Chewing, Rasping, and Drilling: Bioerosion Rates on Reefs

Coral reefs are suffering increasing degradation from coral bleaching and ocean acidification, and now we have discovered that bioerosion on reefs may also increase.

Coral reef maintenance results from the balance between constructive forces – mainly growth and calcification of corals and crustose coralline algae-, and destructive forces. Among destructive forces, biological erosion, also called bioerosion, is the main force of reef degradation. Bioerosion is due to various organisms that comprise grazers (e.g. gastropods, urchins, fishes), macroborers (e.g. sponges, polychaetes, bivalves) and microborers (e.g. cyanobacteria, green algae, fungi). These organisms use diverse mechanical and chemical mechanisms to chew, rasp, drill and dissolve reef carbonate substrates. Grazers are external agents of bioerosion and feed on phototrophic microborers. This synergistic activity, when very intense like in the Galapagos, is extremely efficient and can reduce dead corals into sand particles and dissolved calcium and carbon within a few years. Internal agents – macroborers and microborers- live inside live and dead carbonate substrates and depend on the water column characteristics. In addition to their destructive role, bioerosion agents play an important role in the maintenance of habitat diversity, reef sedimentation, benthic primary production and biogeochemical cycles.

Unfortunately the balance between reef growth and reef erosion is increasingly threatened by climatic and anthropogenic factors such as ocean acidification, warming and eutrophication. These factors tend to reduce calcification of corals, and increase both coral mortality and bioerosion rates. For instance, ocean acidification affects negatively coral growth and calcification while it increases growth of the boring microflora, therefore increases rates of carbonate dissolution by microborers. Eutrophication favors the development of filter feeding macroborers such as sponges and bivalves. Macroborers can thus be very abundant in coastal areas such as on the northern Great Barrier Reef, and in disturbed reefs of Indonesia and in the Caribbean’s. When reef bioerosion exceeds reef accretion, there is physical loss of reef structure putting in jeopardy reef diversity, maintenance and services to local human populations.

Despite the important role played by bioerosion agents in coral reef ecosystem functioning, this process has been less studied than reef accretion. Agents and processes of bioerosion in coral reefs will be presented in the context of global change and anthropogenic activities.