Coral reef, definition

Abstract

Two similar definitions (“coral reef” and coral-algal reef) are provided, one strictly geological and one that specifies living corals as a fundamental defining characteristic of “coral reef.” The definition is then dissected, and allied terms such as “coral community” and “coral reef community” discussed, noting the completely different meaning of these terms in geology and ecology. The importance of ecological processes in keeping up the supply of building blocks (corals and reef debris) is discussed. Differences in local environment and in particular, hydrodynamic setting, lead to different intergenerational performance of coral populations, and also the propensity for reefs to act as sediment importers, producers or exporters, or to be in a state of net loss due to bio-erosion. A place’s environmental regime considered over decadal time scales and up (including “normal” conditions and extreme events) will determine the likelihood of a coral community making the transition (Figure 1) toan“incipient coral reef ” (early stages of development of a wave resistant structure) and a “coral reef” proper. Key determinants are first, whether there is a sufficient density of corals in a sufficiently large area that grow long enough and get large enough to create a wave-resistant structure; and second, whether that structure gets large enough to attenuate and refract waves and currents in a manner that promotes the retention and compaction of its own biogenic sediments within and around itself. Just as the terms “juvenile,”“mature,” and “senescent” are applied to the coral reefs as geological structures, so are they applied to the tracts of corals for which the structure is simply “substratum.” However, the links between coral dynamics and reef development are poorly understood (Perry et al., 2008b). Improved understanding is important, especially in light of serious, persistent, and widespread declines in coral cover and resilience (e.g., Bellwood et al., 2004) and some very bleak projections for the future of coral reefs under global climate change (Veron, 2008).