Zonation of macrobenthic communities along two bays in an insular coral reef ecosystem (Moorea, French Polynesia).

Zonation of macrobenthic communities (corals: algae, molluscs, sponges and echinoderms) on fringing reefs was investigated in two bays in a coral reef ecosystem (Moorea, French Polynesia). Species richness, abundance and cora l cover, and species richness of macroalgae increased from the bayhead to t he bay entrance. For molluscs and sponges, no general trend was observed. S pecies richness and abundance of echinoderms increased from the head to the entrance of Opunohu Bay, but this trend was less pronounced in Cook Bay. T he gradients observed for corals, macroalgae and echinoderms were correlate d with one or several of the following abiotic factors: salinity, turbidity , concentration of silicates in surface waters, and concentrations of organ ic carbon, carhohydrates and amino acids in the sediments. These factors ar e associated with terrestrial run-off via river discharge that occurs at th e bayheads, where the major river is located. The high degree of confinemen t in the bayheads allowed the establishment of only a few tolerant macroben thic species. Although the low diversity and abundance of corals and echino derms seem to be a characteristic of Polynesian bays, a high diversity of c orals can be found in the vicinity of the bayheads in coral reefs of the we stern Pacific. (C) 2000 Academie des sciences/ Editions scientifiques et me dicales Elsevier SAS.