Coral reefs: generation of sand

Coral reefs generate the fine coral sand supplying shores with the white sand characteristic of tropical islands and one of the main attractions in beach tourism (and tourist brochures!). This sand is not only generated from physical forces but also by the biota. Internal bioeroders such as algae, fungi, bacteria (microborers) and sponges (Clionidae), bivalves (Lithophaga), sipunculans (Aspidosiphon), polychaetes (Eunicidae) generate 10-100 micron sized sediment by boring into the reef structure while external bioeroders include urchins (Diadema) and chitons (Acanthopleura). In concert they generate large quantities of rubble, sand, silt, and clay sediments - sea urchins erosion rates have been reported to exceed 20 kg CaCO3 m-2 year-1 in some reefs.

 

Numerous fish species, mainly surgeonfishes and parrotfishes, erode coral while grazing on algae. Triggerfishes, filefishes and puffers fragment colonies while feeding on live coral tissue or in extracting invertebrates from colonies. Parrotfishes, in particular, can produce large amounts of sediment, especially when their population densities are high. They have well developed jaw muscle and tooth armature, and a pharyngeal mill - a gizzardlike organ that grinds up ingested material into sand- sized particles. In Bermuda the annual erosion rate of parrotfish has been measured at 400-600g of calcium carbonate from each square metre of reef. Parrotfish also tend to feed in shallow areas and defecate in deep, providing a net transport of sediment away from its site of removal.

 

Bioeroders usually attack dead coral and therefore, in general, any condition that causes coral death will increase the rates of limestone loss and sediment generation. Coral reefs in the Galapagos Islands, are currently being bioeroded to rubble and sediment following high coral mortality and low recruitment after the 1982-83 El Nino event. The loss of these erect corals (Acropora) eliminates important microhabitats for fishes and may ultimately suppress all coral recruitment.