Download Climate Change and Coral Bleaching

Risingoceantemperaturesmaypushcoralreefstotheirlimits.
ClimateChangeandCoralBleaching
http://www.nova.org.au/coral-bleaching
Essentials
!
Coralsareanimalsthatliveinamutuallybeneficialrelationshipwithatypeofmicroalgaecalled
zooxanthellae
!
Thezooxanthellaeprovideorganiccarbon(energy)tothecorals,andreceiveinorganicnutrients
(fertilizer)fromtheircoralhosts
!
Coralsaresensitivetosmallchangesinwatertemperatures—longperiodsofhighertemperatures
resultinthebreakdownofthecoral-zooxanthellaerelationship,andifwarmforlongenough,will
leadtocoraldeath
!
Coralbleachingoccurswhenthezooxanthellaeleavethecorals—withoutthenutrientsprovided
bythezooxanthellae,thecoralseventuallydieofstarvationanddisease
ImagineyouareonabeautifultropicalislandontheGreatBarrierReef.Youputonyourmask,finsand
snorkelandjumpintothewater.Butinsteadofthefamousbrightlycoloredcoral,youseeadrab
underwaterlandscapedominatedbyblue-greenalgae(cyanobacteria)andseaweed,butnocorals.
Astheworld'soceansbecomewarmer,andmoreacidic,thisscenariowilloccurmorefrequently,onthe
GreatBarrierReefandcoralreefstheworldover.
Coralsareanimals
TheancientGreeksmistakenlybelievedcoralstobeplants.
Theyareactuallysimplemulticellularanimalsthatarerelated
toanemonesandjellyfish.Individualcoralanimals,called
polyps,buildexternalskeletonsmadeofcalciumcarbonate
(similartolimestone).
Hundredsofindividualsjointogethertomakecoralcolonies.
Eachpolypsitsinitsskeletoncup,andhasacentralmouth
surroundedbytentacleswithstingingcellsonthem.Thetiny
tentaclestrappassingplanktonandpushitintoasmallcavity
thatactsasastomach.
Awin-winrelationship
Butittakesmorethanabunchofcoralpolypstomakeahealthycoralreef.Coralsalsoplayhostto
thousandsoftinyorganismscalledzooxanthellae,individualsthatlivewithineachcoralcell.
Zooxanthellaearesingle-celleddinoflagellatemicroalgaethatusephotosynthesistoproduceorganic
carbon(energy)intheformofsugarsandothercompounds.Theypassmostoftheseontotheircoral
hosts.Thesecompoundsmakeupover95percentofthecoral’sfoodandnutritionrequirements.In
return,thezooxanthellaereceiveinorganicnitrogenandphosphorusfromthewasteproductsofthecoral
polypsownmetabolicprocesses.
coral_bleaching.docx
3/16/17
Thezooxanthellaearealsoresponsibleforthebrowncolorofmanycorals.Thebrightpinks,greensand
bluesalsoseenincoralreefsareproducedbyfluorescentcompoundsmadebythecoralhostthatprotect
thezooxanthellaefromgettingtoomuchsunlight,whichcanbeaproblematthemiddleofthedayon
coralreefsinthetropics.
Coralsalsomakecompounds(calledmycosporine-likeaminoacids),whichactasasunscreen,protecting
themfromtoomuchultravioletradiation.Coralscanvarytheconcentrationofbothofthesecompounds
tohelpthemacclimatizetolocalconditions.However,therearelimitstotheabilityofthesepigmentsto
respondandprotectthezooxanthellae.Ifconditionschangetooquicklyorsubstantially,thecoral’sability
toacclimatizewillbeoverwhelmed,leadingtocoraldamageordeath.
Nottoohot,nottoocold...it’sgottobejustright
Coralsarealsofussyaboutthetemperatureofthewaterinwhichtheylive.Somecoolwatercoralsare
happywithwatertemperaturesofaround18°C,whilelotsoftropicalcoralsliveinwaterswith
temperaturesbetween23and29°C.Some,likethoseintheArabianGulf,canexperiencetemperaturesup
to34°Cormore.
Althoughmostcoralscancopewithhighertemperaturesforshortperiodsoftime,theydon’tlikeitif
temperaturesstayhighforlongperiods—acoralthatlivesin27°Cwatermaycopewiththewaterbeing
29°Cforadayortwo,butwouldn’tliketobein31°Cforaweek.
Coralbleaching
Bleachingoccurswhentheclose
symbioticrelationshipbetweenthezooxanthellae
andtheircoralhostsbreaksdown.Thiscanbe
causedbyanumberofenvironmentalstresses,most
commonlyhigherwatertemperatures,particularly
whencombinedwithstill,warmoceanconditions.
Thephotosynthesisprocessinthezooxanthellaeis
disrupted,andtheyproduceanexcessofproducts
thatbecometoxictothemselves.Thisinturn
damagesthemetabolismofthecoralpolyp,which
expelsthebrownzooxanthellae,leavingthecoral
skeletonastark,‘bleached’white.
Thebleachedcoralcanrecover,butonlyifwatertemperaturecoolsagainandthezooxanthellaehave
timetobeabletorepopulatethecellsofthecoralhost.Iftemperaturesremainabovethebleaching
threshold,zooxanthellaewillbelostforsubstantialperiodsoftime.Withoutthemealticketprovidedby
theirzooxanthellae,coralswilleventuallydiefromstarvationanddisease.
Causes
Heatstresswillgenerallyresultinthemassbleachingoflargeareasofcoral.Smallerregionsofcoralreefs
canalsoaffectedbymorelocalizedimpacts,suchas:
! exposuretotoomuchlightandultraviolet(UV)radiation
! decreaseinsalinityfromstormwaterandheavyrainsfloodingthereef
! exposuretocoraldiseases
! sedimentssuchassandordirtcoveringthecoralforextendedperiods
! exposuretochemicalcompoundssuchascyanide,herbicidesandpesticides
coral_bleaching.docx
3/16/17
Temperaturesareontheup
Overthepast100years,thetemperatureofseawaterinalmostalltropicalareashasbeenrisingrapidly.
ComprehensiveseasurfacetemperatureshavebeenmeasuredintheAustralianregionsincetheearly
1980s.These,andhistoricalrecordsshowthatthepasttwodecadeshavebeenthewarmestsincethese
recordsbeganin1870.
Theserecordsarecomplementedbyrecordsfromtheskeletonsoflong-livedcoralssuchasPorites.The
corals’calciumcarbonateskeletonismadefromcalcium,carbonandoxygen.Therearetwomainisotopes
ofoxygenfoundwithinthecalciumcarbonate,andtheratioofthesetwoisotopesdependspartlyonthe
temperatureofthewaterinwhichthecoralgrew.So,asthecoralgrows,layingdownnewlayersof
calciumcarbonate,itcreatesarecordoftemperature.Bystudyinghowtheoxygenisotoperatiovaries
throughacoreofthecoralskeleton,scientistshavebeenabletoreconstructseatemperaturesgoingback
hundredsofyears.Theserecordsshowthatinthecontextofthepastfewhundredyears,recent
temperatureshavebeenunusuallywarm.
Oceantemperaturesarepredictedtocontinuetoriseifwekeepaddingcarbondioxideandother
greenhousegasestotheatmosphere.The2013IntergovernmentalPanelonClimateChange(IPCC)report
predictsanincreaseinthesurfacetemperatureoftropicaloceansof3–4°Cby2100ifnochangeismade
tothecurrentpatternsofgreenhousegasemissions.Australianandinternationalresearchhasshown
thatanincreaseinaverageglobaltemperaturesofjust1°Cabovethepreindustrialperiodwillcausecoral
reefstolosealltheircoralsbymid-century.
Cancoralssurvivebleaching?
Althoughthecoralanimalsthatmakeupmoderncoralreefshaveoccupiedreefsystemsforatleast240
millionyears,thecurrentreefstructuresontheGreatBarrierReefformedaftersealevelsstabilized
around8,000–10,000yearsago.Coralreefsgenerallyoccurinareaswhereaveragewatertemperatureis
only1–2°Cbelowthetemperatureatwhichthecoralsbecomestressedandundergobleaching,soan
averagetemperatureincreaseof3–4°Cbytheendofthiscentury,aspredictedbythe2013IPCCreport,
doesnotbodewell.
Somescientistshavesuggestedthatcoralsmaybeabletoadapttowarmertemperaturesbychangingthe
typeofsymbioticzooxanthellaetheyhost.However,thereislittlescientificevidenceofcoralsbeingable
toswaptheirsymbioticzooxanthellaefornew,warmervarieties.Thereissomeevidencethatsometypes
ofcoralcancopebetterwithwarmertemperaturesthanothertypes.However,thesedifferencesbetween
coraltypeswillonlygivemorethermallytolerantcoralsaleewayof1–2°C.Consequently,these
differenceswillonlyhelpintheshorttermandwillcertainlynotofferalongtermsolutioninthefaceof
continuedincreasingtemperatures.
Aswatertemperaturescontinuetoincrease,goingbeyondthewindow
oftemperaturesthatcoralsliketolivein,theywillfinditharderto
survive.Thestressesofincreasedtemperatureswillbecompounded
bythoseofincreasedextremeweathereventslikecyclonesand
storms,andalsooceanacidification.Coralsinwarmerandmoreacidic
seaswillfindithardertorecoverafternaturaldisturbances,leadingto
aneverdiminishingamountofcoralontheworld’scoralreefs.
coral_bleaching.docx
3/16/17
Coralreefsneedourprotection
Preventingstressfromthingsliketoomuchsediment,pollutionanddestructivefishingwillhelpour
reefs.Ultimately,though,itisthesustainedhighertemperaturesthatclimatechangeisprojectedtobring
thatposethegreatestthreattothewell-beingofcoralreefs.Curbingofgreenhousegasemissionswill
makethebiggestdifferenceinensuringtheirlong-termsurvival.
Whyarecoralreefsimportant?
Coralreefsareimportantformanydifferentreasons.Thelargecalciumcarbonate(limestone-like)reef
structuresthatcoralscreatehelpunderpintheconstructionofislandswhileatthesametimeprotecting
shorelinesfromthedamagingimpactsofwavesandstorms.Reefsalsoprovidecomplexthreedimensionalhabitatsthatareextremelyrichinbiodiversity.It’sthoughtthatasmanyasonemillion
speciesoffishandothermarineorganismsliveinandaroundcoralreefs.Reefssustainlivelihoodssuch
asfisheriesandtourism,whichsupportanestimated500millionpeoplewholivealongtheworld’s
tropicalcoastlines.TheGreatBarrierReefisespeciallyimportanttotheAustralianeconomyand
generatesmorethanfivebilliondollarseveryyear.
Inaddition,long-livingcoralsthatgrowoverhundredsofyearsareveryusefultoscientists,aschangesin
thethicknessandisotopicchemistryofthegrowthbandsintheircalciumcarbonateskeletonscan
provideinformationaboutpastclimatesandgrowthandvitalityofcoralsovertime.
coral_bleaching.docx
3/16/17
ClimateChangeandCoralBleaching
http://www.nova.org.au/coral-bleaching
Name:
Per: Date:
1. Discussthetermcoral.
Explainhowcoralcoloniesareformed.
2. Definemutualismanddescribewhytherelationshipbetweencoralsandalgaerepresentsa
mutualism.
Explaintheroleandimportanceofalgaetothesurvivalofcoral.
3. Whatiscoralbleaching?
Whathappensduringableachingeventandwhyisthisaconcerntoscientists?
coral_bleaching.docx
3/16/17
4. Listfivecausesofcoralbleaching.
a.
b.
c.
d.
e.
5. Explainwhyoceantemperaturesareunusuallywarm?
Describehowclimatechangecouldaffectthecoral-algaemutualism.
6. Cancoralssurvivebleaching?
7. Givefourreasonswhycoralreefsareimportant.
a.
b.
c.
d.
coral_bleaching.docx
3/16/17