Box 2: Coral Reefs and Atoll Formation

Corals are anthozoans, organisms within the phylum Cnidaria. Anthozoans comprise more than 6,000 species and in addition to corals include sea fans, sea pansies, and sea anemones. Stony corals (schleractinians) make up the largest order of anthozoans. Stony corals and coralline algae are primarily responsible for providing the foundation and structure for coral reefs. Schleractinians are colonial organisms composed of individuals called polyps. Most corals contain symbiotic algae called zooxanthellae. The corals provide a protective environment and the compounds necessary for the algae to photosynthesize. The algae in return produce organic products – glucose, glycerol, and amino acids – that the coral use as building blocks in manufacturing proteins, fats, and carbohydrates. In addition, these algae produce organic products that are used by the coral to produce calcium carbonate. Massive reef structures are formed when each coral polyp secretes a skeleton of the mineral aragonite, which is calcium carbonate (CaCO3). Coral reefs can grow from 0.5 to 2 centimeters (0.2 to 0.8 inches) per year under normal conditions and up to 4.5 centimeters (1.8 inches) per year under more favorable conditions (high light exposure, consistent temperature, moderate wave action). Reef building corals are restricted in their geographical distribution due to the environmental conditions needed by the coral polyps. In general, major reef building requires temperatures between 18 to 29o C, salinity between 32 to 42 parts per thousand, and the water must be clear enough for a high degree of light penetration. A concert of these favorable conditions is usually geographically restricted to tropical and subtropical waters between 30o S and 30o N.

Coral reefs begin to form after coral larvae attach to submerged edges of islands or continents. As the corals grow and expand the reef begins to take on one of three characteristic structures – fringing, barrier, or atoll. Fringing reefs are the most common type of coral reef and project seaward from the shoreline forming borders along the shoreline and surrounding islands. Barrier reefs also border shorelines, but at a much greater distance from the shoreline compared to fringing reefs (i.e., Australia’s Great Barrier Reef). Barrier reefs are separated from their adjacent land mass by a lagoon of open water that can be deep. If a fringing reef forms around a volcanic island that subsides completely below sea level as the fringing reef continues to grow toward the surface then an atoll forms. Atolls are usually oval or circular with a central lagoon (see figure below).

Stages of atoll formation (from http://earthobservatory.nasa.gov/Study/Maldives/maldives2.html)

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