There are a number of stresses or environmental changes that may cause bleaching including disease, excess shade, increased levels of ultraviolet radiation, sedimentation, pollution, salinity changes, and increased temperatures. Because the zooxanthellae depend on light for photosynthesis, reef building corals are found in shallow, clear water where light can penetrate down to the coral polyps. Reef building coral communities also require tropical or sub-tropical temperatures, and exist globally in a band 30 degrees north to 30 degrees south of the equator. Reefs are generally classified in three types. Fringing reefs , the most common type, project seaward directly from the shores of islands or continents.
Barrier reefs are platforms separated from the adjacent land by a bay or lagoon.
The longest barrier reefs occur off the coasts of Australia and Belize. Atolls rest on the tops of submerged volcanos. They are usually circular or oval with a central lagoon. Parts of the atoll may emerge as islands. Over 300 atolls are found in the south Pacific.