Coral reefs are considered to be the most diverse and complex marine ecosystem on Earth.
Yet well developed reefs grow under a restricted set of environmental parameters. Factors most significantly influencing coral growth and distribution include temperature, light, and nutrient levels. Optimal conditions for coral growth are warm (18-30 °C), clear, oligotrophic (nutrient-poor) waters.
The classification of coral reefs dates back to Darwin who placed them in three main categories: fringing reefs, barrier reefs, and atolls. Many scientists also recognize bank reefs and patch reefs as distinct reef types. Still other scientists have put forth their own theories or developed their own classifications. Fringing reefs, barrier reefs, atolls, bank reefs, and patch reefs are described below. Some reefs found in the national marine sanctuaries fit obviously into one of these categories. In other cases, the classification scheme does not clearly apply.
Fringing reefs closely follow shorelines at a short distance with at most a narrow, shallow lagoon between reef and land. Barrier reefs also generally follow a shoreline, however, they are separated by a greater distance and a deeper lagoon. The distance may vary from about 1.5 to 40 km and the lagoon may be deeper than 18 m. Atolls are horseshoe or ring-shaped, open sea reefs that surround a lagoon. They are common in the Indian and Pacific Oceans in areas of submerged volcanic islands. Bank reefs are also open sea reefs but have no central lagoon. They are surrounded by deep water and are miles from any land mass. Patch reefs are small, isolated reefs. Their size and the depth at which they are found vary.
In addition to form, reefs vary substantially in the fauna and flora they support. Temperature, water depth, clarity, substrate type, storm frequency and intensity, and other factors determine the communities found on reefs. The National Marine Sanctuary System has four sites with tropical coral reef ecosystems and one with a tropical/temperate reef community. These reefs' locations, physical features, structures, and fauna and flora are described here.
Click the following sanctuary names to read more detail about their specific coral reef habitats: