Factors influencing the stable carbon and oxygen isotopic composition of Porites lutea coral skeletons from Phuket, South Thailand

Transcription

1 Cral Reefs (1996) 15:43-57 ~'~0 NA~~ Cral Reefs Springer-Verlag 1996 ~' /~ Factrs influencing the stable carbn and xygen istpic cmpsitin f Prites lutea cral skeletns frm Phuket, Suth Thailand N. Allisn 1,*, A. W. Tudhpe 1, A. E. Fallick ~ Department f Gelgy and Gephysics, Grant Institute, West Mains Rad, Edinburgh EH9 3JW, UK 2 Istpe Gesciences Unit, Scttish Universities Research Reactr Centre, East Kilbride, Glasgw G75 OQU, UK Accepted: 12 May 1995 Abstract. We determined the 8J80 and 8~3C cmpsitin f the same fixed grwth increment in several Prites lutea cral skeletns frm Phuket, Suth Thailand. Skeletal grwth rate and 81sO are inversely related. We explain this in terms f McCnnaughey's kinetic istpic disequilibria mdel. Annual trends in 8]sO cannt be slely explained by bserved variatins in seawater temperature r salinity and may als reflect seasnal variatins in calcificatin rate. Cral tissue chlrphyll a cntent and 8~3C f the underlying 1 mm f skeletn are psitively related, suggesting that algal mdificatin f the disslved inrganic carbnate pl is the main cntrl n skeletal 8~3C. Hwever, in crals that bleached during a perid f exceptinally high seawater temperatures in the summer f 1991, 613C f the uter 1 mm f skeletn and skeletal grwth rate (ver 9 mnths up t and including the bleaching event) are inversely related. Seasnal variatins in 8~3C may reflect variatins in calcificatin rate, zxanthellae phtsynthesis r in seawater 813C cmpsitin. Bleached crals had reduced calcificatin ver the 9-mnth perid up t and including the bleaching event and ver the event they depsited carbnate enriched in 13C and ]80 cmpared with unaffected crals. Hwever, calcificatin during the event was limited and insufficient material was depsited t influence significantly the istpic signature f the larger seasnal prfile samples. In prfile, verall decreases in 8180 and 813C were bserved, supprting evidence that psitive temperature anmalies caused the bleaching event and reflecting the lss f zxanthellae phtsynthesis. Intrductin The aragnite depsited by scleractinian crals is usually depleted in 13C and 180, relative t equilibrium with *Present address. Department f Envirnmental Sciences, University f Plymuth, Drake Circus, Plymuth PL4 8AA, UK ambient seawater. McCnnaughey (1989a) attributes these istpic disequilibria t kinetic and metablic effects. The frmer, resulting frm kinetic istpe fractinatin during CO 2 hydratin and hydrxylatin, cause simul-taneus depletin f 180 and 13C. Deviatins frm the skeletal 613C expected frm kinetic fractinatin are attri-buted t metablic istpic effects caused by mdificatin f 6J3C f the internal disslved inrganic carbn (DIC) pl by zxanthellae phtsynthesis and by algal and cral respiratin. In additin, a depletin in carbnate 8180 ccurs as temperature increases in inrganic (O'Neil et al. 1969) and bigenic carbnates. Several palae-thermmeter equatins describing this effect have been prpsed fr Prites crals, including: 8c - 8w = T (McCnnaughey 1989a, cral frm Galfipags, wrt PDB) 6c - 8w = T (Chakrabrty and Ramesh 1993, cral frm the Arabian Sea) where 6c = 8180 f CO z released by reactin f CaCO 3 with H3PO 4 at 25 C (wrt PBD), 6w = 6~80 f CO 2 equilibrated with supernatant seawater at 25 C, T = temperature ( C). These tw equatins are in gd agreement and crrespnd t a decrease in skeletal 8~80 f apprximately 0.21% per 1 C seawater temperature increase. Hwever, nt all authrs have bserved the skeletal ~180 variatins expected frm cnsideratin f temperature dependent fractinatin effects (Emiliani et al. 1978; Greau 1977), and the reasns fr this are unclear. The sampling frequencies (4-6 samples per annual grwth band) used by the authrs may have been insufficient t accurately trace skeletal 8180 variatins. Alternatively, temperature effects may have been bscured by changes in seawater 5180 cmpsitin r by sme physilgical effect prduced by the rganism. There are several reprts f a relatinship between skeletal grwth rate and istpic values. Land et al. (1975) bserved greater depletin in skeletal 13C and ~80 in the faster grwing areas f single cral clnies and als in the faster grwing skeletal elements frm the same calice. McCnnaughey (1989a) reprted an inverse

2 44 relatinship between linear extensin and 8~3C and 6~80 in the utermst carbnate f Pavna clavus heads frm Galfipags but nly fr thse parts f the cral grwing at less than 5 mm year -~. Fr mre rapidly grwing parts f the cral, calcificatin rate did nt appear t have a significant effect. The present study investigates the varius cntrls n the 8~3C and 8~80 cmpsitin fprites lutea cral skeletns frm the Phuket area f Suth Thailand. Seasnal variatins in 6~80 and 6~3C are cnsidered in relatin t seawater temperature, and salinity data and the relatinship between the chlrphyll a cncentratin f the verlying cral tissue and the istpic cmpsitin f the skeletn is investigated. Arund Phuket large variatins in the skeletal grwth rates f clnies frm the same site, water depth and reef-frnt lcatin ccur (Scffin et al. 1992), and the effect f this n skeletal istpic cmpsitin is explred. We als examine the effect f a lcal cral bleaching event n skeletal istpic cmpsitin. Materials and methds Descriptin f study area K Phuket is an island n the SW cast f Thailand in the Andaman Sea at latitude 8 00'N and lngitude 98 20'E (Fig. 1). The area has a mnsnal climate, with a dry nrth-east mnsn frm Nvember t March and a wet suth-west mnsn frm April t Octber. Clud cver is lwest and sunshine highest between Nvember and May. Rainfall measured at the Phuket Twn weather statin frm January 1982 t June 1992 ranged frm 0 t 881 mm mnth -~ (Fig. 2). Seawater salinity data were cllected weekly r biweekly frm the pier statin at Phuket Marine Bilgical Statin between March 1982 and Nvember 1986 and between September 1990 and June 1991 (Vudichai Janekarn, persnal cmmunicatin, 1992; Fig. 2). Illustrated data are the mnthly means f the 2 and 4 m water depth measurements, crrespnding t the depth range f the crals sampled. Salinity usually varied by %0 ver the whle depth range measured frm the water surface t the bttm f the reef frnt (at abut 7 m depth). The annual salinity range is 1-2.0%0 but seasnal trends are nt cnsistent. In sme years (1983 and 1985) highest and lwest salinities were recrded between April and June and between Nvember and January, respectively. Hwever, in ther years n seasnal trend is apparent (1984) r the trend appears t be reversed (1986). The few data available fr the perid f this study (Nvember 1988 t June 1992) illustrate an increase in salinity frm September 1990 t January 1991 fllwed by a decrease up t April This is largely expected frm the rainfall data which is lwest between Nvember and February. Hwever, it is unclear why the strngly seasnal rainfall is nt always reflected by seawater salinity variatins. It is pssible that in sme years a time lag ccurs between rainfall and freshwater discharge int the sea. Alternatively, the salinity trend expected frm rainfall may be swamped by the influx f anther water bdy f higher r lwer salinity. Hydrdynamics in the area are principally determined by the tw mnsn seasns. During the suth-west mnsn, airflw results in the mvement f surface waters nrth. On the east side f K Phuket these surface waters mve up int Phangnga Bay and the bttm currents in the Bay mve suth. This situatin is reversed during the nrth-east mnsn (Carr 1992). Mnthly mean surface seawater temperatures fr the 5 latitude by lngitude area, including the sea arund Phuket, ranged frm 27.9 C in January t 29.4 C in May (frm 1951 t 1980, Overseas Develpment Administratin envirnmental data, Fig. 3). These tem-peratures are in reasnable agreement with data cllected frm the pier statin at the Phuket Marine Bilgical Centre. Between March 1981 t December 1986 and July 1990 t June 1992 temperatures ranged frm apprximately 26 C in December and January t 30 C in May and June (Vudichai Janekarn, persnal cmmunicatin, 1992). Temperatures varied by C Over the depth range measured, frm the water surface t the bttm f the reef frnt (at abut 7 m depth). Extensive cral bleaching was bserved in the Phuket area frm early June 1991 nwards and affected many branching and massive crals n the fringing reefs arund the Phuket Marine Bilgical Centre. There were marked differences in the degree f bleaching in adjacent Prites clnies f apparently the same species. Visibly unaffected crals retained their usual dark brwn pigmentatin, while bleached crals became very pale r even white. Partial recvery f pigmentatin was bserved in several Prites clnies twards the end f Jnlv. and ver the fllwing weeks many f the g~'k Miang Similan Islands (SM) 198E 99E1 8":... i ( 198E Z~ ~,A~ ~ ~K Phi Phi K Racha Yai (KR) 99E1 / Fig. 1. Map f the study area in Phuket, Suth Thailand, shwing the reef sampling sites Prites Bay (PB) and Aquarium reef (AQ) and the psitin f Phuket Twn (). The 200 m depth cntur is shwn

3 lo00 I g f il + 'i i ;ii!.r~,ii I I /ii.;i i i1!!/i! i i~'i! i %' i !: J- ~i! " 1' i i!i/ ~. I1~. Ii ", iit~,:l I.,/ If ' '.!i! i I ~!./ "~ I " +1 I' Ii i Ik I,,I ' ' I I',I \ ~ll i i!t Ill. I'l. ~l '!l! i!i iii!!~i t i I ' +t~ i'!, I 1 l \ / I Fig. 2. Mnthly rainfall (frm the Phuket Twn weather statin) and mean mnthly salinity data (frm the pier statin, Phuket Marine Bilgical Centre) G em 3O * Mean '(+/- std deviatin) '~" 29 m " / 28 Fig. 3. Average mnthly seawater temperature data frm the 5 degree lngitude/latitude square cntaining Phuket Island Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nv Dee affected crals recvered t nrmal pigmentatin (Niphn Phngsuwan, persnal cmmunicatin, 1991) Surface seawater temperatures had been up t 1 C abve nrmal since December 1990, and the bleaching event apprximately cincided with the annual maximum f 30.2 C (Fig. 2). In cmmn with many ther bleaching reprts, there is gd circumstantial evidence that this event was primarily temperature induced. Cral specimens were cllected frm the Prites Bay reef n the SE cast f Phuket Island. This is an inshre fringing reef, typical fr the muddy waters f Phuket's SE cast (Brwn et al. 1986; Scffin et al. 1992). The reef cnsists f an inter-tidal reef flat m wide and a near vertical reef slpe leading dwn t muddy (terrigenus clay-rich) fre-reef sediments at 4-6 m water depth. The turbidity f lcal waters is largely due t resuspensin f this fre-reef mud by tidal and wind-driven currents. The reef frnt is dminated by large clnies f Prites, up t 4 m in diameter. The upper surfaces f Prites clnies in this area are hummcky, with prminent knbs r heads having up t 30 cm relief. Clnies were sampled by remval f these heads, in whle r in part, underwater using a hammer and chisel. All samples were cllected frm Prites Bay between 1 and 3 m belw mean lw water levels, within a 15-day perid during July Crals were cllected within 30 m f each ther. In additin ne specimen was cllected frm the Aquarium Reef in July Similar t Prites Bay in mrphlgy, this reef ccurs n the ppsite (suth) side f the headland, apprximately 0.5 km frm Prites Bay. Bigenic carbnate samples are cmmnly treated befre gechemical analysis t remve rganic cntaminatin. Hwever McCnnaughey (1989a) fund that neither vacuum rasting cral samples at C r treating with hydrgen perxide imprved sample reprducibility. Gaffey and Brnnimann (1993) cncluded that sdium hypchlrite was the mst effective slutin at remving rganic material frm bigenic carbnates and we used this t remve rganics frm the samples analysed during the present study. Cral heads were saked in a diluted slutin f sdium hypchlrite (ca. 3-4%) fr 48 h after cllectin and briefly rinsed in tap water. On arrival in Britain, prepared samples were saked in a similar slutin fr 2-3 days and rinsed fr 48 h. This treatment did nt appear t significantly affect stable istpe ratis fr alternate sdium hypchlrite-treated and -untreated samples alng a grwth band (6'sO = -5.66%0 +_ 0.05 and -5.68%0 0.04; 8t3C = -2.84%0 _ and-2.75%0 _ respectively [all standard deviatins are l]). Althugh this pretreatment did nt imprve sample reprducibility it was necessary t remve tissue and ther rganic cntaminatin, particularly in the utermst samples. Crallite skeletal structure was examined by bincular and scanning electrn micrscpy. Only crals exhibiting mrphlgical char-acteristics f Prites lutea (using the keys f Vern 1986) were used in this study. T investigate the cntrls f St3C and 5t80 in the crals f this area we adpted several strategies: 1, Grwth increment experiment. We analysed the istpic cmpsitins f the same grwth increment frm different cral clnies and frm replicate samples within the same clny. Istpic cmpsitin was related t skeletal grwth measured as linear extensin, skeletal bulk density and calcificatin.

4 46 2. Istpic prfile experiment. We cnstructed prfiles f seasnal variatins in 6'3C and 6~+O acrss the three mst recent years f grwth in several cral clnies. Seasnal variatins in 6~+O and 6 ~C were cnsidered with reference t seawater temperature and salinity data, while prfile differences between crals were related t variatins in linear extensin. In additin, data prduced frm prfiles spanning the bleaching episde were used t establish the effects f this event. 3. Analysis f the uter 1 mm f skeletn. We analysed the istpic cmpsitin f the utermst 1 mm f several cral skeletns cllected during the bleaching event. Bleached and unbleached samples were cmpared t assess the effect f the bleaching event. The istpic cmpsitin was als related t the chlrphyll a cncentratin f the verlying cral tissue. Grwth increment experiment Analyses were made n samples frm nine clnies cllected frm Prites Bay in July All clnies appeared unaffected by the bleaching event in prgress at the time. Clnies were sectined alng the axis f maximum grwth int slices f mm thickness, which were phtgraphed under ultravilet radiatin t recrd flurescent banding patterns. Bright-dull flurescent cuplets represent apprxi-mately annual grwth in crals in this area, with bright band accretin beginning in apprximately Nvember (Scffin et al. 1992). In this area, bright flurescent banding des nt crrelate directly with lcal rainfall (Nvember is the start f the dry seasn) which differs frm previus reprts frm ther areas (Isdale 1984; Bt and Isdale 1985). Hwever, alizarin red S grund-truthing and extensive bservatins f the flurescent banding in cral frm the Phuket area, leaves n dubt that bright band depsitin cmmences at abut Nvember. Skeletal density banding was nt used t prduce chrnlgies as it has prved t be ambiguus in the crals f this area with sme crals prducing tw pairs f dense/less dense bands per year (Brwn et al. 1986). Linear grwth, skeletal bulk density and calcificatin were calculated fr each sample fr the grwth increment frm the start f the last bright flurescent band (apprximately Nvember 1990) t the utermst part f the cral skeletn (July 1991). Althugh this grwth increment may nt exactly crrespnd t the 9-mnth perid described, we can be sure that the same time interval was sampled frm each cral, allwing interclny cmparisns. Between ne and seven samples were prepared frm the tp f each cral head. Linear extensin was measured alng the axis f maximum grwth, and the skeletal bulk density f the grwth increment was determined by cutting ut a rectangular blck f cral (apprximately 10 mmx 5 mmx linear extensin) and measuring its weight and vlume (dimensins measured using callipers). Calcificatin fr the time interval was calculated as a prduct f linear extensin and skeletal bulk density. Repeat measurements n same samples indi-cated that errrs in measurement were less than 5%. Sample blcks were analysed fr 6uC and 8~80. Each blck was grund t a fine pwder immediately befre analysis and a subsample f mg weighed. Samples were reacted at 90 C with 100% rthphsphric acid (s.g. 1.9) and the resulting CO 2 was analysed using a VG Isgas Prism mass spectrmeter. The machine was calibrated using a standard marble reference (SM1) and values were cnverted t a PDB standard. The standard deviatin (1~) fr 100 analyses f the standard carbnate (SM1), carried ut ver several mnths, was _+0.068%0 fr 6~80 and %0 fr 6L~C. Duplicate analyses f cral samples gave replicatin (1~) f better than _+0.05%0 fr xygen and _+0.1% fr carbn. Duplicate analyses n the uter-mst parts f the cral skeletn shwed the highest 6 ~3C variability, prbably due t the presence f increased cncentratins f rganic material nt remved by the sdium hypchlrite pretreatment. Reprducibility f ther carbn analyses was frequently better tfian +0.05%0. An annymus reviewer f this manuscript reprts btaining anmalus istpe values fr samples frm the utermst parts f the skeletn which are ccupied by the tissue layer. In ur studies f istpe prfiles, including material taken frm the tissue layer (ntably frm crals sampled in 1990, befre the bleaching event ccurred), we have fund n evidence that material frm the tissue layer prduces anmalus values nce treated t remve rganic material. Istpic prfile experiment Five istpic prfiles were established frm fur crals cllected frm Prites Bay in July 199 I. Cral PB9 appeared unaffected by the bleaching event while crals PB4, PB7 and PB8 all appeared bleached. Cral heads were sectined alng the axis f maximum grwth and phtgraphed under ultravilet radiatin t recrd flurescent banding patterns as befre. Detailed istpic prfiles (8-10 samples per annual grwth band) were cnstructed acrss the three mst recent years f grwth, up t and including the bleaching event. Fr each prfile, a thin strip f skeletal material, apprximately 3 mm x 3 rnm, was cut alng the maximum grwth axis f each cral frm the start f the bright band depsited in apprximately Nvember 1988 t the utermst part f the skeletn (July 1991). Each strip was ultrasnically cleaned, dried at 50 C and divided, using a fine scalpel blade, int samples f measured length. Samples were treated, pwdered, weighed and analysed as befre. A secnd series f samples were prepared frm cral PBg, called PB9b. These samples were prepared frm a grwth axis clse t the base f the cral which had a linear extensin f abut half f that bserved alng the maximum grwth axis. An additinal prfile was als prepared frm clny AQNEW, a cral cllected frm the Aquarium Reef in July 1992, which had apparently been unaffected by the bleaching event 1 year previusly. In this case the prfile spanned the time perid apprximately Nvember 1989 t July Analysis f the uter 1 mm f skeletn Samples were prepared frm nine clnies cllected frm Prites Bay in July Five appeared unaffected by the bleaching event in prgress at the time, while fur were extensively bleached (all virtually white). Five sample pairs were prepared frm the tp f each clny. One half f each pair, apprximately 100 mm 2, was analysed fr chlrphylls within 1 h f cllectin f the cral (see belw) and the ther half, apprximately 10 x 5 x 20 mm (deep), was returned t Britain fr stable istpe analysis. It was nt pssible t measure the rate at which the utermst 1 mm f skeletn was depsited. Hwever heads n eight f the cral clnies (fur bleached, fur unbleached) were treated with alizarin red S stain, which clurs skeletn depsited during stain expsure and acts as a time reference plane. In situ crals were enclsed in plastic bags cntaining the stain fr 6-7 h; the bags were then remved and the crals were cllected after anther 3 weeks. Istpe samples were als examined under ultravilet radiatin and the widths f the uter mst grwth increments (apprximately Nvember 1990 t July 1991) were recrded. Since this grwth increment cntains apprximately 1 mnth f the bleaching event, any majr changes in grwth rate ver the bleaching event wuld manifest themselves as small variatins in net accretin ver this nine mnth grwth increment. Immediately prir t istpic analysis, the utermst 1 mm f the skeletn was scraped away using a fine scalpel blade. Samples were pwdered, weighed and analysed as befre. It is nt pssible t be certain that the same grwth increment was sampled in each cral. Hwever, crals were cllected 4-6 weeks after the bleaching began, s this increment shuld cnsist predminantly f material depsited during the bleaching event (assuming sme skeletgenesis). Each sample fr chlrphyll analysis cnsisted f an area f apprximately 100mm 2 cral tissue and its underlying skeletal material t a depth f abut 5 ram. Samples were analysed accrding t the methdlgy f Jkiel and Cles (1974). In summary, the area f each sample was measured using calipers and the sample was

5 47 then finely grund in 10 ml 90% acetne using a mrtar and pestle. Samples, including all the assciated material, were decanted int glass test-tubes, wrapped in silver fil and stred in a refrigeratr fr 24 h. At the end f this perid the samples were reshaken, centrifuged and the supernatant liquid decanted. The vlume f the supernatant liquid was measured, it was diluted if necessary, and the light extinctin, in 10 mm ceils, was measured in a Tshiba spectrphtmeter at 750, 665,645 and 630 nm. Cncentratins f chlrphylls a, b and c were determined using the frmulae f Strickland and Parsns (1968). Repeat analyses f replicates indicated that standard deviatins f measurement were +-2'/0 fr chlrphyll a and +5% (1~) fr chlrphyll b. Cral zxanthellae cntain n chlrphyll b, s its presence was assumed to riginate frm cntaminatin f the sample by filamentus algae, which infest the cral skeletn belw the layer f living tissue (Jkiel and Cles 1974). Analyses perfrmed n skeletal material cntaining nly filamentus algae and n cral tissue shwed that the chlrphyll a: chlrphyll b rati f these algae was cnstant (chl alchl b = , lc~). Cnsequently the chlrphyll b cncentratin f the cral samples culd be used t determine the relative cncentratins f chlrphyll a due t filamentus algae and t symbitic zxanthellae. The chlrphyll a: chlrphyll c rati in the filamentus algae was nt cnstant, s we were unable t calculate cral chlrphyll c cncentratins. In general the chlrphyll a cncentratins attributed t the filamentus algae were abut a tenth t a twentieth f thse attributed t the zxanthellae. The chlrphyll a cncentratin attributed t9 the zxanthellae was crrected fr differences in measured tissue surface area and supernatant vlume t give a chlrphyll a cncentratin in mg cm -' cral tissue. This methd makes n crrectin fr variatins in the depth f the tissue layer between samples. Results Grwth increment experiment Skeletal grwth and istpic measurements fr the utermst grwth increment are shwn in Appendix 1.6~80 values varied frm t -6.16% and 8'3C values frm t -4.51%0. Regressins were calculated (methd as in Wetherill 1982) between 6'3C and 6~80 values and all the grwth measurements (Figs. 4, 5). Regressin equatins, Pearsn's crrelatin cefficients (r) and levels ~ f significance are superimpsed nt each graph. There are gd crrelatins between all grwth measurements (linear extensin, skeletal density and calcificatin) and 6180 (all significant at the level). Multiple istpic measurements were made n fur f the cral heads (PB3, PB5, PB6 and PB9, see Fig. 6). Only the samples frm clny PB6 exhibit a similar linear extensin versus 8~80 relatinship, within clny. The lack f a clear intraclny relatinship in the ther crals is discussed later. Crrelatins between 6L~C and all the grwth measurements are pr. Istpic prfiles experiment Flurescent banding patterns fr the crals studied are shwn in Fig. 7. There is little variatin in the linear extensin between crals PB7, PB8 and PB9a ver the time perid analysed (45, 43 and 45 mm respectively), but substantial differences ccur in crals PB9b and PB4. Bright flurescent bands frm PB9b and PB4 have similar widths t thse f PB9a at the beginning f the recrd in Nvember 1988 (apprximately 5 ram), but extensin rates subsequently diverge and by the utermst grwth increment band widths in PB9b and PB4 are less than half the width f PB9a (3.5, 4.5 and 10 mm respectively). Cral AQNEW extends by 54 mm ver the time perid studied. Seasnal 613C and 6'80 trends are shwn in Figs. 8 (crals PB9a, PB9b and PB4) and 9 (crals PB7, PB8 and AQNEW). All istpic determinatins are detailed in Appendix 2. The relative psitin f each sample in the annual band has been calculated by dividing the distance f the middle f each sample alng the annual band by the width f the annual band. This allws all the crals t be shwn n the same chrnlgical axis, despite variatins in linear extensin between crals. Representing the data , y = x r = (0.001) (9 y=-5, x r = (0.001) y = x r = (0.001) -L 813c i I f -4.0 G / I -4.5 i Linear Extensin (mm) Skeletal Density (gcm -3) Calcificatin (gcm "2 ) Fig. 4. Regressins f (~13C f the uter skeletal grwth interval (apprximately Nvember 1990 t July 1991) versus linear extensin, skeletal bulk density and calcificatin. Standard deviatins fr istpic and grwth measurements are shwn fr ne pint in the graph at the far fight. Regressin equatins, Pearsn's crrelatin cefficients (r) and levels f significance (in parentheses) are superimpsed nt each graph

6 y = x r = (0.001) -5.2 / \ \ ~ y= x r=-0.70 (0.001),,:,/ \\. \ y = x r = (0.001) 180 l ~'.\ e -5.6 '~.,, ~5.8 e '~" -6.0 ) ~3 \ 7" / // / illll -6.2 i i I " " " t Linear Extensin (ram) Skeletal Density (gem'3) Calcificatin (gem "2) Fig. 5. Regressins f ~80 f the uter skeletal grwth interval (apprximately Nvember 1990 t July 1991) versus linear extensin, skeletal bulk density and calcificatin. Standard deviatins fr istpic and grwth measurements are shwn fr ne pint in the graph at the far right. Regressin equatins, Pearsn's crrelatin cefficients (r) and levels f significance (in parentheses) are superimpsed nt each graph KEY: PB3 * PB5 PB6 D PB El! Linear Extensin (mm) Nv '89 Jul '91 Nv '88 I i PB7 PB8 AQNEW Fig. 6. Plt f 6 ~80 versus linear extensin fr multiple samples (ver the grwth interval apprximately Nvember 1990 t July 1991) frm fur cral clnies. Standard deviatins ( 1 ~) f measurements are shwn n ne pint in this manner makes n allwance fr seasnal variatins in linear extensin rate. Hwever, we assume that these seasnal variatins wuld be similar between clnies. The crals in bth Fig. 8 and Fig. 9 shw similar trends in 6180, with the istpically lightest xygen depsited arund the beginning f the bright flurescent bands (apprximately Nvember) and the heaviest depsited between bright bands (apprximately April t June). Crals PB7, PB8, PB9a and AQNEW exhibit annual 6180 ranges f %. Crals PB8, PB9a and AQNEW depsited similar values f 6180, but cral PB7 generally depsited higher 6780 values (by abut 0.3%). Crals PB4 and PB9b depsited similar 6~80 values t thse f PB9a at the start f the recrd (Nvember 1988), but they shw a greater enrichment in ~80 as the recrd prceeds. By July 1991 these crals depsited 61sO 0.5 t 0.8%0 higher than the faster grwing material in cral PB9a. All crals shw similar 6~3C annual trends, but recrds frm different crals r even frm different axes f the ~ P B 9 b ~ P B 4 Nv '88 Nv '90 Nv '89 PB9a Scale: 2 cm I Fig. 7. Flurescent banding patterns in this crals examined in the present study. Apprximate depsitin dates are shwn (bright band depsitin begins in apprximately Nvember). All crals were cllected in July 1991 with the exceptin f cral AQNEW (cllected in July 1992) same cral shw widely varying values. Each cral exhibits an annual 6~3C range f %0, with the istpically lightest carbn depsited arund the beginning f the bright flurescent bands (apprximately Nvember) and the heaviest depsited between bright bands (apprximately April t June). Crals PB9 and PB7 depsited

7 49 similar values f ~13C, but crals PB8 and AQNEW depsited carbnate relatively enriched in ~3C (by abut %0). Carbnate 813C frm cral PB9b is usually lwer than that at the tp f the cral (PB9a) by t - 1.5%. 613C in cral PB4 is enriched by % cmpared with the material depsited by cral PB9a. The carbnate depsited in the flurescent cuplet cntaining the bleaching event shws a slightly different 8180 and 8~C trend frm that bserved in the previus and 813 C 8180 KEY: -- PB9a... PB9b... PB4-1.0 :3i:!:i:;i:! l standard deviatin i~ ~ / / " \\\/ / -4.5 I standard deviatin ~!i ~Cq\ /// "... -" ~i~i~ :.i -5.0 "...F. 7r~... / -6.0 /,./ \. I I Nv '88 Nv '89 Nv '90 Apprximate date, calculated frm flurescent banding Fig. 8. Seasnal variatins in the 8~3C and 8~sO cmpsitins f tw crals exhibiting widely different grwth rates. Standard deviatins (la) f analysis are shwn. The apprximate perid f the bleaching event is shaded 8 fllwing years. In a 'nrmal' flurescent cuplet a peak f relatively high 8~80 is depsited midway between the start f bright bands, in apprximately April t June. This peak is either absent (AQNEW) r is substantially reduced (PB7, PB8 and PB9) during the year cntaining the bleaching event. The carbn istpic recrd fllws a similar seasnal trend t that f xygen during flurescent cuplets depsited under 'nrmal' cnditins. Cral PB9 fllws this trend during the bleaching event with highest 8~3C values depsited in apprximately April t June. Cral PB8 prduced a small peak f high 6~3C values in apprximately February t April, but then values decrease. Cral AQNEW prduced tw peaks f high 6~3C values during the flurescent cuplet cntaining the bleaching event. Analysis f the uter 1 mm f cral skeletns All the crals treated with alizarin red S incrprated sme f the stain int the skeletn, indicating that despite the high degree f bleaching in sme crals all were still depsiting calcium carbnate. Hwever, extensively bleached clnies incrprated nly small amunts f the stain, indicating extremely limited calcificatin in cmparisn with the apparently unaffected clnies that depsited distinct stain lines. In additin, the unbleached clnies bviusly depsited up t 1 mm f fresh carbnate ver the stain line in the perid between staining and sample cllectin (3 weeks). The unbleached crals appeared pale pink frm the utside at the time f sample cllectin suggesting calcificatin ver this 3 week perid was insufficient t cver the riginal uptake f stain. In summary, the unaffected clnies appearing t be calcifying mre quickly than thse which were extensively bleached. KEY: -- PB7... PB8 (cllected July 1991)... AQNEW (cllected July 1992) 813 C // -,\ [ // \ I / \ /I \\ \ :! / 2::::::::... " ii!iiii... " I standard deviatin -5.2 A J' ', /X/"....- I standard deviatin " XL.. [,%:,,f:/:' '. i Nv '88 Nv '89 Nv '90 Nv '91 Apprximate date, calculated frm flurescent banding Fig. 9..Seasnal variatins in the 8J3C and 8~80 cmpsitins f crals PB7, PB8 /tnd AQNEW. Standard deviatins (la) f analysis are shwn. The apprximate perid f the bleaching event is shaded

8 50 Linear extensin, chlrphyll a cncentratins and 8~3C and 8~80 measurements are shwn in Appendix 3. N chlrphyll a detected in the material cllected frm the fur bleached crals frm Prites Bay was attributed t zxanthellae. Zxanthellae chlrphyll a cncentratins in the unbleached crals varied frm 5.7 t 22.2 mg cm -2. 8~3C and 8~sO in these samples varied frm t -5.50%0 and frm t -6.94% respectively. Regressins f 8~C and ~80 versus chlrphyll a cncentratin are shwn in Fig. 10. The equatin, crrelatin cefficient and level f significance fr each regressin are superimpsed nt each graph. Chlrphyll a cncentratin and 5~3C shwed a brad psitive crrelatin in the unbleached crals (r = +0.64, significant at the level). There is n significant relatinship between 8~80 and chlrphyll a. 5~3C and 8~80 in the bleached cral samples varied frm t -4.68%0 and frm t -5.20%0 respectively. The Mann-Whitney U test (a nn-parametric test sensitive t differences between medians) was emplyed t test the significance f differences in stable istpic cmpsitin between the uter 1 mm f skeletn in bleached and unbleached crals. Mann-Whitney U values and levels f significance are shwn in Table 1. This test was als used t test the significance f differences in the linear extensin between bleached and unbleached crals ver the utermst grwth increment (apprximately Nvember 1990 t July 1991). 6~3C and 8~80 were significantly higher-in the bleached crals (bth at 0.01 levels f significance). Hwever, bleached crals als had significantly lwer rates f linear extensin (0.01 significance level), s lwer 8~80 values shuld be expected in these samples. T separate grwth rate effects frm thse f the bleaching event, regressins between linear extensin and skeletal 813C and 813 C y = x r = (0.001) = x r = e~ Chlrphyll a cncentratin (mgcm -2 ) Fig. 10. Regressins between chlrphyll a cncentratin f cral tissue and stable istpic cmpsitin f the underlying 1 mm f skeletn in samples frm five cral clnies. Standard deviatins (1~) f chlrphyll analysis are shwn n each graph. Deviatins f istpic measurements are abut the same size as the symbls. Regressin equatins, Pearsn's crrelatin cefficients (r) and levels f significance (in parentheses) are superimpsed nt each graph e Table 1. Mann-Whitney U-values and levels f significance between linear extensin (f the uter grwth increment) and 8~3C and 8 ~80 f the uter 1 mm f skeletn frm bleached and unbleached crals frm Prites Bay. Numbers f bleached (BL) and unbleached (UN) crals in each grup are shwn Characteristic Mann-Whitney Significance value Linear extensin (Nv 1990-July 1991) 813C < 0.01 (BL = 20, UrN = 22) 94.0 < (BL = 20, UN = 22) 0 < ~80 are cmpared in bleached and unbleached crals (Fig. 11). Regressin equatins, cefficients and levels f significance are superimpsed nt each graph. A gd crrelatin is bserved between 8~80 and linear extensin in the unbleached crals (r = ). Hwever, the crrelatin between 8~80 and linear extensin in the bleached crals is pr (r = -0.42). The tw regressin equatins were cmpared using the frmulae f Wetherill (1982) but were nt significantly different (t = 0.134, d.f = 38, level f significance = 0.90). N crrelatin is bserved between linear extensin and 8~3C in the unbleached crals (r = ) althugh there is sme evidence f a psitive relatinship in the bleached cral specimens (r = ). Althugh these regressin equatins appear quite separate, the spread f the data is such that they are nt C Unbleached y = x -~-~, ~ 8 ~<~ Bleached r=-0.10(0.70) y= x ;-~_~ ~ ~ r=-0.54(0.01) Unbleached y = x -3.5 r = (0.001)... <~ Bleached y = -4, x e.-.~... e r = (0.01) ~lso ~ ~..., ~..., ~ Unbleached Bleached Linear extensin (mm) Fig. 11. Regressins between linear extensin f the uter grwth increment (apprximately Nvember 1990-July 1991) and 8 ~3C and 8~80 f the uter 1 mm f skeletn in bleached (pale diamnds and dtted lines) and unbleached (dark circles and slid lines) cral specimens. Standard deviatins f istpic analysis are abut the same size as the symbls. Regressin equatins, Pearsn's crrelatin cefficients (r) and levels f significance (in parentheses) are superimpsed nt each graph

9 51 significantly different (t = 2.39, d.f = 38, level f significance ). Discussin Linear extensin and 6~0 Calcificatin and linear extensin bth shw a strng inverse relatinship with 6180 in the data fr the uter grwth increments (apprximately Nvember 1990 t July 1991). This supprts the mdel prpsed by McCnnaughey (1989b) t explain istpic disequilibria. He suggested that xygen istpic disequilibria ccur due t discriminatin against the heavier istpes during CO 2 hydratin and hydrxylatin in the extracytplasmic calcifying fluid (ECF) immediately befre calcificatin. Fr these disequilibria t be maintained in the carbnate, calcificatin rate must be sufficiently high fr the CO~units in the crystal t be buried befre they can istpically equilibrate with the H20 present in the ECF, which is istpically indistinct frm seawater. During slw calcificatin, substantial equilibratin may ccur and the carbnate istpic cmpsitin may apprach equilibrium values. Our 6~sO data frm the uter grwth increments supprt this, with the slwest extending crals depsiting carbnate clsest t equilibrium values. We bserve a better relatinship between skeletal grwth rate and 6180 with the linear extensin data than with the rates f calcificatin. We wuld expect that a cral skeletn prduced by quick linear extensin with a relatively lw density will be depleted in 180 cmpared with a skeletn extended less quickly but f a higher density, which may have a similar calcificatin rate t the first cral. Differences between the xygen istpic prfiles f crals PB9a, PB9b and PB4 supprt this skeletal grwth rate/6~80 relatinship. The decrease i~ linear extensin (and calcificatin) in the slw grwing crals (PB4 and PB9b) may be respnsible fr the relative enrichment f~80 in the skeletn. Linear extensin rates in PB4 and PB9b fr the flurescent cuplet Nvember Nvember 1989 are 14 and 10 mm respectively. In the apprximate 9-mnth interval frm Nvember 1990, linear extensin rates are 4.5 and 3.5 ram, and it seems likely that linear extensin fr the annual band, Nvember 1990 t Nvember 1991, wuld have been apprximately half f that frm Nvember 1988 t Nvember A decrease in annual linear extensin f apprximately 5-8 mm crrespnds with a relative 180 enrichment f % in the skeletn (calculated frm the regressin equatin determined in Fig. 3a). This is reasnable agreement with the bserved shift f %. Hwever, this grwth rate/6~80 relatinship is rarely apparent between multiple samples frm the same clny, and the reasn fr this is unclear. The range f linear extensin rates bserved in each cral is small (1, 1, 2 and 4 mm), and it is nly ver the widest range (in cral PB6) that the grwth rate/5180 relatinship is bserved. We suggest that in the ther crals this grwth range is t small and the scattering f pints may reflect an inherent variability in the degree f istpic equilibriatin reached in each sample. In additin, bundaries between bright and dull flurescent bands are nt sharp, and it is pssible that nt all samples cvered exactly the same time interval. It is als pssible that very lcal differences in grwth rate ccur acrss the clny thrughut the year and this may accunt fr sme f the bserved scatter. Variatins between the prfiles f crals cannt always be explained simply as a functin f grwth rate. Cral PB7 extends at a similar rate t crals PB8 and PBga but generally depsits carbnate enriched in 180 by abut 0.3%0. Cral AQNEW depsits 5180 values similar t crals PB8 and PB9, althugh AQNEW has a substantially higher rate f linear extensin. These ffsets between clnies may reflect sme genetic differences between the crals which alter the extent f istpic disequilibria. Despite these deviatins the differences between the istpic prfiles f the fast- and slw-grwing axes f cral PB9 cnvince us that calcificatin rate has an imprtant influence n 6J80. Interestingly, McCnnaughey (1989a) nly bserved the grwth rate/8180 relatinship in parts f the crals extending at less than 5 mm year -1, while we have bserved it at all linear extensin rates. His wrk was n a cral f a different genera (Pavna clavus), but we currently have n explanatin fr this discrepancy. Seasnal variatin in 1~180 Seasnal variatins in xygen istpic cmpsitin are typically related t changes in seawater temperature and/ r seawater cmpsitin. The annual seawater temperature range in this area is 2-4 C, with lwest and highest seawater temperatures ccurring in January and May respectively. The Observed range f skeletal 5180 ( %) crrelates well with the range predicted frm seasnal variatins in seawater temperature (2-4 C = %, using McCnnaughey's equatin 1989a). Hwever if the bserved seasnal variatin in 8180 were a functin f seawater temperature then the lwest 818Ocarbnate wuld be depsited in May t June, apprximately halfway between the start f each bright flurescent band, nt in abut Nvember as bserved. Given that we are cnfident in dating the flurescent banding chrnlgy, i.e. bright band accretin begins in apprximately Nvember, we cnclude that sme ther factr is influencing skeletal 6180 and effectively masking any trend attributable t seawater temperature. Rainfall, seawater evapratin, freshwater runff and/ r cean circulatin may all alter significantly the xygen istpic cmpsitin f lcal seawater and the subsequently precipitated carbnate. These changes are usually accmpanied by changes in salinity (Lwenstam and Epstein 1957; Swart and Cleman 1980). It is difficult t assess the influence f salinity variatins in this study as n cnsistent seasnal trends in seawater salinity were fund in the area and few data are available fr the perid cvered by the study. Between September 1990 and January 1991 salinity increased frm 32%0 t 33% and then decreased t 32.4% in April This trend in salinity crrelates well with the bserved skeletal 5180 pattern with lwest 5180 values depsited during the

10 52 perid f lwest salinity. Hwever it is unlikely that this salinity change (1%) is sufficient t be entirely respnsible fr the bserved 8180 skeletal signal ( %0). Annual mean measurements f 8180 f atmspheric waters frm Bangkk and Singapre yield values f-5% and -7% respectively (Yurtsever and Gat 1981). Assuming that rainfall at Phuket is -6%, then t prduce a change in seawater 8~80 cmpsitin f 0.6% (the maximum annual 8180 range bserved in the crals) wuld require a 1 in 10 dilutin f this rainwater which wuld be accmpanied by a salinity change f abut 3%0. In reality this is an underestimate f the change required as seawater cmpsitin must alter sufficiently t cause the annual 5180 skeletal trends and t swamp the %0 annual signal expected frm temperature variatins. In the limited data cllected ver the study perid the salinity range is abut 1%0, and in ther years the annual range des nt exceed 2%0. We cnclude that it is very unlikely that the seasnal variatins in skeletal 8180 are caused slely by changes in seawater cmpsitin, and we suggest that anther pssible influence exists. We have bserved an inverse relatinship between 8180 and skeletal grwth rate in the uter grwth increments f crals frm this area and we suggest that the annual skeletal 5180 trend may at least in part reflect seasnal variatins in skeletal grwth rate. There is evidence that seasnal variatins in skeletal grwth rate ccur in crals arund Phuket. Chansang et al. (1992) bserved an apprximate dubling in the extensin rates f massive Prites crals during the dry mnsn (Nvember t March) when they examined crals repeatedly stained with alizarin red S in situ frm reefs t the nrth west f Phuket island. Charuchinda and Hylleberg (1984) examined mnthly variatins in linear extensin rate in branching Acrpra frm the Aquarium Reef site. Extensin rate was highest and lwest in December t February and June t August respectively. It is difficult t infer seasnal variatins in grwth frm the crals in ur study, as the bleaching event in prgress at the time f their cllectin may have affected skeletal grwth rate even in apparently unbleached specimens. Hwever, the bserved enrichment in skeletal 180 values arund the wet seasn in ur study may reflect a higher degree f equilibratin between the carbnate and the extracytplasmic calcifying fluid due t a decrease in skeletal grwth rate. The bserved skeletal 8180 pattern and the pssible influences n it, i.e. seawater temperature, salinity and skeletal grwth rate, are summarised in Fig. 12. An apprximate dubling f skeletal linear extensin between wet and dry seasns frm 5 t 10 mm (causing an annual band width f 15 mm) wuld crrespnd t a depletin in skeletal 180 f %0 (using the regressin equatin in Fig. 5). This acts in psitive cmbinatin with the skeletal variatins ( %0) expected frm salinity variatins (1-2%) and in negative cmbinatin with skeletal variatins ( %0) expected frm seawater temperature (2-3 C) t yield an expected skeletal 8180 range f 0-0.6%0. This is in reasnable agreement with the bserved range f %. There are n previus reprts f seasnal grwth rate variatins affecting skeletal 8180 althugh such a relatinship may explain why a seawater temperature/skeletal 6180 Nvember Observed skeletal trend Trend expected frm temperature data Trend expected frm salinity data Nvember Fig. 12. A simplified representatin f the bserved annual 6~80 skeletal trend and the expected inputs f seasnal variatins in seawater temperature, salinity and skeletal grwth rate crrelatin has nt been fund in sme palaeceangraphic studies. Of curse, several studies have reprted excellent seawater temperature/skeletal 5~80 relatinships, and we can nly cnclude that seasnal variatins in skeletal grwth rate either d nt ccur at these sites r that they are small r are vershadwed by ther influences. Much mre wrk n the seasnality f grwth rate in crals and its effect n skeletal istpic cmpsitin remains t be dne. Factrs influencing 613C N significant relatinship is bserved between 6~3C and any f the grwth measurements in the uter grwth increment samples prepared frm the unbleached crals (r = -0.34). Similarly, crals with similar rates f linear extensin (PB7, PB8 and PB9a) depsited widely varying fi13c values. Departures frm skeletal 813C values expected frm kinetic behaviur are called metablic effects (McCnnaughey 1989a). We bserved a psitive relatinship between the 8~3C f the uter 1 mm f the cral skeletns and the chlrphyll a cncentratin f the verlying tissue in the unbleached cral samples (r = +0.64). 12C is preferentially fixed in phtsynthesis by the enzyme ribulse biphsphate carbxylase leaving the DIC pl, and subsequently the depsited carbnate, relatively enriched in 13C (reviewed in Swart 1983). Frm ur data it appears that fixatin f 12C and mdificatin f the DIC pl is highest in the crals cntaining high cncentratins f chlrphyll a. This is in i

11 53 agreement with the negative relatinship between skeletal 5~3C and chlrphyll a/chlrphyll c rati (which increases as available light decreases) bserved by Weil et al. (1981). We suggest that metablic effects mdify the DIC pl t such an extent in these crals that the grwth rate/513c relatinship is effectively masked. N relatinship was bserved between skeletal 5~80 and chlrphyll a cncentratin f the verlying cral tissue, althugh respired CO 2, prduced by bth the cral and the zxanthellae, is depleted in 13C and 180 relative t the DIC pl. The enzyme carbnic anhydrase accelerates 180 exchange between this respired CO 2 and H20 in the cral tissue which is in equilibrium with seawater (McCnnaughey 1989b). This xygen exchange will remve any relative enrichment f 160 in the DIC pl. A weak inverse relatinship is bserved between 513C f the utermst 1 mm f skeletn and linear extensin (f the uter grwth increment) in the bleached cral specimens. We suggest that the lss f the symbitic algae and/r their pigments in these bleached crals has remved the metablic influences which typically mdify the DIC pl. In this case the DIC pl is mdified by kinetic effects nly and bth 513C and 5180 are inversely related with linear extensin. The regressin cefficients between linear extensin and bth 5~3C (r =-0.54) and 5180 (r = -0.42) are cnsiderably less than that bserved between 6~80 and linear extensin in the uter grwth increment experiment. Hwever, it was impssible t en-sure that the same grwth increment was sampled in each cral in the uter 1 mm experiment, and we suggest that the sampling f slightly different grwth increments, bth between and within crals, has increased the degree f scatter bserved in the plts. Seasnal variatin in 6J3C Seasnal trends in 513C are similar t thse f 5180 with highest and lwest values depsited in apprximately April t June and Nvember respectively. Althugh n significant relatinship is bserved between skeletal 813C and grwth rate in the uter grwth increment experiment, it is still pssible that annual 513C trends reflect seasnal variatins in grwth rate. It seems likely that the 513C/grwth rate relatinship in the uter grwth increment experiment was masked by varying degrees f mdificatin f the disslved inrganic carbn (DIC) pl by the different crals and their algae, i.e. metablic effects. Hwever, if these metablic effects were relatively cn-stant fr each cral thrughut the year, then seasnal trends in 513C wuld reflect variatins in the kinetic effect, i.e. skeletal grwth rate. There is evidence that skeletal grwth varies seasnally. In additin, the similarity be-tween the 513C and 6~80 prfiles supprts the thery that seasnal trends reflect kinetic effects. Carbn and xygen disequilibria will be similarly affected by kinetic effects while metablic effects will influence 8~3C nly. Hwever, annual trends in 513C may als reflect sme seasnal variatins in metablic effects, i.e. phtsynthetic and respiratry mdificatin f the DIC pl. Higher cncentratins f chlrphyll a have been detected in reef flat crals in the Phuket area in the wet seasn (April t Octber) than in the dry seasn (Barbara Brwn, persnal cmmunicatin, 1993). If similar seasnal variatins in chlrphyll cncentratin ccur in crals alng the reef frnt, then increased algal fixatin f the istpically lighter carbn during perids f high chlrphyll cntent (April t Octber) culd accunt fr the relative enrichment f 13C in the cral skeletn (apprximately June t July). Finally, the DIC pl and subsequently skeletal 513C will be influenced by variatins in seawater 513C cmpsitin. Seawater 513C may be affected by freshwater runff r by algal blms which preferentially fix ~2C during phtsynthesis. At present few data are available n seawater prductivity fr the Phuket area s we are unable t speculate n the pssible influence f such variatins. The effect f the 1991 bleaching event n 61sC and 6l~0 Visibly bleached crals had significantly reduced grwth characteristics cmpared with unbleached crals during the 9-mnth perid up t and including the bleaching event. There were n significant differences between the grwth characteristics f the same crals fr the flurescent cuplet depsited befre the bleaching event, i.e. apprximately Nvember 1989 t Nvember 1990 (Tudhpe et al. 1992) s it seems that the reductin in calcificatin in the bleached crals is an effect f the bleaching event and the cnditins leading up t it and is nt a characteristic f crals that are mre susceptible t bleaching. Bleached crals were significantly enriched in 180 and 13C in the uter 1 mm f skeletn cmpared with unbleached specimens. Hwever, as grwth rate appears t inversely affect skeletal 5180, an enrichment in 180 is expected in these slwer grwing crals. In an attempt t separate the effects f skeletal grwth rate and bleaching, we cmpared regressins between linear extensin and 5180 in samples frm the utermst 1 mm f the skeletns frm bleached and unbleached crals. Althugh nt significantly different frm that fr the unbleached crals, the regressin equatin fr the bleached crals is ffset twards mre psitive 5180 values, suggesting that the bleached crals are depsiting carbnate even mre enriched in 180 in the uter 1 mm f skeletn than expected frm cnsideratins f skeletal grwth rate. This may reflect a mre prnunced decrease in the grwth rate f these crals ver the actual bleaching event. Hwever this enrichment in 180 is nt bserved in the istpic prfiles f the bleached crals (PB7 and PB8) up t and including the bleaching event. We suggest that calcificatin in the affected crals ver the bleaching event was very limited and insufficient 180 enriched material was depsited t impact n the prfile samples. In fact all the crals, bleached and unbleached, shw a reduced 180 enrichment fr that time f year. By the time f the bleaching event, seawater temperatures were abut 1 C abve nrmal, which wuld crrespnd t a depletin in 5180 f 0.21% relative t the values nrmally bserved (McCnnaughey 1989a). This depletin in 5180 disguised the usual seasnal trends.

12 54 The bleached cral skeletns depsited significantly higher 613C values in the uter 1 mm f skeletn than apparently unaffected crals. Althugh the remval f the zxanthellae, which preferenti~tlly fix 12C, will increase the prprtin f 12C in the DIC pl, it appears that calcificatin, at this pint, was reduced t such an extent that newly depsited carbnate was able t cme clser t istpic equilibrium with the carbn in the extracytplasmic fluid. This is supprted by the bservatin f a inverse relatinship between ~5~3C f the uter 1 mm f the bleached cral skeletns and linear extensin (f the uter grwth increment). This relatinship is nt bserved in unbleached crals. In summary, in the uter 1 mm f the cral skeletn, algal mdificatin f the DIC pl is the main cntrl n skeletal 613C in the unbleached crals while skeletal grwth rate is the primary influence in the bleached specimens. There is n evidence that the bleaching event affected the seasnal 513C trend in cral PB9 (unbleached). This cral prduced a typical peak f 13C, between the beginning f bright flurescent bands, in apprximately April t June. This trend was nt bserved in crals PB8 (bleached) and AQNEW (unbleached). Bth prduced a relatively small 613C peak early n in the flurescent cuplet. Cral AQNEW als prduced a secnd peak later in the cuplet. These variatins prbably reflect the state f the zxanthellae in the cral tissue ver the bleaching event. Bth crals began t prduce the usual ~3C peak in apprximately February t April but then depsited lighter carbn than expected. If zxanthellae numbers r chlrphyll cntent are reduced then the amunt f 12C fixed by phtsynthesis will als decrease. The DIC pl becmes relatively enriched in 12C and the skeletn depsited frm it cntains relatively lw fi 13C values. Althugh cral AQNEW did nt appear visibly affected by the bleaching event, zxanthellae numbers r chlrphyll cntent may have been reduced frm earlier in the year. A recvery in zxanthellae numbers r chlrphyll cntent twards the end f the bleaching event wuld accunt fr the secnd ~13C peak bserved twards the end f the flurescent cuplet. Cnclusins 1. Skeletal grwth rate has a pwerful influence n skeletal 6180 in Prites lutea crals frm Phuket, Suth Thailand. We suggest that the bserved 6~sO annual trend reflects a cmbinatin f seawater temperature, salinity and skeletal grwth rate variatins. Seasnal and lnger term variatins in skeletal grwth rate must be cnsidered when interpreting palaeenvirnmental recrds frm stable xygen istpic chemistry f cral skeletns. 2. A similar relatinship is bserved between 6J3C and grwth rate in the utermst skeletal material f bleached cral specimens. Hwever, in the unbleached crals metablic mdificatins f the disslved inrganic carbnate pl (DIC) mask any such relatinship. In these crals 513C and cral tissue chlrphyll a cntent are psitively related, suggesting that algal mdificatin f the DIC pl is the main cntrl n skeletal 813C. Seasnal variatins in 813C may reflect variatins skeletal grwth rate, changes in the phtsynthetic activity f the zxanthellae r variatins in the 613C cmpsitin f seawater. 3. Crals that bleached during the perid f elevated seawater temperatures at Phuket in the summer f 1991 had significantly reduced skeletal grwth rates and depsited skeletns significantly enriched in 13C and 180 cmpared with crals that appeared unaffected. This enrichment cincides with a reductin in skeletal grwth rate. 4. The 13C and 180 enrichment bserved in the surface material depsited during the bleaching event was nt bserved when istpic prfiles were prepared frm either the bleached r the unbleached crals at a frequency f 8-10 samples per annual grwth band. Calcificatin at this time was prbably s limited that insufficient material was depsited t impact n the prfile samples. An enrichment in these istpes may have been bserved if a higher sampling frequency had been emplyed. Instead skeletal material depsited in the mnths befre and during the bleaching event was relatively depleted in ~80, supprting the thery that psitive temperature anmalies ccurred during the beaching event and may have been its cause. Whle r partial expulsin f zxanthellae (r lss f their pigments) was manifest as a relative depletin in 13C, reflecting the impact f the algae n skeletal 613C. Acknwledgements. We wish t thank Terry Dnnelly fr technical assistance with the mass spectrmeter at SURRC. We are grateful t the Directr and staff f the Phuket Marine Bilgical Centre fr their cntinued supprt. In particular we thank Vudichai Janekarn fr his genersity in supplying the seawater temperature and salinity data cllected frm the centre. Other seawater temperature data were btained as part f a prject funded by the Overseas Develpment Administratin, Natural Resurces and Envirnmental Department and were cmpiled by Richard Dunne. Rainfall data were supplied by the Phuket Twn weather statin. This manuscript has been imprved by the cmments f several annymus reviewers: This wrk has been supprted financially by the UK Natural Envirnment Research Cuncil (research studentship n. GT4/90/ G3/47 t NA; and grant n. GR3/5857 t AWT). The Istpe Gesciences Unit at SURRC is supprted by NERC and the Scttish Universities. References Bt K, Isdale P (1985) Flurescent bands in massive crals result frm terrestrial fulvic acid inputs t nearshre zne. Nature 315: Brwn BE, Le Tissier MDA, Hward LS, Charuchinda M, Jacksn JA (1986) Asynchrnus depsitin f dense skeletal bands in Prites lutea (Edwards and Haime). Mar Bil 93:83 89 Carr DJ (1992) Quaternary sediments f a trpical estuary and bay: Phangnga Bay, Suth Thailand. Unpublished PhD thesis, University f Edinburgh Chakrabrty S, Ramesh R (1993) Mnsn-induced sea surface temperature changes recrded in Indian crals. Terra Nva 5: Chansang H, Phngsuwan N, Bnyanate P (1992) Grwth f crals under effect f sedimentatin alng the nrthwest cast f Phuket Island, Thailand. Prc 7th Int Cral Reef Symp 1: Charuchinda M, Hylleberg J (1984) Skeletal extensin f Acrpra frmsa at a fringing reef in the Andaman Sea. Cral Reefs 3:

13 55 Emiliani C, Hudsn JH, Shinn EA, Gerge RY (1978) Oxygen and carbn istpic grwth recrd in a reef cral frm the Flrida Keys and a deep sea cral frm Blake Plateau. Science 202: Gaffey S J, Brnnimann CE (1993) Effects f bleaching n rganic and mineral phases in bigenic carbnates. J Sediment Petrl 63: Greau TJ (1977) Cral skeletal chemistry: physilgical and envirnmental regulatin f stable istpes and trace metals in Mntastrea annularis. Prc 1~ Sc Lnd B 196: Isdale P (1984) Flurescent bands in massive crals recrd centuries f castal rainfall. Nature 310: Jkiel PL, Cles SL (1974) Effects f heated effluent n hermatypic crals f Kahe Pint, Oahu. Pac Sci 28:1-18 Land LS, Lang JC, Barnes DJ (1975) Extensin rate: a primary cntrl n the istpic cmpsitin f West Indian (Jamican) scleractinian reef cral skeletns. Mar Bil 33: Lwenstam HA, Epstein S (1957) On the rigin f sedimentary aragnitic needles f the Great Bahama Bank. J Ge165: McCnnaughey T (1989a) ~3C and ~80 istpic equilibrium in bilgical carbnates. I. Patterns. Gechim Csmchim Acta 53: McCnnaughey T (1989b) '3C and *so istpic equilibrium in bilgical carbnates. II. In vitr simulatin f kinetic istpe effects. Gechim Csmchim Acta 53: O'Neil JR, Claytn RN, Mayeda TK (1969) Oxygen istpe fractinatin in divalent metal carbnates. J Chem Phys 51: Scffin TP, Tudhpe AW, Brwn BE, Chansang H, Cheeney RF (1992) Patterns and pssible envirnmental cntrls f skeletgenesis f Prites lutea, Suth Thailand. Cral Reefs 11: 1-13 Strickland JDH, Parsns TR (1968) A manual f seawater analysis. Bull Fish Res Bd Can Swart PK (1983) Carbn and xygen istpe fractinatin in sclerac-tinian crals: a review. Earth Sci Rev 19:51-80 Swart PK, Cleman ML (1980) Istpic data fr scleractinian crals explain their palaetemperature uncertainties. Nature 283: Tudhpe AW, Allisn N, Le Tissier MDA, Scffin TP (1992) Grwth characteristics and susceptibility t bleaching in massive Prites crals, Suth Thailand. Prc 7th Int Cral Reef Symp 1: Vern JEN (1986) Crals f Australia and the Ind-Pacific. University f Hawaii Press, Hnlulu, p 224 Weil SM, Buddemeir RW, Smith SV, Krpnick PM (1981) The stable istpic cmpsitin f cral skeltns: cntrl by envirnmental variables. Gechim Csmchim Acta 45: Wetherill GB (1982) Elementary statistical methds. Chapman and Hall, Lndn Yurtsever Y, Gat JR (1981) Atmspheric waters. In: Gat JR, Gnfiantini R (eds) Stable istpe hydrlgy: deuterium and O-18 in the water cycle. (IAEAA technical reprt series n. 210) pp Appendix 1 Grwth rate measurements and the stable istpic cmpsitin f samples frm the uter grwth increment (apprximately Nvember 1990 t July 1991) f six Prites lutea clnies frm Prites Bay Cral Linear Skeletal Calcificatin 8t3C fi180 sample extensin density (g cm -2) (wrt PDB) (mm) (g cm 3) PBI PB PB PB PB PB PB PB PB PB !67 PB PB PB ' PB PB PB PB PB PB PB PB PB PB PB PB PB PB

14 56 Appendix 2 8~3C and 8~80 f prfile samples frm crals PB4, PB7, PB8, PB9a, PB9b and AQNEW. The relative psitin f each sample in the annual band was calculated by dividing the distance f the middle f each sample alng the band by the width f the band. Samples prefixed by 10 were depsited in the cuplet apprximately Nv 1990 t Nv 1991; by 9 in the cuplet Nv 1989 t Nv 1990 etc. Sample 8 ~3C 8~80 Relative 8 t3c 8 ~80 Relative 8 ~3C 8 ~80 Relative psitin psitin psitin PB4 PB9a PB9b ; , Sample Lst Sample Lst Sample Lst Sample Lst Sample Lst PB7 PB8 AQNEW

15 57 Appendix 3 Linear extensin, cral tissue chlrphyll a and stable istpic measurements f bleached and unbleached crals frm Prites Bay. Linear extensin measurements were perfrmed n the uter grwth increments (apprximately Nvember 1990-July 1991), stable istpic analysis n the utermst 1 mm f the skeletn and chlrphyll a measurements n the verlying cral tissues Cral Linear Chl a 8t3C 6tsO Cral Linear Chl a 8~aC 6~80 sample Grwth mgcm -2 (wrt PDB) sample Grwth mgcm -2 (wrt PDB) nlrn Unbleached crals Bleached crals PBI PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB PB ylin