Bioerosion

Coral reef bioerosion, the fracturing and dissolution of carbonate skeletons by organisms, is inherently high in the biologically productive eastern Pacific region. Disturbances that cause coral mortality can accelerate carbonate erosion by exposing dead surfaces to internal and external bioeroder species. Commonly present internal microscopic bioeroders are cyanobacteria, microalgae, and fungi; among internal macroborers are sponges, polychaetes, and peanut worms, and bivalve mollusks (date mussels). Prominent external bioeroders are hermit crabs, sea urchins (Diadema and Eucidaris), and fish (pufferfishes and triggerfishes). Destructive but less common coral breakage is caused by sharks and turtles in pursuit of prey. Ocean acidification, a result of anthropogenic-generated CO2, depresses carbonate skeletal precipitation and internal cementation thus enhancing bioerosive processes. Internal and external bioerosion can cause rapid carbonate loss and reef framework disintegration. Sea urchin bioerosion after the 198283 El Nino disturbance amounted to a 0.5 m reduction in vertical framework height of a Panamanian reef over a 16 year period, and complete reef frame destruction of a 1 m high patch reef in the southern Galapagos Islands in <5 years. Paradoxically, the bioerosion of reef frameworks, resulting in increasing cover of dead coral rubble, may not depress but increase cryptic species biodiversity on Pocillopora reefs in Panama. Repeated El Nino coral mortality and bioerosion of reef frameworks prevent continued development of coral reefs reaching large size in the Galapagos Islands.