Tropical storm off Myanmar coast sweeps reefs in Ritchie's Archipelago, Andaman

12 Pages·2012·1.23 MB·English

EnvironMonitAssess
DOI10.1007/s10661-012-2948-7
’
Tropical storm off Myanmar coast sweeps reefs in Ritchie s
Archipelago, Andaman
P.Krishnan&Grinson-George&N.Vikas&
Titus-Immanuel&M.P. Goutham-Bharathi&
A.Anand&K. Vinod Kumar&S. Senthil Kumar
Received:23February2012/Accepted:8October2012
#SpringerScience+BusinessMediaDordrecht2012
Abstract The reefs in some islands of Andaman and in the shallow inshore reefs were broken and dis-
Nicobar suffered severe damage following a tropical lodgedbythethrustofthewaves.Significantdamage
stormintheBayofBengaloffMyanmarcoastduring in the deeper regions and offshore reefs were caused
13–17 March 2011. Surveys were conducted at eight by the settlement of debris and sand brought down
sites in Andaman, of which five were located in the from the shallower regions. The fragile branching
Ritchie’s Archipelago where maximum wind speeds corals (Acropora sp.) were reduced to rubbles and
of 11 ms-1 was observed; and three around Port Blair the larger boulder corals (Porites sp.) were toppled
whichlayontheleewardsideofthestormandhadnot over or scarred by falling debris. The reefs on the
experienced wind speeds of more than 9 ms-1. Corals windward side and directly in the path of the storm
windsweretheworstaffected.Theinvestigationexpo-
ses the vulnerability of the reefs in Andaman to the
: : :
PP..KKrriisshhnnaann((**))·.GGrirninsosonn-G-Geeoorgrgee·T.itTuistu-Ism-Immamnauneule·l oceanographic features which generally remain unno-
M.P.Goutham-Bharathi
ticed unless the damage is caused to the coastal
MarineResearchLaboratory,
habitats.
CentralAgriculturalResearchInstitute,
PortBlair744101AndamanandNicobarIslands,India
e-mail:[email protected] Keywords Coralreefs.Tropicalstorm.Surge.
Geostrophiccurrents.Eddies
N.Vikas
DiveIndiaLtd,
Havelock,SouthAndaman744122,India
Introduction
A.Anand
RegionalRemoteSensingCentre-Central,
Coral reefs across the world were affected by the
NationalRemoteSensingCentre(ISRO),
Nagpur440010,India elevated sea surface temperature (SST) during May
2010 and in Andaman the extent of damage was up
K.V.Kumar
to 70 % in some islands (Krishnan et al. 2011). The
DivisionofPhysicalOceanography,
reefsinsomeislandssufferedaseveredamageduring
NationalInstituteofOceanography,
DonaPaula,Goa403004,India March2011followingthetropicalstormoffMyanmar
coastwhichwasobservedduringaroutinereefhealth
S.S.Kumar
monitoring survey. Tropical storms and cyclones fre-
DepartmentofEnvironmentandForests,
quently occur at latitudes 10-30° on both sides of the
AndamanandNicobarAdministration,
PortBlair744101,India equatorandareperhapsthemostobviousandfrequent
EnvironMonitAssess
natural disturbances affecting reef communities (Gard- Hawaiiovera20yearperiodsuggeststhatthereefsare
ner et al. 2005; Done 1992) at various spatial and resilient and persisting through geologic time through
temporal scales. A tropical cyclone hit the Myanmar naturalstressorsbuttheincreaseinanthropogenicactiv-
coastline around Irrawaddy Division and Moon State itiesandtheireffectstendtomaketherecoveryprocess
withwindspeedupto50knotsandwaveheightof4– more difficult (Dollar and Grigg 2003). Hence, even
6mduring14–16March,2011.Around4000fishermen thoughepisodiceventslikestormsandhurricaneshave
were swept into the Andaman Sea from the Pyapon a profound impact on the reefs, other factors like an-
region in Myanmar during the storm (http://www. thropogenic activities and effects of changing climate
mizzima.com/news/inside-burma/5054-up-to-4000- havesignificantbearingonthesurvivalofreefs.
fishermen-from-pyapon-missing-in-storms). Myan-
mar’s Department of Meteorology and Hydrology
reportedonMarch14,2011thattheregionexperienced Materials and methods
heavy winds and rain with thunder and lightning. The
surge associated with the storm which moved south- The study started after one of the routine reef health
wardscausedsignificantdamagetothereefsinitspath. monitoringsurveysinthelastweekofMarch2011at
Anumberofstudieshavedocumentedtheextentof South Button Island, a popular dive site in Andaman,
direct mortality caused by storms at local or regional wherecoralswerefoundcrushedtorubbles.Thislead
scales (Gardner et al. 2005; Done 1992). The ecolog- toapurposivesurveyduringApriltoSeptember2011
ical effects of cyclones on coral reefs have been at eight selected sites in South Andaman viz., South
reviewed by Harmelin-Vivien (1994). The tropical ButtonIsland(12°13'366”N93°01'292”E),asmall
storms cause severe damage to the reefs and their island located towards the northern side of Ritchie’s
impacts include the removal of reef matrix, scouring Archipelago with reefs extending upto a depth of
and fragmentation (Done 1992; Van-Woesik et al. 24 m; Sebastian’s Hill (12° 13' 261” N 93° 02' 404”
1991; Rogers et al. 1991), deposition of loosened E), Minerva Ledge (12° 04' 023” N 93° 09' 066” E),
material onto beaches above sea level or propelling Broken Ledge (12° 06' 791” N 93° 08' 703” E) and
them into deeper sub-reef environments (Done 1992; Jackson’s Bar (12° 07' 410” N 93° 09' 653” E), all
Scoffin1994).Thesedimentstakenintosuspensionby popular offshore underwater dive sites located in the
storm currents lead to burial of reef organisms Ritchie’s Archipelago; Red Skin Island (11° 32'
(Hubbard et al. 1991; Rogers et al. 1991), increase in 23.27” N 92° 35' 03.98” E), a popular tourist site
turbidity, decrease in available light and augmented under Mahatma Gandhi Marine National Park; North
energy allocation towards the removal of sediment Bay(11°42'21.20”N92°45'15.20”E)andChidiya-
particles (Rogers 1983). tapu(11°29'28.00” N 92° 42'40.00” E), sites inPort
Thewavesandswellgeneratedfollowingthe1980 Blairwhichareborderedbyfringingreefsextendingup
hurricaneintheregionsofCoralreefsofWestHawaii toamaximumdepthof10m(Fig.1a).Aftertheinitial
causedextensivedamagetocorals,withthestormsurf surveys,SouthButtonwasselectedforfurtherextensive
destroyingzonationpattern,causingdislodgementand survey.TheIslandwasdividedinto7regionstoinves-
breakage of branching corals and leaving reef slope tigatetheextentofdamagetothereefs(Fig.1b).
area littered with broken plates of Porites lobata Field surveys were conducted using Self Contained
(Dollar 1982). Similar storm related coral reef dam- Underwater Breathing Apparatus (SCUBA), photo-
ages have been documented in French Polynesia in graphs and videos were taken using a digital camera
1982–83 (Harmelin-Vivien and Laboute 1986), (Canon powershot G11) with an underwater case
Jamaica in 1980 (Woodley et al. 1981) and in Great (Ikelite). Data with respect to coral species that were
Barrier Reef of Australia by Cyclone Joy in 1990 damagedanddepthsofdamageofreeforganismswere
(Van-Woesik et al. 1995) and tropical cyclone Ingrid collected. Similar data with respect to the study sites
in2005(Fabricius et al.2008). collected prior to the incidence of storm were used to
The Caribbean reefs which were reduced to 17 % determinetheextentofdisturbance.Basedonthequal-
followingaHurricanedidnotrecovertopre-stormstate itative observations made, extent of damage was cate-
for at least 8 years after impact (Gardner et al. 2005). gorizedintonil,moderateandsevere,whereonascale
Thestudyonthedamageandrecoveryofcoralreefsin often,0–3isnil;4–7ismoderateand7–10issevere.
EnvironMonitAssess
Fig. 1 a Map of Andaman
andNicobarIslandsshow-
ingthestudysitesatRit-
chie’sArchipelago;(1)
SouthButtonIsland;(2)
Sebastianhill;(3)Minerva
ledge;(4)Brokenledge;(5)
Jacksonbar;(6)RedSkin;
(7)NorthBay;(8)Chidya-
tapu.bSpecificlocations
surveyedaroundSouthBut-
tonIsland
EnvironMonitAssess
Satellite Altimetry-based Map of Sea Level Weather Research and Forecasting (WRF) model-
Anomaly (MSLA) data of Near Real Time (NRT) ling system was employed for predicting the storm
available with the Archival, Validation and Inter- surge dynamics during the study period. The model
pretation of Satellite Oceanographic Data (AVISO) domainextends from 90°E to95°E and 9°N to 15°N,
(http://www.aviso.oceanobs.com/en/data/products/ covering the Andaman group of Islands and the sim-
index.html) was used to study the Sea Level ulationwascarriedoutfrom10–19March2011witha
Anomaly and Surface Geostrophic currents in the resolution of 9 km. The tide pattern during March
region during 13–23 March 2011. These data sets 2011 was obtained from the automatic tide gauge
are prepared by AVISO using Altimetry data from installedat Marine Hill, Port Blair.
various sensors (Multi-mission products). Sea level
anomaly and surface geostrophic current vector
data were processed to generate sea level anomaly Results
and current maps using ARC-GIS software with
the data for the period 13–23 March 2011. Eddy Large scale disturbances were observed in all the
Kinetic Energy (EKE) was computed from the selected study sites in Ritchie’s Archipelago (Fig. 2),
Zonal and Meridional Geostrophic current vectors. while thereefs around PortBlairviz., RedskinIsland,
Fig.2 Damagetocoralsduetothestorm.aAlargetablecoral (Helioporacaeurulea);ecoralrubblecollectedat24mdepth;
measuring3mturnedover;bbranchesbrokenoffatablecoral; fatoppledbouldercoral(Poriteslobata);gbleachedTubastrea
c debris strewn on top of submassive coral (Montipora sp.) micrantha,revealingthewhiteskeleton;htablecoralpartially
damaging its branches; d broken branches of blue coral bleachedattheedges
EnvironMonitAssess
North Bay and Chidiyatapu were unaffected. Among broken; Acropora chesterfieldensis was scarred;
the islands affected, there was variation in depths of Tubastrea micrantha had bleached and most colonies
damage between the islands and also between the had died (Fig. 2g) and the table coral, Acropora hya-
locationswithinanisland.Surveysrevealedextensive cinthus was found to be flipped over. At Minerva
coral damages up to 15 m depth in the north-eastern Ledge several boulder corals (Porites spp.) were bro-
region of South Button Island. The dead corals from ken off at their bases and rolled onto their sides at
thepreviousbleachingevents,alongtheverticalwalls depthsrangingfrom9–18m.Thescarsonlivingcoral
of South Button were dislocated. surfaces were attributed to the debris. Some corals
Table corals (Acropora spp.) were found to have belonging to Acroporidae were partially bleached
been toppled (Fig. 2a) and some had broken edges (Fig.2h),whichcouldbeattributedtoreducedsalinity
(Fig. 2b). The outer-shelf reefs were damaged by the levels due to the incessant rains as reported by Van-
current-swept debris which broke and scarred the Woesik et al. (1995) and Hoegh-Gulberg and Smith
branchingandplatecorals(Fig.2c,d).Thebranching (1989).
coralswhichwerebleachedduringMay2010haddied The storm caused significant damage to the reef
subsequently and these dead corals along with those associated organisms also. The barrel sponge
few which were recovering also were washed off by (Xestospongiatestudinaria)(Fig.3a)andsoftcoralsof
the storm and were piled up at deeper (24 m) zones Nepthidae (Fig. 3b) at Jackson Bar were partially
(Fig. 2e). At Sebastian Hill, massive corals (Porites bleached. Among the survey sites, storm-associated
lobata)measuringover2minsizeandfoundatdepths bleaching was common at Jackson Bar, however, it
of 9–11 m were toppled (Fig. 2f). Except for the was observed only in few individuals and most of the
removal of dead branching corals at 19–22 m, no partiallybleachedspecieshadastrongchanceofrecov-
significant damage was observed at Jackson Bar. At ery.Therockoysters(Ostreidae),usuallycoveredwitha
deeper waters (30 m), sand and rubble were piled thickmatofalgae,appearedtohavebeenscrubbedby
underoneoftheledges.SurveysconductedatBroken thestorm(Fig.3c).Thesurge/stormalsodislodgedthe
Ledge in depths ranging from 17–23 m showed that weakreefassociatesfromtheirhabitatandcarriedthem
the storm has left different patterns of damage on the to the deeper zones along with the coral rubbles
corals of shallower regions viz., Millipora sp. was (Fig.3d).
Fig. 3 Effects of storm on
coral-associatedorganisms.
aPartiallybleachedbarrel
sponge(Xestospongiatestu-
dinaria);bpartiallybleached
softcoral(Nephthidae);c
rockoysters(Ostreidae)
cleanedoffalgae;dfeather
star(Crinoidea)dislodged
fromitsreefhabitat
EnvironMonitAssess
Thetidepatterndidnotshowsignificantchangein direction of the storm. The shape of the Island might
the amplitude during the period of storm, as recorded have partially protected the Northeast reefs where
from the ATG installed at Port Blair (Fig. 4). The damage at both the surveyed sites was comparatively
WRF model showed that wind velocities were less lesser.Beingontheleewardside,thereefofsouthern
than 7 ms-1 till 13 March and gradually increased regionwascompletelysparedofanymechanicaldam-
through 14–15 March 2011 up to about 8 ms-1. The age by the storm (Mumby 1999). A detailed account
highest wind speeds were recorded on 16 and 17 on the site-wise status of the reef is provided in
Marchreachingupto11ms-1and10ms-1respective- Table 2. The reefs around Port Blair remained unaf-
ly.The speed reduced drastically to4 ms-1and below fected as the energy of storm would have dissipated
during18–19March2011indicatingthedissipationof andalsoduetothefactthatthesitessurveyedwerein
thestormwinds(Table1).Studiesonwindspeedand theleewardside ofthe storm.
direction revealed that the prominent wind direction Mesoscale eddies are quite common in the seas
was north-easterly over the Ritchie’s Archipelago surrounding the Andaman and Nicobar Islands, how-
(Fig. 5). evertheirpresenceinsuchcloseproximitytothecoast
Map of sea level anomalies (MSLA) and Surface as observed in this event has not yet been recorded.
Geostrophic current maps revealed the presence of Eddies occurring in coral reef areas are known to
mesoscale warm core (anti-cyclonic) eddy close to cause thermal stress related bleaching due to the up-
theshoresofSouthandLittleAndamanencompassing welling associated with the eddy circulation (Jokiel
the areas where damage to reefs is documented. Sur- andColes1990).Modelsevaluatingthehydrographic
face Geostrophic current speeds ranging between 20 effects of eddy on island waters (Farmer and Berg
and 30 cms-1 were observed in the periphery of the 1989) have explained the dispersal of larvae due to
eddy(Fig.6).Theeddypersistedduringtheperiodof high-velocity shear currents generated by the
13–23March,2011.The EddyKinetic Energy(EKE) approaching eddy. Cross-frontal advection has been
was high inRitchie’s Archipelago (Fig. 7). documented for cold core and warm core eddies
(Wiebe and Flierl 1983; Wroblewski and Cheney
1984). Fast currents generated by eddies, tidal and
Discussion ocean currents and gyres, quite close to coral islands
are considered as physical factors that induce local
The comprehensive survey around the South Button water movements, flush toxins and remove thermal
Island showed the progression of storm clearly. The stratification in coral reef locations and hence are
damage was more in the northern and northwestern assumed as high reliability factors of resistance to
side of the Island, which corroborated with the coral bleaching (West and Salm 2003). Physical
Fig.4 TidepatternduringMarch2011asrecordedfromtheautomatictidegaugeinstalledatPortBlair
EnvironMonitAssess
Table 1 Wind-model estimates over Andaman as determined eddyshowshighkineticenergyintheareaofRitchie’s
byWRFModel
Archipelago (Fig. 7). Although eddies with high ki-
Date MeanWind Direction netic energy alone are insufficient to cause such ex-
Speed(ms-1) tensivedamage,thepresenceofsuchintensivecurrent
activity coupled with the other factors like storm
10Mar2011 6–7 Easterly
winds and shallow bathymetry in the coral reef areas
11Mar2011 5–7 Easterly
seem to have acontributory effectespecially in trans-
12Mar2011 4–5 North-easterly
portofthesedimentsandsmotheringofcoralreefsas
13Mar2011 4–5 North-easterly observed extensivelyin this case.
14Mar2011 7–8 North-easterly Thelocationofthereef,coralcommunitytypeand
15Mar2011 7–8 North-easterly succession stage of coral development are the factors
16Mar2011 9–11 NorthandNorth-easterly which play an important role in determining the fra-
17Mar2011 9–10 North-easterly gility orrobustness of thereef (Fabricius etal.2008).
18Mar2011 3–5 North-easterlyandEasterly The spatial factors determining vulnerability of a par-
ticular coral community are the position of a reef
damagetothecoralreefstructuresduetoeddiesisnot across the continental shelf and their location within
yet documented. The EKE computed for the present areef.Mostofthecoralswerestrippedofftheinshore
Fig.5 Winddirectionandspeedduring12–18March2011derivedfromtheWRFmodelshowingtheoccurrenceofstormsurgein
SouthAndamanon16March2011
EnvironMonitAssess
Fig.6 Sealevelanomaly(SLA)andgeostrophiccurrentsaroundAndamanon13–18March2011
reefs(upto15m)ofSouthButton,whichmainlycon- Acropora spp. and foliose corals (Montipora spp. and
sisted of taller branching and larger table corals of Echinopora lamellosa). This damage might be due to
EnvironMonitAssess
Fig.7 Eddykineticenergy(EKE)mapshowinghighkineticenergyzonesinthevicinityoftheRitchie’sArchipelago
lower skeletal density and weaker reef substrata (more the effect of the storm was confined to the shallower
loosely assembled and poorly cemented) of inshore regionswhileinthedeeperregions,thedamagewasdue
coralsthanthoseonoffshorereefsduetogreaterinternal to the settlement of debris and sand that had been
bioerosionandfewercrustosecorallinealgae(Perryand broughtdownfromtheshallowerregions.
Smithers 2006). It was also observed that the robust The type of coral community is another factor that
offshorereefsofSebastianHill,Jackson’sBar,Broken determines the degree of vulnerability of coral reefs.
Ledge and Minerva Ledge had comparatively lesser The habitat of coral communities vary across the in-
damages to the inshore reefs. In almost all the sites, shore–offshoregradientinwaveexposure,bathymetry,
EnvironMonitAssess
Table2 Thestatusofreefs
aroundtheSouthButtonIsland Site Depth(m) Statusofreefs Extentof
damage
Site1 5–6 AlltheAcroporaandotherbranchingcoralsweretotally Severe
strippedofftheirsubstratumandreducedtorubbles.
Site2 8–10 ThecoralsmainlybelongingtoAcroporaspp.havebeen Severe
brokenandstrewnintheentirereefregion.Afewlive
corals(PoritidaeandFavidae)werefound.
Site3 8–9 MassivecoralsofthegenusPoritespredominate.The Nil
deadcoralskeletonsofAcroporidaewerefoundintact.
Site4 7–9 FavidaeandPoritidaewerethedominantcorals. Nil
Site5 6–8 MainlycoveredwithshortbranchedAcroporidae.Few Moderate
wereobservedtohavebeenstrippedoffthesubstratum;
brokencoralrubblecollectedat2–4mdepth
Site6 7–9 Acroporidaepredominate.Theskeletalremainsofthe Moderate
branchingcoralswhichdiedfollowing2010mass
bleachingweremostlyintactwithminordamages.
Site7 5–7 ShortbranchedandsmallTablecoralsofthegenus Severe
Acroporawerethepredominantcoralspresenthere.
Mostoftherockshavebeencompletelycleanedoff
allcoralcover.
waterclarityandlight(Done1982).Bouldercoralsand been reported (Massel and Done 1993). Some of the
submassivecoralsarelessvulnerabletowaveassociated coralswithlargerbaseswhichshowedresistancetothe
damage.EvenAcroporaspp.withrigid,corymboseand waveswerescarredbydebrisfallingonthem.
submassivegrowthforms,complementedbysomelow, Stage of development of the coral community also
compact branching forms are capable of withstanding influencesthedegreeofvulnerabilityofreef(Fabricius
thethrustofwaves,howeverinastorm,thelightskeletal etal.2008).Areefthathasbeendenudedinrecentyears
structure and branchesrenderthemfragile and suscep- (e.g. by cyclonic waves, bleaching, or outbreaks of
tible.SouthbuttonreefwaspredominatedbyAcropora crown-of-thornsstarfish)andisoccupiedbynewlyset-
spp. particularly up to 7 m depth and hence was ad- tledyoungcoloniesofferslittlehydrodynamicdragand
versely affected by the storm. Catastrophic damage to would berelatively invulnerable(Madinand Connolly
delicatecoralsandthesurvivalofrobustmassivecolo- 2006). The Andaman reefs which witnessed mass
nies is common during major storms (Edmunds and bleaching due to the elevated SST during May 2010
Witman 1991) and has important implications be- (Krishnan et al. 2011) were dead but were still able to
cause massive corals build the framework of coral shelterseveralfishesandotherorganismsintheirstruc-
reefs (Sheppard 1982). turalcomplexes.Whenthestormwaveslashedatthese
Massivecoralassemblagesaffectedbystormshave coralstructurestheywerenotabletoresistthosewaves,
tworesponsesviz.,dislodgementandresistance(Done much like the live branching corals; dead corals being
1992;Van-Woesiketal.1991;DoneandNavin1990). even more vulnerable due to the action of several bio-
It was observed in the current study that the massive eroders (Glynn 1993). Though the storm left the reef
corals,Poriteslobata,P.luteaandP.solidawereless denuded,substrateforcolonizationbynewcoralrecruits
affected in all the reefs. However, in some offshore increased(Rogersetal.1991).
reefs, corals measuring up to 2 m in diameter were Disturbanceplaysacentralroleinthedynamicsof
foundtohavebeendislodgedandoftenpusheddown- awidevarietyofecologicalcommunities(Karlsonand
slope. Examination of basal areas of toppled corals Hurd 1993; Krummel et al. 1987). Extreme distur-
suggested that they were already unattached or were bancecanreducediversityandaltercommunitystruc-
only weakly attached to the substratum prior to dis- ture by eliminating species (Petraitis et al. 1989;
lodgement.Tissuesbroughtintocontactwiththesub- Pickett et al. 1989). In contrast, moderate levels of
stratumduringthefallhaddiedbutremainingareasof disturbance may promote local diversity by reducing
thecolonysurvived.Similarobservationshavealready the abundance of competitively dominant species and