Reef formation |
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| The prime reef builders are the stony or hard corals (subphylum Anthozoa). Reefs are formed by the skeletons of the tiny coral polyps. A coral polyps consist of a fleshy sack with a ring of tentacles which sits in a limestone skeletal case, secreted by the polyp.
Reef-building corals have evolved an indispensable symbiotic relationship with a type of brown algae called zoxanthellae (Symbiodinium microadriaticum). Millions of these single-celled algae are living as symbionts within their tissues (hermatypic corals). Zooxanthellae produce sugars and oxygen through photosynthesis thus helping the coral in the process of producing limestone or calcium carbonate. Corals grow up to three times faster with the help of the zooxanthellae. Zooxanthellae give the corals their characteristic greenish color. A change of environmental conditions such as higher temperatures or a change in salinity but also disease can cause the polyps to expel the algae. The coral becomes totally white (= coral bleaching). If the coral regains some algae it might survive, but bleaching can be irreversible and then the coral dies. |
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Coral reefs on earth |
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| Corals need warm water, if possible temperatures should be between 18°C and 30°C. If the water is colder, the reefs are poorly developed or nonexistent. Most coral reefs lie between the latitudes of 30 degrees north and south where sea temperatures are warmest.
Corals need access to sunlight (the algae living in their cells need light for photosynthesis) so the water where they grow needs to be shallow and clear. Too much sediment in the water can smother the coral polyps and too much fresh water kills them. That is why coral reefs don’t grow close to the mouths of rivers. |
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Map of the reefs on earth
Orange = atolls / Red = fringing reefs / Green = barrier reefs |
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Types of reefs |
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| Coral reefs are divided into four main types: fringing reef, platform reefs, barrier reefs and atolls. | |||||||||||||||
| Fringing reef | Fringing reefs are relatively young. They can develop in shallow waters along the coast of tropical islands or continents. The corals grow upwards to sea level or just below and outwards towards the open ocean. Fringing reefs are generally narrow platforms a short distance from shore and don’t contain a substantial lagoon.
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Platform reef |
Platform reefs usually lie in sheltered seas and quite far offshore. They are flat-topped with small and very shallow lagoons. | ||||||||||||||
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Barrier reef
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A coral reef growing parallel to the coastline and separated from it by a lagoon is called a barrier reef. The lagoon may develop between the fringing reef and the land. As the reef continues to grow further and further offshore it eventually reaches the edge of the continental shelf. Barrier reefs can also originate offshore if the depth of the seabed out there is shallow enough to allow corals to grow.
The most famous barrier reef is the Great Barrier Reef in Australia. It stretches over 2300 km and covers over 200’000 km2. It lies between 24 and 240 km from the main continent. |
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Atoll
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Atolls are rings of reef, often encircling an island (sand and coral rubble). They typically have a shallow, sandy, sheltered lagoon in the middle. Access to the open sea beyond is through a number of channels. These provide fresh and colder water for the lagoons. Corals atolls are on the top of submarine mountains. These mountains are remnants of volcanos. Once there were fringing reefs around the volcano. As it slowly submerged the corals continued to grow up to the surface of the water. What remained after the volcano became invisible is a ring of coral reefs surrounded by deep ocean. However some atolls were probably formed by rising sea levels rather by the sinking of islands. Other theories suggest that corals colonized eroded limestone formations, so called karstic saucers. |
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a fringing reef s
urrounding an active volcano – subsidence of the seafloor or rising sea levels shrinking of the island – an atoll results when the island has disappeared |
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Atoll |
Platform reef |
Bay with sandy beach |
Lagoon |
Lagoon |
Inner lagoon |
Fringing reef |
Fringing reef |
Reef flat |
Island with small reef |
Island with channels from run off water |
Fishing village with channel |
Reef morphology
A) Mud flats close to shore with tide pools – B) Mangroves – C) Seagrass bed – D) Patch reefs on inner reef slope – E) Inner lagoon – F) bottom with mud – G) Pinnacle – H) Outer lagoon – I) Fine sand – K) Acropora corals – L) Algae ridge – M) Sand and rubble – N) Gorgonians and black corals – O) Cave or overhang
A = coarse sedimentation B = fine sedimentation C = daily variation of temperatures D = number of different species (according to H. Schumacher 1967)
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