Coral Reefs Ecosystems

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Coral Reefs Ecosystems


The world’s oceans are made up of different ecosystems. Among them the coral reefs occupy a small percentage of the total area covered by the oceans but host a diverse array of marine species. However, coral reefs are extremely sensitive to changes in the ocean surface water temperature changes and pH levels making them very vulnerable. Further, due to the diversity of species and the large number of animal inhabitants in coral reefs, they are among the most productive ecosystems in the world. Nonetheless, their function in nature goes beyond just being a habitat for other species as they form barriers that protect the shore line against tidal waves. Besides, they are a source of livelihood for people across the world dependent on them for fishing and tourism. However, due to human and environmental factors, coral reefs across the world are threatened by climate change, sedimentation, pollution, and over-exploitation.

Coral Reefs

The Coral reef is actually a colony made up of thousands of tiny marine organisms called polyps that grow in saline water like oceans. Corals are invertebrate animals belonging to the class Anthozoa of phylum Cnidaria in the animal kingdom. In the water, the polyps build their exoskeleton from calcium carbonate and since they live in colonies, their exoskeletons get connected and forms reefs that grow continuously under ideal climatic conditions.  The coral reefs are home to many species of plants and animals. Although corals can capture their food from the sea water, they have evolved a symbiotic relationship with a group of single-celled algae known as zooxanthellae. The algal cell prepares food by the process of photosynthesis (Hughes et al., 2017) which is consumed by the coral polyps. In return, the coral provides the algae shelter and minerals for their growth. In addition, the coral varieties acquire their brilliant colors from the symbiotic partner the zooxanthellae algae that grow fast and form part of the reef structure. Consequently, any effect on one symbiotic partner affects the other partner too.

All living organisms reproduce to ensure continuity of the species. As such, the corals have evolved different reproduction strategies that not only increase their numbers but also guarantee genetic diversity. Since corals can exist as three sexual forms: either male or female or a hermaphrodite state having both the sexes, hence, they can reproduce both sexually and asexually. During sexual reproduction, corals produce eggs which are fertilized with sperms from a different colony, and the emergent larvae can swim and establish independent polyps in ideal substrates. Nonetheless, the process of sexual reproduction varies from internal to external sexual reproduction depending on the species. During external sexual reproduction, corals synchronize their spawning and release a large number of eggs and sperms that float to the surface of the ocean and fertilization occurs externally. However, in internal sexual reproduction, the corals release sperm into the water while the egg is brooded internally. Thus, as the sperms sink, the polyps take them in and fertilize the eggs internally. As a consequence, the genetic variability of the coral colonies is enhanced. Apart from sexual reproduction, corals also reproduce asexually (Kayanne, 2016) through budding or fragmentation.  Corals can produce their clones and continue multiplying throughout their lifetime by the asexual reproduction method.



Types of Corals

Based on their growth and colony formation, coral reefs are divided into three different types namely; fringing reefs, atolls, and barrier reefs.

Fringing Reefs

These types of reefs extend from the shoreline inwards forming borders along the shoreline. As such, they do not form a large lagoon and are the most common type of coral reefs in most of the regions where corals grow.


Atolls are coral reefs that grow and surround a sunken volcanic island and in the process are either oval in shape or circular. Further, there is a large lagoon in the middle of this type of coral reef. However, since they are usually established on sunken islands, they are found far from the mainland.

Barrier Reefs

Barrier reefs grow parallel to the shore line but do not originate from it and are usually separated from the shoreline by a large expanse of water. As such, they form a large lagoon between the reef and the shoreline. Although they are similar to the fringing reefs, they do not extend from the shoreline. In addition, when they originate from continental shelves, these reefs can grow to form ‘shelf barrier reefs.’ For instance, the Great Barrier Reef in Australia and the Belize Barrier Reef originate from the edge of continental shelves.

Benefits of Coral reefs

Ecological Benefits

Regardless of their type, coral reefs are significant both ecologically and economically. Coral reefs are amongst the most diverse ecosystems in the world ecologically. According to Spalding, Ravilious, and Green (2001), coral reefs occupy 234,300 square kilometers of the world oceans in the tropical areas. However, they are not evenly distributed, and the Indo-Pacific region accounts for almost 92% of all of the world’s coral reefs. Nonetheless, coral reefs are home to approximately 25% of the oceans flora and fauna biodiversity (Spalding, Ravilious and Green, 2001). As such, coral reefs are not only important to ocean ecological health but also offer other benefits to the marine species. For instance, they provide shelter for several species and due to their physical characteristics; coral reefs are an ideal breeding ground for other species.

Apart from being a perfect habitat for other marine species, coral reefs provide other ecological advantages. For instance, they act as barriers and protect the shorelines against tidal waves thus minimizing erosion. A study by Ferrario et al. (2014) found that coral reefs provide a defense against storm surges and other natural hazards by lowering the wave energy by up to 97%. In addition, the lowering of the wave’s power ensures the waves are calmer and reduces erosion of the shoreline. As such, sedimentation is also minimized, and the coral reefs’ health is maintained.

Apart from breaking wave energy, coral reefs protect a large human population against the natural hazards emanating from the oceans by providing a natural barrier which is far cheaper than a constructed artificial defense. Ferrario et al. (2014), opined that the average cost of building one meter of marine defense is 19, 791 US Dollars while the cost of coral reef rehabilitation was 1290 US Dollars. As such, it is much cheaper to preserve the natural defenses that coral reefs provide rather than building artificial barriers and countries facing natural marine hazards are investing more on the nature based solutions which are cheaper (Temmerman et al., 2013). Consequently, as more evidence emerges that natural solutions are more cost-effective, many countries with coastlines are investing in protecting the coral reefs in their area.

Economic Benefits

Coral reefs are a source of economic benefit to millions of people directly and indirectly. Coral reefs harbor a rich biodiversity and not only provide food, but they are also a source of income for many coastal communities who depend on them as their only source of livelihood (Halpern et al., 2015). As such, where properly managed to prevent over exploitation, coral reefs can offer long-term source of income through the marine products.

Apart from extractive benefits obtained by fishing and other activities that remove coral reef resources, they are also an important source of tourism income. Due to the diversity of species that are found in coral reefs, they are ideal sites for tourism activities that include snorkeling and underwater photography. As such, they employ all the support industries for tourism including hotels, boat services, equipment hire, guide services, and entry fees charged for the site seeing activities. For instance, in Australia, the Great Barrier Reef contributed 6.4 billion dollars in direct tourism revenue in the year 2012. In addition, a further 641 million was generated through recreation, commercial fishing, and scientific research. Further, an additional 5.7 billion dollars in value addition was created in the four streams mentioned above which also generated a total of 68, 979 jobs (Great Barrier Reef Marine Park Authority [GBRMPA], 2013). As such, due to their attractive nature and high biodiversity, coral reefs are a source of revenue and jobs.

Threats to Coral Reefs

Coral reefs are sensitive to changes in various parameters in the ocean environment. As such, they are threatened due to climatic change and other factors occasioned by human activities. Among the main variables that negatively affect the coral reefs are: rise in ocean temperatures due to global warming and climate change, change in pH, sedimentation, pollution, and over fishing.


Rise in Temperatures

Coral reefs are mainly found in the tropics. As such, they thrive within temperature ranges of (64-84° Fahrenheit) with few varieties being able to withstand higher temperatures for a short period. However, rising ocean temperature due to climate change is affecting the survival of many coral reefs. According to Smith et al., (2017), climate change is threatening the symbiotic relationship between zooxanthellae and the corals by raising the surface temperatures in the oceans beyond the tolerance levels of the zooxanthellae. As the temperature rises, the zooxanthellae leave the coral tissue, and the corals are bleached by the loss of color as well as starved of food and begin to die.  For instance, temperature stress was responsible for the bleaching event that hit the GBR in 2016 which affected 91% of individual reefs (Hughes et al., 2017). As a result, climate change is one of the biggest threats affecting coral reefs around the world.

Change in Ocean pH

Apart from temperature, corals are also sensitive to the change in potential hydrogen [pH] ion concentration of the ocean water caused by the process known as ocean acidification. The change in ocean pH is precipitated by the absorption of large amounts of anthropogenic carbon dioxide [CO2] by the ocean (Bates, 2007). As such, the increased uptake of CO2 makes the water slightly acidic which reduces the calcification of coral reefs rendering them more brittle (Eyre, Andersson and Cyronak, 2014). Consequently, human activities that are contributing to the increased CO2 in the atmosphere are contributing to the death of the world’s largest ecosystem.


The symbiotic relationship observed in coral reefs with zooxanthellae requires certain quality of water for the relationship to thrive. The clarity of water is critical to allow availability of sunlight for photosynthesis by the zooxanthellae. However, anthropogenic activities are contributing towards impeding the recruitment of new polyps into coral colonies. As the ability to photosynthesize is directly correlated with the clarity of water, human activities that lead to sedimentation are affecting the ability of coral reefs to acquire food and energy through their symbiotic partner zooxanthellae (Fournay and Figueiredo, 2017). Anthropogenic activities like dredging and forest clearing contribute to sedimentation following sea bed disturbances and erosion that negatively affects the coral reefs.



Human activities are a source of pollution that negatively affects coral reefs resilience to reproduce and recruit young polyps in the colonies and threatens the well being of the mature colonies. Human activities like oil spills and farming are contributing to pollution and affecting the coral reefs. Water pollution occasioned by leached nutrients leads to the growth of other algae that .affects the water clarity and prevents the zooxanthellae in coral from photosynthesizing. Although local human population density has not been correlated with coral reef degradation (Bruno and Valdivia, 2016), human agricultural activities have been correlated with nutrient pollution that has inadvertently been observed to encourage coral-algae competition and in the process affects the resilience of the coral colonies (Zanaveld et al., 2016). Consequently, human activities that increase the leaching of nutrients in the water ways that finally end up in the oceans need to be curtailed. However, there is need for a concerted international effort since the effectiveness of one country’s efforts is limited by the global nature of the problem.


The health of an ecosystem is dependent on the maintenance of equilibrium. Though coral reef ecosystems are biodiversity rich and thus a source of food and income for many coastal communities, they are under pressure from over exploitation. According to Rasher et al., (2017), the presence of predators in the coral reefs creates “hot spots” of fear which leads the reef herbivores to stop feeding in these spots and the process encourages the growth of sea weeds. This trophic cascade is also recreated when one species is over exploited in the coral reef leading to emergence of increased competition between the corals and the other organisms. In addition, the loss of certain species of organisms due to overfishing may lead to imbalance in the population dynamics and the emergence of diseases thus reducing the coral reefs resilience.

Apart from overfishing, the fishing methods used in the coral reefs also have a bearing on their well being. Fishing methods that destroy the integrity of the coral reefs are being practiced in many places around the world. For instance, cyanide fishing is very common in South East Asia and is responsible for at least 15% of all coral fish offered for sale in the European Union for marine aquariums (Vaz et al., 2017). As a result, more fish are being caught thus destabilizing the coral ecosystems.


Coral reefs are some of the most productive and diverse ecosystems in the world. However, they face serious threats to their continued survival. Climate change, sedimentation, pollution, and over exploitation are some of the factors affecting their well being. Nonetheless, coral reefs are source of food and income for millions of people around the world forming the basis for their economic benefits. They also provide ecological benefits by controlling storm surges and in the process minimizing shoreline erosion.




















Works Cited

Bates, N. R., 2007. Interannual variability of the oceanic CO2 sink in the subtropical gyre of the North Atlantic Ocean over the last 2 decades. J. Geophys. Res.-Oceans., 112: C09013.

Bruno, J. F., Valdivia, A., 2016. Coral reef degradation is not correlated with local human population density. Scientific Reports, 6: 29778.

Eyre, B. D., Andersson, A. J., Cyronak, T., 2014. Benthic coral reef calcium carbonate dissolution in an acidifying ocean. Nature Climate Change., 4: 969–976.

Ferrario, F., Beck, M. W., Storlazzi, C. D., Micheli, F., Shepard, C. C., Airoldi, L., 2014. The effectiveness of coral reefs for coastal hazard risk reduction and adaptation. Nat. Commun. 5:3794

Fournay, F., Figueiredo, J., 2017. Additive negative effects of anthropogenic sedimentation and warming on the survival of coral recruits. Scientific Reports, 7: 12380.

Great Barrier Reef Marine Park Authority [GBRMPA], 2013. Economic Contribution of the Great Barrier Reef. Deloitte Access Economics., 1-52.

Halpern, B. S., Potapenko, J., Casey, K. S., Koenig, K., Longo, C.,Lowndes, J. S., Rockwood, R. C., Selig, E. R., Selkoe, K. A., Walbridge, S., 2015. Spatial and temporal changes in cumulative human impacts on the world’s ocean. Nat. Commun. 6:7615.

Hughes, T. P., Kerry, J. T., Alvarez-Noriega, M., Alvarez-Romero, J.G., 2017.Global warming and recurrent mass bleaching of corals. Nature., 543:373-377.

Kayanne, Hajime., 2016. Coral Reef Science: Strategy for Ecosystem Symbiosis and Coexistence with Humans Under Multiple Stresses. Japan : Springer.

Rasher, D. B., Hoey, A. S., Hay, M. E., 2017. Cascading predator effects in a Fijian coral reef ecosystem. Scientific Reports, 7: 15684.

Temmerman, S., Meire, P., Bouma, T. J., Ysebaert, T., De Vriend, H. J., 2013. Ecosystem based coastal defence in the face of global change. Nature., 504: 79-83.

Smith, E. G., Vaughan, G. O., Ke3tchum, R. N., McPalard, D., Burt, J. A., 23017. Symbiont community stability through severe coral bleaching in a thermally extreme lagoon. Scientific Report., 7:2428

Vaz, M. C. M., Esteves, V. I., Calado, R., 2017. Live reef fish displaying physiological evidence of cyanide poisoning are still traded in the EU marine aquarium industry. Scientific Reports, 7: 6566.

Zaneveld, J. R., Burkepile, D. E., Shantz, A. A., Pritchard, C. E., McMinds, R., Payet, J.P., Welsh, R., Correa, A. M. S., Lemoine, N. P., Rosales, S., Fichs, C., Maynard, J. A., Thurber, R. V., 2016. Overfishing and nutrient pollution interact with temperature to disrupt coral reefs down to microbial scales. Nature communications, 7: 11833.


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