Reefs at the edge: coral community structure around Rapa,southernmost French Polynesia |
| |
| Authors: | Mehdi Adjeroud Carden C Wallace Pauline Bosserelle Claude Payri Jean‐Louis Menou Michel Pichon |
| |
| Institution: | 1. Institut de Recherche pour le Développement, UMR 9220 ENTROPIE & Laboratoire d'Excellence CORAIL, Perpignan, France;2. Queensland Museum, South Brisbane, Qld, Australia;3. USR 3278 CNRS EPHE, Centre de Recherches Insulaires et Observatoire de l'Environnement, Moorea, French Polynesia;4. Institut de Recherche pour le Développement, UMR 9220 ENTROPIE & Laboratoire d'Excellence CORAIL, Centre IRD de Nouméa, Nouméa, New Caledonia;5. School of Tropical and Marine Biology, James Cook University, Townsville, Qld, Australia |
| |
| Abstract: | Rapa (27°36′ S, 144°20′ W) is a small (~40 km2) volcanic island isolated in the Southern Austral Archipelago, where direct anthropogenic stressors are extremely limited. Here, we present the results of the first quantitative survey of coral community structure across habitats and depths around the island. Despite its geographical isolation in the depauperate South Central Pacific, its small size and unfavourable environmental conditions (competition with macroalgae, low sea surface temperatures, reduced reef accretion), the diversity of scleractinian corals at Rapa is particularly high (112 species from 32 genera, including 37 species of Acropora) in comparison to other French Polynesian islands and subtropical Pacific locations. Our results indicate that the abundance (>100 colonies per 10 m2 recorded at nine of the 17 sampling stations) and cover (>40% at four stations) of corals are relatively high for a marginal reef location. Strong spatial heterogeneity was found, with high variation in diversity, abundance, cover and community composition among stations. Variation in community composition was related to habitat types, with distinct assemblages among fringing reefs within bays, reef formations at bay entrances, and those on the submerged platform surrounding the island. On the platform, a depth gradient was detected, with generic richness, abundance and cover generally greater at deeper stations (18–20 m depth) compared with medium‐depth (10–12 m) and shallow (1–3 m) stations. A gradient was also recorded along bays, with increasing coral diversity and abundance from the bay heads to the bay entrances. The coral community at Rapa was characterized by the presence of several taxa not found in other French Polynesian archipelagos and the rarity of others that are common and abundant in the Society and the Tuamotu islands. Another distinctive feature of reef communities at Rapa is the high cover and dominance of macroalgae, particularly in the shallower parts of the surrounding platform, which probably explains the lower densities of coral colonies recorded there. These characteristics of the diversity and biogeographical composition of coral assemblages at Rapa provide considerable ecological grounds for its conservation. |
| |
| Keywords: | Austral Islands community structure coral reefs marginal environments scleractinian corals spatial patterns subtropical reefs |
|
|
