Global Distribution

Global Distribution of Coral Reefs

The vast majority of large reefs created by corals in shallow waters (< 50 m water depth) are located within a tropical zone located between 30 º N and 30 º S latitude (Figure 3.8) with a preferred temperature range of approximately 22 º to 29 º C. Corals also grow best in areas with little suspended sediment in the water, so large coral reefs systems are not common to locations where there is a large input of sediment to the coastal zone by river systems. Although there are cold, deep water types of coral present in the ocean basins, they do not create large nearshore reef structures that affect adjacent coasts.

Figure 3.8: Red dots indicate the global distribution of coral reefs. Reef systems built primarily out of calcium carbonate secreting organisms.
Credit: NASA: Wikimedia Commons: "Coral reef locations" (Public Domain)

In total there are three main types of shallow water coral reef structures: 1) barrier reefs, 2) fringing reefs, and 3) coral atolls. These three types are differentiated on the basis of proximity to land, the overall scale of the reef structure, and the shape of the reef.

1. Barrier reefs are typically large-scale, linear features that extend parallel to a shore with a lagoon between the reef and the mainland (Figure 3.9).
2. Fringing reefs are directly attached to the shore with no well-developed lagoon between the reef structure and the mainland (Figure 3.10).
3. Coral atolls are circular reefs that often start out as fringing reefs attached to a volcanic island. As the volcanic islands subside, the reef grows upward and a lagoon develops behind the reef and inside the submerging island. Eventually, the island can subside below the water level, and a ringlike coral reef structure remains (Figure 3.11).

Figure 3.9: Satellite image of a portion of the Great Barrier Reef on the east coast of Australia. Note the parallel trend of the reef to the coast and the lagoon that separates the reef from the mainland. The arrows identify sediment plumes that are entering the lagoon by the inflow of water. There are substantial concerns because such plumes can also deliver excess nutrient and pollution loads that are deadly to the coral organisms.
Credit: NASA: EO: River Plumes Threaten Great Barrier Reef

Figure 3.10: Australia's largest fringing coral reef, Ningaloo Reef, on the western shore of Australia. Close examination of the image reveals that the reef is in close contact to the land and there exists a little lagoon between the reef and the mainland.

Credit: NASA: EO: Ningaloo Reef

Figure 3.11: Wake Island atoll in the central Pacific Ocean is located 4,000 km west-southwest of Hawaii. The central blue lagoon area is approximately the outline of the now submerged crater of the volcano around which the atoll developed. Note the aircraft landing strips and other structures that are maintained by the U.S. Department of the Interior for U.S. Air Force and Army operations in this remote Pacific location.
Credit: NASA: Wake Island, Pacific Ocean

Voluntary Readings and Video

For more information on coral and coral reefs check out these links:

• http://en.wikipedia.org/wiki/Coral_reef
• http://ocean.si.edu/corals-and-coral-reefs
• http://en.wikipedia.org/wiki/Atoll
• https://www.youtube.com/watch?v=vrD1O5hex6Y