Quantitative Study of the Scleractinian Coral Communities of a Fringing Reef of Reunion Island (Indian Ocean)

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Abstract
A fringing reef situation on the leeward coast (northwest) of Reunion Island (Mascarene Archipelago, Indian Ocean) was studied. A boat channel, a compact reef flat and a reef flat with transverse stripes were successively found between shore and reef front. Two structures were defined on the outer reef slope: the coral-built spurs-and-grooves zone (0-20 m) and the lower sloping platform, a volcanic flagstone with low biogenic coverage (20-30 m). Seawards, this flagstone is completely covered by a field of free organic nodules. Scleractinian coral communities were studied with quantitative sampling methods (quadrat and transect techniques) in terms of species composition, zonation and diversity patterns. On the basis of these data, the coral communities were divided into 3 assemblages: a community of the boat channel and the compact reef flat; the emersion of this reef flat with transverse stripes appears as a transition zone with the 2nd community; a community of the spurs-and-grooves zone of the outer reef slope, divided into 3 subzones, i.e., upper level, influenced by strong wave action, characterized by spatial competition between scleractinian corals and encrusting calcareous algae, intermediate level dominated by a small number of acroporid and poritid species, and lower level, where the decrease in light intensity reduces the building activity of Scleractinia; and a community of the lower part of the outer slope, extending to the nodules field, characterized by settlement of an important number of deep species and by high species richness. Forty-two genera and 120 spp. of Scleractinia reveal a remarkable faunistic richness for the reef studied.