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Development of a Regional Habitat Classification Scheme for the Amirante Islands, Seychelles

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Development of a Regional Habitat Classification Scheme for the Amirante Islands, Seychelles Western Indian Ocean J. Mar. Sci. Vol. 9, No. 1, pp. 43 - 55, 2010 © 2010 WIOMSA Development of a Regional Habitat Classification Scheme for the Amirante Islands, Seychelles Sarah Hamylton, Annelise Hagan and Tom Spencer Cambridge Coastal Research Unit, Department of Geography, University of Cambridge, Downing Place, Cambridge, CB2 3EN, UK. Keywords: Remote sensing, Geographical Information Systems, coastal management, habitat. Abstract—A collaborative expedition between Khaled bin Sultan Living Oceans Foundation, Cambridge Coastal Research Unit and Seychelles Centre for Marine Research and Technology – Marine Parks Authority (SCMRT-MPA) was conducted to the southern Seychelles, western Indian Ocean, in January 2005. This resulted in a series of habitat maps of the reefs and reef islands of the Amirantes Archipelago, derived from remotely-sensed Compact Airborne Spectrographic Imager (CASI) data. The procedures used in map development, image processing techniques and field survey methods are outlined. Habitat classification, and regional-scale comparisons of relative habitat composition are described. The study demonstrates the use of remote sensing data to construct digital habitat maps for the comparison of regional habitat coverage, a key function for coastal management. INTRODUCTION diversity and ecological status, allow comparison of the status within and Surveys of coastal environments between ecoregions, and facilitate provide information on the the detection of changes in coastal distribution and abundance of shallow ecosystems relative to established water benthic communities, as well baselines (English et al., 1997). In the as local topography and bathymetry. shallow water environments of the Standardised survey protocols permit tropics and sub-tropics, standardised assessment of regional biological surveys can be applied to ecosystems Corresponding author: SH E-mail: [email protected] 44 S. HAMYLTON ET AL. such as coral reefs, seagrasses and temporally dynamic benthic surfaces mangroves, which collectively provide as discrete units. This requires valuable ecological functions and some form of classification scheme. services in the marine environment. Habitat classification schemes play Remote sensing instruments an important role in standardising provide a synoptic portrait of the the thematic content of regional Earth’s surface by recording numerical benthic maps, facilitating their use information on the radiance measured as a common baseline against which in a series of picture elements (pixels) regional subsets can be interpreted. In across a number of spectral bands a spatial context, classes are assigned (Green et al., 2000). When mounted to homogenous patches of surface on an airborne platform, they sample that differ in appearance from their habitat cover over extensive spatial surroundings but may vary widely in ranges. The ability to survey detailed size, shape, type, relative heterogeneity information from relatively large areas and boundary characteristics (Forman is particularly valuable at the coast and Godron, 1986). Ecological because changing weather, locations units can be divided hierarchically which are inaccessible or difficult to to accommodate user requirements. access, and the logistical challenges Top-level descriptions often identify of carrying out underwater fieldwork geomorphological context, while all compromise the ability to survey lower tiers describe the relative cover a representative sample of these of benthic organisms discriminated shallow water environments directly. from ground-verified data (e.g. Remote sensing data benefit the Mumby and Harborne, 1999). A mapping process by enabling accurate hierarchical classification scheme that extrapolation of information to broader subsumes both geomorphological and scales, providing information on areas ecological characteristics is systematic not accessible in the field. In this way, in structure with tier-level descriptors cost savings can be achieved. Mumby that cannot be used interchangeably. et al., (1999) found the combined use Geomorphological classes (e.g. fore- of remote sensing and field surveys to reef slope coral spur) can, therefore, be a cost-effective means of acquiring be coupled with ecological ones (e.g. meaningful survey data on the Caicos high cover of calcareous algae) within Bank when compared to a solely field- the same tier level. based approach. Many coastal mapping strategies employ coarse mapping at the Hierarchical classification schemes regional scale, augmented by finer for coastal zone management resolution maps at locations of To map coastal habitats, it is necessary specific interest (e.g. Borstadt et to represent spatially continuous and al., 1997). Despite the potential use DEVELOPMENT OF A REGIONAL HABITAT CLASSIFICATION SCHEME 45 of habitat classification schemes in discrete data structure on communities regional comparisons, most coastal that often present themselves as a habitat mapping has been conducted continuum of changing densities, on an ad hoc basis and displays such data formats lend themselves little consistency in terminology. well to analysis within Geographical This limits the interpretation of map Information Systems (GIS) (Burrough products, particularly where regional and McDonnell, 1998). Such an comparisons would be of value. The approach to data storage provides a use of digital habitat maps derived computationally efficient means by from remotely-sensed imagery is also which to carry out statistical analyses limited by these difficulties as they on landscape-scale datasets, e.g. range considerably in resolution. extracting coverage values for the Habitat maps are commonly different habitats. compiled from vector data, which partition an area in such a way that STUDY AREA each location falls into a polygon The Amirantes Archipelago, which assigned a value that is assumed to be lies SW of the extensive, shallow homogenous for all locations within water Seychelles Bank in the western that polygon. Although this imposes a Indian Ocean, comprises a group of (i.) Seychelles Figure 1. (i.) Location of the Seychelles Islands, western Indian Ocean. 46 S. HAMYLTON ET AL. (ii.) 53º00 E African Banks Remire D’Arros St. Joseph Sand Cay Desroches Poivre Etoile 06º00 S Boudeuse Marie Louise Desnoeufs Alphonse 07º00 S Bijoutiere and St. Francois Figure 1. (ii.) The Islands of the Amirantes Bank, Seychelles. carbonate islands and islets extending at sea level with varying degrees of over a distance of ~152 km, from subaerial sand cay and coral island o 4°52’S (African Banks) to 6 14’S development. They have evolved (Desnoeufs) (Fig. 1). The majority over the last 6,000 years since the of the islands are coral reef platforms post-glacial sea level approached its DEVELOPMENT OF A REGIONAL HABITAT CLASSIFICATION SCHEME 47 Table 1. Aerial coverage of CASI data. cyclonic gyre to the south (between 40–15°S) and reversing monsoon Number of islands surveyed 13 gyres north of 10°S. The northern Area flown 268 sq kms boundary of the subtropical gyre Flight lines flown 110 lines is formed by the South Equatorial Current (SEC). During the northern Data volume (raw) 65 Gbytes hemisphere summer the SEC is Data volume processed (estimated) 150 Gbytes displaced northwards as far as present level on the Amirantes Bank 6°S, and thus into the region of the (Stoddart, 1984). The Amirantes Bank Amirantes Archipelago, with typical -1 is an elongate structure, measuring current speeds of 0.25 m s and an approximately 180 km by 35 km, estimated transport rate of 50 Sv 6 3 -1 deepest in its centre (up to ~70 m) with (where 1 sverdrup (Sv) = 10 m s ). a marginal rim 11-27 m deep. With These climatic patterns influence the exception of the islands of Etoile both the structural form and surface and Boudeuse, the reef platforms lie communities of the reefs of the towards the eastern margin of the Amirantes, which display notable Bank. The atolls of Alphonse and leeward-windward contrasts in reef Bijoutier/St François which form the platform development (Spencer et Alphonse Group are approximately 95 al., 2009). km further south. Of the 14 islands, 13 were mapped, the exception being METHODS Desroches, a shallow submerged atoll, The primary aim of this collaborative 19-21 km in diameter, lying 16 km to expedition was to use a Compact the east of the Amirantes Bank. Airborne Spectrographic Imager The climate of the western Indian (CASI) remote sensor onboard a Ocean is humid and tropical, with seaplane to conduct large-scale mean monthly temperatures always >20°C and an annual rainfall >700 mapping of the reefs and islands of the mm. Seasonal and inter-annual Amirantes Bank. The sites comprised climatic variability is determined laterally extensive shallow water by i) the SE Asian Monsoon and the landforms, which were ideally suited associated seasonal reversal of winds; to airborne mapping. Concurrent field ii) monsoon-related movements of surveys were conducted alongside the Inter-Tropical Convergence Zone (ITCZ); iii) changes in the position the airborne surveys. Data were and intensity of the South Indian collected on the terrestrial and marine Ocean subtropical high pressure; and habitats. Results from the CASI image iv) variations in ocean circulation processing provided the first detailed and sea surface temperature. maps of the distribution of shallow The surface ocean circulation marine habitats for each of these of the western Indian Ocean is characterised by a subtropical, anti- locations. 48 S. HAMYLTON ET
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