cora - liebe pur
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this is a song of cora! I love this song!
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Coral reefs are underwater structures made from calcium carbonate secreted by corals. Corals are colonies of tiny living animals found in marine waters containing few nutrients. Most coral reefs are built from stony corals, and are formed by polyps that live together in groups. The polyps secrete a hard carbonate exoskeleton which provides support and protection for the body of each polyp. Reefs grow best in warm, shallow, clear, sunny and agitated waters.
Often called “rainforests of the sea”, coral reefs form some of the most diverse ecosystems on earth. They occupy less than one tenth of one percent of the world ocean surface, about half the area of France, yet they provide a home for twenty-five percent of all marine species, including fish,molluscs,worms, crustaceans, echinoderms, sponges, tunicates and other cnidarians. Paradoxically, coral reefs flourish even though they are surrounded by ocean waters that provide few nutrients. They are most commonly found at shallow depths in tropical waters, but deep water and cold water corals also exist on smaller scales in other areas.
Coral reefs deliver ecosystem services to tourism, fisheries and shoreline protection. The annual global economic value of coral reefs has been estimated at $30 billion. However, coral reefs are fragile ecosystems, partly because they are very sensitive to water temperature. They are under threat from climate change, ocean acidification, blast fishing, cyanide fishing for aquarium fish, overuse of reef resources, and harmful land-use practices, including urban and agricultural runoff and water pollution, which can harm reefs by encouraging excess algae growth.
* Fringing reef – a reef that is directly attached to a shore or borders it with an intervening shallow channel or lagoon
in the Maldives.]] in the Maldives]]
Other reef types or variants are:
* Cays – small, low-elevation, sandy islands formed on the surface of a coral reef. Material eroded from the reef piles up on parts of the reef or lagoon, forming an area above sea level. Plants can stabilize cays enough to become habitable by humans. Cays occur in tropical environments throughout the Pacific, Atlantic and Indian Oceans (including the Caribbean and on the Great Barrier Reef and Belize Barrier Reef), where they provide habitable and agricultural land for hundreds of thousands of people.
* When a coral reef cannot keep up with the sinking of a volcanic island, a seamount or guyot is formed. The tops of seamounts and guyots are below the surface. Seamounts are rounded at the top and guyots are flat. The flat top of the guyot, also called a tablemount, is due to erosion by waves, winds, and atmospheric processes.
Most coral reefs were formed after the last glacial period when melting ice caused the sea level to rise and flood the continental shelves. This means that most coral reefs are less than 10,000 years old. As coral reef communities were established on the shelves, they built reefs that grew upwards, keeping pace with the rise in sea level. Reefs that didn't keep pace could become drowned reefs, covered by so much water that there was insufficient light for further survival. Coral reefs are also found in the deep sea away from the continental shelves, around oceanic islands and as atolls. The vast majority of these ocean coral islands are volcanic in origin. The few exceptions have tectonic origins where plate movements have lifted the deep ocean floor on the surface.
In 1842 in his first monograph, The Structure and Distribution of Coral Reefs Charles Darwin set out his theory of the formation of atoll reefs, an idea he conceived during the voyage of the Beagle. His theory was that atolls were formed by the uplift and subsidence of the Earth's crust under the oceans. Darwin’s theory sets out a sequence of three stages in atoll formation. It starts with a fringing reef forming around an extinct volcanic island as the island and ocean floor subsides. As the subsidence continues, the fringing reef becomes a barrier reef, and ultimately an atoll reef.
Darwin predicted that underneath each lagoon would be a bed rock base, the remains of the original volcano. Subsequent drilling proved this correct. Darwin's theory followed from his understanding that coral polyps thrive in the clean seas of the tropics where the water is agitated, but can only live within a limited depth of water, starting just below low tide. Where the level of the underlying land stays the same, the corals grow around the coast to form what he called fringing reefs, and can eventually grow out from the shore to become a barrier reef.
Ocean Education Service. Retrieved 9 January 2010.]]
Where the land is rising, fringing reefs can grow around the coast, but coral raised above sea level dies and becomes white limestone. If the land subsides slowly, the fringing reefs keep pace by growing upwards on a base of dead coral, forming a barrier reef enclosing a lagoon between the reef and the land. A barrier reef can encircle an island, and once the island sinks below sea level a roughly circular atoll of growing coral continues to keep up with the sea level, forming a central lagoon. Barrier reefs and atolls don't usually form complete circles, but are broken in places by storms. Should the land subside too quickly or sea level rise too fast, the coral dies as it is below its habitable depth. As sea level rose, the water and the corals encroached on what had been hills of the Australian coastal plain. By 13,000 years ago sea level had risen to lower than at present, and the hills of the coastal plains were, by then, continental islands. As the sea level rise continued, water topped most of the continental islands. The corals could then overgrow the hills, forming the present cays and reefs. Sea level on the Great Barrier Reef has not changed significantly in the last 6,000 years, Although the Great Barrier Reef formed along a continental shelf, and not around a volcanic island, Darwin's principles apply. The Great Barrier Reef development stopped at the barrier reef stage, since Australia is not about to submerge. It formed the world's largest barrier reef, from shore, stretching for .
Healthy coral reefs grow horizontally from per year, and grow vertically anywhere from per year; however, they grow only at depths shallower than due to their need for sunlight, and cannot grow above sea level.
Coral reef ecosystems contain distinct zones that represent different kinds of habitats. Usually three major zones are recognized: the fore reef, reef crest, and the back reef (frequently referred to as the reef lagoon).
All three zones are physically and ecologically interconnected. Reef life and oceanic processes create opportunities for exchange of seawater, sediments, nutrients, and marine life among one another.
Thus, they are integrated components of the coral reef ecosystem, each playing a role in the support of the reefs' abundant and diverse fish assemblages.
Most coral reefs exist in shallow waters less than fifty metres deep. Some inhabit tropical continental shelves where cool, nutrient rich upwelling does not occur, such as Great Barrier Reef. Others are found in the deep ocean surrounding islands or as atolls, such as in the Maldives. The reefs surrounding islands form when islands subside into the ocean, and atolls form when an island subsides below the surface of the sea.
Alternatively, Moyle and Cech distinguish six zones, though most reefs possess only some of the zones.
. The water waves at the left travel over the off-reef floor until they encounter the reef slope or fore reef. Then the waves pass over the shallow reef crest. When a wave enters shallow water it shoals, that is, it slows down and the wave height increases.]]
* The reef surface is the shallowest part of the reef. It is subject to the surge and the rise and fall of tides. When waves pass over shallow areas, they shoal, as shown in the diagram at the right. This means that the water is often agitated. These are the precise condition under which coral flourish. Shallowness means there is plenty of light for photosynthesis by the symbiotic zooxanthellae, and agitated water promotes the ability of coral to feed on plankton. However other organisms must be able to withstand the robust conditions to flourish in this zone.
* The off-reef floor is the shallow sea floor surrounding a reef. This zone occurs by reefs on continental shelves. Reefs around tropical islands and atolls drop abruptly to great depths, and don't have a floor. Usually sandy, the floor often supports seagrass meadows which are important foraging areas for reef fish.
* The reef drop-off is, for its first 50 metres, habitat for many reef fish who find shelter on the cliff face and plankton in the water nearby. The drop-off zone applies mainly to the reefs surrounding oceanic islands and atolls.
* The reef face is the zone above the reef floor or the reef drop-off. "It is usually the richest habitat. Its complex growths of coral and calcareous algae provide cracks and crevices for protection, and the abundant invertebrates and epiphytic algae provide an ample source of food."
Coral reefs are estimated to cover , which is just under one tenth of one percent of the oceans' surface area. The Indo-Pacific region (including the Red Sea, Indian Ocean, Southeast Asia and the Pacific) account for 91.9% of this total. Southeast Asia accounts for 32.3% of that figure, while the Pacific including Australia accounts for 40.8%. Atlantic and Caribbean coral reefs account for 7.6%.
Although corals exist both in temperate and tropical waters, shallow-water reefs form only in a zone extending from 30° N to 30° S of the equator. Tropical corals do not grow at depths of over . The optimum temperature for most coral reefs is , and few reefs exist in waters below . However reefs in the Persian Gulf have adapted to temperatures of in winter and in summer.
Deep water coral can exist at greater depths and colder temperatures. Although deep water corals can form reefs, very little is known about them.
Coral reefs are rare along the American west coast, as well as along the African west coast. This is due primarily to upwelling and strong cold coastal currents that reduce water temperatures in these areas (respectively the Peru, Benguela and Canary streams). Corals are seldom found along the coastline of South Asia from the eastern tip of India (Madras) to the border of Bangladesh and Myanmar.
Live coral are small animals embedded in calcium carbonate shells. It is a mistake to think of coral as plants or rocks. Coral heads consist of accumulations of individual animals called polyps, arranged in diverse shapes. Polyps are usually tiny, but they can range in size from a pinhead to a foot across. Reef-building or hermatypic corals live only in the photic zone (above 50 m depth), the depth to which sufficient sunlight penetrates the water for photosynthesis to occur. Coral polyps do not themselves photosynthesize, but have a symbiotic relationship with single-celled organisms called zooxanthellae; these organisms live within the tissues of polyps and provide organic nutrients that nourish the polyp. Because of this relationship, coral reefs grow much faster in clear water, which admits more sunlight. Indeed, the relationship is responsible for coral reefs in the sense that without their symbionts, coral growth would be too slow for the corals to form significant reef structures. Corals get up to 90% of their nutrients from their zooxanthellae symbionts.
Reefs grow as polyps and other organisms deposit calcium carbonate,the basis of coral, as a skeletal structure beneath and around themselves, pushing the coral head's top upwards and outwards. Waves, grazing fish (such as parrotfish), sea urchins, sponges, and other forces and organisms act as bioeroders, breaking down coral skeletons into fragments that settle into spaces in the reef structure or form sandy bottoms in associated reef lagoons. Many other organisms living in the reef community contribute skeletal calcium carbonate in the same manner. Coralline algae are important contributors to reef structure in those parts of the reef subjected to the greatest forces by waves (such as the reef front facing the open ocean). These algae deposit limestone in sheets over the reef surface, thereby strengthening it.
The colonies of the one thousand coral species assume a characteristic shape such as wrinkled brains, cabbages, table tops, antlers, wire strands and pillars.
Internally fertilized eggs develop in the polyp for a period ranging from days to weeks. Subsequent development produces a tiny larva, known as a planula. Externally fertilized eggs develop during synchronized spawning. Polyps release eggs and sperm into the water en masse, simultaneously. Eggs disperse over a large area. The timing of spawning depends on time of year, water temperature, and tidal and lunar cycles. Spawning is most successful when there is little variation between high and low tides. The less water movement, the better the chance for fertilization. Ideal timing occurs in the spring. Release of eggs or planula usually occurs at night and is sometimes in phase with the lunar cycle (3–6 days after a full moon). The period from release to settlement lasts only a few days, but some planulae can survive afloat for several weeks. They are vulnerable to predation and environmental conditions. The lucky few who attach to substrate next confront competition for food and space.
Coral reefs cover less than one tenth of one percent of the surface of the world’s ocean, yet they support over one-quarter of all marine species. This huge number of species results in complex food webs, with large predator fish eating smaller forage fish that eat yet smaller zooplankton and so on. However, all food webs eventually depend on plants, which are the primary producers. Coral reefs' primary productivity is very high, typically producting 5-10g C m−2 day−1 biomass.
One reason for the startling clarity of tropical waters is that they are deficient in nutrients and drifting plankton. Further, the sun shines year round in the tropics, warming the surface layer, making it less dense than subsurface layers. The warmer water is separated from the cooler water by a stable thermocline, where the temperature makes a rapid change. This keeps the warm surface waters floating above the cooler deeper waters. In most parts of the ocean there is little exchange between these layers. Organisms that die in aquatic environments generally sink to the bottom where they decompose. This decomposition releases nutrients in the form of nitrogen (N), phosphorus (P) and potassium (K). These nutrients are necessary for plant growth, but in the tropics they are not directly recycled back to the surface.
Plants form the base of the food chain, and need sunlight and nutrients to grow. In the ocean these plants are mainly microscopic phytoplankton which drift in the water column. They need sunlight for photosynthesis, which powers carbon fixation, so they are found only relatively near the surface. But they also need nutrients. Phytoplankton rapidly use nutrients in the surface waters, and in the tropics these nutrients are not usually replaced because of the thermocline. Zooxanthellae can provide up to 90% of a coral’s energy requirements. In return, as an example of mutualism, the coral shelter the zooxanthellae, averaging one million for every cubic centimetre of coral, with and provide a constant supply of the carbon dioxide it needs for photosynthesis.
they host]]
Corals also absorb nutrients, including inorganic nitrogen and phosphorus, directly from the water. Many corals extend their tentacles at night to catch zooplankton that brush them when the water is agitated. Zooplankton provide the polyp with nitrogen, and the polyp shares some of the nitrogen with the zooxanthellae, which also require this element. The varying pigments in different species of zooxanthellae give corals their different colours. Coral which loses its zooxanthellae becomes white and is said to be bleached, a condition which unless corrected can kill the coral.
Sponges are another key to explaining Darwin’s paradox. They live in crevices in the coral reefs. They are efficient filter feeders, and in the Red Sea they consume about sixty percent of the phytoplankton that drifts by. The sponges eventually excrete nutrients in a form the corals can use.
The roughness of coral surfaces is the key to coral survival in agitated waters. Normally a boundary layer of still water surrounds a submerged object, which acts as a barrier. Waves breaking on the extremely rough edges of corals disrupt the boundary layer, allowing the corals access to nutrients. Turbulent water thereby promotes rapid reef growth and lots of branching. Without the nutritional gains brought by rough coral surfaces, even the most effective recycling would leave corals wanting in nutrients.
Cyanobacteria provide soluble nitrates for the reef via nitrogen fixation.
Coral reefs also often depend on surrounding habitats, such as seagrass meadows and mangrove forests, for nutrients. Seagrass and mangroves supply dead plants and animals which are rich in nitrogen and also serve to feed fish and animals from the reef by supplying wood and vegetation. Reefs in turn protect mangroves and seagrass from waves and produce sediment for the mangroves and seagrass to root in.
Reefs are home to a large variety of organisms, including fish, seabirds, sponges, Cnidarians (which includes some types of corals and jellyfish), worms, crustaceans (including shrimp, cleaner shrimp, spiny lobsters and crabs), molluscs (including cephalopods), echinoderms (including starfish, sea urchins and sea cucumbers), sea squirts, sea turtles and sea snakes. Aside from humans, mammals are rare on coral reefs, with visiting cetaceans such as dolphins being the main exception. A few of these varied species feed directly on corals, while others graze on algae on the reef.
A number of invertebrates, collectively called cryptofauna, inhabit the coral skeletal substrate itself, either boring into the skeletons (through the process of bioerosion) or living in pre-existing voids and crevices. Those animals boring into the rock include sponges,bivalve mollusks, and sipunculans. Those settling on the reef include many other species, particularly crustaceans and polychaete worms. The algae population consists of turf algae, coralline algae, and macroalgae.
Coral reefs are dying around the world. In El Nino-year 2010, preliminary reports show global coral bleaching reached its worst level since another El Nino year, 1998, when 16 percent of the world's reefs died As a result of excessive water temperature. In Indonesia's Aceh province, surveys showed some 80 percent of bleached corals died. In July, Malaysia closed several dive sites after virtually all the corals In some areas were damaged by bleaching.
In order to find answers for these problems, researchers study the various factors that impact reefs. The list of factors is long, including the ocean's role as a carbon dioxide sink, atmospheric changes, ultraviolet light, ocean acidification, biological virus, impacts of dust storms carrying agents to far flung reefs, pollutants, algal blooms and others. Reefs are threatened well beyond coastal areas.
General estimates show approximately 10% world's coral reefs are already dead. About 60% of the world's reefs are at risk due to destructive, human-related activities. The threat to the health of reefs is particularly strong in Southeast Asia, where 80% of reefs are endangered.
Biosphere reserves are other protected areas that may protect reefs. Also, Marine parks, as well as world heritage sites can protect reefs. World heritage designation can also play a vital role. For example Belize's Barrier reef, Chagos archipelago, Sian Ka'an, the Galapagos islands, Great Barrier Reef, Henderson Island, Palau and Papahānaumokuākea Marine National Monument have been designated as world heritage sites.
In Australia, the Great Barrier Reef is protected by the Great Barrier Reef Marine Park Authority, and is the subject of much legislation, including a Biodiversity Action Plan.
Inhabitants of Ahus Island, Manus Province, Papua New Guinea, have followed a generations-old practice of restricting fishing in six areas of their reef lagoon. Their cultural traditions allow line fishing but not net and spear fishing. The result is that both the biomass and individual fish sizes are significantly larger in these areas than in places where fishing is unrestricted.
Organizations which promote interest, provide knowledge bases about coral reef survival, and promote activities to protect and restore coral reefs:
In large restoration projects, propagated hermatypic coral on substrate can be secured with metal pins, superglue ormilliput. Needle and thread can also attach A-hermatype coral to substrate.
During accretion, the settled corals display an increased growth rate, and size, and density, but after the process is complete, growth rate and density return to levels that are comparable to naturally growing corallites, and are about the same size or slightly smaller.
Beginning a few thousand years after hard skeletons were developed by marine organisms, coral reefs emerged. The times of maximum development were in the Middle Cambrian (513–501 Ma), Devonian (416–359 Ma) and Carboniferous (359–299 Ma), due to Order Rugosa extinct corals, and Late Cretaceous (100–65 Ma) and all Neogene (23 Ma–present), due to Order Scleractinia corals.
Not all reefs in the past were formed by corals: Early Cambrian (542–513 Ma) reefs resulted from calcareous algae and archaeocyathids (small animals with conical shape, probably related to sponges) and rudists, a type of bivalve, built Late Cretaceous (100–65 Ma) reefs.
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