coral bleaching in the southern seychelles during the 1997–1998 indian ocean warm event

Coral Bleaching in the SouthernSeychelles During the 1997±1998 IndianOcean Warm EventTHOMAS SPENCER *, KRISTIAN A. TELEKI , CLARE BRADSHAWà and MARK D. SPALDING ,§ Cambridge Coastal Research Unit, Department of Geography, University of Cambridge, Cambridge CB2 3EN, UKàPort Erin Marine Laboratory, University of Liverpool, Port Erin, Isle of Man IM9 6JA, UK§World Conservation Monitoring Centre, 217 Huntingdon Road, Cambridge CB3 0DL, UK

Coral bleaching shows complex spatial-temporal dynam-ics at several scales. Recent developments in ocean surfaceremote sensing technologies allow for both a better ap-preciation of these dynamics and an opportunity to placemore local studies of coral bleaching and bleaching-re-lated coral mortality into wider oceanographic and bi-ogeographic contexts. Coral bleaching is described at fourcoral reefs in the southern Seychelles (sea area 6±10°S45±54°E) during March±May 1998. Bleaching intensityvaried between locations, between environments at thewithin-reef scale, and between coral growth forms. Thesedata are compared with bleaching reports and sea surfacetemperature statistics for eight further stations in thewestern Indian Ocean to establish a link betweenbleaching and the unprecedented warming of the IndianOcean during 1997/98. Implications for long-term reefhistory, and coral reef futures, in the western IndianOcean are discussed. Ó 2000 Elsevier Science Ltd. Allrights reserved.

Keywords: ENSO; El Ni~no; sea surface temperature;ocean warming; global environmental change; IndianOcean.

Introduction

Space±time dynamics of coral bleachingThe last 10 years have been characterized by a

growing interest in the assessment of coral reef conditionand the threats, both natural and anthropogenic, that

reefs face at present and in the near future (e.g. Gins-burg, 1993; Wilkinson, 1996; Bryant et al., 1998). Oneprocess of particular concern has been the phenomenonof coral bleaching and bleaching-induced coral mortal-ity (e.g. Wilkinson, 1998). The term Ôcoral bleachingÕ isused to describe the whitening of corals which resultsfrom the degeneration and/or loss of symbiotic algae,the zooxanthellae, and/or the loss of cells containingzooxanthellae from coral tissues (Brown and Ogden,1993; Brown, 1997). When corals bleach they typicallylose 60±90% of their zooxanthellae and remainingzooxanthella may lose 50±80% of photosynthetic pig-ments (Glynn, 1996). Controlled laboratory experi-mentation (e.g. Hoegh-Guldberg and Smith, 1989;Glynn and D'Croz, 1990) and ®eld programmes linkingin situ measurements of environmental variables withbleaching episodes (e.g. Thailand: Brown et al., 1996;Brown, 1997; Papua New Guinea: Davies et al., 1997;French Polynesia: Gleason, 1993; Hoegh-Guldberg andSalvat, 1995; E Paci®c: Glynn, 1984, 1988) now stronglysuggest that the combined e�ect of elevated tempera-tures and high solar irradiance at UV wavelengths drivebleaching processes by overcoming coral photo-protec-tive mechanisms (Glynn, 1996; Schick et al., 1996;Brown, 1997; Jones et al., 1998).

One problem with the assessment of coral bleaching isthat there is no standardized method to quantify thelevel of bleaching that has occurred (Glynn, 1993). It ispossible, therefore, that a heightened awareness ofbleaching has led to inexperienced observers overesti-mating the scale and severity of local bleaching impacts.Nevertheless, it is di�cult to ascribe the reporting of amarked upturn of extensive bleaching from all majorreef provinces in the 1980s to this process (Glynn, 1993).As a result, bleaching statistics have been used both asan early signal of global environmental change in thetropical oceans (e.g. Glynn, 1991) and as an indicator of

Marine Pollution Bulletin Vol. 40, No. 7, pp. 569±586, 2000

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E-mail address: [email protected] (T. Spencer).

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a range of non-climatic stresses, often human-related, intropical shallow marine environments (e.g. Bryant et al.,1998). These arguments have arisen in part because thephenomenon under study shows intra- and inter-speci®cdi�erences within coral communities, spatial variabilityat within-reef, between-reef and biogeographical reefprovince scales and complex temporal patterns (Wil-liams and Bunkley-Williams, 1990; Glynn, 1996;Brown, 1997; Huppert and Stone, 1998). Part of theexplanation of bleaching susceptibility is probably dueto interaction with ¯uctuating zooxanthellae popula-tions (Rowan et al., 1997; Fagoonee et al., 1999). Inaddition, however, this biological variability must itselfinteract with variations in the key physical controls ofsea surface temperature (SST) and solar irradiance. Asseen above, a large number of reports have con®rmedthe importance of these controls at reef site to wholereef scales. However, these processes are themselvesnested within larger scale patterns of ocean conditions,determined by large-scale climatological and oceano-graphic dynamics.

A much better knowledge of the space±time dy-namics of these large-scale physical controls has be-come possible in the last two decades with there®nement of ocean surface remote sensing technologiesand the deployment of in situ monitoring arrays, mostnotably the equatorial Paci®c TOGA±TAO network(NOAA, 1998a). In particular, large-scale, repeat sat-ellite imagery of the worldÕs oceans has allowed theidenti®cation and tracking of SST `hotspotsÕ. Theseanomalies are then related to thermal tolerancethresholds to identify potential loci for coral bleachingepisodes (Goreau et al., 1997; Strong et al., 1997;Strong, 1998; NOAA, 1997, 1998b). This methodologysuggests that each large scale bleaching event between1983 and 1991 has been preceded by a +1°C SSTanomaly in the warmest month (Goreau and Hayes,1994). Furthermore, such analyses have con®rmed theimportance of large-scale, El Ni~no Southern Oscillation(ENSO)-related heating, cooling and migrations ofocean water masses as a prime determinant of massbleaching episodes, as hypothesized initially by Wil-liams and Bunkley-Williams (1990; and see also Stoneet al., 1999), and suggested by the links between coralbleaching episodes and shallow water conditions in the1980s in the eastern Paci®c Ocean (e.g. Glynn, 1988),Caribbean Sea (Williams, 1987; Goreau and Macfar-lane, 1990) and western Paci®c Ocean (Brown andSuharsono, 1990).

ENSO variability and the 1997/98 El Ni~noENSO events have been traditionally de®ned by ref-

erence to a series of linked atmosphere-ocean processesin the equatorial regions of the Paci®c Ocean (Fig. 1(a)and (b), Webster and Palmer, 1997). Records of atmo-spheric pressure changes and eastern Paci®c SST vari-ations have shown individual ÔwarmÕ phase oscillations,or El Ni~no events, to vary considerably in the timing of

their inception, course of development and decay, andpeak ÔstrengthÕ (Philander, 1990). The multivariateENSO index (MEI; Climate Diagnostics Center, 1998)recognizes 13 El Ni~no warmings in the period 1950±1998 (Fig. 2). The 1982/83 and 1997/98 El Ni~no eventswere probably the strongest such events since 1877/78(Kiladis and Diaz, 1986). On the basis of the MEI indexthe stronger of the two events was that of 1982/83(Wolter and Timlin, 1998), although the 1997/98 eventwas of very similar strength, exhibited the most rapidinitial warming on record (McPhaden, 1999a), andshowed an extended, double peak, with maximum de-velopment in both July/August 1997 and February/March 1998. Indeed, other indices have shown the 1997/98 event as the strongest this century (Slingo, 1998;McPhaden, 1999b).

Fig. 1 (a) Schematic view of Walker Cell Circulation along theequator (after Bigg, 1996); (b) Ocean ± atmosphere character-istics of the El Ni~no ± Southern Oscillation phenomenon. (i)Cool, or `La Ni~na', phase (ii) Warm, or `El Ni~no', phase (afterNOAA, 1998c); (c) ENSO-related precipitation patterns (afterRopelewski and Halpert, 1987). Timing of anomalies refers tothe classic de®nition of El Ni~no events as encompassing a twoyear period, commencing in the July prior to the year in whichthe maximum warming in the Paci®c Ocean occurs (i.e. fromJuly (year )1) to July (year +1); Rasmussen and Carpenter,1982).

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ENSO events in the Indian OceanThe periodic reorganizations of ocean water masses

which are a key part of the ENSO phenomenon lead tochanging patterns of enhanced rainfall and droughtwhich extend well beyond the boundaries of the Paci®cOcean (Fig. 1(c); e.g. Ropelewski and Halpert, 1987).Due to their signi®cance for rain-fed agriculture insouth-east Asia, the search for linkages between Paci®cand Indian Ocean climatology and oceanography andthe Asian monsoon has a long history (Allan et al.,1996). The northern summer south-west monsoon typ-ically weakens during an El Ni~no episode as the mainascending limb of the Walker Circulation (Fig. 1(a)),and its associated high convective rainfall totals, ismoved eastwards into the central Paci®c Ocean. Somerecent studies of the inter-annual variability of IndianOcean SSTs have suggested a periodic ENSO signal,with the dominant mode of variability in phase with thepeak El Ni~no warm phase o� the west coast of SouthAmerica or lagged some six to nine months behind it(Tourre and White, 1995, 1997; Nicholson, 1997).However, other research (e.g. Webster et al., 1999; Sajiet al., 1999) has argued that the oscillations in sea sur-face temperatures, precipitation and winds between theeastern and the western Indian Ocean ± a tropical dipolemode ± are the result of internal ocean ± atmospheredynamics and not a direct response to external ENSOforcing. Whatever the explanation, as in the Paci®cOcean, the Indian Ocean temperature signal begins topropagate from a western ocean `warm pool', variouslyidenti®ed along the coast of East Africa (Tourre andWhite, 1995, 1997; Slingo, 1998) and between the Hornof Africa and Southern India (Nicholson, 1997), twoyears before the El Ni~no peak on the Paci®c coast ofSouth America. However, the Indian Ocean SSTanomaly develops and propagates eastwards along theequator more slowly (15±25 cm sÿ1) than is the case inthe Paci®c Ocean (Meehl, 1987; Kiladis and van Loon,1988). High SSTs are centred at 10°S, near NE Mada-gascar, after one year and then, at the peak of the Paci®cwarm phase, in the central Indian Ocean (Tourre and

White, 1995, 1997; Nicholson, 1997). As the warm phasedissipates, the temperature anomaly in the Indian Oceancontinues to migrate towards the Timor Sea, ®nallymerging with the now re-established western Paci®cwarm pool two years after the eastern Paci®c peakanomaly.

The 1997/98 El Ni~no and the southern SeychellesRecent reports of widespread and catastrophic coral

bleaching in the Indian Ocean have intimated a linkageto the 1997/98 El Ni~no event (e.g. Hsieh and Ormond,1998; Wilkinson, 1998, Linden and Sporrong, 1999;Wilkinson et al., 1999). This paper examines theseclaims further, evaluating coral bleaching impacts onfour reefs in the southern Seychelles within the contextof wider climatological and oceanographic changes inthe Indian Ocean in 1997/98. Speci®cally, it (a) providesthe ®rst modern descriptions of reef morphology andcommunity structure at three locations in the remotesouthern Seychelles; (b) quantitatively assesses bleach-ing impacts on these reefs, and at Aldabra Atoll, duringthe 1997/98 Indian Ocean warm event; (c) describes andexplains the pattern of SST temperature change at se-lected stations in the western and central Indian Oceanduring this event; and (d) thus supplies the linkagesbetween wider scale environmental processes and site-speci®c coral bleaching and bleaching-related coralmortality in this region in more detail than has been thecase hitherto.

The Southern Seychelles, Western Indian Ocean

The coral reefs of the Seychelles can be classi®ed intothree groups: fringing reefs (typical of the high graniticislands of the Seychelles Bank); platform reefs; andatolls. Within each of these types, raised reefs are alsofound (Stoddart, 1984). In this study we concentrateupon the sea-level atoll of Alphonse Atoll, the reefssurrrounding the raised platform reef of St Pierre, thecarbonate bank of Providence±Cerf, and the reefs sur-rounding the raised atoll of Aldabra Atoll (Fig. 3).

Alphonse Atoll (9°00S, 52°450E), at the southern endof the Amirantes ridge, is a small (6� 4 km) atoll lo-cated 415 km south of Mah�e, the largest island in thegranitic Seychelles. The peripheral reefs, 0.7±1.9 km inwidth, enclose a simple dish-like lagoon of 5.4 km2,reaching depths of ca 10 m at its centre (Fig. 3). St Pierre(9°190S, 50°430E) is a small (1.7 km2) raised reef island,250 km south of Alphonse Atoll (Fig. 3). It consists oflate Quaternary reef limestones, ca 5 m above presentsea level, previously extensively quarried for islandphosphates (Gardiner, 1926±1936; Stoddart, 1967). The300 km2 Providence±Cerf Bank lies 40 km to the east ofSt. Pierre. It is a north±south orientated, 40 km long, 1±10 km wide bank whose surface is characterized by ex-tensive seagrass beds, subtidal sand channels and, inplaces, tidal ¯ats and sandbanks which dry at low tide.The land area of the bank covers 2.3 km2 (Stoddart,

Fig. 2 Multivariate ENSO Index for the six strongest El Ni~no eventssince 1957 and the 1997/98 event (after Climate DiagnosticsCenter, 1998).

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1984): the island of Providence (9°150S, 51°E) occupiesthe northern extremity of the bank and a series ofsmaller sand cays form the Cerf islands at its southernmargin (Fig. 3). Aldabra Atoll (9°240S, 46°200E) is alarge (34 km long, up to 14.5 km wide) raised atoll whichlies 525 km south-west of Providence±Cerf and 420 kmnorth-west of Madagascar (Fig. 3). Late Quaternaryraised reef limestones, averaging 2 km in width and upto 8 m above sea level, rim a shallow central lagoon. Thelagoon is linked to the ocean by two major and onesmaller channel and by several smaller reef passages(Stoddart, 1984).

Methods

Experimental designQualitative observations of reef morphology, coral

community composition and reef health in the southernSeychelles were made between 27 March and 6 May1998 over the course of 275 dives at the four locations.These observations were supplemented by quantitativedescriptions of coral communities using (a) the LinePoint Intercept (LPI) method (English et al., 1997;shown to be the most e�cient technique for yieldingspecies abundance estimates similar to those of the true

Fig. 3 Coral reefs of the southern Seychelles. (a) General locationmap (after Stoddart, 1970); (b) Alphonse Atoll (from map se-ries D.O.S. 204P, 1979); (c) St Pierre and Providence±Cerf(D.O.S. 304P, 1979); and (d) Aldabra Atoll (Y852 D.O.S. 304P,1978). Solid circles indicate coral bleaching assessment sites.

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reef (Ohlhorst et al., 1988)) and (b) videographic imag-ery, subsequently analysed using the AIMS 5-dotmethod (Osborne and Oxley, 1997).

Seven locations (21 sampling transects) were surveyedat Alphonse Atoll, including three locations within thelagoon; 11 locations (65 transects) at St. Pierre; fourlocations (21 transects) along the Providence±CerfBank; and seven locations (30 transects) at AldabraAtoll (Fig. 3, Table 1). Repeat surveys were possible onthe outer reef slope at Alphonse Atoll, locations ob-served on 29 March being visited again ®ve weeks lateron 4 May 1998.

At each location at least one transect was establishedparallel to the shoreline; this was set at a depth of 15 m.This depth identi®es the seaward margin of the reefslope at Alphonse Atoll and St Pierre, falls within thezone of most abundant coral growth at Aldabra Atoll(6±28 m; Barnes et al., 1971) and marks the limit ofactive coral growth on the outer slopes of the fringingreefs of Mah�e, northern Seychelles (Lewis, 1968). Whereand whenever possible, however, additional shore-par-allel transects were set out at water depths of 5, 10, 20 mand occasionally 25 m. LPI transects of 25 m lengthwere surveyed to yield 250 point measurements per siteand over 25 000 observations in total. Video transectsvaried in length from 25 to 50 m, averaging between 25and 30 m at the di�erent locations. This methodologyproduced an additional 20 000 observations. Substrateswere classi®ed into six categories: coral rubble; macro-algae; turf algae; recently dead coral (white skeletonwith evidence of ®lamentous algal overgrowth);bleached coral; and live coral. Within each of the coralcategories, growth form (massive; branching; tabular;encrusting; and foliose) was recorded. The occurrence ofMillepora, Heliopora and Halimeda spp on transect lineswas also recorded. A comparative analysis was madebetween data extracted from videotape and that re-corded directly by the LPI method for six transectlengths at St Pierre. Correspondence was good, withmean estimates within �3.5% for substrate coverage ofunbleached coral, �2% for bleached coral and � < 2%for dead coral.

Coral bleaching reportsInternet reports for 1997±1998 archived at the Coral

Health and Monitoring worldwide website (NOAA,1997, 1998b) were accessed to determine IndianOcean-wide patterns of coral bleaching; these reportswere augmented by further personal communications(see acknowledgements). Additional country-by-coun-try reports have been collated by Wilkinson (1998),Wilkinson et al. (1999) and Linden and Sporrong(1999).

Sea surface temperatures and temperature anomaliesThe evolution of SST anomalies in the Indian Ocean

at weekly intervals between end December 1997 andJune 1998 was analysed using the NOAA/NESDIS SSTanomaly coral bleaching hotspot charts (NOAA,1998b). Night-time SSTs are routinely obtained fromAdvanced Very High Resolution Radiometer (AVHRR)measurements from polar-orbiting NOAA satellites at36 km resolution, interpolated to 50 km (Strong et al.,1997). Such data have been used to generate a compositeglobal chart of monthly mean maximum SST at all gridpoint locations (Strong et al., 1997). The anomalies be-tween this chart and the night-time SST ®eld (on weeklyto monthly timescales) are then used to produce thecoral bleaching hotspot charts mentioned above(NOAA, 1998b).

SST data were extracted from the GISST2.3b (Rayneret al., 1996; for monthly means for the period 1961±1998) and the MOHSST6D (for monthly mean tem-perature anomalies (1961±1990 baseline)) databases heldby the Hadley Centre for Climate Prediction and Re-search, UK Meteorological O�ce. Statistics were cal-culated from amalgamated data collected on a 1°� 1°grid for the area enclosing the islands of interest in thesouthern Seychelles (6±10°S 45±54°E) and for areassurrounding selected individual sites in the western In-dian Ocean (see Fig. 11 for details of areas sampled).The base year for analyses was taken as 1961; SST dataare available for earlier periods but the record containsmissing values.

TABLE 1

Experimental design for coral reef status surveys in the southern Seychelles.

Locationa Survey dates(1998)

No. of locationsa No. of sampling transects (LPI/Video)b

Water depth (m) Total

5 10 15 20

Alphonse Atoll 29/3±1/4; 4/5 7 2 (2/0) 5 (3/2) 11 (3/8) 3 (2/1) 21St Pierre 3/4±7/4 11 65 (59/6) 65Providence±Cerf 13/4±14/4; 1/5 4 9 (4/5) 4 (0/4) 8 (5/3) 21Aldabra Atoll 22/4±28/4 7 9 (9/0) 13 (13/0) 8 (8/0) 30

Total 29 11 18 97 11 137

a See Fig. 3.b LPI: line point intersect sampling method; Video: underwater videography. See text for details of methods.

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Results

Site-speci®c substrate characteristics in the southernSeychelles

Alphonse Atoll. The fore-reef is characterized by arocky pavement, with low relief spur-and groove to-pography immediately seaward of the reef ¯at margin; a50±150 m wide fore-reef slope between 5 and 15 m waterdepth; and a steep slope or drop-o� below 15±20 m, withexposed rock surfaces and coral rubble accumulations.Coral cover rises from less than 30% in shallow water toover 40% at intermediate depths before declining againat 20 m water depth. The reef community is dominatedby Pocillopora spp, Acropora spp and Stylophora sppwith widely spaced large colonies of Pavona spp andPorites spp In some areas of the outer reef, monospeci®cstands of the blue octocoral Heliopora coerulea arepresent.

St Pierre. Coral communities on the western, leewardside of the island are found in water depths of less than20 m within 100±200 m of the shoreline; o�shore of thispoint, substrate angle increases rapidly, forming a steepslope to 90 m in places. Leeward reefs are characterizedby a typical coral cover of ca. 60% in which Pocilloporaspp, Acropora spp and Millepora spp are dominant. Bycontrast, the south-eastern, windward coast is charac-terized by a low angle slope which only steepens at 20±30 m water depth at 1±2 km o�shore. Coral cover islower (ca 50%) and dominated by Millepora spp (par-ticularlyMillepora tenella) with low percentage coverageof Acropora spp and Pocillopora spp Substrate coverageby coral rubble (23%), bare rock (19%) and Halimedasands (12%) is correspondingly greater than on leewardcoasts.

Providence±Cerf. At the northern and southern limitsto the Providence±Cerf bank, cli�ed and bio-erodedbedrock surfaces, with minimal living coral cover, arecharacteristic in water depths of 5±25 m. At interveningsites along the western side of the bank more gentlyshelving subtidal slopes are typical. Sand, bedrock andrubble substrates account for over 60% of the bottomtype on the bank and bank margins. In addition, 20% ofthese environments are characterized by dense beds ofThalassodendron ciliatum (Selin et al., 1992) withinwhich small colonies of Stylophora pistillata are com-mon. Elsewhere, coral communities are localized andaccount, on average, for only 7% of substrate cover. Onboth the bank margin and within the deeper channels onthe bank itself species of the coral genera Pocillopora,Acropora, Porites and Favia are present. Soft coralsaccount for some 2% of substrate coverage.

Aldabra Atoll. Barnes et al. (1971) proposed that thereef front areas at Aldabra Atoll can be classi®ed intofour reef morphological categories based on exposure towave and storm action. This study focussed on the least-

exposed western coast and the moderately exposednorthern coast. Typical coral cover between 10 and 25 mon these coasts is 35%. The western reefs are charac-terized by a 460 m ± wide reef ¯at, a reef ridge marginand reef front slopes of 20±45°. The northern reefssupport a narrower reef ¯at and reef front slopes of 30±45° whose margin at ca 25 m is characterized by massive,often vertically sided Ôreef bastionsÕ separated by sand-and rubble-®lled channels. On the basis of a photo-transect (Drew, 1977), western coasts are typi®ed bybranching and columnar corals in 0±6 m water depth,followed by a dominance of soft corals (6±14 m), mas-sive corals, particularly faviids, and Halimeda (14±28 m)and encrusting corals and gorgonians (28±42 m). On themore exposed northern coast, these zones are translateddownwards, with branching and columnar coralsreaching 20 m and massive corals over 30 m water depth(Barnes et al., 1971).

Patterns in bleaching and bleaching-related coralmortality in the southern Seychelles in 1998

Sampling for patterns of bleaching incidence and re-lated coral mortality is made di�cult by the operation ofthe spatially variable bleaching process across substratesof variable coral/non-coral cover, community composi-tion and structure, and topography (Oliver, 1985).Furthermore, the di�culty of undertaking sustained andsimultaneous observations at remote sites means thatany large scale spatial survey must, to some extent, re-sult in sampling di�erent stages in the cycle of impact-from initial bleaching to coral recovery and/or algalovergrowths on dead coral skeletons- at di�erent sites.These di�culties, both spatial and temporal, results inhigh levels of variability in coral bleaching datasets.Nevertheless, it is possible to see some general rela-tionships in the 1998 bleaching statistics from thesouthern Seychelles.

Location and aspect. Bleaching impacts were severe atthree out of the four sites investigated (Fig. 4), partic-ularly at St. Pierre and the Providence±Cerf bank wherebleached and recently dead corals accounted for over80% of the corals sampled. Of the 39% coral cover onthe outer reef slopes at Alphonse Atoll, 74% was re-corded as being bleached or recently dead. However,impacts were noticeably less at Aldabra Atoll, with only41% of corals sampled being so classi®ed (Table 2). Thereasons for these di�erences can be explained by refer-ence to the dynamics of SST changes in 1997±1998which are discussed in more detail.

Bleaching was not exclusive to hermatypic corals.Incidences of bleaching were widespread in alcyona-ceans, non-scleractinian coelenterates (Stichodactyla sppand Heteractis spp) and bivalves (Tridacna spp). Somealcyonaceans were completely bleached (Lobophytumspp and Sinularia spp) and at all sites mortality andsubsequent necrosis and disintegration of their growthform was in an advanced state. At Providence±Cerf, for

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example, 76% of soft corals surveyed showed tissuenecrosis and soft coral communities that had alreadydied were marked by the large bare patches which re-mained following their death and disintegration.

At a within-island scale, there is some evidence tosuggest that levels of bleaching were mediated by aspect.From the large dataset assembled for St Pierre at 15 m

water depth, bleaching impacts were seen to be lower(77% bleached or recently dead) on windward coaststhan on leeward coasts (87% bleached or recently dead)(Fig. 5, Table 3). This may be due to greater watermovement on more exposed reefs, perhaps operatingthrough localized wind-driven upwelling of deeper,cooler waters along windward island margins.

Fig. 5 Coral condition by aspect at St Pierre.

Fig. 4 Coral condition by location.

TABLE 2

Bleaching impacts at 15 m water depth by location, southern Seychelles, March±May 1998.

Location Mean coral cover (%) at 15 mwater depth

Status of coral cover

Unbleached �%�1 SD� Bleached �%�1 SD� Recently dead �%�1 SD�

Alphonse Atoll 39 26:6� 19:0 53:6� 17:6 19:8� 12:1St Pierre 47 17:7� 23:8 27:5� 23:8 54:8� 25:8Providence±Cerf 6 18:4� 20:3 48:6� 33:7 33:0� 33:0Aldabra Atoll 37 59:1� 34:5 30:4� 27:8 10:5� 16:1

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Depth. Where su�ciently large datasets are available,there is some evidence from transects grouped at dif-ferent depths to suggest an increased susceptibility tobleaching and bleaching-induced mortality with in-creasing depth (Fig. 6, Table 4), although the 15 m depthdata from Aldabra are anomalous. Additional evidence

comes from shallow waters. At Alphonse Atoll, lagoo-nal study sites were visually less impacted than fore-reefenvironments. In the shallow subtidal environments ofthe surface of the Providence±Cerf bank many of theindividual colonies encountered, primarily Porites andPavona spp, displayed no evidence of bleaching. And atAldabra Atoll, most of the coral species found in thechannels between the lagoon and the open ocean wereliving and displayed no obvious signs of a bleachingevent. Corals of the genera Galaxea, Seriatopora,Acropora and Pocillopora were seen to be completelybleached, but this e�ect only became pronounced withdistance seaward from the channels and the tidal ex-change associated with them. Lagoonal patch reefs andindividual heads of massive coral species in the lagoonat Aldabra displayed only limited bleaching.

By analogy with studies undertaken elsewhere (e.g.Cook et al., 1990; Hoeksma, 1991; Sheppard, 1999), itcan be hypothesized that corals in shallow (3±10 m)waters in 1997±1998 were more tolerant to temperature¯uctuations than corals in deeper water (10±20 m) whichusually experience a more constant temperature regime.As temperature changed, corals in deeper waterbleached ®rst, only followed later by shallow water coralcolonies. This argument is supported by data from thetwo visits to Alphonse Atoll, between 29 March±1 Apriland on 4 May 1998. In March/April, 35% of corals wereunbleached at 10 m water depth whereas the equivalent®gure at 15 m was 27%. In the subsequent ®ve weeksprior to the second, repeat survey, the percentage ofunbleached corals at 10 m declined by over one third, to22%, and the percentage of dead corals almost doubled,to 64% (Fig. 7, Table 5). By contrast, at 15 m waterdepth, the bleaching episode appeared resolved by May,leaving either dead corals or corals which appeared to

TABLE 3

Bleaching impacts at 15 m water depth by reef aspect, St Pierre, southern Seychelles, April 1998.

Location Mean coral cover (%) at 15 mwater depth

Status of coral cover

Unbleached �%�1 SD� Bleached �%�1 SD� Recently dead �%�1 SD�

Windward reefs 49 23:0� 26:5 30:3� 24:9 46:7� 23:1Leeward reefs 58 12:9� 20:2 25:0� 22:9 62:1� 26:2

TABLE 4

Bleaching impacts by water depth, southern Seychelles, March±April 1998.

Location and date Coral status Water depth (m)

10 15 20 25

Alphonse Atoll (March 1998) Unbleached �%�1 SD� 35:1� 21:6 26:6� 19:0 22.1Bleached �%�1 SD� 26:8� 23:7 53:6� 17:6 69.1

Recently dead �%�1 SD� 38:1� 3:5 19:8� 12:1 8.8Aldabra Atoll (April 1998) Unbleached �%�1 SD� 40:4� 33:7 51:2� 38:1 35:1� 47:2 0.0

Bleached �%�1 SD� 29:3� 33:2 26:4� 27:8 51:2� 40:9 73:3� 37:7Recently dead �%�1 SD� 30:4� 26:8 9:1� 15:4 13:8� 24:3 26:7� 37:7

(a)

(b)

Fig. 6 Coral condition by depth at (a) Aldabra Atoll and (b) Al-phonse Atoll.

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have recovered from bleaching. A similar pattern wasseen at St Pierre where coral colonies found in the 10±20m depth range appeared to be in a more advanced stageof bleaching-related mortality than those in the shal-lower 3±10 m depth range. In addition, the Alphonsedatasets suggest that in the long term the overall impactof this bleaching episode is likely to have been greater in

shallow water, with a proportion of the 14% of bleachedcorals at 10 m in May 1998 being added subsequently tothe stock of dead coral substrate (Table 5).

Coral growth form. Ultimately, patterns of bleachingimpact result from the interaction of the ÔpositionalÕcontrols referred to above with the varying susceptibilityto bleaching of di�erent coral growth forms. Thus highlevels of bleaching, and in many instances mortality, ofbranching coral species contrasted with the lack ofbleaching in encrusting corals (Table 6). At AlphonseAtoll in March the majority of Acropora and Pocilloporaspp displayed evidence of recent mortality and at StPierre most branching corals were bleached or partiallybleached with many showing clear signs of ®lamentousalgal overgrowth. In particular, bleaching incidencelevels of 95±100% characterized large stands of Acro-pora spp at shallow depths (3±10 m) on leeward shoresat this locality. Live Millepora spp were rare at all thestudy locations, with evidence of extensive stands havingsu�ered mass mortality prior to the mortality of othercorals. At St Pierre, no live colonies of Millepora sppwere found at any of the sites surveyed. By comparison,massive corals, particularly Porites spp and Pavona spp,displayed partial and `patchy' bleaching throughout thearea. At Aldabra, for example, bleaching was at somesites con®ned to a single side of a massive coral colony.Finally, at St Pierre, many of the smaller massive (i.e.Favia pallida) and encrusting coral colonies at windwardsites displayed no evidence of bleaching in spite of thehigh general levels of incidence at this location. Theseobservations con®rm similar patterns of morphology-related bleaching incidence documented for earlier epi-sodes in other locations (e.g. Brown and Suharsono,1990; Glynn, 1990; Jokiel and Coles, 1990; Williams and

(a)

(b)

Fig. 7 Coral condition over time at Alphonse Atoll at (a) 10 m and (b)15 m water depth for March and May 1998 survey periods.

TABLE 6

Susceptibility to bleaching by di�erent coral growth form: summary statistics from all survey sites in the southern Seychelles.

Coral growth form Unbleached (%) Bleached (%) Recently dead (%)

Branching 23 52 73Massive 34 37 11Tabular 3 4 8Encrusting 39 6 5Foliose 1 1 3

TABLE 5

Changing bleaching impacts with water depth over time, Alphonse Atoll, March±May 1998.

Time of survey Coral status Water depth (m)

10 15 20

March 1998 Unbleached �%�1 SD� 35:1� 21:6 26:6� 19:0 22.1Bleached �%�1 SD� 26:8� 23:7 53:6� 17:6 69.1

Recently dead �%�1 SD� 38:1� 3:5 19:8� 12:1 8.8May 1998 Unbleached �%�1 SD� 21:9� 11:8 41:3� 45:6 67:0� 24:0

Bleached �%�1 SD� 13:7� 0:3 0.0 6:0� 8:5Recently dead �%�1 SD� 64:4� 12:1 58:7� 45:6 27:0� 32:5

577


Bunkley-Williams, 1990; Glynn and Deweerdt, 1991;Glynn, 1993) and for impacts of the 1997/98 event (e.g.Chagos Archipelago: Sheppard, 1999).

Regional aspects, contexts and explanations forsouthern Seychelles coral bleaching

The surface circulation of the Indian Ocean is char-acterized by reversing monsoon gyres north of 10°S anda subtropical, anti-cyclonic gyre to the south. In thenorth, the direction of surface currents is determined bythe prevailing winds. The north-east monsoon normallycommences in November and reaches its maximumstrength in the period from December to February. Atthis time, the west-¯owing North Equatorial Currentand the South Equatorial Current are both well devel-oped. The North Equatorial Current feeds the south-¯owing Somali Current; this in turn feeds the east-¯owing Equatorial Counter Current between 0 and 8°S(Tomczak and Godfrey, 1994; Fig. 8(a)). The south-westmonsoon typically begins in May and becomes strongest

between July and August. North of the Equator, surfacewaters ¯ow east in this period; thus the North Equato-rial Current reverses to form the east-¯owing south-westMonsoon current (Molinari et al., 1990), before ¯owingsouth in the eastern Indian Ocean to feed the SouthEquatorial Current. The Somali Current reverses and¯ows north along the coast of Somalia and Oman, beingfed by the South Equatorial Current (Fig. 8(b)). Strongupwelling, both coastal and o�shore (Brock et al., 1992),develops in this region; between 5 and 11°N this resultsin the replacement of warm (25±29°C) surface waters bycold (20±22°C) water masses at this time of year (Raoand Gri�ths, 1998).

As a result of the re-organization of the Walker Cir-culation (Fig. 1(a)) during an El Ni~no episode, easterlywind anomalies develop over the equatorial IndianOcean (Nigam and Shen, 1993). This wind ®eld weakensboth the south-west Monsoon current and the north-¯owing Somali Current and leads to a diminution ofupwelling along the coast of Somalia and Oman. Thussubstantial SST anomalies develop along the coast ofEast Africa. These unusually high temperatures triggerpersistent convection which in turn maintains the eas-terly winds and reinforces the SST anomaly which mayreach +2°C or more. In addition, the south-east tradewinds which migrate north over the northern summermonths, typically show greater than usual displacementtowards the Equator, further strengthening the easterlywind anomalies. In the 1997/98 El Ni~no period, easterlywind anomalies began to develop along the Equator inearly June 1997 and strengthened signi®cantly in theperiod September±November, exceeding 5 m sÿ1 byDecember 1997 (Webster et al., 1999). These anomalieswere further intensi®ed by unusual changes in the sur-face wind ®eld. During June±December 1997, the south-east trades were displaced much closer to the Equatorthan normal; indeed in November 1997, the northernedge of this wind belt was located north of the Equator.Thereafter, the normal seasonal southward retreat of thesouth-east trades did not take place. The result of thesechanges was to weaken the tradewinds further south.With less latent heat release with lower windspeeds, theIndian Ocean outside the equatorial zone warmed con-siderably into 1998. Oceanographic adjustments tookthe form of the re-appearance of easterly propogatingKelvin waves and west-moving Rossby waves (Cham-bers et al., 1999) which had been absent from the seasurface height record since July 1997; alongside the cli-matological factors, these processes allowed the abnor-mal warming to spread to the entire basin (Yu andReinecker, 1999).

From end December 1997 to beginning June 1998,patterns of SST anomalies in the Indian Ocean werecomplex, with dynamic hotspots and areas of localizedcooling (Fig. 9). Nevertheless, several clear trends areapparent. Signi®cant potential for coral bleaching ®rstbegan to be seen in the Indian Ocean in December 1997,developing into a coherent north-west±south-east band,

Fig. 8 Surface currents in the Indian Ocean in (a) March±April (latenorth-east monsoon) and (b) September±October (late south-west monsoon).

578


Fig. 9 Evolution of SST anomaly/coral bleaching hotspots, 30December 1997±2 June 1998 (from NOAA 1997; see text formethodology).

579


from north of Madagascar to western Australia. Thispattern strengthened through January 1998, extendingto a broad triangle delimited as far south as 45°S bymid-February, and persisting until a period of disinte-gration and transition in mid-March. The subsequentpattern of abnormally high SSTs was a west±east bandlying between the Equator and 15°S. This pattern dis-appeared ®rst in the western Indian Ocean in mid-April,heralding the progressive migration of the SST anomalytowards the north-eastern Indian Ocean from the be-ginning of May. By the beginning of June, bleachinghotspots were restricted to areas of relatively smallspatial extent in the Gulf of Aden, the Straits of Hor-muz, the Bay of Bengal, the west coast of Java and partsof the South China Sea (Fig. 9).

In the area encompassing the islands described in thispaper (6±10°S, 45±54°E), monthly mean SSTs typicallyreach their maximum between February and April witha minimum in August. In 1997±1998 temperatures werewell in excess (> �1 standard deviation) of long-termmeans (Fig. 10). Firstly, from November 1997 to March1998 SST anomalies in the area were greater than +1°Cabove the long-term mean monthly maximum SST.Secondly, the maximum excursion of +1.84°C (Febru-ary 1998) is noteworthy. Thirdly, SSTs did not fall be-low 30°C for the entire period from February to April

1998. It was the combination of these three factors, withperhaps the additional agent of a reduction in the sea-water attenuation coe�cient as the result of low wind-speeds and calm seas (Davies et al., 1997), that led to theextensive and pervasive coral bleaching event andbleaching-related mortality reported above for thesouthern Seychelles.

This record can be disaggregated into the SST dy-namics for individual sites within the southern Sey-chelles. At Providence±Cerf and St Pierre, the warmingtrend which became apparent in December 1997 con-tinued until February 1998 when SSTs were +1.94°Cabove the long-term mean monthly maximum, reaching30.56°C. It was not until May 1998 that SSTs began todecrease; even at this time recorded SSTs were still over1°C above the long term mean. The duration of thismarked warm event ± in excess of ®ve months ± helpsexplain, therefore, the severity of the coral bleachingepisode at these two locations. At Alphonse Atollslightly less severe bleaching impacts (Table 2) may beexplained by the fact that warming was delayed: thesteep increase in SST did not take place until February1998. This warming was 1.8°C greater than the longterm mean monthly maximum SST and was followed bya 2.5 month period with SSTs greater than 30°C.Finally, at the surveyed site least a�ected (Table 2) in thesouthern Seychelles, Aldabra Atoll, the peak warmingwas 0.5°C less than at the other three locations. Here arapid increase in SSTs occurred from November 1997with a +1°C SST anomaly by January 1998. Peak SSTs(30.65°C) were reached in March, representing a+1.31°C anomaly above the long term mean maximumSST for this month. The +1°C anomaly persisted untilApril 1998.

Similar linkages between SST anomalies and the in-cidence of coral bleaching episodes in 1997/98 can beseen at other stations in the Indian Ocean (Fig. 11,Table 7). The pattern of incidence of both processesvaried, however, at di�erent sites as a result of the largescale dynamics of warm water changes described above(Fig. 9). To the north, at Socotra and in The Maldives,positive temperature anomalies were of relatively lowmagnitude but sustained: an anomaly of +1°C wasreached in September 1997 and thereafter ¯uctuatedbetween +0.5°C and +1.5°C until July 1998. On theKenyan coast and the Seychelles Bank, excursions weremuch greater, reaching +2.5°C, but of shorter con-tinuous duration above the +1°C anomaly level. AtMayotte, and particularly in the Chagos Archipelago,anomalies built over time to exceed the +1°C level.Finally, to the south, anomalies of over +1°C wereonly brie¯y maintained (and see also Turner, 1999).Clearly both peak temperature and duration ofanomalously high temperatures are important in de-termining the level of bleaching incidence; Table 7suggests, however, that duration above a +1°Cwarming provides the preconditions for catastrophicbleaching.

(a)

(b)

Fig. 10 SSTs for the area 6±10°S 45±54°E, July 1997±July 1998. (a)Monthly means (GISST2.3b; Rayner et al., 1996). (n) Long-term monthly mean�1S.D. for 1961±1997; (�) monthlymeans for 1997±1998. (b) Monthly SST anomalies (MO-HSST6D). Note +1°C anomaly.

580


Discussion

Longer-term analysis of the GISST2.3b and theMOHSST6D datasets for the study area suggests thatthe 1997±1998 warming was the highest for the last 37years, both in terms of absolute temperatures (Fig. 12(a))and, particularly, in the magnitude of the observedtemperature anomaly, the latter being almost doublethat previously recorded (Fig. 12(b)). These recordscon®rm the exceptional nature of this event. To whatextent, however, is it possible to reconstruct a history ofprevious SST-related bleaching episodes in the IndianOcean? The relatively sparse documentary evidenceavailable con®rms that bleaching events are associatedwith El Ni~no years, but only in general terms and only

as far back as the 1982±1983 event (Fig. 13). This ®ndingis consistent with Webster et al.'s (1999) argument thatwestern Indian ocean warming may occur even in theabsence of ENSO extrema.

Of these sites, closer examination of SSTs duringpreviously observed bleaching events at Mayotte o�ersthe possibility of identifying a regional threshold tem-perature at which corals bleach in this region. Fig. 14and Table 8 suggest a temperature of, or close to, 30°C.If this threshold holds good for the southern Seychellesin general, then bleaching may have taken place at thefour study sites in 1969, 1983 and 1986/7 (Fig. 12(a)).Interestingly, a high proportion of massive coralsbleached in the 1997/98 event at Aldabra Atoll dis-played signs of previous die-o�, indicated by algal

Fig. 11 Monthly SST anomalies (MOHSST6D), July 1997±July 1998for selected 1°� 1° grid cells at eight sites in the central andwestern Indian Ocean. +1°C anomaly is widely regarded as akey coral bleaching threshold (Goreau and Hayes, 1994).

581


TABLE

7

Tim

ing,strength

anddurationofSSTanomalies

andreportsofcoralbleachingandbleaching-relatedcoralmortality

ateightcentralandwestern

IndianOceansites,July

1997±July

1998.

Location

aMonth

ofpeak

anomaly

in1998

bMaxim

um

anomaly

(°C)b

Longestcontinuous

periodof>

+1°C

anomaly

(months)

b

Totalcontinuous

periodof>

+1°C

anomaly

(months)

b

Bleachingreported

Bleaching

(%)c

Coralmortality

(%)c

Reference

Socotra

March

1.15

46

April±May

)`H

igh'

Hsieh

andOrm

ond(1998),Wilkinson

(1998)andKem

p(pers.comm.,1998)

Maldives

May

1.41

35

April±May

80±95(S)

80±90+(S)

Hsieh

andOrm

ond(1998),Wilkinson

(1998)andWilkinsonet

al.(1999)

30±40(D

)Mombasa,

Kenyancoast

May

2.43

77

March±April

90±100(S)

50±90(S)

Wilkinson(1998)andWilkinsonet

al.

(1999)

50+

(D)

50+

(D)

SeychellesBank

February

2.54

36

January

andMarch±

May

40±90

� x�

75;50±95

Hsieh

andOrm

ond(1998),Wilkinson

(1998)andWilkinsonet

al.(1999)

Mayotte

March

1.19

23

April±August

upto

80

`High'

Hsieh

andOrm

ond(1998),Wilkinson

(1998),Wilkinsonet

al.(1999)

Chagos

February

1.41

66

� x�

84;max.100

onseaward

reefs;

75in

lagoons

Sheppard

(pers.comm.,1999)

Rodrigues

January

1.23

22

Mauritius/St.

Brandon

February

1.11

11

April

1±15

`Slight'

Wilkinson(1998)andWilkinsonet

al.

(1999)

aForlocationsseeFig.11.

bFrom

MOHSST6D

database.

cFrom

Wilkinsonet

al.(1999)S:shallow

(<10m),D:deep(20m

+).

582


overgrowths and the presence of encrusting and boringinvertebrates.

At longer timescales, exploratory work on the link-ages between the isotopic chemistry of the skeletons ofmassive, century or more old Porites heads on the coastof Oman (Tudhope et al., 1996), in the Amirantes(Charles et al., 1997) and in the southern Red Sea (Kleinet al., 1997), SST and rainfall patterns, and inter-annualmonsoon dynamics suggest that these atmosphere-ocean dynamics have a�ected Indian Ocean reefs overhistorical timescales. Furthermore, and by analogy withthe geologically thin reefs of the eastern Paci®c (e.g.Glynn and Colgan, 1992), consideration should be givento the role of ocean warming events in long-term reefdevelopment in the Indian Ocean. It is suggested thatthe relatively poor degree of Holocene reef developmentat several sites in the western Indian Ocean, includingthe presence of drowned reefs and banks, on the Sey-chelles Bank, Mascarene Ridge and the Amirantes, maybe indicative of periodic high temperature controls onreef construction and maintenance.

Finally, the correlation between coral reef health andENSO dynamics raises speculation as to reef futures inthe Indian Ocean. The periodicity of ENSO ¯uctuationsappears to have shortened from ca 5 years to ca 4 yearsafter 1965, with further changes after the mid-1970s(Nicholls et al., 1996). Both instrumental and proxyenvironmental records suggest that the period from ca1920 to 1940/50 was characterized by relatively few

ÔmoderateÕ to ÔstrongÕ warm phase ENSO episodes.Furthermore, the two major El Ni~nos of this century(1982/83, 1997/98) have occurred recently and at aninterval of 15 years, rather than the typical gap of 30±40years (Caron and O'Brien, 1998). Since 1976/77 therehave been more frequent warm phase (El Ni~no) eventsand few cold phase (La Ni~na) events (Wang, 1995) andthis trend has been further reinforced since 1989. Thelatest period has been highly unusual in showing con-sistently negative values of the Southern OscillationIndex. This has no analogue in the 120 years of atmo-spheric pressure readings for the South Paci®c region(Trenberth and Hoar, 1996; Rajagopalan et al., 1997).One e�ect of this pattern has been to keep SSTsanomalously high and thus increase the likelihood ofbleaching events taking place. This may be due to along-term trend in ocean warming, identi®ed by Parkeret al. (1995) for the tropical oceans. The temperatureanomalies for both the Southern Seychelles (Fig. 12(b))and Mayotte (Fig. 14(b)) both indicate a warming trendof 0.108°C per decade since the 1960s, similar to the®gure of 0.126°C reported from the eastern IndianOcean (Brown et al., 1996). Alternatively, this period of

Fig. 13 Previous bleaching events described from the Indian Oceanand their relation to the ENSO chronology. ENSO intensityclassi®cation ((n n n) `strong'; (n n) `moderate'; (n) `weak') byseason from SST for the tropical Paci®c along the Equatorfrom 150°W to dateline (NOAA, 1998c).

Fig. 12 Long-term monthly SSTs for the area 6±10°S 45±54°E, 1961±1998. (a) Monthly means (GISST2.3b; Rayner et al., 1996).(b) Monthly SST anomalies (MOHSST6D), using 1961±1990baseline. Regression line indicates warming trend of 0.108°Cper decade.

583


high temperatures may be the result of the interaction ofa series of ocean-atmosphere processes with varyingperiodicities (McPhaden, 1999b). The answer is, as yet,not clear. Finally, modelling of future ENSO dynamicswith increases in global mean temperature remainsproblematical because of the di�culties of resolvingsome of the key processes involved, such as equatorialupwelling (Meehl and Washington, 1996). Recent

modelling (Timmerman et al., 1999), however, suggestsa change to more frequent El Ni~no episodes and moreintense La Ni~na events in the near future. If this work issubsequently con®rmed, then attention will need to beturned towards the implications of such changes in theequatorial ocean-atmosphere system for long-term coralreef vitality in the Indian Ocean.

The authors acknowledge the assistance of Jude Bijoux, Emily Hu-gues-Dit-Ciles, Elke Talma, and Alve Henricson (Captain) and DanielSimonsson, Pierre Guander and Niklas Roselius of the r/v Searcher ingathering the ®eld data reported in this paper. We are grateful to TheNational Research and Development Council of the Seychelles andThe Island Development Company for permission to undertake thisresearch in the Southern Seychelles. Mr G Savy, Mr J Morgan, Mr MLoustau Lalanne, Dr J Collie, Dr J Nevill, Dr J Mortimer, Mr MNicette, Mr G Fotherby and Mr D Rowat assisted considerably withlogistics and local information. We are grateful to The Director,Hadley Centre for Climate Prediction and Research, UK Meteoro-logical O�ce for long-term SST data and to J Arnott, Hadley Centre;to Dr J Turner, University of Wales Bangor, Dr C Sheppard, Uni-versity of Warwick, and Dr J Kemp for bleaching reports; and toProfessor BE Brown, University of Newcastle upon Tyne, for en-couraging this approach to the bleaching problem.

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Fig. 14 Long-term monthly SSTs for Mayotte (lat/long) 1961±1998.(a) Monthly means (GISST2.3b; Rayner et al., 1996). Arrowsrecord previously reported bleaching episodes in May±June1983 (Faure et al., 1984), May±July 1986 (Thomassin inWilliams and Bunkley-Williams, 1990), February±April 1987(Faure in Williams and Bunkley-Williams, 1990) and April1998 (Thomassin in Wilkinson, 1998). (b) Monthly SSTanomalies (MOHSST6D), using 1961±1990 baseline. Regres-sion line indicates warming trend of 0.108°C per decade.

TABLE 8

Temperature±bleaching relationships for the Seychelles Bank, Mayotte and southern Seychelles, 1969±1998.

Locationa 1969Bleachingevent

1969Maximumtemperature

(°C)b

1983Bleachingevent


(°C)b

1987Bleachingevent


(°C)b

1998Bleachingevent


(°C)b

Mayotte 29.87 Xc 29.97 Xc 30.37 Xc 30.12Seychelles Bank 30.12 29.81 29.90 Xc 30.88Southern Seychelles:Alphonse Atoll 30.16 29.78 30.14 Xc 30.94Providence±Cerf/StPierre

29.91 29.73 30.04 Xc 30.51

Aldabra Atoll 30.10 30.25 30.48 Xc 30.65

a See Figs. 3 and 7.bSource: GISST2.3b (Rayner et al., 1996)cX: Bleaching Event Reported. Mayotte: 1983 (Faure et al., 1984), February±April 1987 (Faure in Williams and Bunkley-Williams, 1990) and April1998 (Thomassin in Wilkinson, 1998). Seychelles Bank: January and March±April 1998 (Bradshaw et al., in Wilkinson 1998). Southern Seychelles:March±May 1998 (this paper).

584


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coral bleaching in the southern seychelles during the 1997–1998 indian ocean warm event

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