Spatial Patterns in Biodiversity of a Fringing Reef Community Along Opunohu Bay, Moorea, French Polynesia

Opunohu Bay, located on the North coast of Moorea Island (Society Archipelago), is approximately 3 km long and partially edged by a small fringing reef no wider than 100 m. The main objective of this first quantitative survey concerning bays in French Polynesia, was to examine the spatial pattern of diversity of the macro-benthic fringing reef community between the land-end and the ocean-end of the bay. This study considered the conspicuous benthic organisms (Cnidaria, Echinodermata, Mollusca, Porifera and seaweeds) more than 1 cm long and attached to the substratum. A strong gradient in percent cover and species richness was found for corals and echinoderms, between the land-end where the two taxa are almost absent and the ocean-end where they are dominant. Species richness of macroalgae, molluscs and sponges, and macroalgal coverage are high in the middle part of the bay. At the ocean-end, dominance (in terms of coverage) of the coral genus Porites is responsible for the decrease in diversity (H'c) and evenness (J'c). Factorial correspondance analysis and ascending hierarchical clustering of species composition correspond to the land-end, the middle part and the ocean-end. The absence of corals and echinoderms at the land-end, where only a few tolerant macro-benthic species occur, can be explained by low salinity and high turbidity which occur after heavy rains during the wet season.