Sedimentation and Coral Reef Development in Turbid Water: Fanning Lagoon

Abstract

Lack of light and excessive sediment deposition rates are factors limiting coral reef development. The presence of very turbid water and muddy bottom does not mean, however, that coral growth is prohibited. Fanning Lagoon has a turbid water area (visibility, 2 m) and a clear water area (visibility, 10 to 15 m). Both areas have a muddy bottom. Because of the shallow depth and the light-scattering effect of the suspended CaC03, relative light intensity at the bottom is greater than 5 percent. The cleaning mechanism of the corals is sufficient to handle the deposition of sediment. Live corals cover 62 percent of the clear-water area and 31 percent of the turbid. Reefs in the turbid water are ecologically different from the ones in clear water, but they are still living reefs. Ramose corals make up 55 percent of the individuals in the turbid water and only 10 percent of those in the clear water. This difference is reflected in the structure of the reefs; those in clear water are massive and steep-sided, while those in the turbid water have gentler slopes and are more open with sediment infill. Fanning Lagoon is an example of penecontemporaneous formation of reef and intervening muddy sediment with bathymetric relief never more than 8 m.