Reef Coasts

Reef Coasts mjg8
  • What is the global distribution of coasts lined with coral reefs?
  • Why are reefs globally distributed in this fashion?
  • What are the different types of reef systems we see on a regional scale of kilometers or more?

Historically, the exact definition of a reef has been a bit controversial, and different definitions would be provided if you were to ask a geologist or a biologist. For the purpose of this discussion, a reef can be considered any organic framework that is wave resistant and modifies the environment around it because of organic growth. Keep in mind that this definition offers no information about the type of organism creating the reef. Corals, shellfish such as oysters, and even some types of worms can create reefs, although the scale of reefs created by oysters and certain types of worms is much different than the scale of reefs that can be created by corals. The Great Barrier Reef of northeastern Australia extends for nearly, 2300 km and is the only organic structure that is visible from space, whereas oyster reefs are typically only several 10s of meters in length. Fundamentally, reefs rise above the substrate that they are sitting upon and thus modify or alter the speed and direction of currents and waves.

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Image from space: The Great Barrier Reef offshore of northeastern Australia (barrier reef shows up as the white trend located offshore of the coastline). The Great Barrier Reef is constructed primarily from calcium carbonate secreted by corals that build a structure above the surrounding substrate to remain in the photic zone below water. The reefs build vertically into the photic zones so that algae, which are symbiotic with the coral polyps, can complete photosynthesis.
Credit: NASA: Australia's Great Barrier Reef

Global Distribution of Coral Reefs

Global Distribution of Coral Reefs azs2

The vast majority of large reefs created by corals in shallow, sunlit waters (< 50 m water depth) are located within a tropical zone located between 30º N and 30º S latitude with a preferred temperature range of approximately 22º to 29º 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.

Map showing coral reefs: Atlantic, Pacific, and South Asian Islands, Caribbean, Indian Ocean and Red Sea coast, Malta, Shetland Islands.
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.
  2. Fringing reefs are directly attached to the shore, with no well-developed lagoon between the reef structure and the mainland.
  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.
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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 reef can only be reached by boat from the coast. 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 (Public Domain)
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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. You can swim from the beach to the reef.
Credit: NASA: EO: Ningaloo Reef (Public Domain)
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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 (Public Domain)

Recommended Readings and Video: For more information on coral and coral reefs, check out these links:

Learning Check Point

Learning Check Point mjg8

Take a few minutes to think about what you just learned, then answer the questions below.