 3

North Aceh coast North Aceh coast 4 Northern Islands Pulau Nasi Besar 5

Pulau Buro 6 Pulau Weh 7 Pulau Rondo 8 Pulau Bunta 9

Table1.RegionsandsurveysitesasshownonFigure1.


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METHODS

Twoprimarysurveymethodswereusedduringtheexpedition:theMantaTowmethodandtheReefCheckPlusprotocol(Hodgsonetal.,2005).TheMantaTowmethodisarapidvisualassessment,enablingaverylargeareatobesurveyedinshortperiodoftime.Itinvolvedasnorkellerholdingontoa‘MantaBoard’beingtowedbehindaboat(Englishetal.,1997).Thesnorkellerrecordedavisualassessmentofthereefobserved(i.e.,percentagecoveroflivecoral,rock,rubble,etc.).TheReefCheckPlusmethodsfocussedonamuchsmallerareaofreefbutthesurveysweremoredetailed,surveyingthebenthic,fishandinvertebratecommunitiesalonga100mtransectline.Typically,shallow(3-5m)anddeep(8-10m)ReefCheckPlussurveyswereconductedateachsite.Thesetwomethodshavevariousadvantagesanddisadvantagesbutbyemployingthemincombinationtheadvantagesweremaximizedandthedisadvantageswereminimized.ThesetwosurveymethodsenabledgeneralcharacteristicsofthereefsofnorthSumatratoberecorded.Inadditiontothesestandardmethods,particularnotewasmadeoftsunamidamageonthereefs.Tsunamidamagewasidentifiedas:

1) Mechanicaldamage:Brokenpiecesofcoral2) Overturned/rolledcoral3) Sedimentation:Run-offfromlandbeingwashedontoreef

Theleveloftsunamidamageobservedwasalsorecordedas‘low’,‘medium’or‘high’byestimatingthenumberofoverturnedand/orbrokencoralpiecesobservedduringeachMantaTow.0–10piecesindicated‘low’tsunamidamage,10-30piecesindicated‘medium’tsunamidamageand30+piecesindicated‘high’tsunamidamage.A‘pieceofcoral’wasdefinedasbeinglessthan15cmindiameteralongitslongestaxis.

Intotal,nineoffshoreislandsites(Karang,Bangkaru,Baleh,Bagu,NasiBesar,Buro,Rondo,WehandBunta)andonemainlandsite(northcoastofAcehProvince,eastofBandaAcehcity)weresurveyed(Fig.1).

RESULTS

ReefCharacteristics

Benthicsurveyresultshavebeencombinedintothreegroups:Banyakregion,northcoastofAcehandnorthernislands(Table1;Fig.2).Banyakregionreefswereshowntobedominatedbyhardcoralcover(39%cover)androck(29%cover)withmoderateamountsofrubbleandsand.Recentlykilledcoralrepresentedonly0.1%coverintheBanyakregion.Reefsofthenorthernislandweredominatedbyrock(37%cover)andrubble(29%cover),followedbyhardcoralcover(25%cover).Recentlykilledcoralrepresentedonly0.3%coverinthenorthernislands.ReefsofthenorthAcehcoastlineshowedmarkeddifferencescomparedtotheothertwoareas.Herethereefwasdominatedbyrock(35%cover),andalthoughhardcoralcoverwasidentical(25%cover)tothatrecordedinthenorthernislands,softcoralswerealsoevidentinthecoralcommunity(11%cover).ThenorthAcehcoastlinedisplayedahigherproportionofrecentlykilledcoral(3%cover)butamuchlowerproportionofrubble(11%cover)comparedtotheothertwosites.


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EarthquakeDamage:BanyakRegion

TwomajorearthquakesoccurredinthewatersoffshoreofAcehinDecember2004andMarch2005.EarthquakedamagewasobservedatPulauBangkaru(uplift),PulauBaleh(subsidence)andPulauBagu(subsidence)inthePulauBanyakgroup.Alargeareaoflargelyintact(littleerosionwasobservedandmostbranchingcoralswereunbroken)reef-flat,approximately500minwidth,hadbeencompletedraisedbyapproximately+2m,killingthecoralsthroughsubaerialexposure(Fig.3).Thecoralshadnotyetbeenerodedandcouldeasilybeidentifiedtogenuslevel,indicatingthattheupliftwasrecent.ManydeadPoritesmicroatollswerepresent,aswerecoloniesofbranchingAcroporaandPocilloporaandmanyemptygiantclamshells.

Incontrast,terrestrialobservationsattheislandsofBalehandBagu,twoislandswhichlielessthanonekilometerapartfromoneanotherintheBanyakgroup(Fig.1,sites4and5)indicatedthatsubsidencehadoccurredasadirectresultofanearthquake.Terrestrialtsunamidamagewashighlyevident.Low-lyingvegetationclosetotheshorewasbrownanddead(Fig.4a),presumablyasaresultofsalt-waterinundationandmanybuildingshadbeenremovedfromthecoastline(Fig.4b).

0%

20%

40%

60%

80%

100%

Banyak region N. Aceh coast Northern Islands

Perc

enta

ge C

over

OtherSpongeSandAlgaeSoft CoralRubbleHard CoralRockRKC

Figure 2.SummaryofpercentagecoverbybenthiccategoryforthreeregionsofAceh.ResultsfromReefChecksurveys(RKC=RecentlyKilledCoral).

Figure 3.UpliftedreefatPulauBangkaru,observedonOctober19,2005.


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Thehousesremainingalongtheseafrontwerenotedtoallhaveaclearbrownmarkatapproximately80cmheightuptheirwalls(Fig.5).Itseemedstrangeforthiswatermarktoremainsoclear10monthsafterthetsunamihit,buthavingspokentotheislandersitbecameapparentthatthiswasaneffectoftheearthquakeasopposedtothetsunamiwave.TheislandhadsubsidedasaresultoftheDecember2004earthquakeandasaresult,thebuildingsalongtheseafrontarenowinundatedwithupto1mofwaterduringeachhightide.Presumablythecoralreefssurroundingtheseislandsmustalsohavesubmergedbyasimilaramount,convertingintertidalreef-flatcommunitiesintosubtidalones.

TsunamiDamage:NorthCoastofAcehProvince

ThenorthcoastofAceh,approximately13kmeastofthetownofBandaAceh,exhibiteddifferingdegreesoftsunamidamage.Allsurveyswereshallow(3-5mdepth)asthereefdidnotextendbelow5mwaterdepth,butinsteadgavewaytoasandybottom.Thefivesiteswerefoundtoharbordifferenttypesofreefcommunitiesandexhibitedvaryingdegreesoftsunamidamage.MantaTowsindicatedthattsunamidamagewasgenerally‘low’atthissitewithonly4outof39towsindicating‘high’damageand4outof39towsindicating‘medium’damage.Rockwasestimatedtodominatethesubstrate,representing44%coveralthoughlivecoralcoverrepresentedanaverageof31%andrubblerepresented25%cover.ThefirstReefCheckPlussurveywasconductedattheheadland‘UgBatukapal’,asiteidentifiedbytheMantaTowteamashavinggoodlivecoralcover.Indeedthesubstratetransectwasdominatedbylivecoralcover(32%)withamoderateamountofrockandsand(25%coverforeach).Interestingly,softcoralsmadeup14%ofthetotalsubstrateatthissite,acategorythathadbeenlittleobservedelsewhere.

a b

Figure 4. TerrestrialtsunamidamageatPulauBalehshowing(a)deadvegetationalongthecoastand(b)foundationsofbuildingsthathavebeenwashedawaybythetsunamiwave.


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Thesurveyconductedattheheadlandadjacentto‘UgBatukapal’indicatedaverydifferenttypeofreefcommunity.Herethereefwascomposedoflargeflatsolidplatesoflimestone‘coralpavement’(50%oftotaltransect)interspersedwithsoftcorals;specificallyofthegenusSinulariaandwhipcorals(Fig.6aandb).Therewerefewhardcorals(hardcoralcoverwasonly6%)comparedtothenumberofsoftcoralspresent,whichaccountedfor27%ofthetotalsubstratealongthetransectline.

Figure 5. WatermarkonhouseonPulauBaleh;markrepresentsthedailyheightofwaterinundationathightide.Thishouseisapproximately70minland.

Figure 6.Reefdominatedbycoralpavementinterspersedwithsoftcoralsat3mwaterdepth:(a)Sinulariaspp.(b)Sinulariaspp.anddelicateseawhipsJunceella fragilis,northAcehcoast,October27,2005.

a b


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Movingeastward,twosurveyswereconductedinalargebayarea.Oneofthesesurveyswasofparticularinterestasitidentifiedconsiderabletsunamidamage,specificallyoverturneddeadAcroporatables(Fig.7),overturnedlivePoritesspp.(Fig.8)andtreedebris(Fig.9).

Figure 7. OverturneddeadAcroporasp.tableat4mwaterdepthonthenorthcoastofAcehProvince,October27,2005.

Figure 8. OverturnedlivePoritessp.colonyat4mwaterdepthonthenorthcoastofAcehProvince,October27,2005.


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Thereefwascharacterisedby31%rockandequalproportionsoflivecoralcoverandrubble(21%each).Movingfurthereast,thefinalsurveyinthisareadisplayedminimalsignsoftsunamidamage.Althoughonetreebranchwasobservedonthereef,nocoralhadbeenkilled,brokenoroverturnedandthereefdisplayedlargestandsofhealthybluecoralHeliopora coerulea(Fig.10)andPoritesspp.Livecoralcoveraccountedfor35%ofthesubstrate.

Figure 9. Treedebrisonthereefat5mwaterdepth;northcoastofAcehProvince,October27,2005.

Figure 10.LargestandsofhealthybluecoralHeliopora coeruleaat3mwaterdepthoffthenorthcoastofAcehProvince,October27,2005.


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TsunamiDamage:NorthernIslands

PulauWeh(markedbythemaintown‘Sabang’onFig.1)liesoffthenorthcoastofAcehProvinceandisthelargest(153km2)andmostpopulated(population~28,500)ofthenorthernoffshoreislands.AvisitashoreonthenorthcoastofPulauWehconfirmedthattherehadbeensignificantimpactsfromthetsunamiwaveonland(Fig.11).

AsinglesitewassurveyedonthesouthwestcoastofPulauWeh.MantaTowsindicatedthattherewas44%rockcoverand23%livecoralcoverwiththerestofthesubstratebeingsplitequallybetweenrubbleandsand.Halfthetowsindicated‘low’tsunamidamageandhalfindicated‘medium’tsunamidamage.TheReefChecksurveyat6mdepthindicatedthatalthough38%ofthesubstratewaslivecoralcover,thisfigurewasequalledbyrubblecover.Rockrepresented22%ofthetransectline.Althoughsomepatchesofreefwereintact(Fig.12),therewasclearevidenceoftsunamidamageonthereefalongthistransect.

ManymassivePoritesspp.colonieshadbeensplitintoverticalfragmentsoroverturned(Fig.13aandb)andlargecoloniesofthebluecoralHeliopora coeruleahadbeenoverturnedandshatteredintosmallpieces(Fig.14aandb).

Figure 11. LumbaLumbadiveshoponPulauWeh;arrowindicatesmaximumheightofwaveaction(~5mabovesealevel)onDecember26,2004.


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Figure 13.(a)SplitPoritessp.colonyand(b)overturnedPoritessp.colonyat4mwaterdepth,TelukBalohan,PulauWeh,October28,2005.

Figure 12. Intactcoralreefat4mwaterdepthclearlyshowinghealthyPoritesspp.(farleftandfarright)andHeliopora coerulea(centrefront)coloniesalongtransectlineatTelukBalohan,PulauWeh,October28,2005.Transectlineshownbackleft.

Figure 14. (a)OverturnedbluecoralHeliopora coeruleaand(b)shatteredbluecoralHeliopora coeruleaat3mwaterdepth,TelukBalohan,PulauWeh,October28,2005.

a b

a b


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PulauRondo(Fig.1,site13)isasmall,uninhabitedislandsituatednorth-westofPulauWehandisthemostnortherlypointofIndonesia.MantaTowsandshallowanddeepReefCheckPlussurveyswereconductedattwositesatPulauRondo;oneoffthewestcoastandoneofftheeastcoast.TheMantaTowsindicatedthatsomeareasdisplayed‘low’tsunamiimpactandsomeareasdisplayed‘high’tsunamiimpact,butthemajorityshowed‘medium’tsunamiimpact.Livecoralcoverwasestimatedtobe50%,withafurther15%ofthesubstratebeingreportedas‘recentlykilledcoral’.

OnthewestsideofPulauRondo,theshallowReefChecksurveywasdominatedbyrock(41%),withareasonableamountoflivecoralcover(37%).ThedeepsiteonthewestsideofPulauRondoshowedsometsunamidamage,specificallyoverturned

Acroporaspp.tables,bothalive(Fig.15)anddead(Fig.16aandb)andoverturnedPoritesspp.,butthemajorityofthereefwasunaffected(Fig.17).Livecoralcoverwas39%,althoughthisfigurewasequalledbytheproportionofrubblealongthetransectline.

Figure 15.OverturnedliveAcroporasp.tableat10mwaterdepthatPulauRondo,October29,2005.

Figure 16.(a)OverturneddeadAcroporaspp.tablesat10mwaterdepthatPulauRondo,October29,2005.(b)overturneddeadtablecoralsurroundedbybranchingcoralrubble.

a b


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OntheeastsideofPulauRondotheshallowsurveywasdominatedbyrock(50%cover)withrubblerepresenting30%ofthesubstrateandlivecoralcoveronly17%.Thedeepsurveywasdominatedbyrubble(70%cover),withlittlelivecoralcover(18%).Tsunamidamagewasobservedinthisarea;specificallyoverturneddeadAcroporaspp.tablesandalargetreetrunk(over6minlength)hadbeendepositedonthereefbetween15mand18mwaterdepth(Fig.18).

AtPulauBunta(Fig.1,site14),MantaTowswereconductedaroundtheentirecircumferenceoftheisland.Theresultssuggestedveryhightsunamidamage,withthereefbeinglitteredwithsmallcylindricalbranchingcoralfragmentsandafewoverturneddeadAcroporaspp.tablesbeingobserved.Rockwasestimatedtoaccountfor56%ofthesubstrateobservedandrubble31%,withonly4%ofthesubstratebeingrepresentedbylivecoral.Fivecoconutpalmtrunkswereobservedonthereefatdepthsofbetween4and6m.WhileconductingtheMantaTows,observationsontheislandconfirmedthattherehadbeensignificanttsunamiimpacthere.Manycoconutpalmshadfallenandmuchofthelow-lyingvegetationhadbeenkilled.Althoughthisislandissmall(0.16kmx0.38km),approximatelysevenbuildingswereobserved.Clearlythesebuildingswereverynewandpilesofbuildingdebriswereobserved,indicatingthatthetsunamiwavemusthavedestroyedthebuildingswhichpreviouslystoodthere.PulauBuntawouldhavebeenoneofthefirstislandsoffthenorthcoastofAcehProvincetobehitbythetsunamiwaveasitprogressednorthwardsfromtheepicenter(Fig.1).

Figure 17. Healthyreefcommunitiesat10mwaterdepthatPulauRondo,October29,2005.

Figure 18.Treetrunkonreefatwaterdepthof18m,PulauRondo,October29,2005.


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DISCUSSION

RegionalReefCharacteristics

ReefsoftheBanyakregionandnorthernislandsdisplayedverysimilarbenthiccharacteristics,withcombinedvaluesofrock,hardcoralandrubblecontributingtobetween85-90%oftheoverallbenthos.Highestamountsofrubblewererecordedinthenorthernislands,whichmaysuggestthattheseoffshoreislandsareexposedtoahighenergyenvironmentduetooceanicswellgeneratedthousandsofkilometersawayintheIndianOcean(Tomasciketal.,1997).ThereefsofthenorthcoastofAcehweretypifiedbybarecoralpavementwithlittlerubble,andthesereefsdisplayedasoftcoralcommunitythatwasnotobservedatanyothersite.

Evidenceofboatanchordamageanddynamitefishingwasobservedatnearlyallsurveysites,suggestingthatcontinuouslyhighlevelsofanthropogenicstressonthereefsofSumatraishavingamoresignificantimpactoncoralreefhealththanthatwhichresultedfromtheDecember2004tsunami.

EarthquakeDamage

Earthquakedamageresultedinthreemajoralterationstothereefenvironment.Firstly,extensivemortalityofreef-flatcoralsoccurredduetoupliftatPulauBangkaru.Thecoralsthatwereupliftedandsubsequentlykilledthroughsubaerialexposurewerethoseontheshallowreef-flat,andduetothenaturallyharshnatureofthereef-flatenvironmentthesecoralswouldhavebeenmoreresistanttonaturalenvironmentalstress(e.g.higherwatertemperaturesandsolarradiation)thanothercoralsfurtherdownthereefslope.Large(>2mdiameter)microatolls,massivecoralstypicallywithadead,flatuppersurfacesurroundedbyalivingmargin(ScoffinandStoddart,1978),wereupliftedapproximately1.5mabovesea-levelonthesouthwestcoastofSimeulueisland(Sieh,2005)andsmallerraisedmicroatollswereobservedatPulauBangkaru.Astheupwardgrowthofmicroatollsisconstrainedbysealevelthroughprolongedexposureatlowspringtides,microatollsactasnaturalrecordersofsealevel(ScoffinandStoddart,1978;WoodroffeandMcLean,1990;Zachariasenetal.,1999).Inregionaltermsithasbeensuggestedthata1,000kmstretchofreefalongtheplateboundaryfromtheAndamanandNicobarislandstoSumatrahassufferedupliftorsubmergenceasaresultoftheDecember2004earthquake(Bilham,2005).Consequentlyahugenumberofreef-flatcoralsandmicroatollshavebeenkilledinthisregion.Therearefewcoralspeciesthatarecommontobothreef-flatareasandreefslopeareasinthisregion,themostdominantbeingPorites lutea(Brown,2005,pers.comm.;PhongsuwanandBrown,2007),andthelossofsomanyotherreef-flatcoralspeciesislikelytohaveseriousimplicationsforthere-populationofthereefsoftheregion.

Secondly,reefupliftatPulauBangkaruhasbroughtreef-frontcoralsintothereef-flatzone.Thecoralsthatoncethrivedatdeeperdepthsonthereefhavenowbeenupliftedtowithinafewmetersofthesurfaceandonlytimewilltellhowwellthesecoralswillsurviveafterexperiencingsucharadicalverticalshiftinenvironments.Although


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itisconceivablethatthesecoralswilladapttotheirnew,warmerwatertemperatureandassociatedincreasedsolarradiation,typically,suchadaptationsareonlysuccessfulthroughgradualchangeoverlongtimeperiods.However,itmustalsobeconsideredthatsomespeciesmaybemoreabletoadaptthanothers,whichmayalterthecoralcommunitycomposition.

Thirdly,movingfurthernorth,effectsoftheearthquakewereobservedattheislandsofPulauBalehandBagu,butunlikeatPulauBangkaruwherethereefshadbeenuplifted,theseislands,andthusthesurroundingreefs,hadbeensubmergedasaresultoftheearthquake.Althoughlittlestructuraldamagewasobservedasaresultofthetsunamionthereefsoftheseislands,thedisplacementofshallowreefstodeeperzonesduetothistectonicplateshiftmay,overtime,haveimplicationsforthereefecosystem.Coralsareextremelysensitiveandverysusceptibletovariationsintemperature.Consequently,averticalshiftofevenaslittleasametercouldhavesevereconsequencesforthecoralcommunity.

TsunamiDamage

Awidespectrumoftsunamidamagewasobservedoveralargedistance(650km)inashortperiodoftime.Typically,itwasonlypossibletosurveyoneortwositesateachislandvisited,yieldingonlyasnap-shotoftheoverallreefenvironment.Therefore,generalisationsofthedegreeoftsunamiimpactatdifferentsitesmustberegardedwithduecaution.

NodiscernabletsunamidamagewasobservedonthereefsofPulauKarangorPulauBangkaru,themostsoutherlyislands(Fig.1,sites2and3).ItispossiblethatthereefsoftheseislandswereshelteredfromthetsunamiwavebythelargeislandofSimeulue,whichlies44kmsouthoftheearthquake’sepicenter(Fig.1).SometsunamidamagewasobservedonthereefsofthenorthernoffshoreislandsandonthenorthcoastofAcehProvince.ThemostfrequentlyobserveddamagewasoverturnedAcroporaspp.tables,overturnedmassivePoritesspp.coloniesandtreedebrisonthereef.Tsunamiimpactwasexhibitedaspocketsofdamage(althoughlargerareasthandisplayedasaresultofdynamitefishing)asopposedtohugeareasofthereefbeingcompletelydestroyed.Duetothelimitedamountofsurveysundertaken,itisnotpossibletodiscussvariationsintsunamidamagewithrespecttodepthoraspect.Forexample,atsomesitestsunamidamagewasobservedonthedeeptransectbutnotontheshallowtransectandviceversa,anditisnotclearwhythismayhavebeenthecase.

ThereefareaobservedtobemostaffectedbythetsunamiwasonthenorthcoastofAceh,asiteinthecentreofalargebaybetweentwoheadlands.Althoughduetotherandomanddispersednatureofthesurveysitisdifficulttomakeanycommentonthepatternoftsunamidamage,itcouldbesuggestedthatherethetsunamiwavesmayhavebeenrefractedofftheheadlandseithersideofthebayandcompoundedinthecentreofthebaycausingextensivedamageatthiscentralbaysite.Similarresultshavebeenreportedelsewhere,forexample,moreextensivetsunamiinducedreefdamagewasobservedinbayareasofSriLanka(Rajasuriya,2005).


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CONCLUSIONS

Insummary,54%ofthesitessurveyedshowednotsunamidamage,31%showedlowtomoderatedamageand15%showedhighlevelsofdamage.Althoughminimalcoralrecruitmentsubsequenttotheearthquakeandtsunamiwasobserved,typicallythereefsofSumatradisplayedbetween30%and65%livecoralcover(Fig.2).SomeoftheoverturnedcoralsobservedinSumatrawerestillalivebutothersweredead.However,someofthesedeadcoralswerewelleroded,suggestingthattheymayhavebeendeadbutstillstandingpriortothetsunamievent.Deadstandingcoralsarefarmoresusceptibletotsunamidamageduetotheirweakattachmentontothesubstrate.Itfollowslogicthatreefswhicharealreadysubjectedtohighanthropogenicstressarelikelytosufferthemostasaresultoftsunamiimpact(Bairdetal.,2005).

So,howlongwillittakethereefsoftheseAcehislandstorecoverfromthetsunamiimpact?Whentalkingaboutreefrecovery,itisimportanttolookatthetypeofdamageobserved.Manyoftheoverturnedcoralsthatwereobservedcontainedlivetissue.Althoughitisunreasonabletoassumethattheportionoflivecoralnowrestingontheseabedwillsurvive,thecolonyshouldgraduallyspreadacrossthebaresubstratewhichwasoncethebasearea.Thesurvivingpartsofthesecolonieswillalsobeanimportantlarvalsourceforre-populatingthereef.Therecoveryratesareexpectedtotakesignificantlylongerinthoseareaswherecoralswerekilledasaresultofthetsunami.Forexample,atPulauRondosomeoverturnedAcroporaspp.tableswerealreadydead(Fig.16aandb)andtheamountofbranchingcoralrubbleontheeasternsideoftheislandsuggestedthattherewasconsiderabletsunamidamagehere.However,itisimportanttonotethaneveninareaswhereseveretsunamidamagewasobservedandcoralswerekilledasaresultoftheevent,therewerestilllargeareasofhealthycoralpresent,whichwillservetorepopulatethedamagedreef.

Post-tsunamireefstudiesinThailandfoundthat66%ofthe174sitessurveyedshowednoorverylittledamage,withonly13%exhibitingseveredamage(>50%ofcoloniesaffected)(Brown,2005;PhongsuwanandBrown,2007).Ithasbeensuggestedthatthesereefswillrecoverfromthetsunamieventwithinthenext5-10years(Brown,2005;PhongsuwanandBrown,2007).ThereefsofSumatraappeartohavesufferedsimilarlevelsofdamagefromtheDecember2004tsunamitothatreportedfromthesurveysinThailand.ItcanthereforebereasonablysuggestedthatrecoverytimeswillbesimilarforthereefsofSumatra,thatis,thereefsarelikelytorecoverwithinapproximately5yearsandfullrecoveryofevenseverelydamagedreefswilloccurwithinthenextdecade.

ACKNOWLEDGEMENTS

Thethreeorganizationsresponsibleforthissurveyare:TheKhaledbinSultanLivingOceansFoundation,ReefCheckInternationalandIUCN(WorldConservationUnion).PrimaryfundingforthisprojectwasgraciouslyprovidedbytheKhaledbinSultanLivingOceansFoundation.OursinceregratitudegoestotheIndonesianGovernmentforgrantingpermissionforthisresearchtobeconducted.


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