Barrier reefs are platforms separated from the adjacent land by a bay or lagoon. The longest barrier reefs occur off the coasts of Belize and Australia. Atolls rest on the tops of submerged volcanoes. They are usually circular or oval with a central lagoon. Parts of the atoll may emerge as islands.
Coral reefs provide habitats for a large variety of organisms.
These organisms rely on corals as a source of food and shelter. Coral reefs and their associated communities of seagrasses, mangroves and mudflats are sensitive indicators of water quality and the ecological integrity of the ecosystem. They tolerate relatively narrow ranges of temperature, salinity, water clarity, and other chemical and water quality characteristics. Reefs thus are excellent sentinels of the quality of their environment. Proper monitoring of reefs can identify changes in water quality or impacts from land-based activities.