Ask a Scientist

Corals are a type of marine invertebrate that lives in warm, tropical ocean water. They are closely related to other marine invertebrates with stinging tentacles, such as sea anemones and jellyfish. If you viewed a coral polyp up close, you may mistake it for an anemone. It consists of a soft tubular body with a ring of tentacles on top. Unlike anemones, stony coral polyps secrete a calcium carbonate “skeleton,” which they sit on top of as they grow. Most species of coral are colonial, and hundreds of thousands of coral polyps can cover a large a head of coral. Corals are important reef builders because the calcium carbonate skeleton that corals create becomes the structure of the coral reef that many organisms rely on for their homes.

Most corals use their tentacles to feed on microorganisms floating in the water. Corals also rely on another organism called zooxanthellae, a single-celled algae that lives within the coral, to meet their nutritional and physiological needs. These two organisms participate in a symbiotic relationship, where they benefit from each other. The coral is provided with nutrients, oxygen, and up to 95% of the food it needs to survive, and in exchange, the zooxanthellae receive nutrients, carbon dioxide, and a safe place to live. Corals could not survive without this symbiotic relationship.

There are many reasons why coral reefs are dying. Both global and local disturbances are having severe impacts on coral reefs. Some of the main reasons include impacts from climate change, overfishing, land-based pollution, habitat destruction, and storm damage. Most of these impacts can be linked to humans as our actions are directly contributing to these disturbances. For example, overfishing is both a global and a local problem that is impacting coral reefs. Overfishing occurs when more fish are removed from the reef than can be reproduced naturally. If too many fish are removed from the reef, not only can it lead to a loss of certain fish species, but it can also have widespread effects on the entire reef ecosystem. In nearly every country where we worked, coral reef fish are being overfished. Fishers not only target important reef fish, but also those that live most of their adult life in the open ocean. Some commercial fishers use fishing gear to collect large schools of fish. This gear also captures bycatch, or fish that were not the target species for which they were fishing. Depending on the type of gear they use, this bycatch can include organisms like sea turtles, sharks, rays, and other fish that play an important role in the reef ecosystem. At a local level, most coastal communities fish for food, but also to support local tourism or to sell or export their catches to generate income.

Local, land-based pollution carries nutrients, sediment, and other toxins to the ocean. These excess nutrients cause a decline in water quality, which can harm coral reefs. Runoff can come from agriculture (fertilizers and livestock), sewage, industrial waste, and coastal development, to name a few. Excess nutrients that are washed into the ocean can be damaging to coral reefs because it can promote the overgrowth of algae on the reef and in the water column.  This can lead to algal blooms that reduce water quality and block light from reaching the reef. When macroalgae grows too quickly, it can take up valuable space on the reef floor needed for new coral growth. The overgrowth of these organisms can lead to a phase shift from a coral dominated reef to an algal dominated reef which can impact the entire ecosystem.  

Some of these disturbances, such as excess nutrient runoff and overfishing can be remedied through improved management by communities and governments. Many coral reefs are combating multiple disturbances at the same time. By alleviating the pressure from one of these stressors, it allows the reefs to have a better chance of surviving the disturbances that are not as easily managed.

Coral bleaching is when corals lose their symbiotic algae. Coral bleaching occurs when the corals ore the algae are stressed out, typically because of changes to environmental conditions such as temperature, nutrients, salinity, and light. A rapid rise in water temperature, or sustained high temperatures, is the most common cause of bleaching events and occurs when the ocean becomes too warm or cold for corals to live. This change in water temperature can be as little as 1°C (2° F) above or below average. When the water becomes too warm, the most frequent cause of coral bleaching, corals will expel, or release, zooxanthellae that live within their tissue. If the water cools down to within the coral’s normal temperature range, the corals can reabsorb the zooxanthellae back into their tissue and continue to grow and reproduce; however, if the water temperature remains above average for too long, the coral will reject the zooxanthellae and eventually perish. The zooxanthellae are what gives the corals their color. When zooxanthellae are released, the corals often appear white due to their calcium carbonate skeleton underneath. This is the reason why the process is referred to as bleaching.

Coral bleaching, due to warming water temperature, is caused by climate change. As the climate continues to warm the ocean, corals are being forced to live at the maximum threshold temperature for which they can survive, and bleaching events will continue to become more frequent.

Climate change is caused by the increase of greenhouse gasses, especially carbon dioxide (CO₂), being released into the atmosphere. This excess carbon dioxide acts like a blanket trapping heat in the atmosphere which is changing conditions of the climate globally.

The ocean covers over 70% of the Earth’s surface, and because of this, it plays a significant role in controlling the conditions of the atmosphere. As CO₂ is released into the atmosphere, much of it is absorbed by the ocean. This is causing ocean acidification. Ocean acidification occurs when the excess carbon dioxide in seawater reacts with carbonate, a compound commonly found in the ocean. This reaction causes the pH of the ocean to decrease and become more acidic (more H+). This can have repercussions on many organisms throughout the ocean but is especially detrimental to coral reefs. Corals use carbonate to create their calcium carbonate skeletons. As carbonate reacts with the excess carbon dioxide, it. becomes less available for corals to use. This causes coral skeletons to become more fragile and more susceptible to damage from things like storms and waves.

Additionally, as the Earth warms, typhoons, cyclones, and hurricanes are becoming more frequent and severe. This is caused by the increase in ocean temperatures and changes in the Earth’s climate. These more severe and frequent storms bring strong waves that can damage reefs and already fragile corals. Large amounts of rain can also erode shorelines, transporting sediment and land-based pollution to coral reefs which negatively impacts this ecosystem.

If there are large reductions in carbon emissions, it is possible that some corals can adapt to climate change. Many scientists are studying this problem to identify which coral species and zooxanthellae may be able to adapt to the new conditions caused by climate change, but with the rapid rate at which the climate is changing, most corals won’t be able to adapt quick enough.

Coral reefs are one of the most important and valuable ecosystems on earth. It is estimated that one billion people worldwide rely on coral reefs for their livelihood. They provide food, jobs, recreation, protection from storms, and play an important cultural role to many people around the world. Additionally, coral reefs host 25% of the marine species found in the world’s oceans. Globally, coral reefs cover less than 1% of the sea floor, yet most ocean animals visit them at some point in their life stage. Coral reefs are some of the most productive ecosystems on the planet and are often referred to as the “rainforests of the sea.” With over one million species estimated living on coral reefs, many of these organisms provide important food sources and are being studied to develop new medicines to cure illnesses like cancer, arthritis, bacterial and viral infections, and other diseases.

The Khaled bin Sultan Living Oceans Foundation uses science, education, and outreach to provide people with science-based solutions to improve coral reef conservation. Over the course of 10 years, we completed the Global Reef Expedition, where we mapped and surveyed coral reefs in 16 countries around the world. This allowed us to collect valuable data on coral reefs so we could provide accurate conservation recommendations to communities. This will allow them to make informed decisions, based on science, to monitor and manage their marine resources well into the future. We also teach the public about coral reefs and ways they can help protect them. Through our unified approach, we help teach people of all ages and genders the importance of this critical ecosystem.

I am hopeful that with better conservation and management of human-caused disturbances, coral reefs will be able to recover. Realistically, they will never look the same. We have already lost 50% of coral reefs in the past 30 years, and many have experienced a reduction in the number of species seen on reefs. Additionally, some reefs have seen a shift in from coral to macroalgae that will be difficult to reverse. But by swiftly addressing climate change and reducing other localized disturbances such as overfishing and excess nutrient runoff, these coral reefs can persist and recover to a more stable state. If we act now, we can help save coral reefs for future generations.

Even though you may not live near a coral reef, or you may have never visited one, you are impacting this vital ecosystem. Almost everyone is contributing to climate change, which can cause coral bleaching, ocean acidification, and stronger storms. All water eventually flows to the ocean, carrying land-based pollution from rivers and streams to the sea. Being a globally connected economy, you likely purchase items that were delivered from overseas. Large barges are used to ship these items around the world, emitting enormous quantities of carbon dioxide, polluting the ocean, and even running aground, destroying coral reefs and other ocean ecosystems. Some of the seafood that you eat may be unsustainably caught from a coral reef and shipped to your local seafood markets, groceries, or restaurants.  

It is important for the youth to learn about coral reefs and share their knowledge with their peers and guardians of all ages. Anyone, regardless of where you live, can become an ocean steward. Here are a few actions that you can take:

— reduce the amount of plastic and waste we use
— encourage people to use safe and sustainable fertilizers that are nontoxic for the environment
— reduce your carbon footprint by walking, biking, or taking public transportation places when safely possible
— know where your seafood comes from and only consume seafood caught sustainably
— buy locally
— look into the materials and practices of where your clothes and other goods are coming from and choosing to buy from the most sustainable brands

You can also take an active role in conservation by hosting trash clean-up events around your neighborhood or volunteering with local conservation organizations. Additionally, get out in nature with your friends and family to teach them the importance of respecting and protecting the natural world and discussing how their actions can even affect far away ecosystems, like coral reefs.

There are many other actions that you can take to become an ocean steward. We encourage you to learn additional ways to live a more sustainable life.

Are you still interested in learning more about coral reefs? Visit our Education Portal where you can find a wealth of knowledge about coral reefs. Click on the different course tabs at the bottom of the Education Portal to begin your journey exploring this important ecosystem.

If you have additional questions that you would like answered, please fill out the form below. We can’t guarantee that every question will be answered, but we want to hear what you want to know about coral reefs.

What would you like to ask a scientist about coral reefs?

Question(s) *

Submit