detriot tigers caps - Formation
See also: Fringing reef, Atoll reef,?and The Structure and Distribution of Coral Reefs
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 Charles Darwin published his first monograph, The Structure and Distribution of Coral Reefs. There he 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 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 theory starts with a volcanic island which becomes extinct
As the island and ocean floor subside, coral growth builds a fringing reef, often including a shallow lagoon between the land and the main reef
As the subsidence continues the fringing reef becomes a larger barrier reef further from the shore with a bigger and deeper lagoon inside
Ultimately the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon
A fringing reef can take ten thousand years to form, and an atoll can take up to 30 million years
A small atoll in Maldives.
Darwin predicted that underneath each lagoon would be a bed rock base, the remains of the original volcano. Subsequent drilling has 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. 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.
In general, the two main variables determining the geomorphology, or shape, of coral reefs are the nature of the underlying substrate on which they rest, and the history of the change in sea level relative to that substrate.
As an example of how coral reefs have formed on continental shelves, the current living reef structure of the Great Barrier Reef began growing about 20,000 years ago. The sea level was then 120?metres (390?ft) lower than it is today. As the sea level rose, the water and the corals encroached on what had been the hills of the coastal plain. By 13,000 years ago the sea level was 60?metres (200?ft) lower than at present, and the hills of the coastal plains were, by then, continental islands. As the sea level rise continued most of the continental islands were submerged. The corals could then overgrow the hills, forming the present cays and reefs. The sea level on the Great Barrier Reef has not changed significantly in the last 6,000 years, and the age of the present living reef structure is estimated to be between 6,000 and 8,000 years. Although the Great Barrier Reef formed along a continental shelf, and not around a volcanic island, the same principles apply as outlined by Darwin's theory above. The Great Barrier Reef development has stopped at the barrier reef stage, since Australia is not about to submerge. It has formed the world's largest barrier reef, 3001000 metres (330-1100 yards) from shore, and 2,000?kilometres (1,200?mi) long.
Healthy coral reefs grow horizontally from 1 to 3 centimetres (0.39 to 1.2?in) per year, and grow vertically anywhere from 1 to 25 centimetres (0.412?in) per year; however, they are limited to growing above a depth of 150?metres (490?ft) due to their need for sunlight, and cannot grow above sea level.
Types
The three principal reef types are:
Fringing reef a reef that is directly attached to a shore or borders it with an intervening shallow channel or lagoon.
Barrier reef a reef separated from a mainland or island shore by a deep lagoon.
Atoll reef a more or less circular or continuous barrier reef extending all the way around a lagoon without a central island.
Other reef types or variants are:
Patch reef an isolated, comparatively small reef outcrop, usually within a lagoon or embayment, often circular and surrounded by sand or seagrass. Patch reefs are common.
Apron reef a short reef resembling a fringing reef, but more sloped; extending out and downward from a point or peninsular shore.
Bank reef a linear or semi-circular shaped-outline, larger than a patch reef.
Ribbon reef a long, narrow, somewhat winding reef, usually associated with an atoll lagoon.
Table reef an isolated reef, approaching an atoll type, but without a lagoon.
Inhabited cay in the Maldives
Microatolls certain species of corals form communities called microatolls. The vertical growth of microatolls is limited by average tidal height. By analysing the various growth morphologies, microatolls can be used as a low resolution record of patterns of sea level change. Fossilized microatolls can also be dated using radioactive carbon dating. Such methods have been used to reconstruct Holocene sea levels.
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 for them to be habitable by humans. Cays occur in tropical environments throughout the Pacific, Atlantic and Indian Oceans (including in 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. Their surrounding reef ecosystems also provide food and building materials for island inhabitants.
When a coral reef cannot keep up with the sinking of a volcanic island, a seamount or guyot is formed. Seamounts and guyots are below the surface of the ocean and may host many species, depending on their location and depth. 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.
Distribution
Locations of coral reefs.
Boundary for 20 C isotherms. Most corals live within this boundary. Note the cooler waters caused by upwelling on the south west coast of Africa and off the coast of Peru.
This map shows areas of upwelling in red. Coral reefs are not found in coastal areas where colder and nutrient rich upwellings occur
Coral reefs are estimated to cover 284,300?square kilometers (109,800?sq?mi), which is just under one percent of the surface area occupied by the world oceans. 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 only 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 50?meters (160?ft). The optimum temperature for most coral reefs is 2627?C, and few reefs exist in waters below 18?C. However reefs in the Persian Gulf have adapted to temperatures of 13?C in winter and 38?C in summer.
Deep water coral is more still exceptional since it 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. They are also rare along the coast around north-eastern South America and Bangladesh due to the freshwater release from the Amazon and Ganges Rivers respectively.
Principal coral reefs and reef areas of the world
The Great Barrier Reef - largest coral reef system in the world, Queensland, Australia.
The Belize Barrier Reef - second largest in the world, stretching from southern Quintana Roo, Mexico along the coast of Belize to the Bay Islands of Honduras.
The New Caledonia Barrier Reef - second longest double barrier reef in the world, with a length of about 1,500?kilometers (930?mi).
The Andros, Bahamas Barrier Reef - third largest in the world, following the east coast of Andros Island, Bahamas, between Andros and Nassau.
The Red Sea Coral Reef - located off the coast of Israel, Egypt, Sudan, Eritrea, Djibouti, Somalia, Jordan, Saudi Arabia, and Yemen.
Pulley Ridge - deepest photosynthetic coral reef, Florida.
Numerous reefs scattered over the Maldives.
Ghe Raja Ampat Islands in Indonesia's West Papua province offer the highest known marine diversity.
Biology
Anatomy of a coral polyp.
See also: Coral
Live coral should be thought of as small live animals embedded in calcium carbonate. It is a mistake to think of coral as plants or rocks. Coral consists 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. Reefs grow as polyps along with other organisms deposit calcium carbonate, the basis of coral, as a skeletal structure beneath and around themselves, pushing the coral's "head" or polyps upwards and outwards. Waves, grazing fish (such as parrotfish), sea urchins, sponges, and other forces and organisms break down coral skeletons into fragments that settle into spaces in the reef structure. 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.
Reef-building or hermatypic corals are only found 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 photosynthesize, but have a symbiotic relationship with single-celled organisms called zooxanthellae; these cells within the tissues of the coral polyps carry out photosynthesis and produce excess organic nutrients that are then used by the coral polyps. 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 impressive reef structures. Corals get up to 90% of their nutrients from their zooxanthellae symbionts.
Table coral
Close up of polyps arrayed on a coral, waving their tentacles. There can be thousands of polyps on a single coral branch.
Corals can reproduce both sexually and asexually. An individual polyp may use both reproductive modes within its lifetime. Corals reproduce sexually by either internal or external fertilization. The reproductive cells are found on the mesentery membranes that radiate inward from the layer of tissue that lines the stomach cavity. Some mature adult corals are hermaphroditic; others are exclusively male or female. A few even
detriot tigers caps change sex as they grow.
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 simultaneously. Eggs disperse over a large area. Spawning depends on four factors: 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 larvae usually occurs at night and is sometimes in phase with the lunar cycle (36 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 (7, 14). They are vulnerable to heavy predation and adverse environmental conditions. For the lucky few who survive to attach to substrate, the challenge comes from competition for food and space.
There are about one thousand species of coral, which build different shapes such as wrinkled brains, cabbages, table tops, stag antlers, wire strands and pillars.
Brain coral
Staghorn coral
Spiral wire coral
Pillar coral
Darwin's paradox
Darwin's paradox
Coral... seems to proliferate when ocean waters are warm, poor, clear and agitated, a fact which Darwin had already noted when he passed through Tahiti in 1842.
This constitutes a fundamental paradox, shown quantitatively by the apparent impossibility of balancing input and output of the nutritive elements which control the coral polyp metabolism.
Recent oceanographic research has brought to light the reality of this paradox by confirming that the oligotrophy of the ocean euphotic zone persists right up to the swell-battered reef crest. When you approach the reef edges and atolls from the quasi-desert of the open sea, the near absence of living matter suddenly becomes a plethora of life, without transition. So why is there something rather than nothing, and more precisely, where do the necessary nutrients for the functioning of this extraordinary coral reef machine come from?? Francis Rougerie
During his voyage on the Beagle, Darwin described tropical coral reefs as oases in the desert of the ocean. He reflected on the paradox that tropical coral reefs, which are among the richest and most diverse ecosystems on earth, flourish when they are surrounded and supported by tropical ocean waters that provide hardly any nutrients. It has been a challenge for scientists to explain this paradox.
Coral reefs cover just under one percent of the surface of the world 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. And the primary productivity on a coral reef is very high, resulting in a typical biomass production of 5-10g C m2 day1.
Tropical waters are often described as crystal clear. This is because they are deficient in nutrients and drifting plankton. The sun shines year round in the tropics, warming the surface ocean layer so it is less dense than subsurface layers. The warmer water is separated from
detriot tigers caps 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. 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, phosphorus and potassium. These nutrients, N, P and K, are necessary for plant growth, but in the tropics they are not directly recycled back to the surface.
Plants are the base of the food chain, and need sunlight and nutrients if they are to grow. In the ocean these plants are mainly a type of plankton, microscopic phytoplankton which drift in the water column. They need sunlight for photosynthesis, which powers carbon fixation, so they are found only in the surface waters. But they also need nutrients. Phytoplankton rapidly use any nutrients in the surface waters, and in the tropics these nutrients are not usually replaced because of the thermocline.
Coral polyps
The situation with coral reefs is different. The lagoons that are formed by the upward growth of coral reefs fill in with material eroded from the reef and the island. They become havens for marine life, providing protection from waves and storms.
Most importantly, nutrients are recycled, and not lost like they are in the open ocean. In coral reefs and lagoons, the producers include phytoplankton as well as marine worms, seaweed, and coralline algae, especially small types called turf algae, which pass their nutrients to the corals. The phytoplankton are eaten by fish and crustaceans, who also pass nutrients along the food web. Recycling ensures that fewer nutrients are needed overall to support the community.
Corals harbour numerous symbiotic organisms. In particular, there is a remarkable symbiosis between coral and a microscopic algae, the single cell dinoflagellate known as a zooxanthella. The zooxanthella forms an endosymbiosis
detriot tigers caps with a coral polyp, that is, it lives within the tissues of the polyp. There it absorbs solar energy with special pigments, using photosynthesis to supply the polyp with organic nutrients in the form of glucose, glycerol, and amino acids. Zooxanthellae can provide up to 90% of a coral energy requirements. In return, as an example of mutualism, the coral provides the zooxanthellae, averaging one million for every cubic centimetre of coral, with a relatively safe place to live and a constant supply of the carbon dioxide it needs for photosynthesis.
Corals are nocturnal feeders. Here, in the dark, coral polyps have extended their tentacles to feed on zooplankton
The colour of corals depends on the type zooxanthella 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 provides the polyp with nitrogen, and the polyp shares some of the nitrogen with the zooxanthellae, which also require this element. The 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 lead to the death of the coral.
A 2001 paper reported that sponges are another key to explaining Darwin paradox. These sponges 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 nutrients the sponges absorb from the phytoplankton are then excreted in a form the corals can use.
Researchers in 2002 explained why coral thrives better in agitated waters. They found the roughness of coral surfaces is the key. Normally there is a boundary layer of still water around a submerged object which acts as a barrier. But when waves break on the extremely rough edges of corals the boundary layer is disrupted, allowing the corals access to the few nutrients that are there. The researchers claim that turbulent water promotes rapid reef growth and lots of branching. Although coral ecosysemss are great at recycling, with the wastes of one species becoming the food of another, the researchers also claim that, without the nutritional gains achieved by rough coral surfaces, even the most effective recycling would leave corals wanting in nutrients.
In 2004, another symbiotic organism, a bacteria called Cyanobacteria, was discovered to provide soluble nitrates for the reef via nitrogen fixation.
