Inhibition of reef framework by frequent disturbance: examples from the Arabian Gulf, South Africa, and the Cayman Islands

Abstract

The environmental processes that prevent reef framework development in areas with a diverse and dense cover of coral were investigated in the southern Arabian Gulf (Dubai, UAE), the southwestern Indian Ocean (South Africa) and the central Caribbean (Cayman Islands, BWI). In Dubai, temperature anomalies predicted to recur every 10–15 yr cause repeated mass mortalities of Acropora spp., the dominant potential framebuilding species. Subsequent bioerosion of the dead coral skeletons renders them brittle and susceptible to fragmentation during winter storms. As a consequence, regrowth of the next generation of framebuilders, which takes about 10 yr, has to initiate on the original bedrock surface. This cycle of mass mortality, framework break-down and regrowth inhibits in situ framework accumulation. In South Africa, the formation of coral frameworks is prevented by annually recurring high wave energy events, which stunt coral to such small sizes that no framework is built. The situation is further aggravated by the fact that branching coral fragments, which could serve as asexual propagules, are washed off the reefs and lost. In the Cayman Islands, no coral framestones of in situ skeletons are formed in areas shallower than about 5 m. In these areas, coral frameworks formed by Acropora palmata are removed by hurricanes, broken up and deposited in the form of a rubble accumulation further inland on the island shelf. Thus, although a coral deposit remains, it is not an in situ, grown framework. However, since the build-ups in the upper 5 m are mostly made up by coral fragments, such structures would likely be identified as reefs if encountered in a fossil outcrop situation. These three models can help to explain the paleoecology of allochthonous, peri-reefal or non-reefal coral deposits (i.e. coral rudstones, floatstones, bindstones). They also demonstrate that at least in recent coral systems, species richness and high coral coverage alone need not necessarily translate into high in situ framebuilding potential.