10 Biggest Threats to Coral Reefs

Threats to Coral reefs have been an important issue discussed over time, despite its importance to humans and the environment the reefs have been under serious and severe threats.

Coral reefs are colonies of individual animals called polyps, which are related to sea anemones. The polyps, which have tentacles to feed on plankton at night, play host to zooxanthellae, symbiotic algae that live within their tissues and give the coral its color.

The coral provides CO2 and waste products that the algae need for photosynthesis. Coral reefs, the “rainforests of the sea,” are some of the most biodiverse and productive ecosystems on earth.

They occupy less than 1% of the ocean floor, yet are home to more than a quarter of all marine species: crustaceans, reptiles, seaweeds, bacteria, fungi, and over 4000 species of fish make their home in coral reefs.

With a global economic value of approximately $375 billion a year, coral reefs provide food and resources for more than 500 million people in over 100 countries and territories. But tragically, coral reefs are in crisis and critically endangered.

Coral reefs are endangered by a variety of factors, including natural phenomena such as acidification of the ocean, predators, and diseases; human threats such as overfishing, destructive fishing techniques, pollution, careless tourism, etc.

Coral Reef

10 Biggest Threats of Coral Reefs

Human-induced or anthropogenic activities such as pollution, overfishing, destructive fishing practices, and natural factors are major threats to coral reefs. These have been seen to damage reefs all around the world every day.

Here are some major threats to coral reefs in the environment:

  • Introduction of Pollutants
  • Uncontrolled Tourism
  • Climate Change
  • Natural Disasters
  • Sedimentation Increase
  • Careless Fishing Techniques
  • Ocean Acidification
  • Diseases
  • Predators
  • Over-fishing

1. Introduction of Pollutants

Major pollutants that are released from various sources, mainly due to careless human activities, pose a serious threat to coral reefs and the wide array of marine flora and fauna species that are solely dependent on them.

Coral reefs are affected by pollution from land including leaking fuels, anti-fouling paints, and coatings, hot water releases from power plants, pathogens, trash, and other chemicals that enter the water.

These pollutants are either directly dumped into the oceans or by runoff that flows from the land into the ocean through rivers and streams thereby endangering the coral reefs.

Petroleum spills do not always appear to affect corals directly because the oil usually stays near the surface of the water, and much of it evaporates into the atmosphere within days.

 However, if an oil spill occurs while corals are spawning, the eggs and sperm can be damaged as they float near the surface before they fertilize and settle.

So, in addition to affecting water quality, oil pollution can disrupt the reproductive success of corals, making them vulnerable to other types of disturbances.

Furthermore, When some pollutants enter the water, nutrient levels can increase, promoting the rapid growth of algae and other organisms that can smother corals.

Marine pollution is dangerous not only for coral reefs but for other marine organisms too.

2. Uncontrolled Tourism

Coral reefs provide protection to shores and are also a great attraction for tourists. Tourism has been assessed as a major threat to coral reefs by the fact that a greater loss of coral reefs has been faced in shallow corals around 10m depths.

Tourism, while relying on the appeal of coral reefs, can be damaging when careless divers trample on corals or break off pieces as souvenirs.  

As with globalization, tourism has increased by huge amounts in some nations. It has gone as high as to contribute 60% of the country’s Gross Domestic Product, like in Maldives.

Corals are also harvested with tropical fishes for aquarium trade and jewelry. Over-harvesting of species disrupts the ecosystem and destroys the local coral habitat.

3. Climate Change

One of the biggest threats to coral reefs is climate change. Increasing temperatures and changing climate patterns have put incredible stress on the reefs.

Coral reefs worldwide face the impacts of climate change since human-caused global warming has led to the heating of the earth’s atmosphere and the elevated surface temperature of ocean waters.

With changing weather patterns caused by various factors, like El Niño; there is an increase in ocean temperatures as well. This temperature rise kills algae, exposing the white calcium skeleton of corals beneath. This phenomenon is referred to as coral bleaching.

Coral bleaching puts corals at increased risk of death due to reduced nutrients. It also makes coral reefs more vulnerable to other factors. It has been estimated that the optimal water temperature facilitating coral growth is around 20-28°C.

With global warming continuing to heat the planet unabatedly, coral bleaching is expected to become more severe.

Other than changing temperatures, low tides for long periods of time also exposes the coral heads in shallow waters. This causes great damage.

Also, when corals are exposed during the daytime, they are exposed to high amounts of ultraviolet radiation from the Sun, which can increase the temperature and removes moisture from the coral’s tissues.

This puts Corals in physiologically stressed conditions; leading to disruption of symbiotic relationship with zooxanthellae algae, then bleaching and eventually death.

4. Natural Disasters

Strong storms like cyclones and hurricanes are a very common threat to shallow coral reefs causing a whole lot of damage to the coral reefs. The waves from these storms break the reef into fragments by tearing them apart or flattening the reef.

Storms rarely kill entire colonies of coral. However, these storms provide an opportunity for the algae to grow faster than the slow-growing corals can recover from damage.

These algae negatively impact the growth and recruitment of reefs, making it difficult for them to recover now.

5. Sedimentation Increase

With rising development for varying reasons like recreation, the sediment runoff in coastal regions has dramatically increased over the past few years.

This has been exaggerated by increasing deforestation and soil erosion. Sediments can enter water bodies through various coastal development activities like mining, farming, logging, and building projects, and urban stormwater runoff.

Sediments that are deposited on coral reefs can smother corals, thereby impeding coral growth and reproduction, posing a severe threat to the health of the coral reefs, impeding growth and reproduction of coral reefs. The sediments in the runoff affect corals in two ways.

Firstly, the sediments are suspended in the water and effectively block sunlight, thereby reducing photosynthesis. Secondly, the sediments settle to the bottom and bury the corals. They effectively clog the coral mouths. This leads to reduced nutrition for the corals and affects benthic organisms.

This means there is an increased risk of corals becoming threatened and subsequently endangered.

Furthermore, nutrients (nitrogen and phosphorous) from agricultural and residential fertilizer use, sewage discharges (including wastewater treatment plants and septic systems), and animal waste are generally recognized as beneficial for marine ecosystems;  when in excess can lead to the growth of algae that blocks sunlight and consumes oxygen corals need for respiration.

This often results in an imbalance affecting the entire ecosystem. Excess nutrients can also support the growth of microorganisms, like bacteria and fungi, that can be pathogenic to corals.

6. Careless Fishing Techniques

In many areas, coral reefs are destroyed when coral heads and brightly colored reef fishes are collected for the aquarium and jewelry trade.

Careless or untrained divers can trample fragile corals, and many fishing techniques can be destructive. Blast fishing, is practiced in around 40 countries, it is the use of dynamite or other heavy explosives to startle fish out of hiding places.

This practice kills other species and can crack and stress corals so much that they expel their zooxanthellae and leading to large-scale destruction of the reefs.

Another abrupt technique implored is Cyanide fishing, which involves spraying or dumping cyanide onto reefs to stun and capture live fish, this also kills coral polyps and reduces the reef habitat. More than 15 countries have reported cyanide fishing activities.

Other damaging fishing techniques include muro-ami netting, where weighted bags are pounded to startle fish out of crevices this directly devastates and breaks coral colonies and deep water trawling, which involves dragging a fishing net along the sea bottom, this technique is common and used in many countries.

Often, fishing nets left as debris can be problematic in areas of wave disturbance. In shallow water, live corals become entangled in these nets and are torn away from their bases.

In addition, anchors dropped from fishing vessels onto reefs can break and destroy coral colonies.

7. Ocean Acidification

One major disastrous consequence of industrialization has been the rise of greenhouse gases like carbon dioxide (CO2) in the atmosphere.

Ocean acidification is the increase in carbon dioxide levels due to the excessive burning of fossil fuels which leads to ocean waters becoming increasingly acidic. This in turn lowers the pH of ocean water, thereby affecting the coral reefs around the world.

Each year, the ocean absorbs about one-quarter of the carbon dioxide emitted from the burning of fossil fuels (oil, coal, and natural gas). Since the Industrial Revolution, ocean acidity has increased by about 30%, a rate that is more than 10 times what has previously occurred for millions of years.

Further, ocean acidity levels are expected to increase by an additional 40% above present levels by the end of this century.

CO2 is absorbed by the oceans directly. It’s also absorbed by the rainwater that joins those oceans. Both of these result in reduced pH or acidification of water.

The carbonic acid formed as a result of this acidification process negatively impacts the availability of salts in the corals to build their calcium carbonate exoskeletons alongside the availability of ions.

In extreme cases, this can even directly lead to the dissolution of calcium skeletons. Consequently, coral growth and reef growth can be slowed or even death of the reef is observed, with some species affected more than others.

If acidification becomes severe, coral skeletons can actually dissolve. On a local level, nutrient enrichment due to run-off from human activities on land can also cause increased acidity in coastal waters, exacerbating the effects of ocean acidification.

8. Diseases

A newly emerging threat that is exacerbated by both natural and human activities is the coral disease. Coral diseases have increased quite significantly over the last decade, raising the coral mortality rate.

These diseases have been a result of deteriorating water conditions and the growth of pathogens caused by pollution and stress induced by natural factors like Ultraviolet radiation and high temperatures.

The intrusion of bacteria, fungi, and viruses has led to the spread of various diseases like Black-band disease, red-band disease, and yellow-band disease. These diseases damage the living tissues, exposing the limestone skeleton. The limestone skeleton is a breeding ground for algae.

Without adequate attention and proper cure for any of these diseases (except black-band disease), it means the corals rarely survive after being infected.

9.  Predators

Along with natural disasters, corals are also susceptible to natural predators. These predators can cause significant damage during population growth or outbreak.  

Predators of coral reefs include Fish, marine worms, barnacles, crabs, snails, and sea stars. The predators feed on the inner soft tissues of coral polyps.

Also, this predation increases the bio-erosion of coral reefs. Bio-erosion results in loss of coral cover and topographic complexity. This drives a phase shift from coral to algal dominance, which leads to reduced growth of coral reefs.

10. Over-fishing

Coral reefs face the biggest threat from overfishing.  Due to increasing consumption demands by humans, continuous fishing practice is upheld so as to satisfy the increasing demand. 

Coral reefs are very fragile ecosystems that are highly dependent on inter-species interaction throughout the ecosystem.

Reduction or damage to any species can reduce the stability of the whole ecosystem.

Overfishing can alter the food-web structure and cause cascading effects, such as reducing the number of grazing fish that keep corals clean of algal overgrowth.

Coral harvesting for the aquarium trade, jewelry, and curios can lead to over-harvesting of specific species, destruction of reef habitat, and reduced biodiversity.


All of these threats have significantly impacted coral numbers throughout the world. We can only hope that significant research is performed to relieve corals from these threats.

We need to save the coral reefs because they are home to millions of marine organisms and they also have essential benefits for humans and the environment.

To this effect, there should be adequate education for people who are living within and without the coast on how important they are and why they should be protected.


Environmental Consultant at Environment Go! | + posts

Ahamefula Ascension is a Real Estate Consultant, Data Analyst, and Content writer. He is the founder of Hope Ablaze Foundation and a Graduate of Environmental Management in one of the prestigious colleges in the country. He is obsessed with Reading, Research and Writing.

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