in BOUCHET P., LE GUYADER H. & PASCAL O. (Eds), The Natural History of Santo. MNHN, Paris; IRD, Marseille; PNI, Paris. 572 p. (Patrimoines naturels; 70). Algal and Seagrass Communities from Santo Island in Relation to Habitat Diversity Claude E. Payri The coral reef communities of Vanuatu five species from Santo, i.e. Cymodocea rotundata, have been little studied and nothing Halodule uninervis, H. pinifolia, Halophila ovalis and has been previously published on Thalassia hemprechii. the benthic algal flora from Espiritu Santo Island. Some marine algae The present algal flora and seagrass investigation of from Vanuatu have been found in Santo was conducted during August 2006 as part of the British Museum collections (BM) the "Santo 2006 expedition". This work is a com- and are mainly Sargassum species and panion study to that of the Solomons and Fiji and is Turbinaria ornata. In their report on intended to provide data for ongoing biogeographic Vanuatu’s marine resources, Done and work within the West and Central Pacific. Benthic Navin (1990) mentioned Halimeda opuntia as occurring in most of the Extensive surveys have been carried out in most sites investigated and noted the high encrustation of the habitats recognized in the southern part of of coralline algae in exposed sites. More work has Santo Island and in the Luganville area, including been done on seagrass communities; earlier authors islands, shorelines, reef flats, channels and deep reported a total of nine species from Vanuatu including outer reef slopes. SAMPLING SITES AND METHODS The 42 sites investigated are shown in figure 409 species list for the southern part of Santo. and are distributed from Palikolo in the north- Specimens were sampled to make a taxonomical ernmost part of the study area, down to Urepala collection for the area. islet located in the southern part including the Segond Channel, the Malo passage and Abokisa All the collected specimens were pressed and dried Island on the east coast of Aore Island. Sites were for herbaria; fragments of specimens were preserved selected to include the largest possible range of in a solution of buffered formalin in seawater (5 %) environments. for further anatomical studies. Tissues from vari- ous taxa were also preserved for further phylog- Most of the sites were surveyed by SCUBA divers eny and molecular analysis and all the herbarium from the surface down to 60 m deep. The shallow specimens were air dried (without formalin) which areas, including fringing reef flats, reef channels allows for extra DNA analysis if necessary. The and rocky shorelines, were sampled by snorkelling collection is currently housed in the phycological and walking on the reef. The sampling effort was herbarium at IRD Nouméa (IRD-NOU) and will standardized and inventory duration at each site be transferred to the Muséum national d’Histoire was fixed to 80 minutes. A species inventory was naturelle in Paris (PC). Part of the collection will compiled in order to create a more comprehensive be deposited at USP in Suva, Fiji. MARINE MACROPHYTES IN SANTO: GENERAL INSIGHTS Macrophyte communities on coral reefs are In addition, Vanuatu’s reefs have a complex tec- generally distributed in assemblages that more tonic history, having experienced several emer- or less reflect reef zonation. The distribution gence and subsidence events. These have resulted of the marine flora on a coral reef is influenced in some features that are typical of many reefs by various factors including sunlight, salinity, with rocky shorelines, and recent tectonic dis- water turbulence and currents, the nature of placements and uplifts may have affected some the substratum, depth, exposure to air, geo- of their benthic assemblages. However, we did morphology, topography, herbivore pressure not observed a recent influence of tectonic dis- and biological competition with other benthic placement on benthic communities in the study organisms. areas and the most significant disturbances we 337 ... Marine Ecosystems ....... Santo 2006, marine algae habitats ( sampling sites � wrecks 32 ( Reef geomorphology Mainland Shelf patch-reef complex Ocean-exposed fringing reef main land subtidal reef flat deep terrace Inner-seas patch-reef complex Inner-seas exposed fringing reef fore reef enclosed lagoon or basin forereef reef flat forereef pinnacle shallow terrace reef flat reef flat Shelf slope subtidal reef flat shallow terrace shelf slope Cartographie : C. Chauvin IRD 2008. 15 ( Source : Payri Santo 2006, Andréfouët Millenium Coral Reef Mapping Project. (37 Paikolo 38 ( ( 2 (1 ESPIRITU SANTO Luganville ( ( 41 ( 39 40 ( � ( �4 Segond Channel ( Coolidge Million Dollar Point 13 30 ( ( 34 31 ( (14 8 ((9 (29 12 ( 6 ( Tutuba 28 Aore ( 35 ( 5 ( 36 3 ( 11 ( ( 26 ( 7 (( 42 10 21 Abokisa ( Brigstocke 23 ( ( ( 25 20 ( ( ( 33 ( 18 ( ( 19 16 ( 24 17 Bruat Channel Urelapa 22 27( ( ( (44 45 ( 46 � 43 ( 0 2,5 5km Malo Datum : WGS 1984 Projection : UTM Zone 58 S Figure 409: Map of sampling sites in the Luganville area, southern part of Santo Island. observed were the result of recent cyclone activity are generally distributed parallel to the shorelines and bleaching events over the past few years. and reef margins. However, environmental factors are not homoge- This synthesis comprises an overview of the rep- neous across the reefs, and gradually change from resentative biotopes investigated and describes the the shoreline to outer reef slopes through lagoons different macrophyte (algae and seagrasses) commu- and reef flats. The rather patchy zonation patterns nities associated with the different identified habitats. ...338 ....... The Natural History of Santo REPRESENTATIVE BIOTOPES The main features of the Santo coral reef complex The 42 sites surveyed have been classified into 12 major are the absence of a barrier reef and associated habitat groups which include geomorphology, topog- habitats. Most of the structures comprise narrow raphy and major benthic communities. Schematic fringing reefs, outer reefs, patch reefs in shallow diagrams (profiles) are given in figures 411-422, the water, sheltered and open embayments, deep chan- list of the symbols used in the profiles are given figure nels and shallow passes, exposed outer reef slopes 410. Descriptions of the profiles are as follows. and reef walls or drop-offs. … Segond Channel (Fig. 411) Most of the sites have reef habitats compressed into This long channel runs between Santo and Aore Island. narrow coastal margins and exposed to ocean influ- Around the Luganville area and down to the south ences. On unsheltered coasts the coral reefs are there are few reef formations and coral communities wave-beaten structures that are heavily encrusted and these are mainly developed on sandy slopes and by coralline algae as well as by coral species that rubble. Narrow reef flats are present, mostly at both are well-adapted to strong turbulence. entrances to the Channel and along the Aore Island coast. Coral communities are mostly Acroporidae and The great ocean depths, large fetches and the refrac- occasional massive Porites. There is also evidence of tion of swells around the small islands adjacent damaged coral in the high proportion of coral rubble. to Santo mean that the open coasts on all sides Large beds of the green macrophyte Halimeda opuntia are subject to strong wave forces, and this limits intermixed with sponges colonise the hard substratum. the types of reefs that can establish. Less robust In the northern area, the middle part of the channel forms of corals and other benthic communities are is deep (70 m depth) and muddy, marked with ghost however able to develop in more sheltered embay- shrimp (Calianasseae) hummocks and small benthic ments. The islands around the Segond Channel communities including some Nephtheidea, Dophleina provide significant shelter from the open ocean, and Asthenosoma urchins. The benthic communities and the channel accumulates siltation originating of the channel environment are dominated by sponges from the terrestrial erosion of the adjacent island and octocorallians (soft corals and sea fans). On the of Santo; the channel supports a range of habi- shallow muddy flats adjacent to the shore of Santo tats with conditions ranging from intermediate to island seagrasses such as Halodule and Cymodocea abundant shelter, and muddy substrata. form sparse patches. Living massive coral Sponges Living branched coral Dead corals Living tabular coral Coral rubble Living foliaceous coral Ghost shrimp hummock Tubastrea Gorgonian Mangrove Sarcophyton Lobophyton Seagrass Coconut tree Halimeda Caulerpa Trees Rhodophyta Native houses Figure 410: List of symbols used for figures 411-422. 339 ... A B C I Marine Ecosystems H D E F G ....... ... L J K 340 0 0 10 10 mudflats andmangrove Depth (m) Depth (m) 20 20 30 30 40 shoreline 40 50 50 seagrass bed outward channel slope channel bottom inward channel slope 0 Distance from shore (m) 1800 Santo Is. Segond Channel Aore Is. Figure 411: Schematic diagram of the topography and reef communities of the Segond Channel. A: Mangrove trees on the shoreline. B: Typical gentle slope with dense Halimeda bed. C: Typical mixed community on channel slope dominated by Neiphteidae. D: Diffuse seagrass bed of Halodule pinifolia and Halophila ovalis. E: Halimeda minima forming thick mats with sponges and other organisms on the channel slope. F: Typical community of invertebrates and algae on the deep channel edge. G: Mixed seagrass bed of Halodule uninervis and Cymodocea serrulata. H: Physogyra. I: Gorgonian Juncella. J: Fromia milleprorella. K: Dophleina. L: Gomophia watsoni. (Photos J.-L. Menou & J.-M. Boré IRD Nouméa). A B D E F C H I J ....... G 341 ... K 0 0 10 10 Depth (m) Depth (m) 20 20 reef flat 30 30 The Natural History of Santo rocky reef flat outward channel slope channel bottom inward channel slope 40 40 0 Distance from shore (m) 650 Santo Is. Segond Channel Aore Is. Figure 412: Schematic diagram of the topography and reef communities of Channel Pass (southern entrance of the Segond Channel).