Coral Reefs: Formation, Types, Theories, and Their Impact on Coastal Ecosystems"
Discover the fascinating world of coral reefs, their types, formation process, economic and ecological significance, and their impact on the environment. Learn how coral reefs support marine life, protect coastlines, and contribute to tourism and fishing industries in East Africa and beyond.
What is Coral Reef.
A coral reef is a ridge of rock consisting of limestone. It is formed from skeletons of tiny marine organisms called coral polyps. The coral polyps live in millions of colonies and form their skeletons from the calcium carbonate extracted from the seawater. When they die the softer parts of their Principles of Physical Geography
bodies' decay and are washed away, but the skeletons are cemented into solid rock to form coral reefs. Built up layer by layer by living corals growing on top of the skeletons of past generations, coral reefs grow upward at a rate of 1 to 20 cm per year. The increase in the number of the coral polyps causes the masses of coral to grow to enormous size, forming coral reefs and coral islands.
Types of coral reefs.
Three distinct types of coral reef are recognized:
Fringing reef
This type of reef consists of a platform of coral which is connected to, and which is built out from a coast. It is separated from the coast by a shallow, narrow lagoon of less than 1 km in width.
The surface of the platform is usually flat or slightly concave and its outer edge drops away steeply to the surrounding sea floor. Fringing reefs can be seen at Mombasa, Kilifi and Tiwi along the Kenyan coast and at Dar es Salaam and Tanga along the coast of Tanzania.
Barrier reef
This is similar to a fringing reef in that it is connected to and built from a coast. However it differs from a fringing reef in that it is situated several kilometers off the coast and it is separated from it by a deep, wide, water lagoon.
The largest barrier reef in the world is the Great Barrier Reef in Australia off the Queens land coast, which is 2028km in length with a varying width of 30-50 km. south of the Tanzanian coast is the Mayotte reef and at the north end of the Mozambique Channel is the Comoro Island reefs.
Atolls reefs.
Atolls are in the shape of a ring, horseshoe or circular, enclosing a mass of water to form a lagoon. However, atolls are broken at various intervals by channels leading to the deep wide lagoon as shown
They are usually found in the middle of oceans. The Aldatra atoll is found 700 km off the East African coast. The most important groups of atolls in the Indian Ocean are however t Maldives and the Laccadive Islands, while the most important groups in the Pacific Ocean are the Caroline Islands, the Marshall Islands, the Gilbert Islands and Ellice Islands. The Maldive Island inthe Indian Ocean has a lagoon of about 64 kilometres in width.
Conditions favouring the formation of coral reefs.
- The temperature of the water must be high ranging between 20-30°C. This means they are confined to tropical and near-tropical seas within about 30° North and South of the equator. Extensive corals have formed on the eastern sides of landmasses where warm currents flow near the coasts. They do not develop on the western coasts in these latitudes because of the cool currents, which flow along these coasts leading to a fall in temperature.
- Presence of minute sea organisms called polyps which die and whose skeletons pile and accumulate to form the coral landforms.
- Corals do not form in fresh water. The water must be salty. This is because the polyps develop their hard skeletons from the calcium carbonate (salt) extracted from the seawater.
- Coral polyps cannot grow out of water. They are therefore formed below the level of the low tide.
- Coral polyps require a shallow water depth not exceeding 30-60 metres deep. This is because they need sunlight and oxygenated water. The sunlight supports the algae that live within their tissues. The depth of 30-60 metres guarantees both the light and oxygen required by the polyps.
- The water must be clear and not silt-laden if the water is muddy because it is full of silt brought down by rivers, the coral polyps cannot survive and the reef does not grow. This explains why where a river flows into the sea, there is a break in the reef as there will be a lot of silt deposited by it, hence creating conditions unfavorable for coral polyp growth.
- The availability of large quantities of microscopic life or plankton, which act as food for the coral polyps. Food supplies are usually most plentiful on the seaward side of a growing reef, so that the coral tends to grow more rapidly outward.
- Absence of strong currents, which would prevent the accumulation of skeletons of the coral polyps.
- Presence of a shallow continental shelf and oceanic ridges, which allow easy penetration of light ideal for the growth of polyps.
- Variation in the water level i.e. high and low water tide levels. These allow death and accumulation of coral shells.
- Presence of submerged islands which offer bases for the growth of especially the atoll reefs.
THE ORIGIN OF CORAL REEFS
The origin of fringing reefs is now understood that they grow from the shallow continental coasts seawards. The origin of barrier and atoll reefs, is however not yet fully understood. This is partly because some of these corals grow from very great depth of over 60 meters deep where conditions favouring their formation do not exist. Some atolls have also been found to stand isolated in very deep waters where coral growth could not normally take place.
Many theories have been put forward to explain the origin of coral reefs, and in common with so many theories explaining the possible origin of other landforms, no one can say which the true one is. All that we can do is to examine these theories and accept that which sounds the most reasonable. The main theories put forward to explain the origin of coral reefs are:
a) Subsidence theory or Darwin's theory.
b) Antecedent theory or Murray's theory.
c) Glacial control theory or Daly's theory.
The subsidence theory
This theory was put forward by Charles Darwin and independently supported by Dana and others. According to the theory, originally there was a fringing reef growing around a coast of a volcanic Island in the ocean. It was separated from the coast by a shallow, narrow lagoon. As the volcanic cone slowly subsided, the corals continued to grow upward keeping pace with the subsidence. Coral growth is more vigorous on the outer side of the reef so that it forms a higher rim, while the inner part comprises an increasingly deep and wide lagoon.
Illustration
Fact to support Darwin's theory
Some coasts within close proximity to barrier and a toll reef revealed features of submergence such as drowned valleys like. along coasts of Indonesia and the edge of the coastal plain of Queens land. Even along the East African coast, there are features of submergence such as rias and estuaries.Corals can grow upwards at a rate of 1-2 cm a year hence keeping pace with the rate of subsidence.
Facts to disapprove Darwin's theory.
· Some coral reefs have been found in areas where there is no evidence at all of submergence but where uplifting has taken place.
· The theory does not explain why some corals are found at great depth far beyond 30-60 meter depth.
The antecedent theory
This theory was put forward by Sir John Murray and supported by Agassiz, Semper and others. He refuted the idea of subsidence as put forward by Charles Darwin. He explains the formation of the barrier and a toll reefs in the following ways;
Formation of barrier reefs
Originally, there was a fringing reef growing along a coast and separated from the coast by a shallow narrow lagoon. This reef was attacked by waves and eroded, and the masses of coral fragments were gradually deposited and accumulated on the seaward side of the reef.
As coral tends to grow more strongly on the seaward side, it builds out further and further on the deposited fragments while this is happening, the corals on the inner side of the reef are deprived of food and so die. Murray also supposed that much of this dead coral was dissolved in water, so forming a deep wide lagoon. A reef separated from the coast by a deep wide lagoon which is a barrier reef thus formed.
Illustration
Understanding the Formation of a toll reefs
Murray's idea of an atoll formation is that the base of a reef consists of a submarine hill or plateau rising from the ocean floor. The sub-marine hill or a plateau is usually of volcanic origin. These eminencies, all reaching about 55 metres of the surface consist either of sub-surface volcanic peaks, wave-worn stumps or an accumulation of various pelagic deposits (sand and silt). It is on these that corals begin growing. The corals grow more on the outer seaward side where there is plenty of food. The inner corals lacked food and died dissolving in water to form a deep wide lagoons hence the atoll reefs.
Facts to support Murray's theory.
· Through borings put down through the reefs, confirm that most, if not all, a toll do rest on truncate volcanic cones.
· Through borings, sediments were found underneath at a depth of 600 metres on the Bikini Island.
Facts to disapprove Murray's theory
· Coral reefs once formed are very hard and cannot be easily eroded as Murray suggested.
· The lagoons formed by barrier and a toll reefs are so deep that they are unlikely to be formed by the dissolving of the corals.
· The theory does not explain why some corals are found so deep where conditions favouring their formation do not exist.
The glacial control theory
This theory was put forward by Daly (1934). The theory is based on the changing level of these during and after the last ice age (glaciation and deglaciation). According to him, the formation of barrier and atoll reefs was due to quaternary oscillations of climate and sea levels. According to the theory, temperatures were normal at one time and corals grew at the coast. Later the conditions changed from warm to cold especially during the ice age.
The polyps that existed either died remained dormant. All the pre-glacial reefs and other islands were planned down by marine erosion to the sea level of that time. The platforms provided bases for upward growth of coral when the temperatures warmed up. The increase in the temperature led to the melting of ice sheets, releasing large quantities of water to the sea hence causing a rise in sea level. As the water level rose, corals grew upwards and outwards on the platforms keeping pace with the rising sea level to form fringing, barrier and later atoll reefs.
Facts to support Daly's theory
· The theory helps to account for the narrow, steep-sided reefs, which compose most of the atolls.
· Corals grow upwards at a rate of 1 meter in 30 years. There is therefore no doubt of their ability to keep pace with the rising sea level.
· The theory explains why some coral reefs occur at great depth.
· Features of glaciation have been discovered on the islands in the Pacific ocean e.g. the Hawaii.
· We have gone through four ice ages. The level of the sea has therefore been rising and falling as Daly suggested.
Fact to disapprove Daly's theory
Old corals are not easily eroded once formed. If they are it is at a very slow rate of about 2 mm a year. It is therefore doubtful that the sub-marine wave-cut platforms could have formed by erosion.
In conclusion, no theory satisfactorily explains the origin of coral reefs. However, Daly's theory is nearer to the answer as it explains many factors, which are not explained by other theories.
Importance of Coral Reefs: Economic, Environmental, and Ecological Benefits
Coral reefs are among the most valuable ecosystems on Earth, offering economic, environmental, and ecological benefits. From supporting tourism and fishing industries to protecting coastlines, coral reefs play a crucial role in marine biodiversity and human livelihoods. Below are the key benefits of coral reefs, with examples from East Africa, including Kenya, Tanzania, Zanzibar, and Seychelles.
1. Source of Limestone for Cement Production
Coral reefs are rich in limestone, making them an essential resource for cement manufacturing.
- Example:
- Limestone from coral reefs is mined and used in cement production at Bamburi Cement Factory near Mombasa (Kenya) and Wazo Hill Cement Factory in Tanga (Tanzania).
2. Coral Reefs as a Building Material
- In some coastal regions, coral reefs are quarried and used as construction materials for buildings and homes.
- Example:
- In Zanzibar and Pemba, coral stone is commonly used in traditional building construction.
3. Major Tourist Attractions
Coral reefs are known for their stunning shapes, colors, and diverse marine life, making them prime tourist destinations.
- Tourists visit coral gardens for snorkeling, scuba diving, and glass-bottom boat tours.
- Examples:
- The Seychelles coral reefs are world-famous for their breathtaking underwater beauty.
- Malindi Marine Reserve (Kenya), established in 1967, was designated a World Biosphere Reserve in 1979.
- Kenya’s coral reefs along the coastline support a thriving marine tourism industry, attracting thousands of visitors annually.
4. Coastal Protection Against Strong Waves and Sharks
Coral reefs act as natural barriers, protecting coastlines from strong ocean waves, tidal currents, and dangerous sharks.
- The lagoons formed by coral reefs provide safe recreational areas for swimming and water sports.
- Example:
- Coral reefs off Kenya’s coastline and the Seychelles help block sharks from reaching shallow waters, making coastal areas safer for tourism.
5. Coral Reefs Reduce Coastal Erosion
Coral reefs serve as natural sea walls, absorbing wave energy and reducing erosion along coastlines.
- Examples:
- Leven and Andromache Reefs in Mombasa and Kitoka Reef in Kilifi (Kenya) help protect coastal communities from coastal erosion and storm surges.
6. Fishing Grounds and Marine Biodiversity
Coral reefs provide habitat for diverse fish species, supporting fishing industries and local livelihoods.
- Lagoons formed by coral reefs act as nurseries for fish, making them important fishing grounds.
- Example:
- Kenyan and Tanzanian coastal fisheries depend on coral reef ecosystems for a steady supply of fish.
7. Coral-Derived Soils for Agriculture
Weathered coral rocks contribute to soil formation, which supports tree crops like coconut, cashew nuts, and citrus fruits.
- Example:
- In Zanzibar and Pemba, coral-derived soils are used to grow coconut palms, cashew nut trees, and citrus fruits, sustaining local agriculture.
8. Research and Scientific Studies
Coral reefs are important for marine research, helping scientists study climate change, biodiversity, and ecosystem health.
- Universities and research institutions use coral reefs for marine biology studies and conservation projects.
Disadvantages of Coral Reefs in East Africa
1. Obstacle to Navigation and Shipwrecks
- The Indian Ocean coastline of East Africa has extensive coral reefs, making navigation difficult.
- In Kenya and Tanzania, reefs restrict ships from reaching ports directly, requiring deep channels or dredging.
- Example:
- The Kilindini Harbour (Mombasa, Kenya) had to be deepened through dredging due to coral formations blocking large vessels.
- Zanzibar and Mafia Island (Tanzania) have experienced shipwrecks due to coral reefs, especially during monsoons when visibility is low.
2. Formation of Infertile Soils (Coral Wanda) in Coastal Areas
- In Zanzibar, Pemba, and the coastal regions of Tanzania, coral rock weathers into coral wanda (infertile, porous soil).
- Effects:
- Poor agricultural yields force locals to import food from mainland Tanzania or engage in fishing instead of farming.
- Land degradation leads to food insecurity, especially in rural parts of Zanzibar.
- Example:
- In Pemba Island, coral wanda covers large areas, limiting crop production and forcing people to rely on coconut, cassava, and fishing instead of staple grains.
3. Hindrance to Marine Fisheries
- Coral reefs trap fishing nets and hinder boat movement, making deep-sea fishing difficult in East Africa.
- Example:
- Fishermen in Lamu, Kenya, and Bagamoyo, Tanzania, struggle with damaged nets due to coral entanglement.
- Mozambique’s Quirimbas Archipelago has reefs that obstruct larger fishing vessels from moving efficiently.
4. Health Risks Due to Mosquito Breeding in Reef-Formed Lagoons
- Fringing reefs create lagoons with stagnant water, which serve as breeding grounds for mosquitoes.
- Example:
- In Tanga, Tanzania, and Malindi, Kenya, residents near reef-enclosed lagoons suffer high malaria cases due to mosquito breeding.
- In Zanzibar, malaria has historically been a major health problem, worsened by reef-formed lagoons trapping stagnant water.
- Government efforts like spraying insecticides and draining lagoons have been necessary to control malaria.
Environmental Disadvantages of coral reefs in East Africa
5. Increased Coastal Erosion in Some Areas
- Coral reefs protect coastlines, but when they degrade, coastal erosion intensifies.
- Example:
- In Dar es Salaam (Tanzania), Watamu (Kenya), and Beira (Mozambique), reef destruction has led to stronger waves hitting the shore, washing away beaches.
- The erosion is worsened by sand mining and climate change, which further weakens the coast.
6. Release of Carbon Dioxide (CO₂) During Coral Bleaching
- Coral bleaching in Kenya’s Mombasa Marine Park and Tanzania’s Mnazi Bay releases stored carbon into the atmosphere.
- Example:
- In 2016, widespread coral bleaching in Tanzania and Kenya due to rising sea temperatures released significant CO₂ into the environment, worsening climate change effects like droughts.
7. Competition for Space with Seagrass and Mangroves
- Coral reefs compete with mangroves and seagrass, affecting marine biodiversity.
- Example:
- In Mida Creek, Kenya, expanding corals reduce the area for seagrass and mangroves, which are important breeding grounds for fish.
- This negatively affects local fishermen, who rely on these ecosystems for fish stock.
8. Interference with Natural Water Circulation
- In areas where coral reefs restrict water movement, pollution and oxygen depletion occur.
- Example:
- In Zanzibar’s Chwaka Bay, coral reefs trap pollutants from hotels and fishing villages, leading to water quality degradation and fish die-offs.
Socio-Economic Disadvantages of coral reefs in East Africa
9. Barrier to Infrastructure Development
- Coral reefs make port expansion costly and difficult in Mombasa, Dar es Salaam, and Beira.
- Example:
- In Mombasa, Kenya, the expansion of the Mombasa Port required extensive dredging of coral reefs, increasing costs.
- In Tanzania, the Tanga Port faces similar challenges, limiting trade and industrial growth.
10. Threat to Livelihoods Due to Unpredictable Coral Health
- Many people in Kenya, Tanzania, and Mozambique depend on reef-based tourism and fishing, but coral decline reduces incomes.
- Example:
- The decline of corals in Watamu (Kenya) and Mnemba Atoll (Zanzibar) has reduced the number of tourists, causing revenue losses for local communities.
11. Increased Costs for Marine Conservation and Management
- Governments must invest heavily in coral conservation.
- Example:
- Kenya and Tanzania have marine protected areas (MPAs) in Malindi, Kisite-Mpunguti, and Mafia Island, but they require large budgets for monitoring and patrols.
12. Damage from Invasive Species
- Crown-of-thorns starfish destroy corals in East Africa.
- Example:
- In Mafia Island (Tanzania) and Lamu (Kenya), these starfish have been increasing, causing reef destruction.
13. Coral Mining and Overexploitation for Commercial Use
- Coral is mined for construction materials and souvenirs in East Africa, leading to environmental damage.
- Example:
- In Mombasa, Kenya, coral blocks are extracted for building homes, causing reef degradation.
- In Zanzibar, illegal coral harvesting for souvenirs threatens marine biodiversity.
Conclusion
While coral reefs are valuable, their disadvantages in East Africa include navigation challenges, infertile soils, fishing difficulties, disease risks, erosion, climate effects, infrastructure barriers, economic losses, and conservation costs. Sustainable marine management and strict regulations are essential to balance their benefits and challenges.
