Coral reef

A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.

Coral belongs to the class Anthozoa in the animal phylum Cnidaria, which includes sea anemones and jellyfish. Unlike sea anemones, corals secrete hard carbonate exoskeletons that support and protect the coral. Most reefs grow best in warm, shallow, clear, sunny, and agitated water.

Sometimes called rainforests of the sea, shallow coral reefs form some of Earth's most diverse ecosystems. They occupy less than 0.1% of the world's ocean area, about half the area of France. Yet, they provide a home for at least 25% of all marine species, including fish, mollusks, worms, crustaceans, echinoderms, sponges, tunicates and other cnidarians. Coral reefs flourish in ocean waters that provide few nutrients.

Formation

Further information: Atoll, Fringing reef, and The Structure and Distribution of Coral Reefs

Most coral reefs formed after the Last Glacial Period when melting ice raised sea levels and flooded continental shelves. These reefs are usually less than 10,000 years old. As they grew, they kept up with the rising sea levels. Coral reefs can also be found deep in the ocean near oceanic islands and atolls.

In his book The Structure and Distribution of Coral Reefs, Charles Darwin described how atoll reefs form. He suggested that the shape of atolls comes from the movement of Earth's oceanic crust. Darwin explained that a fringing reef starts around a volcanic island. Over time, if the island and ocean floor sink, the reef grows and changes into a barrier reef, and finally into an atoll.

Types

Main article: Fringing reef

Main article: Atoll

Coral reefs come in many shapes and sizes. One common type is the fringing reef, which grows right next to the shore. These reefs can stretch for many kilometers along coastlines. Another type is the barrier reef, which lies far from the shore, separated by a deep channel or lagoon. The largest example is the Australian Great Barrier Reef.

Atolls are ring-shaped reefs that form around a lagoon, often where a volcanic island has sunk below the sea. Other types include platform reefs, which can form in the open ocean, and patch reefs, small reefs found within lagoons. Each type of reef provides a unique home for many sea creatures.

Zones

Coral reefs have different areas called zones, each with unique plants and animals. The three main zones are the fore reef, the reef crest, and the back reef, also known as the reef lagoon. These zones are connected, allowing water, nutrients, and sea creatures to move between them.

Coral reefs are usually found in shallow waters less than 50 meters deep. They can be near continents, like the Great Barrier Reef, or around islands, like in the Maldives. Each part of a reef, such as the reef surface or reef face, provides a special home for many sea creatures, helping them find food and shelter.

Locations

Coral reefs cover about 284,300 square kilometers of the ocean, mostly in the Indo-Pacific region. This includes areas like the Red Sea, Indian Ocean, Southeast Asia, and the Pacific. Shallower reefs grow best in warm water close to the equator, but some reefs can survive in colder places like the Persian Gulf or even near Norway.

Some of the world's most famous reefs include the huge Great Barrier Reef off Australia, the Mesoamerican Barrier Reef System near Mexico and Honduras, and the Red Sea reefs. Other important reefs are found around the Maldives, the Philippines, and the Raja Ampat Islands in Indonesia.

Coral

Main article: Coral

Corals are colonies of tiny animals called polyps that live inside calcium carbonate shells. These polyps work together to build reefs, with some growing as large as 12 inches across. Reef-building corals can only live in shallow, sunny waters where they can get enough light.

Coral polyps have a special friendship with tiny algae called zooxanthellae that live inside them. These algae give the coral food and help it grow quickly, while the coral gives the algae a safe home and carbon dioxide. If a coral loses its algae, it turns white in a process called bleaching, which can be harmful to the coral.

Gallery of reef-building corals

Coral reefs are beautiful underwater ecosystems made by tiny animals called coral polyps. These polyps live in groups and build hard homes from a material called calcium carbonate, which forms the structure of the reef. These reefs are built mostly by stony corals, which are the main builders of the reefs we see today.

Other reef builders

Corals are the most important builders of reefs, but many other sea creatures help too. These include coralline algae, some sponges, and bivalves. Together, these different kinds of animals create reefs over many years.

Coralline algae are tiny plants that add strong limestone to reefs, especially in rough waters. Sea sponges can also form reefs, like those found off the coast of British Columbia. Long ago, ancient sponges were the first animals to build reefs. Oyster reefs are groups of oysters that grow together, adding layers over time. These reefs were once common along coastlines but have changed over the years. Some snails also help by gluing themselves to rocks and adding to the reef structure.

Darwin's paradox

In The Structure and Distribution of Coral Reefs, published in 1842, Darwin described how coral reefs were found in some tropical areas but not others, with no obvious cause. The largest and strongest corals grew in parts of the reef exposed to the most violent surf, and corals were weakened or absent where loose sediment accumulated.

Tropical waters contain few nutrients yet a coral reef can flourish like an "oasis in the desert". This has given rise to the ecosystem conundrum, sometimes called "Darwin's paradox": "How can such high production flourish in such nutrient poor conditions?"

Coral reefs support over one-quarter of all marine species. This diversity results in complex food webs, with large predator fish eating smaller forage fish, which eat yet smaller zooplankton, and so on. However, all food webs ultimately depend on plants, which serve as primary producers. Coral reefs typically produce 5–10 grams of carbon per square meter per day (gC·m⁻²·day⁻¹) biomass.

One reason for the unusual clarity of tropical waters is their nutrient deficiency 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 deeper, 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, which 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 do not return directly 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.

Around coral reefs, lagoons fill in with material eroded from the reef and the island. They become havens for marine life, protecting them from waves and storms.

Most importantly, reefs recycle nutrients, which happens much less in the open ocean. In coral reefs and lagoons, producers include phytoplankton, seaweed, and coralline algae, especially small types called turf algae, which transfer nutrients to corals. The phytoplankton form the base of the food chain and are eaten by fish and crustaceans. Recycling reduces the nutrient inputs needed overall to support the community.

Corals also absorb nutrients, including inorganic nitrogen and phosphorus, directly from water. Many corals extend their tentacles at night to catch zooplankton that pass near. Zooplankton provide the polyp with nitrogen, and the polyp shares some of the nitrogen with the zooxanthellae, which also require this element.

Sponges live in the crevices of reefs. They are efficient filter feeders, and in the Red Sea they consume about 60% of the phytoplankton that drifts by. Sponges eventually excrete nutrients in a form that corals can use.

The roughness of coral surfaces is key to coral survival in agitated waters. Normally, a boundary layer of still water forms around a submerged object, acting as a barrier. Waves breaking on the extremely rough edges of corals disrupt the boundary layer, allowing the corals access to passing nutrients. Turbulent water thereby promotes reef growth. Without access to nutrients brought by rough coral surfaces, even the most effective recycling would not suffice.

Deep nutrient-rich water entering coral reefs through isolated events may have significant effects on temperature and nutrient systems. This water movement disrupts the relatively stable thermocline that usually exists between warm shallow water and deeper colder water. Temperature regimes on coral reefs in the Bahamas and Florida are highly variable, spanning temporal scales from minutes to seasons and spatial scales across depths.

Water can pass through coral reefs in various ways, including current rings, surface waves, internal waves, and tidal changes. Movement is generally created by tides and wind. As tides interact with varying bathymetry and wind mixes with surface water, internal waves are created. An internal wave is a gravity wave that moves along density stratification within the ocean. When a water parcel encounters a different density, it oscillates and creates internal waves. While internal waves generally have a lower frequency than surface waves, they often form as a single wave that breaks into multiple waves as it hits a slope and moves upward. This vertical breakup of internal waves causes significant diapycnal mixing and turbulence. Internal waves can act as nutrient pumps, bringing plankton and cool nutrient-rich water to the surface.

The irregular structure of coral reef bathymetry may enhance mixing, producing pockets of cooler water and variable nutrient levels. Arrival of cool, nutrient-rich water from depths due to internal waves and tidal bores has been linked to growth rates of suspension feeders and benthic algae as well as plankton and larval organisms. The seaweed Codium isthmocladum reacts to deep water nutrient sources because their tissues have different concentrations of nutrients dependent upon depth. Aggregations of eggs, larval organisms, and plankton on reefs respond to deep water intrusions. Similarly, as internal waves and bores move vertically, surface-dwelling larval organisms are carried toward the shore. This has significant biological importance to cascading effects of food chains in coral reef ecosystems and may provide yet another key to unlocking the paradox.

Cyanobacteria provide soluble nitrates via nitrogen fixation.

Coral reefs often depend on surrounding habitats, such as seagrass meadows and mangrove forests, for nutrients. Seagrass and mangroves supply dead plants and animals that are rich in nitrogen and 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 in which the mangroves and seagrass can root.

Biodiversity

Coral reefs are some of the world's most productive ecosystems, providing complex and varied marine habitats that support a wide range of organisms. Over 4,000 species of fish live in coral reefs, along with many other animals such as sponges, cnidarians, worms, crustaceans, mollusks, echinoderms, sea squirts, sea turtles, and sea snakes. These reefs offer many hiding places, which help support such a rich variety of life.

Reefs also support many seabirds, such as the Laysan albatross and black-footed albatross, especially in places like Midway Atoll in Hawaii. Sponges are important too, helping to recycle nutrients and provide homes for small creatures. Different animals use the reef at different times, with some hiding during the day and coming out at night.

Ecosystem services

Coral reefs provide important services that help people and nature. They support tourism, fishing, and protect coastlines from waves. The value of these services is huge, estimated to be between $29.8 billion and $375 billion each year. About 500 million people benefit from what coral reefs offer.

Coral reefs also protect shorelines by reducing wave energy. They can cut wave energy by up to 97%, helping to prevent damage to property and loss of life. This protection is especially important for small islands. Reefs can be just as good as, or even better than, man-made structures for protecting coasts. Many people living near reefs get safety benefits from them.

Threats

See also: Environmental issues with coral reefs, Coral bleaching, and Environmental threats to the Great Barrier Reef

Coral reefs have faced many threats over millions of years, including disease, invasive species, and natural hazards. Today, human activities add new dangers. From 2009 to 2018, coral reefs around the world declined by 14%.

Human actions like coral mining, fishing that damages the sea floor, and pollution can harm reefs. Chemicals in some sunscreens can also hurt corals. A bigger threat comes from greenhouse gas emissions, which raise sea temperatures and cause coral bleaching. Climate change can lead to stronger storms and changes in ocean currents, which can damage reefs. While these changes are serious, some corals can adapt to new conditions and become more resistant to heat.

Protection

Marine protected areas (MPAs) are special zones in the ocean designed to protect sea life and promote healthy habitats, including coral reefs. They help manage fishing and protect natural environments. While the success of MPAs can vary, they often bring benefits such as supporting biodiversity and providing economic opportunities for local communities.

Protecting coral reefs also involves stopping overfishing, especially of important species like parrotfish, and reducing human impacts such as pollution from sewage and too many visitors. Healthy reefs need careful management and community involvement. Efforts to reduce carbon dioxide emissions, like limiting deforestation and controlling pollution, also help protect reefs from damage caused by ocean acidification. Countries around the world use laws and conservation strategies to safeguard these vital ecosystems.

Restoration

See also: Aquaculture of coral, Artificial reef, and Restoration ecology

Coral reef restoration has become more important over the past few decades because reefs around the world have been dying. Stressors like pollution, warmer ocean temperatures, extreme weather, and overfishing threaten reefs and the benefits they provide, such as protecting coastlines and supporting fish populations. To help reefs recover, scientists developed coral restoration in the 1970s and 1980s.

One key method is coral aquaculture, or coral farming. In this process, tiny coral pieces, called seeds, are grown in special nurseries until they are strong enough to be moved to damaged reefs. This helps young corals avoid dangers like predators and rough conditions. Coral farming has become the most common way to restore reefs because it works well and is easier than moving whole coral colonies, which can harm existing reefs.

Another way to help reefs grow is by adding special surfaces, called substrates, where corals can attach. These substrates can be made from old tires, ships, subway cars, or even concrete shapes. Some scientists use a process called biorock, which uses tiny electric currents to create a substance that helps corals grow faster and stronger. By carefully choosing where and how to add these surfaces, scientists can create conditions that encourage new coral growth and support healthy reef ecosystems.