ANNELISE B. HAGAN1, SARAH HAMYLTON2 & TOM SPENCER
Cambridge Coastal Research Unit, Dept. of Geography, University of Cambridge, UK [email protected], [email protected] waves were not amplified around the islands due to their position in the western Indian Ocean and Indian Ocean reefs were badly affected by the global surrounding deep water. Amirantes reefs are 1997-98 ocean warming event, suffering up to 90% recovering in terms of live coral cover following the coral mortality. This paper reports the present status 1997-98 ocean warming event, but on-going of reefs of the Amirantes and Alphonse groups of the monitoring is required to gauge time-scales for these southern Seychelles. Fifteen islands and surrounding reefs to regain their former coral diversity. reefs, covering a total area of 270 km2, were mapped in January 2005 using airborne remote sensing. Benthic surveys were conducted at selected localities using video transect techniques and percentage cover The Seychelles archipelago, western Indian Ocean (5o- for ten benthic categories was calculated. A 10oS; 45o-56oE) is made up of 42 granitic islands and hierarchical classification scheme was devised for the 74 coralline islands. The total land area of 455 km2 islands. Results are presented as large-scale habitat lies within an Exclusive Economic Zone (EEZ) of maps. Typically, reef-flats of the Amirantes were 1,374,000 km2 (Jennings et al., 2000). The Amirantes dominated by Thalassodendron ciliatum and Thalassia islands lie immediately to the south-west of the hemprichii seagrass communities and fore-reef slopes ancient (~750 Ma) granitic microcontinent of the were dominated by bare substrate, with substantial Seychelles Plateau (Plummer, 1995). The Group coverage of macroalgae (Halimeda spp.). Live coral comprises 24 islands and islets (including the atolls of cover on the fore-reef slope ranged between 7-26% St. Joseph and Poivre), stretching ~155 km from and was dominated by Porites and Pocillopora. The 4o53’S (African Banks) to 6o14’S (Desnoeufs) on the large sizes of many Porites colonies present indicates arcuate Amirantes Ridge, which separates the that these survived the 1997-98 ocean warming event Amirantes Trough and Somali basin to the west from and the high abundance of Pocillopora is a typical the Amirantes Basin to the east (Plummer, 1996). A response following a large scale disturbance event. The basalt sample recovered from the western flank of the southern Seychelles islands and surrounding reefs were Ridge has been dated at 82 ± 16 Ma (Mid – Late not seriously affected by the 2004 Asian Tsunami; the Cretaceous) (Fisher et al., 1968); this volcanic
Obura, D.O., Tamelander, J., & Linden, O. (Eds) (2008). Ten years after bleaching - facing the consequences of climate change in the Indian Ocean.CORDIO Status Report 2008. Coastal Oceans Research and Developmen in the Indian Ocean/Sida-SAREC. Mombasa. http//:www.cordioea.org 71
The Seychelles lie in the South Equatorial Current and Seychelles reefs are influenced by the persistent south-east trade winds, with typical windspeeds of 6-9 m s-1 for 8-10 months of the year (Stoddart, 1984b). This influence is particularly marked south of 10oS (Sheppard, 2000), resulting in the strong differentiation of windward and leeward reef structures at higher latitudes. Mean annual rainfall in the Seychelles decreases in a south-westerly direction, with the north-eastern islands experiencing approximately twice as much rain as the south-western islands (Walsh, 1984). The reefs of the southern Seychelles receive few terrestrially-derived nutrients, and although the surrounding oceanic waters show basement is overlain by shallow water carbonates of typically low primary productivity, the atolls may less than 1 km in thickness (Stoddart, 1984b). Water induce upwelling and thus enhanced local productivity depths are greatest (~70 m) in the centre of the Ridge, (Littler et al., 1991). Sea surface temperatures in the shallowing to a peripheral rim at -11 to -27 m. The Seychelles show a typical annual range of 26oC to majority of reefs and shoals lie along the eastern and 31oC and open ocean salinities vary from 34.5 ppt to southern margins of the Ridge. The emergent islands 35.5 ppt (Jennings et al., 2000). Many of the are of Holocene age (< 6 ka; Stoddart, 1984a) and Amirantes islands are uninhabited and several, such as composed of ‘bioclastic sands thrown up by wave Etoile and Boudeuse, are protected as bird reserves. action from reef platforms which have accumulated to The islands of Desroches and Alphonse have small sea-level’ (Braithwaite, 1984: 27). In some cases, hotels (20 and 25 rooms respectively) and tourist dive however, these cays record more complex histories centres. Anthropogenic impacts on reefs of the with the presence of raised reefs, bedded calcareous Amirantes are therefore relatively low. However all sandstones and extensive beachrock (Baker, 1963). Seychelles reefs were severely affected by the 1997-98 The atoll of Desroches lies to the east of the main ocean warming event which induced ~60% coral ridge (Stoddart and Poore, 1970) and 95 km further mortality in the Amirantes (Spencer et al., 2000). south are the atolls of Alphonse and St. François which A collaborative expedition between the Khaled bin form, with Bijoutier, the Alphonse Group (Fig. 1). Sultan Living Oceans Foundation, the Cambridge
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Airborne remote sensing data were acquired over 15 islands in the Amirantes from the seaplane ‘Golden Eye’, covering an area of 270 km2 across 133 pre- determined parallel survey lines. Reflectance data were collected over the 430-850 nm region of the electromagnetic spectrum using a Compact Airborne Spectrographic Imager (CASI) sensor. Raw data were geocorrected by collecting ground control points on areas of strip overlap and applying a Coastal Research Unit, University of Cambridge and first order polynomial model to correct for the linear the Seychelles Centre for Marine Research and offset, with nearest neighbour resampling. Strips were Technology – Marine Parks Authority to the southern mosaiced and a band-wise linear colour balancing Seychelles was conducted onboard M.Y. Golden model was applied to minimize across-track variance, Shadow from 10th – 28th January 2005 with histogram matching to adjust for radiance offsets. Training areas were used to derive reflectance measures over a number of spectral subclasses in order to build up a statistical population of each reef habitat class in feature space. A total of 910 signatures were Quantitative underwater surveys were conducted in collected and evaluated for the dataset as a whole before January 2005 at four islands (Marie-Louise (Plate 1), being merged into 24 habitat class populations. A Boudeuse, Poivre and Alphonse (Plate 2)) using well- maximum likelihood classification assigned each pixel established video transect methods (Christie et al., of the image to the most likely class on the basis of 1996). The video data recorded was a plan view of a statistical probabilities (Mather, 2004). rectangular section of the benthic reef community A habitat scheme with a hierarchical structure measuring 20 m x ~ 0.3 m; by recording both sides of (Table 1) was developed to accommodate user the transect line, double this area was covered (i.e. 20 requirements, field data availability and the spatial and m x ~ 0.6 m). Transects were variously placed at spectral resolution of the CASI sensor. shallow (5 m), mid-depth (10 m and 15 m) and deep (20 m) water depths, as allowed by local bathymetry. Video transect footage was analysed using the AIMS 5-dot analysis method (Osborne and Oxley, 1997). Ten benthic categories were identified: sand; At reefs on the south-east fore-reef slope of Marie- rubble; bare substrate; dead standing coral; pink Louise (Fig. 1, Plate 1), live coral cover was 21% at a calcareous algae on bare substrate; pink calcareous depth of 15 m and 16% at a depth of 10 m. algae on dead standing coral; scleractinia; non- Macroalgae (principally Halimeda spp.) dominated the scleractinia; macroalgae; and others (e.g. zooanthids, benthos at both depths (31-36% cover), followed by molluscs, bivalves). Scleractinia, non-scleractinia and bare substrate (24%). Coral rubble accounted for only macroalgae were identified to genus level and the 2-6% of the benthos. The coral community was relevant genera recorded. Percentage cover was comprised of 13 genera. Pocillopora accounted for calculated for each of the ten benthic categories. 40% of the coral community at the 15 m site and 60%
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First tier Second tier 1. Terrestrial vegetation: trees 1.1 Coconut woodland and shrubs 1.2 Other trees and shrubs 2. Herbs and grasses 3. Saline pond 4. Cleared/ bare ground 5. Littoral hedge 6. Mangrove woodland (Plate 3) 7. Coarse beach material & 7.1 Coral sandstone/ Raised reef rocks 7.2 Coral boulders 7.3 Beachrock The fore-reef slope off the west coast of Poivre (Fig. 1) displayed higher live coral cover at deeper sites 8. Beach sand (19% cover at 20 m, 26% cover at 15 m) than at 9. Rock pavement 10. Reef-flat sand shallower sites (9% cover at 10 m, 11% cover at 5 m). At 20 m, 15 m and 5 m depths, the coral community 11. Seagrass (Plate 4) 11.1 Low density seagrass/ macroal- gae was dominated by Porites (39%, 48% and 24% cover 11.2 Medium density seagrass respectively) and Pocillopora (21%, 20% and 19% cover respectively). At 10 m water depth the blue coral 12. High density seagrass Heliopora coerulea dominated (35% cover), followed 13. Lagoon patch reef by Pocillopora (28% cover). 14. Lagoon sand The fore-reef slope of Alphonse displayed an 15. Fore-reef slope material or 15.1 Coral rubble with coralline algae average of 22% live coral cover and only 1% structure. Not sand. 15.2 Fore-reef slope coral spurs with coralline algae 15.3 Rocky fore-reef slope 15.4 Fore-reef slope rubble and sand 15.5 Fore-reef slope with coral 16. Fore-reef slope sand 1999 2001 2003 of the coral community at the 10 m site. Porites was Sand 7.4 10.2 11.0 the second most dominant genus, accounting for 20% Rubble 30.5 26.7 18.4 of the live coral cover at 15 m and 10% of the live Bare Substrate 11.1 13.8 18.6 coral cover at the 10 m depth. Dead Standing 1.0 2.6 0 At Boudeuse (Fig. 1), live coral cover typically accounted for only 7% of the total benthic coverage, Pink CA on BS 25.3 17.7 18.1 being comprised largely of Porites (33%), Favites Pink CA on DS 2.4 2.2 0 (16%), Montipora (9%) and Acropora (9%). At this Scleractinia 10.5 12.6 23.4 site the dominant benthic category was bare substrate Non-Scleractinia 3.2 9.1 9.2 (32%), followed by rubble (23%), sand (15%) and Macroalgae 8.5 4.7 1.3 macroalgae, specifically Halimeda (15%). Other 0.1 0.4 0.1
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macroalgal cover at sites with water depths of 5 to 17 m in 2005. Repeat surveys at Alphonse between 1999 and 2003 indicate a good level of recovery following the bleaching event, with average live coral cover increasing from 10% of total benthic coverage in 1999, to 12-17% in 2001/02, and to 23% in 2003 (Hagan and Spencer, 2006). Over the same period, both non-scleractinian and macroalgal cover decreased. Thus whilst macroalgal cover almost equalled scleractinian cover in 1999, by 2003 it only represented 1-2% of benthic community coverage (Table 2). Between 2003 and 2005, although percentage period and macroalgal cover remained minimal (Fig. cover of bare substrate remained constant, 2). scleractinian cover at Alphonse increased. At one In 2005, Porites was the dominant genus at all example site on the north-east fore-reef slope, survey sites and all depths, except for one 5 m depth scleractinian cover increased by 7% in this two year site off the south-west of the atoll where the dominant
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77 genus was Montipora (50% of live coral cover). On the west side of the atoll, Porites accounted for 55%, 65% and 39% of the live coral community at 15 m, Seagrass beds were a conspicuous and often dominant 10 m and 5 m respectively. On the east side of the feature of the habitat maps produced for the islands of atoll, Porites accounted for 49%, 57% and 63% of the the Amirantes Ridge and the Alphonse Group. live coral community at 15 m, 10 m and 5 m Thalassodendron ciliatum and Thalassia hemprichii respectively. Pocillopora accounted for 11% of the are common in subtidal areas at water depths of up to coral community at 15 m depth on the west side and 33 m throughout the Seychelles (Green and Short, 30% of the coral community at 15 m depth on the 2003). The two species are typically found at densities east side. No Pocillopora was recorded at shallower of 540-627 and 1123-1761 shoots per m2 respectively survey sites on the west side of the atoll, but (Ingram and Dawson, 2001). Appropriate conditions Pocillopora accounted for 19% and 14% of the live for seagrasses include an adequate rooting substrate, coral community at 10 m and 5 m respectively on the water depths that preclude subaerial emergence at low east side. tide and light levels to maintain growth (Hemminga and Duarte, 2000). Within these broad environmental limits, organisation of seagrass communities on the Little variation was apparent within the defined classes islands of the Amirantes ranged from closed canopy and the output maps from the 2005 survey had an meadows, commonly found on the reef flat and average thematic producer’s accuracy of 75.7% for covering deeper platforms, to sparse seagrass patches first tier habitats (Congalton, 1991). Overall, the in areas of greater water movement and shallower maps provided a clear representation of the water depths. heterogeneity apparent in the raw imagery (Fig. 3). The outer fore-reefs of the Amirantes and Seagrass (Plate 4) was the most well-represented Alphonse Group were dominated by bare substrate habitat class, encompassing low and medium density and macroalgae, with low-coverage scleractinian communities, as well as various macroalgal species. communities being dominated by a small number of Fore-reef slope material, reef flat sand and lagoon sand genera. The 1997-98 coral bleaching event had a very coverage was also considerable. Alphonse had the severe impact on the reefs of the Indian Ocean. The highest number of tier 1 habitat classes, with Bijoutier high level of both macroalgal cover and bare substrate and St. François and D’Arros and St. Joseph also on reefs of the Amirantes suggests that this recent supporting a wide range of habitats. Conversely, Sand bleaching event may have led to a benthic community Cay, Remire and Etoile supported lower numbers of with reduced scleractinian cover and increased habitat classes, with a dominance of fore-reef slope macroalgal cover, as has been hypothesised elsewhere material, seagrass and high density seagrass (e.g. Done, 1999). However, although the granitic respectively (Fig. 4). Of the islands mapped, 7 were Seychelles islands in the north suffered over 90% coral vegetated, primarily with coconut woodland or a mortality during the 1997-98 ocean warming mixture of trees and shrubs; three of these vegetated (Wilkinson, 2000), the southern islands were less islands, Marie-Louise, Boudeuse and Desnoeufs were severely affected, with an average mortality of around situated upon an intertidal and shallow subtidal rock 60% (Spencer et al., 2000). It is suggested that this pavement. difference was due to the moderating influence of the The maps remain provisional at the present time, South Equatorial Current at the southerly locations, in being subject to validation by experts in the contrast to the heating of shallow waters, and long Seychelles. It is the intention that final versions of the water residence times, on the Seychelles Plateau. maps will be available in an electronic format in due At some sites in the southern Amirantes, such as course. Marie-Louise, it is surprising that there was little 78
rubble present on these reefs during the 2005 surveys, event in the Amirantes Group. Pocillopora colonies in as coral rubble is a typical sign of recent coral the Amirantes typically measured 10-30 cm in mortality (Rasser and Riegl, 2002). The lack of rubble diameter, sizes which could have been attained in the present suggests that pre-1998, some of these reefs 7 years following the bleaching event. Conversely, the were most probably dominated by massive, rather than presence of Porites as the most dominant coral genus branching, corals. This certainly appears to have been at Poivre, Boudeuse and Alphonse and the second the case in January 1993 when the Netherlands Indian most dominant genus at Marie-Louise suggests t hat Ocean Programme expedition conducted one SCUBA these slow-growing, massive colonies survived the transect on the north-west fore-reef slope at Alphonse. 1997-98 bleaching event. Where patch reefs occur They found acroporids constituted only 1.8% of the within lagoons, at Alphonse Atoll for example, little scleractinian community, pocilloporids constituted evidence of ocean warming related mortality was 10.8% and massive Porites spp. constituted 80% (van observed, suggesting that these shallow water corals der Land, 1994). These results from Alphonse are were already acclimatised to waters warmer than occur consistent with data from 2005, where Porites was the on the outer fore-reef slope. In cases such as this, dominant genus, constituting up to 65% of the live lagoon corals may therefore act as larval refugia, and coral community. may be an important component in reef regeneration Further north in the Amirantes, coral cover was ~ following a major disturbance event in the region. 20% or more in water depths of 15 – 20 m at Poivre. The January 2005 expedition to the southern Furthermore, the presence of large amounts of coral Seychelles was conducted shortly after the 2004 Asian rubble suggests that the 1997-98 bleaching event may Tsunami which devastated islands and reefs have led to post-bleaching reef framework throughout the Indian Ocean basin (Obura, 2006). disintegration of the coral community at this location. However, no physical damage from this event was Interestingly, The Netherlands Indian Ocean observed in either the terrestrial or marine Programme reported 41-50% live coral cover on the environments at any of the islands visited (Hagan et northern reef-slope at Poivre in December 1992. al., in press). The littoral hedge remained intact and Although Porites was the dominant genus recorded in there was no evidence of beach sediment movement or these surveys, there was a significant presence (20- water inundation at island margins. Underwater there 40% of the coral community) of branching acroporids was no evidence found of tsunami-related mechanical and pocilloporids (van der Land, 1994). damage on the reef. Thus, for example, no physical Elsewhere in 2005, the two most prevalent damage to the branching corals (principally scleractinian genera at 15 m and 10 m at Marie-Louise Pocillopora) that are prevalent on these reefs and no and at 20 m, 15 m and 5 m at Poivre were Porites and coral toppling was observed. The islands of Alphonse, Pocillopora At Boudeuse at 10 m, Porites dominated, D’Arros, Desroches, Marie-Louise and Poivre are with Pocillopora ranking 5th in dominance. Likewise, inhabited. In all cases, island personnel said that there van der Land (1994) reported 38% Porites spp., 24% had not been any impact caused by the tsunami and pocilloporids and 6.1% Stylophora mordax at the they hardly noticed the event. The lack of noticeable neighbouring island of Desnoeufs in 1993. impacts within the southern islands compared to Pocillopora damicornis has been described as an islands further north appears to be related to both opportunistic species, due to its rapid reproductive reduced tsunami wave heights to the south (due to the cycle, widespread larval dispersal and fast growth rate ocean basin-scale refraction of the wave from the east on settling, enabling it to quickly occupy any newly – west axis of maximum impact at 0 - 5oN (Spencer, available space (Endean and Cameron, 1990) such as in press)) and to differences in regional bathymetry, that available following the 1997-98 coral bleaching the tsunami being accentuated by the shallow shelf
79 seas of the Seychelles Bank in the north and not amplified around the southern islands which are surrounded by deep water. Baker BH (1963). Geology and mineral resources Pre-1998 reef status data is not available for reefs of the Seychelles archipelago. Geol Surv Kenya Mem of the Amirantes but the time-series available for 3:1-140. Alphonse shows significant recovery in terms of live coral cover following the mass bleaching event (see Braithwaite CJR (1984). Geology of the above, Table 2; Hagan and Spencer, 2006). In order Seychelles. In: Biogeography and Ecology of the for reef recovery to continue and the natural Seychelles Islands. Stoddart, D.R. (ed.) Junk succession of the coral community to progress, it is Publishers, The Hague, The Netherlands pp. 17-38. important that further reef degeneration does not occur. The reefs of the Amirantes have the advantage Christie CA, Bass DK, Neal SL, Osborne K and of being subjected to minimal anthropogenic Oxley WK (1996). Surveys of Sessile Benthic pressures but ongoing monitoring is essential to Communities Using the Video Technique. Long- gauge time-scales involved in these reefs regaining term monitoring of the Great Barrier Reef, Standard their levels of coral diversity. Operational Procedure Number 2. Australian Institute of Marine Science, Townsville, Australia 42p. Online: http://www.aims.gov.au/pages/ research/reef-monitoring/ltm/mon-sop2/sop2- Observations in the Republic of Seychelles were 00.html. supported through a collaborative expedition between the Khaled bin Sultan Living Oceans Congalton R (1991). A review of assessing the Foundation, Cambridge Coastal Research Unit, accuracy of classifications of remotely sensed data. University of Cambridge and Seychelles Centre for Remote Sensing of the Environment 37:35-46. Marine Research and Technology – Marine Parks Authority (SCMRT–MPA). Fieldwork was Done TJ (1999). Coral community adaptability conducted onboard the Khaled bin Sultan Living to environmental change at the scales of regions, Oceans Foundation vessel, the M.Y. Golden Shadow. reefs and reef zones. American Zoologist 39:66-79. We are grateful to Prince Khaled bin Sultan for his generous financial support of the expedition and use Endean R and Cameron AM (1990). Trends and of the M.Y. Golden Shadow; to Capt. P. Renaud, new perspectives in coral-reef ecology. In: Executive Director, Khaled bin Sultan Living Oceans Ecosystems of the World 25: Coral Reefs. Dubinsky, Foundation for extensive logistical support; and to Z. (ed.) Elsevier, Oxford, U.K. pp. 469-492. the Seychelles Island Development Company for permission to visit islands in the southern Seychelles. Fisher RL, Engel CG and Hilde TWC (1968). We also graciously acknowledge the encouragement, Basalts dredged from the Amirante Ridge, Western collaboration, and logistical support provided by the Indian Ocean. Deep Sea Research 15: 521-534. Seychelles Government and the Seychelles Islands Foundation. Special thanks are due to Rolph Payet, Green FP and Short FT (eds.) (2003). World Jude Bijoux, Rodney Quatre and Justin Prosper for Seagrass Atlas. UNEP, UCP, Berkeley. 286p. assistance with fieldwork. Jennings S, Marshall SM, Cuet P and Naim O (2000). The Seychelles. In: Coral Reefs of the Indian
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