Community structure and zonation of microalgae and hermatypic corals on a fringing reef flat of magnetic island (Queensland, Australia)

Abstract

The community structure and zonation pattern of the two dominant benthic components (macroalgae and hermatypic corals) on a fringing reef flat at Geoffrey Bay (Magnetic Island, North Queensland) were investigated quantitatively, using a line transect intercept method. Data collected on species composition, distribution and abundance at three sampling locations on the reef flat were analysed and discussed in relation to the prevailing environmental factors.Two major communities are recognized: an Inner Sedimentary Accumulation community, and a Reef Flat community. Within the latter, several ecological units are distinguished in a seawards sequence: an Inner Reef Flat facies, a Middle Reef Flat facies and a Reef Flat-Reef Slope transition zone. Each of these units corresponds to particular morphological, sedimentological and hydrodynamic conditions which determine the distribution of species and species groups on the reef flat.Macroalgae are the dominant community component, by virtue of their widespread distribution and high abundance. In contrast, corals are restricted to the outer reef flat and seagrasses mainly to the inshore sedimentary substrates both being of relatively minor importance as structural elements in the communities. Macroalgal group analysis indicates that the low-growing turf forms are the most significant cover contributors on the reef flat. However, the large fleshy macroalgae, in particular tall Phaeophytes such as Sargassum spp., dominate community structure. The erect calcareous alga, Halimeda opuntia (Linnaeus) Lamouroux is an important component of the algal flora, particularly on the inner reef flat areas, notably contributing to reef sediments. Conversely encrustation on the reef by calcareous algal development is extremely limited.The zonation of organisms on the reef flat at Geoffrey Bay is discussed and compared with that of local (Great Barrier Reef) and other Indo-Pacific fringing reefs. The observed patterns appear typical of fringing reef flats associated with high islands in sheltered conditions.