Coral reef

A coral reef is formed by a hermatypical ( reef-building ) cnidarians reef structure in the sea, which is large enough to exert a significant influence on their physical and ecological environment. They are the largest structures created by living creatures of the earth. The total area of ​​today's coral reefs is 600,000 km ², in the Maldives, the reefs rise up to 2200 meters above the sea floor.

Coral reefs are composed essentially of corals from the group of stony corals ( Scleractinia ), also carry additional fire coral ( Millepora ) and, in the tropical Indo-Pacific, the Blue Coral ( Heliopora coerulea ) to Riffbildung at. Stony corals colonize both the deep and the shallow water up to the water surface. They build up over many centuries from their lime skeletons on the reef structure.

Coral reefs are complex marine ecosystems. They are the habitat ( habitat ) for a biological community ( community ) of plants and animals, such as worms, mollusks, sponges, echinoderms and crustaceans. A great importance coral reefs as a " nursery " for pelagic fish.

A coral island is caused by long-term changes in the water level. Since the coral can grow up to the water surface forms after later lowering of sea level or raising of the ground an island or series of islands, often in the form of an atoll.

• 2.1 fringing reefs
• 2.2 barrier reefs
• 2.3 platform reefs
• 2.4 atolls

• 9.1 Notes and references
• 9.2 Literature
• 9.3 External links

Types

According to their geographical distribution can be divided coral reefs into two types: tropical coral reefs and the deep-water reefs.

Tropical coral reefs

Reef-building corals from tropical coral reefs can only survive in water temperatures below 20 ° C only very rarely. For these reasons, the emergence limited to a range approximately between 30 ° north and 30 ° south latitude. In the Atlantic, the northernmost coral reefs on the coast of Bermuda (32 ° 30'N ), the southernmost just north of Rio de Janeiro (23 ° S). Although two types usually find riffbildender stony corals at Cape Hatteras (34 ° N ) on the east coast of the United States, but they form there no more reefs. The situation is similar in the Pacific in Tokyo Bay (38 ° N). Here tropical coral reefs come only from the Ryukyu Islands off at 30 ° N. The southernmost foothills of the Great Barrier Reef is located north of Brisbane in the Moreton Bay ( 27 ° 30 ' S ), another very south reef located in the Pacific at Rapa Iti, at the same height. When Lord Howe Island, a warm ocean current allows the coral reef education nor at 31 ° 30 ' S. In the Red Sea coral reefs coming up in the northern reaches of the Gulf of Aqaba ( 29 ° 30' N) ago. The southernmost coral reef world lies in the Indian Ocean during the Houtman Abrolhos Archipelago, on the west coast of Australia (29 ° S). On the west coast of the Indian Ocean rich coral reefs to Inhaca Island in the Maputo Bay (26 ° S, Mozambique ).

Since the vast majority of stony corals live in symbiosis with zooxanthellae, they are dependent on the sunshine, which rapidly decreases with increasing water depth. The total area of ​​all coral reefs is about 600,000 km ². 640 million tons of reef limestone are deposited annually on average.

There are two main categories of tropical coral reefs:

• Littoral ( coastal ) reefs are found in the shallow waters of the continental shelves. By freshwater introduction these reefs are rich in nutrients as neritide reefs, making such reefs are often dominated by soft corals and algae.
• Neritide reefs are found away from the continents, if caused by volcanic activity islands above the water surface. Neritide reefs are found primarily in the tropical Pacific (Hawaii, Tahiti). Here is to be found due to lower nutrient content than in intertidal reefs a higher biodiversity, especially in hard corals.

Deep water reefs

Many stony coral species can live at temperatures below 20 ° C even in cooler and cold water. From the Mediterranean are at least 37 hard coral species are known, but which do not form reefs. The majority of hard coral species in cool and cold water are solitary corals from the families of Caryophylliidae and Dendrophylliidae which grow very slowly, remain small and therefore do not form reefs. Unlike their tropical relatives, they do not use sunlight as the primary energy source, but feed on zooplankton.

An exception is the genus Lophelia, which is colony -forming and together with Madrepora oculata extended, tail -like coral reefs. The largest Lophelia reefs reach a height of 45 meters and a length of two kilometers. They grow in comparison with tropical coral reefs very slowly. The growth is on average 7.5 mm / year, a maximum of 20 to 25 mm / year. Very large reefs must therefore be several thousand years old. A belt of this coral reef extends along the European continental margin of the Iberian Peninsula to the North Cape. Deep-water reefs are typically found in water depths of 200 to 600 meters. Norwegian fjords they are found from 52 m depth.

These cold-water reefs are threatened by human influences in their existence. In addition to the pollution of the seas provide modern fishing methods, the greatest threat to this habitat represents a single bottom trawl can destroy in a few minutes a reef that has grown several thousand years.

Types of tropical coral reefs

Coral reefs are fundamentally different in their form and origin.

Fringing reefs

A fringing reef extends along the coast of the mainland or an island. It is the most widely used Rifftyp and the Red Sea almost the only one. Fringing reefs extend along coasts and may be many kilometers long. Their width is usually less than 100 meters, but can amount to several hundred meters. Fringing reefs are initially formed directly on the banks at the height of the low tide line and expand as they grow more and more seaward from. Your final width depends on where the sea floor drops steeply. The surface of the fringing reef is always kept at the same level just below the waterline. In older fringing reefs, the outer areas have pushed far into the sea, the inner part is deepened by erosion and eventually forms a lagoon. Saumrifflagunen can be over 100 meters wide and several meters deep. As the fringing reef extending parallel to the coast.

Barrier reefs

Barrier reefs are similar to the late stages of a Lagunensaumriffs. They differ from these mainly by their size and their origin. Its lagoon can be several kilometers wide and 30 to 70 feet deep. Especially the advanced Außenriffkante is not a reef, which has previously been located on the shore and then expanded with the growth continues to the open sea, but it has been left there from the beginning. Similar to an atoll here is a lowering of the seabed or a raising of the sea level requirement for Riffentstehung. There thus needs to Riffentstehung a geological process and the Riffbildung compared with that of the fringing reef takes much longer to barrier reefs are much rarer. The most famous barrier reef is the Australian Great Barrier Reef ( Great Barrier Reef ). Other large barrier reefs are the Belize Barrier Reef and the New Caledonian barrier reef. Barrier reefs are also found on the coasts of Providencia, Mayotte, the Gambier Islands, on the southeastern coast of Kalimantan, on parts of the coast of Sulawesi, Southeast New Guinea and the southern coast of Louisiade archipelago.

Platform reefs

While the previously presented reef types always occur on the coasts of islands or continents to platform reefs, both on the continental shelf, as also form in the open ocean, wherever the seabed so far up projects toward water level that the growth zooxantheller, riffbildender coral possible is. In the southern Great Barrier Reef, there are some platform reefs that Swain and Capricorn- group away on the continental shelf, about 100 to 200 km from the coast. Some platform reefs of the northern Mascarene are even several thousand kilometers from the mainland. Platform reefs grow, in contrast to the hem and barrier reefs that extend seaward only, on all sides. You can reach quite different sizes, from a few hundred meters to several kilometers. Most shape is oval to strongly elongated. Parts of the reefs can reach the surface, and form sandbars and small islands to be able to form their own fringing reefs. In the middle of a platform reef, a lagoon may be located. There are platform reefs also within atolls. Here they are called patch reefs and only reach a diameter of a few dozen meters. Rise platform reefs on an elongated structure such as a defunct, former Barrier Reef, they can be arranged in a row. This is the case, for example, on the east coast of the Red Sea in Jeddah. For very old platform reefs can be so severely eroded the inner part, they form a pseudo- atoll. From true atolls they are only accurate tests and possibly to distinguish holes. Some platform reefs of the Lakshadweep islands have by wind and water flow a U-shaped form.

Atolls

Atolls usually arise from fringing reefs around volcanic islands. Over time, the island is removed through erosion and sinking below sea level. A drop in the ocean floor or sea-level rise leads to the formation of atolls. What remains is a ring of reefs that enclose a lagoon. The Maldives consists of 26 atolls such.

Zoning

Each reef can be divided into different zones where, due to the prevailing conditions, different plants and animals live. These zones are less pronounced in the different reef types, and even within a Rifftyps the structure may vary. Subsequently, the typical structure of a fringing reef is explained.

• The beach zone adjoins directly to the mainland. At low tide it falls during the day twice for several hours dry. The beach area is mainly inhabited by crabs and snails. Below the water level at low tide algae, found small shells and hermit crabs.

• In the sea joins the reef flat, the horizontal part of the reef where. In the inner regions dominated by brown and blue-green algae, further seaward followed by red algae. In this area only a few algal species live. Most common are some urchins species. Also, brittle stars and crabs can be encountered. Few fish live in this zone, mainly small gobies and blennies. In the outer regions of the algal zone grow various soft corals, hard corals are also occasionally encountered.
• In the transition zone, there is a high species richness. The water here is deeper and clearer water movement more than further landward. Here you will find larger colonies of hard corals that can form so-called micro- atolls. These areas can occasionally still fall dry.
• The outer area between Innenriffhang and Außenriffkante never falls dry. He is an average of 10 meters wide and has a water depth of 40 cm to one meter. Prevents a strong wave movement that sand and other loose material deposited. They form strong flow tunnels and canals. This area is dominated hard and soft corals, algae, however, one finds only a few. Of fish are found mainly damselfish, wrasse and surgeonfish.

Artificial coral reef

For several years, artificial coral reefs is trying to establish. Starting from the observation that in the sea sunken large objects, such as ships and aircraft, were settled within a few years, one artificial structures in the sea began deliberately to install. There were major failures, such as the Osborne Reef from millions of old car tires, and successes. Thus were used, for example Riffballs and set steel structures according to the Biorock technology under weak direct current, there to be able to deposit minerals in a short time, which are often colonized by coral polyps. Also, salt-water aquariums one is occupied with the artificial reproduction of coral reefs.

Importance

The number of coral reefs is greatly reduced; one fifth of all reefs have disappeared. Reasons include the dynamite fishing and cyanide fishing, overfishing in general, industrial pollution, construction activities and runs aground ships.

Coral reefs are 800 million people in Southeast and South Asia, East Africa and the Caribbean, especially as an excellent source of protein is important. They have a great material value, because they protect beaches from erosion and storm damage. From tourists the reefs are valued for their beauty. Coral reefs also home to an abundance of marine organisms and for the existence of great importance.

According to oceanographers about 20 percent of the coral reefs were destroyed in April 2007 worldwide, another 50 percent at serious risk.

Geology

Even in earlier geological eras coral reefs were formed. The Dolomites and the High Dachstein and other mountains of the Northern Limestone Alps consist mainly of coral limestone and fossil coral reefs that were raised in the course of Alpidic orogeny. When Hohen Goll in the Berchtesgaden Alps, you can still distinguish the zones of a reef including a lagoon.

The first, already produced in the Precambrian of living beings reefs, stromatolites were not created by cnidarians, but are formed by trapping and binding of sediment particles due to the growth of microorganisms, especially cyanobacteria. In the lower Cambrian small reefs were formed by the Archaeocyaths that a specialized group of sponges may have been, along with stromatolites. They reached lengths of up to 30 meters and a width of up to three meters.

From the Ordovician to Permian tabulata and the Rugosa are the first Riffbildner among the flowers animals. From then on, one can speak of coral reefs. In the Silurian and Devonian reefs but were not only built of coral, but also of stromatoporoids mentioned creatures that are possibly related to the sponges. With the mass extinction at the Permian - Triassic boundary, which also includes the Rugosa and the tabulata fell victim to the Riffbildung first came to a halt. From the Middle Triassic, the first stone corals appeared and took over the Mesozoic becoming. In the upper Mesozoic belonging to the shells rudists other important reef builders are. They are dying out at the Cretaceous - Tertiary boundary. Since then, the hard corals are the main Riffbildner.

Examples

• Great Barrier Reef, the world's largest reef and World Heritage
• Belize Barrier Reef, one of the two second largest barrier reefs
• New Caledonian barrier reef, the other of the second largest barrier reefs
• The coral triangle, the Philippines, East Timor, parts of Indonesia, New Guinea and the Solomon Islands includes with Raja Ampat on the Vogelkop peninsula in the center. 2006, 20 new species of coral have been discovered in Raja Ampat.