South Island Reef is a coral reef extending over 1.8 km between South Island and Bird Islet, immediately south of Lizard Island. The reef is exposed to southeast trade winds but is protected from heavy swell by the outer Barrier Reef lying some 12 miles offshore. Benthic zonation and
community structure of South Island Reef were studied, with special reference to two of the most conspicuous benthic groups, which are also two of the most important primary producers, scleractinian corals and algae. Quantitative information was obtained for scleractinian corals by the use
of the line transect intercept method. Numerical analysis of the data collected indicates that the coral assemblages of the slopes are markedly different from those of the reef flat. A zonation pattern is defined on the reef flat. The various coral assemblages show very unequal development,
and most of the reef flat corresponds to a unit in which the subdivisions suggested by the numerical analysis do not represent drastically different coral communities. Owing to the frequent layering of algae in several stories, two indexes are defined: ground cover and line cover. The relative
importance at each station of the algal groups (erect calcareous algae, fleshy macro algae, crustose coralline algae and turf) is discussed, and the variations of abundance of these groups throughout the reef flat are utilized to define and to characterize the zonation of the reef flat.
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