Coral reefs are an integral part of the world’s oceans; although they are quite rare (covering only about 0.089% of the world’s ocean) they provide a habitat for a huge amount of biota including about a third of the world’s marine fish species (McAllister, 1991). Coral reefs are extremely complex environments and have extremely high productivity, although the waters they inhabit are usually nutrient poor (Grigg et al., 1984). They are also connected to other marine systems such as mangrove forests, seagrass beds and the open ocean (Moberg & Folke, 1999).
Corals are central to coral reef ecosystems and the growth of reef-building corals in tropical seas is responsible for the framework of coral reef systems. Although other organisms hold the structure together (e.g. calcareous red algae) and populate it (fish, algae, invertebrates, bacteria) corals have been the fundamental reason for the structure of coral reef ecosystems for millions of years; they have built the primary structures of entire reefs, islands and such massive oceanic barriers as the Great Barrier Reef (Hoegh-Guldberg, 2001). The framework of coral reefs is formed from calcium carbonate which is mainly deposited by calcareous algae and stony corals and the large, branching or encrusting carbonate skeletons of corals provides habitat and food resources for many other reef organisms.
There are four main types of coral reefs: fringing reefs, barrier reefs, atolls and platform reefs. Fringing reefs are the most common; they develop from the upward growth of a calcium carbonate platform from a shelving coastline adjacent to the shore usually along rocky coasts of uplifted islands or along the shores of exposed limestone islands (Cesar, 2002). Barrier reefs are usually older structures rising up from a deeper base; they generally develop away from the coastline. Atolls are usually circular or horse-shoe shaped surrounding an oceanic volcanic seamount and enclosing a wide central lagoon (McGinley & Emmett Duffy, 2010); they are usually found away from the continental shelf. Platform reefs are simple physical structures with a variety of origins. They are essentially reefs with no obvious link to a coastline but without the clear structure of a barrier reef or atoll (Spalding et al., 2001).
Under natural conditions, a healthy coral reef can usually recover from stress or disturbance in time, but when subjected to severe, frequent or prolonged stress, whether due to human activity or climate change, recovery may be impossible thus resulting in the death and decline of coral reef ecosystems.