|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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|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.
Map of the reefs on earth
Orange = atolls / Red = fringing reefs / Green = barrier 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.
|Platform reefs usually lie in sheltered seas and quite far offshore. They are flat-topped with small and very shallow lagoons.|
|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.
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.
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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|Reefs are often referred to as the “rainforests of the oceans”. Coral reefs host an extraordinary variety of marine plants and animals (perhaps up to 2 million) including one quarter of all marine fish species. The wide variety of habitats depends a lot on the morphology of the coral reef. The differences in temperatures, light, exposure to waves and tides, currents and the amount of food available result in different habitats and niches.|
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
|Coastal bays:||Mangroves often live along the edge of bays and river mouths. The water is often turbid with silty bottoms. Mangroves provide shelter with their root system and are a nursing ground for many species of coral reef fishes.|
|Lagoons:||Depths my vary from less than a meter to 90m. If there is good circulation of water there may be many patch reefs or just large stands of acropora corals growing there. There may be seagrass beds or mud flats. Lagoons can trap many fish varieties as tide recedes.|
|Channels:||These connect the lagoons with the open water and are important to provide fresh and colder water for the lagoons. Tidal currents provide ideal conditions for plankton-feeding animals, so channel walls are often overgrown with soft and hard corals.|
|Inner reef slope:||Sometimes there are be patch reefs. In an atoll these areas might be extensive and interesting for their rich life.|
|Reef flat:||The shallow, flattest part of a reef, often partly eroded and uncovered at low tide. They can range from a few meter to a few kilometer. Depending on wave action and temperatures the surface may be just featureless or a complex maze of interconnecting channels, tide pools and sand patches. Coral growth is usually poor.|
|Reef front:||Here the reef takes the full force of wave action. Growth of corals is restricted and usually there is a lot of rubble and sand. Visibility might be bad too because of the sediments raised by the tide and waves.|
|Outer reef slope:||The seaward edge of a reef is fairly steep and slopes down to deeper water. Since the water is generally clearer, corals may grow to the depths of 50m depending on light available. There might be interesting caves, overhangs or gullies.|
|Drop-offs or walls:||Some outer reef slopes are nearly vertical. Sand and sediment will only collect in terraces or shelves. There might be vertical chutes with sand. Currents are either horizontally along the wall or vertically up or down along the chutes.|
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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