Coral Communities on a Seaward Reef Slope, Fanning Island

Abstract

The coral community on a 1 x 100 meter long vertical section of the leeward ocean reef slope at Fanning Island was quantitatively investigated with SCUBA and a quadrat transect technique. Forty-seven species of corals were noted, and coral cover averaged over 60 percent on the transect. Common coral species were restricted to certain depth regimes, which resulted in pronounced vertical zonation. The community as a whole could be objectively divided into three assemblages with respect to depth. The deepest assemblage (30 to 35 m) was characterized by low coral cover, small average colony size, and high species diversity. Some of the species were specialized types not found elsewhere on the transect. Environmental conditions appeared stable with respect to wave action but suboptimal with respect to light and sediment cover. At intermediate depths (20 to 25 m) the coral assemblage showed higher cover and larger average colony size, but lower diversity values due to dominance by a few species. Environmental conditions appeared to be both optimal and stable, with biological interactions determining the nature of the assemblage. At shallow depths (8 to 15 m) the assemblage showed slightly lower cover, moderately higher diversity, and moderately smaller average colony size. Wave action may periodically disrupt the environment, which is otherwise optimal for coral development. In the shallowest environments near shore reef substrates are dominated by coralline algae, and corals are rare. Environmental conditions are probably both suboptimal and unstable, resulting in the inhibition of coral development. The structure and probable factors controlling the structure of the Fanning coral community are similar to those of other reef slope communities recently studied, particularly those in the Red Sea.