Disappearance Of Coral Reefs In The Ocean


 |  Biodiversity/Conservation

The Starfish

What are corals and coral reefs?

A common thing that fascinates ocean divers and people who are curious about the ocean are the colourful structures they see. These complex organisms are called coral reefs. No, they are not accumulated sediments or rocks, rather, they are structures made up of many small animals called coral polyps that are clustered together to form a large animal called coral (Spalding et al., 2001).

These small animals build these complex structures we see by working together to take out calcium carbonate ions from the sea water, which they use to build their outer protective coverings known as exoskeleton. Because coral polyps are clustered together, the exoskeletons they form accumulate and are held together to become a reef.

One feature of coral polyps is that they have a mouthlike opening that is surrounded by flexible body parts known as tentacles. The tentacles contain cells called nematocyst, which they use to sting and poison an animal prey for food. Food enters the stomach of each polyp through the mouth.

Although there are different kinds of corals in the ocean, only the ones that can form reefs are called reef-building corals (Spalding et al., 2001).

Characteristics of coral reefs

Corals contain an algae called zooxanthellae that live within their tissues (Knowlton, 2001). These two organisms benefit from each other through the substances they produce. The coral provides a protected environment as well as carbon dioxide gas and nitrogenous wastes for the algae to use. In return, the algae uses the carbon dioxide gas through the process of photosynthesis to produce carbohydrates which the corals use for food. The algae also helps the coral remove water products from its body. This type of relationship between the coral and the algae is known as mutualism. It allows corals to grow fast and build reefs, without which, reefs cannot be formed.

Where can coral reefs be found?

Coral reefs can be found in oceans close to the equator such as the Pacific ocean, the Caribbean sea, the Red sea, the Indian ocean, and the Persian Gulf. Reef-building corals are commonly located in the shallow parts of these oceans because of the high amounts of sunlight the photosynthetic algae receive. This allows the algae to undergo photosynthesis to produce nutrients necessary for the corals’ growth and to use the calcium carbonate ion taken up by the coral polyps to form reefs. Anything that affects light availability will decrease the corals’ ability to form reefs, which is detrimental to other marine organisms that depend on this ecosystem.

Not only are reef-building corals restricted to the shallow parts of these oceans, but they also require water temperatures between 18 -30℃ (Spalding et al., 2001) for the algae to function optimally. If the ocean temperature is above 30℃, it will negatively affect reef-building corals by causing them to remove the algae from their tissues, giving rise to a phenomenon known as coral bleaching.

Example of coral bleaching

Benefits of coral reefs

Coral reefs provide crucial benefits for both marine ecosystems and humans. In the ocean, they serve as a habitat for different types of animals and plants. Reefs contain approximately 600,000 to 9 million plant and animal species (Birkeland, 1997). This high level of biodiversity ensures the coral reef ecosystem to keep functioning even when a keystone species becomes absent (Moberg & Folke, 1999). Keystone species are organisms whose presence or absence will either increase or decrease the population of the organism it consumes. 

In coral reefs, sharks are the major keystone species. When sharks are absent, the population of the fishes it consumes will increase, causing them to eat a large amount of the seaweeds and decrease their population. If the population of seaweeds decreases, all the organisms that depend on it will decline in population, causing them to go extinct. To prevent this from occurring, another animal in the reef will take the position of the shark and consume the fishes to prevent their population from increasing, thereby preserving the population of the seaweeds.  

Also, scientists can use some of the organisms on coral reefs to manufacture pharmaceutical products to treat diseases. For example, scientists have used the compounds in some of the plants and animals on coral reefs to make drugs for treating cancer, viral and heart diseases, bacterial infections etc.

Coral reefs can change the pH level of the ocean. When coral polyps take up calcium carbonate ion from the seawater to build their exoskeleton, this causes the pH of the seawater to decrease and become acidic (Birkeland, 1997). When this occurs, the ocean will release a small amount of carbon dioxide gas back into the atmosphere. This carbon dioxide released by coral reefs is part of the normal carbon dioxide cycle that is necessary to maintain the atmospheric temperature in a range that can support life on earth.

It is worth noting that coral reef ecosystems also provide benefits for humans such as food source, a site for tourist attraction, income, and employment opportunities for people living close to these oceans.    

The problems reefs are facing worldwide

Globally, approximately 50% of coral reefs are almost lost due to the combination of human and natural causes. However, human activities such as burning of fossil fuels, scuba diving and practices that increase the amounts of nutrients in the ocean, are the major causes of coral reef decline.

When fossil fuels in the form of coal, natural gas and petroleum are burned, carbon dioxide gas is released into the atmosphere. Industries are constantly burning fossil fuels to produce most of the items we use in our day-to-day lives such as toiletries, cosmetics, plastics, medicines etc. Over the years, this has increased the concentration of carbon dioxide gas in the atmosphere to a value of 383 parts per million by volume (Guinotte & Fabry, 2008). As more carbon dioxide gas is released into the atmosphere, it accumulates and absorbs most of the sun rays. The more sun rays are absorbed, the more heat is trapped, which will cause the planet to become warmer and the atmospheric temperature to increase. Overtime, the atmospheric temperature of the earth will become very hot, leading to a phenomenon known as global warming.

The warm atmospheric temperature not only affects humans , land plants and animals, but also affects the organisms that live in water bodies. The ocean will absorb some of the heat from the atmosphere, and this will increase the seawater temperature above the normal range that the marine organisms can tolerate. As a result, the ocean pH will decrease and the seawater will become acidic, leading to ocean acidification. Ocean acidification occurs when the seawater pH decreases and becomes acidic due to the increase in seawater temperature above the normal range.

For coral reef ecosystems, a warmer seawater temperature will cause corals to remove the algae from their tissues through a process known as coral bleaching. The absence of the algae will cause the corals to die because the algae will no longer be available to provide food and nutrients  for corals to grow and survive. Ocean acidification also prevents coral polyps from taking out calcium carbonate ions from the seawater to build their exoskeletons, preventing them from forming reefs. The marine organisms that cannot survive in acidic seawater will die and go extinct due to ocean acidification. Additionally, the acidic water will reduce the amount of carbonate ions that are available to form a strong exoskeleton, thereby causing weak exoskeletons and reefs to form (Guinotte & Fabry, 2008). As such, corals will not be able to grow, and the weak reefs constructed will be easily destroyed by erosion or other natural disturbances. Ultimately, the marine organisms that depend on coral reefs will be deprived of shelter, food, and other important benefits that reefs provide.

Scuba diving is another human activity contributing to the loss of coral reefs. Scuba diving is a sport that involves divers exploring the underwater world out of curiosity and fascination for unique marine creatures. Every year, tourists go to countries with reefs for leisure and recreation. In return, these countries capitalize on this benefit to generate income and employment opportunities for its citizens. However, when divers step on the reefs to take pictures or swim poorly and accidentally hit the reef, they damage it (Barker & Roberts, 2004). Stepping on the reef destroys the exoskeletons of the corals and the protective coverings of other reef inhabitants, which exposes them to diseases and harmful organisms that can kill them (Barker & Roberts, 2004).

When we apply chemical fertilizer or manure on the soil or dispose of untreated sewage, it increases the amount of nutrients in the ocean and contributes to the disappearance of coral reefs. Because the human population on the planet keeps increasing, farmers are under pressure to produce large amounts of food to feed the population. This has caused farmers to use chemical fertilizers to provide essential plant nutrients to support the growth of food crops. Large amounts of chemical fertilizers are frequently applied on the soil with the aim of increasing the performance and output of the plants. Unfortunately, these fertilizers runoff into water bodies after a heavy rainfall and increase the amount of dissolved inorganic nutrients such as nitrogen and phosphorus in the water (Szmant, 2002). 

Apart from chemical fertilizers, some farmers use animal wastes as manure on the soil to also provide nutrients for the plants. Some of it also ends up in the ocean after a heavy rainfall and causes eutrophication. Similarly, just dumping sewage into the ocean without properly treating it will also increase the amounts of nitrogen and phosphorus in the water and cause eutrophication.

Eutrophication is the excessive growth of algae and plant species in water bodies due to the increased amounts of an essential nutrient that was not previously available. Unlike other ecosystems on earth, corals can survive in water that is very low in nutrients. However, when high amounts of nutrients become available in the ocean, the population of algae on the reef will increase much more than that of the corals and outcompete the corals for space and other essential growth factors (D’Angelo & Wiedenmann, 2014). During the competition, algae can produce compounds that can kill the corals, thereby preventing them from building their exoskeletons. This high algal population can also form thick layers over the coral reefs, which will smother the organisms on the reef and prevent corals from growing. If corals do not grow, they will die, and reefs will not be formed. 

Elevated amounts of nutrients in the ocean also increases the population of marine plants known as phytoplankton (D’Angelo & Wiedenmann, 2014). When this occurs, light will not be able to penetrate the ocean, and zooxanthellae within the coral tissues will not be able to carry out photosynthesis and provide the necessary substances  required by the corals. As a result, corals will not grow and very few reefs will be formed over time. Also, when the increased population of marine plants die, lots of plant wastes will be produced, and their decomposition by microorganisms will pollute the ocean and expose the corals to bacterial and fungal infections.

Not only do the human activities mentioned above negatively affect marine organisms, but humans also suffer the consequences. The global decline in coral reefs will contribute to the loss of fishes and other marine organisms that act as food sources for people living close to these oceans, and loss of income and employment for people who depend on coral reefs as a source of livelihood.

What can we do to prevent the loss of coral reefs?

Given the negative effect high amounts of carbon dioxide in the atmosphere has on coral reef ecosystem, we can reduce further carbon emission by switching lights off when not in use and minimize the use of petroleum-based products such as bringing reusable bags to shop instead of using plastic bags. We can also drive less to decrease carbon emission from cars etc. Since industries, transport and energy supply sectors release the highest amount of carbon to the atmosphere, they can switch to use other technologies that reduce carbon emissions and implement other methods of reducing carbon dioxide emissions.

We can encourage farmers to use natural fertilizers instead of chemical fertilizers to reduce the chances of nutrient runoff into the ocean. Natural fertilizers include the use of human urine, cyanobacteria, and agricultural waste as compost. They contain high amounts of plant nutrients such as nitrogen, phosphorus, and potassium, and can help recycle nutrients to ensure they are always available to plants.

Additionally, diving companies should educate tourists about the skills and knowledge required to explore the ocean, and how certain underwater behaviours destroy reefs. More so, divers can reduce the occurrence of damage to the reef by diving in groups under the supervision of an experienced dive leader.

Now that you are aware of the consequences of losing coral reefs, you can contribute to saving this important marine ecosystem by sharing this knowledge with your friends and family. 


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