This paper gives general information on: (1) sedimentary environments and carbonate producers in some Maldivian atolls (North Malé Atoll and Felidu Atoll); (2) theAcanthaster planci outbreaks in North Malé Atoll and (3) the geological evolution of the Maldives.
The distribution and the main features of the reefs are described. The rim of the atolls can be erosional or bioconstructed; the seaward side is often erosional while coral reefs thrive in the lagoonward side and in the lagoonal patch reefs. Attention is paid to the distribution of the main hard coral genera in the different environments of the atolls (oceanward rim, lagoonward rim, passes, lagoonal patch reefs and faro lagoons). GenusDistichopora (Idrozoa) andZoopilus (Scleractinia) are reported in the Maldives for the first time; many sclearactinian species not prerviously reported are illustrated. Evidence of the importance ofTydemania (Codiacea) as a Maldivian producer of fine-grained carbonates is provided.
General inforomation aboutA. planci and its outbreaks in the Maldivian atolls is given. The spawning of the MaldivianA. planci occurs in the spring during the monsoon inversion withChaetodon falcula (a butterfly fish) being an important predator ofA. planci eggs. The skeleton of the Maldivian variety ofA. planci is briefly described.Plerogyra, Physogyra andEuphyllia are reported for the first time among the scleractinians surviving after theA. planci invasion.
DSDP and ODP data were used to calculate the subsidence rate of the Maldivian atolls (from 70 to 30 m/my); this value is much lower than the presumed upward growth rate of a thriving reef (7–8 mm/yr). Only the glacio-eustatic sea-level rise (similar to the post-Wurmian rise) has attained this value.
These observations suggest that the drowning of an atoll or of a shallow water carbonate platform may mainly be due to ecological crises that impair the carbonate production of the framebuilding communities. Crises can arise from changes in climate or in oceanic circulation, from emersion followed by sudden sea-level rise or from the invasion of predators.