Coral Reefs : A Unique Marine Habitat
In tropical seas, many types of coral animals and marine organisms such as coral polyps, calcareous algae, shell-forming creatures and lime-secreting plants live in large colonies. Though they are very tiny creatures, their ability to secrete calcium carbonate within their tiny cells has given rise to a particular type of marine landform. The landforms are popularly known as coral reefs. Coral reefs have numerous species of many forms, colours, and shapes.
Under favourable conditions, the colony of corals grows in profusion just below the water level. Among the numerous organism that forms the part of the coral habitat, the polyps are the abundant species. Each polyp resides in a tiny cup of coral made of calcium carbonate and when a polyp dies the cup cement together to form the reef structure.
There are also non-reef building species such as the 'precious corals' of the Pacific Ocean and the 'red coral' of the Mediterranean Sea which may survive in the colder and even the deeper waters. As a rule, they thrive well only in the warmer tropical seas.
There are two major types of corals: hard corals and soft corals, such as sea fans and gorgonians. Only hard corals build reefs. While the majority of coral reefs are found in tropical and sub-tropical waters, there are also deep water corals in colder regions. According to the United Nations Environment Programme, there are more cold-water coral reefs worldwide than tropical reefs. The largest cold-water coral reef is the Rost Reef off Norway.
The reef-building corals survive best under the following conditions:
- The surface water temperature of the ocean must not be less than 20 degrees Celsius. The condition virtually limits the areal distribution of corals to the tropical, and sub-tropical zones.
- Again they will not flourish where cold current flows and around the cold upwelling zones, as the surface temperature of the water is below the ideal requirement. Since most of the cold current flows along the western border of continents, it explains the absence of corals from western coasts. The presence of warm current in the temperate ocean raises the temperature of ocean water in these places; as a result, corals grow in these places in spite of the fact that the ambient temperature is below the threshold requirement.
- Zooxanthellae a type of algae live in a symbiotic relationship with coral polyps, algae produce food through photosynthesis and provide energy to polyps. So, the survival of corals is dependent on photosynthesis. As a result, the corals are found at a depth not exceeding 180 feet. Below 180 feet depth, the sunlight is too faint for photosynthesis to take place.
- The water should be salty and free from sediment. Corals, therefore, survive best in the moving ocean water well away from the silty coasts or muddy mouths of streams and rivers. The seaward side of the coral reefs is best developed because of the supply of oxygenated water by the constant flow of waves and tides.
Types of Coral Reefs:
The three main types of coral reefs include,
- Fringing Reefs: Fringing reefs are found along the coastline of the islands and continents. They are the most common type of reef structure found in the ocean. Sometimes they are separated by a shallow lagoon. Fringing reefs develop on the wave cut platforms along the continents and Islands. Their outer edge grows rapidly due to the availability of oxygenated water and food supply by constant wave currents.
- Barrier Reefs: A barrier reef is separated from the coast by a much wider and deeper channel or lagoon. The reef is partially submerged. Where it lies above the water level. The barrier reefs have narrow gaps at several places to allow the water from the enclosed lagoon to return to the open ocean. Such gaps are very useful for shipping and provide the only entrances for ships to enter or leave the lagoon. The best-known barrier reef is the Great Barrier Reef off the coast of Queensland, Australia. It is 1200 miles (around 2000 km) long, separated from the coast by a channel 100 miles (160 km) wide in places and over 200 feet (60 m) deep.
- Atolls: Atolls are similar to barrier reefs except that they are circular, enclosing a shallow lagoon without any land in the centre. The encircling ring is usually broken in a few places to allow the free flow of water. Some of the large atolls include Sudadiva in the Maldives, Bangaram atoll in Lakshadweep.
Bleaching refers to the paling of coral colour. It occurs when
- the density of zooxanthellae residing inside the reefs declines.
- The concentration of photosynthetic pigments within the zooxanthellae fall.
When the bleaching of corals occurs, they usually lose around 60-90 percent of their zooxanthellae, and each zooxanthella may lose around 50-80 percent of its photosynthetic pigments. If the stress-causing bleaching is not too severe and if it decreases with time, the affected corals usually regain their symbiotic algae within several weeks or a few months. If the loss of zooxanthellae is prolonged i.e., if the stress continues and depleted zooxanthellae populations do not recover, the coral host eventually dies.
High temperature and irradiance stressors have been implicated in the disruption of enzyme systems in zooxanthellae that offer protection against oxygen toxicity. Photosynthesis pathways in zooxanthellae are impaired at temperatures above 30 degrees Celsius, and this could activate the dissociation of coral/algal symbiosis.
Causes of Bleaching
Coral reef bleaching is a reaction to the stress applied on the coral polyps. It can be induced by a variety of factors, alone or in combination. It is therefore difficult to unequivocally identify the causes for bleaching events. The following stressors have been implicated in coral reef bleaching events.
- Temperature: Corals live in a narrow temperature range and seasonal spikes or lows can affect this ideal temperature zone which can result in coral bleaching. Bleaching events also occur during sudden temperature drops accompanying intense upwelling episodes, seasonal cold-air outbreaks etc.
- Solar radiation: It is believed to play a role in coral bleaching. Both photosynthetically active radiation (400-700 nm) and ultraviolet radiation (280-400 nm) are found to be responsible.
- Subaerial Exposure: Sudden exposure of reef flat corals to the atmosphere during events such as extreme low tides, El Nino Southern Oscillation (ENSO) - related sea level drops or tectonic uplift can potentially induce bleaching.
- Sedimentation: Relatively few instances of coral bleaching have been linked solely to sediment. It is possible but has not been demonstrated, that sediment loading could make zooxanthellae species more likely to bleach.
- Fresh Water Dilution: Rapid dilution of reef waters from storm-generated precipitation and runoff has been demonstrated to cause coral bleaching. Generally, such bleaching events are rare and confined to relatively small, nearshore areas.
- Inorganic nutrients: Rather than causing coral bleaching, an increase in ambient element nutrient concentrations (e.g. ammonia and nitrate) actually increases zooxanthellae density 2-3 times. Although eutrophication is not directly involved in zooxanthellae loss, it could cause secondary adverse effects such as lowering of coral resistance and greater susceptibility to diseases.
- Xenobiotics: Zooxanthellae loss occurs during exposure of coral to elevated concentrations of various chemical contaminants such as copper, herbicides, and oil. Because high concentrations of xenobiotics are required to induce zooxanthellae loss, bleaching from such sources is usually extremely localized and transitory.
- Epizootics: Pathogen-induced bleaching is different from other types of bleaching. Most coral diseases cause patchy or whole colony death and sloughing of soft tissues, resulting in a white skeleton but not bleached corals. Protozoans have been identified as some of the pathogens responsible for such conditions (translucent white tissues).