Coral reefs are dynamic systems, a “healthy reef” is a balance between reef growth and reef destruction. Reef destruction consists of physical erosion by storms, chemical erosion with calcium carbonate dissolving into the water column, and biological erosion or bioerosion.
Coral reef showing bioerosion
Photographer: Public Domain © U.S. National Oceanic and Atmospheric Administration
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Rates of physical erosion increase with wave activity: Figure 1 (see right) shows the surf pounding onto the front of the reef and this is where the maximum rates of erosion occur.
Biological erosion consists of two processes, grazing and boring:
- Grazing of both live coral and dead coral substrate occurs by scarids (parrotfish) (Fig 2 & Fig 3) echinoids and chitons, they scrape off the surface of the substrate in order to collect the endolithic algae which live just under the surface of the coral (Fig 4), using modified mouth parts.
- Boring occurs by a range of organisms including worms, molluscs and sponges as well as micro-organisms such as algae, and fungi (Fig 5). As reefs become increasingly impacted by anthropogenic activity this balance between reef growth and reef destruction shifts with rates of destruction far exceeding rates of growth.
Pat Hutchings has spent time studying both healthy and impacted reefs. Working with her French colleagues Dr Mireille Peyrot-Clausade, they have studied rates of bioerosion using experimental blocks which are attached to the substrate and recovered at intervals (Fig 6). After 6 months of exposure at Faaa, Tahiti, dense populations of echinoids have removed substantial amounts of the reef framework (Fig 7) as the water quality here is low which encourages the development of endolithic algae on which the echinoids feed, and overfishing has removed all the predators of the echinoids, and little coral recruitment is occurring. If this continues the entire reef will be lost together with the protection which the reef affords to nearby low lying areas during storms.
Pat Hutchings became interested in bioerosion as polychaetes are some of the earliest borers when corals die and this interest has developed into studying the entire process of bioerosion both on modern and fossil reefs.
For additional information on coral reef erosion through calcium carbonate dissolving into the water column, see Ocean Acidification.
Mr Martin Pueschel , Scientific Illustrator