Coral reefs are among the most diverse of all known marine ecosystems. As many as 1 in 4 of all ocean species depend on reefs for food and shelter, at some point in their lives. Even though reefs cover less than 1 percent of the earth’s surface and less than 2 percent of the ocean floor. Due to their biodiversity of plants and marine animals, coral reefs are known as the “rainforests of the sea”.
Like mangroves, coral reefs also provide significant shoreline protection during storms and even tsunamis. This is because reefs absorb over 90 percent of a wave’s energy, thus minimizing the destructive power of storms, and preventing loss of life and damage to property and land. Reefs also reduce coastal erosion caused by strong currents. 1
The coral reef biome also plays a vital role in the marine economy. More than 500 million people depend on coral reef ecosystems for their livelihood, as well as for food, and other benefits.  Scientists assess the total global value of coral reefs at between 30 billion and 172 billion U.S. dollars each year, in respect of food, tourism, coastal stabilization and protection, and medical benefits. 2
Ironically, the main beneficiaries of coral reefs (humans) also pose the greatest threat to them. Because the main threat to corals is global warming – a phenomenon created by man-made greenhouse gas emissions that trap heat in the lower atmosphere. About 93 percent of this excess heat is absorbed by the ocean 3 resulting in significant chemical changes and loss of biodiversity in every area of the world. In fact, according to the IPCC’s Special Report on Global Warming of 1.5°C (2018), 1.5 degrees of warming will wipe out 70-90 percent of all warm water coral reefs, and 2 degrees will wipe out 99 percent. 4
- What Are Corals?
- Corals and Their Symbiotic Zooxanthellae
- How Do Coral Reefs Grow?
- How Do Corals Reproduce?
- Where Are Reef Building Corals Found?
- What Are the 4 Different Types of Coral Reefs?
- When Did Coral Reefs First Appear?
- Effect of Climate Change on Coral Reefs
- Bleaching of Coral Reefs
- Recent Coral Bleaching Due to Global Warming
- Ocean Acidification
- IPCC Special Report on Oceans
- Other Threats to Coral Reefs
- Effect of Sunscreen Chemicals on Corals
What Are Corals?
Corals are tiny marine organisms, called polyps, who live mostly in large colonies. A coral polyp begins its life as a microscopic, free-swimming larva, about the size of the head of a pin. Coral reefs form when large colonies of coral larvae attach themselves to submerged rocks or other hard surfaces on the fringes of islands or continents. Once settled on its rock support, each polyp stays there for the remainder of its life.
The coral polyp is a very simple marine organism, whose body is like a small container with an opening, or mouth, at one end. It takes in food and excretes waste through this mouth. Tentacles around the mouth, armed with poisonous stingers (nematocysts), help to catch prey and deal with excretion. Corals are carnivores in the marine food web preying mostly on zooplankton (like copepods and other tiny larvae). Food digestion takes place inside the polyp’s body cavity (which is mostly one big stomach), on the surfaces of specialised digestive filaments or mesenteries, which emit enzymes that rapidly reduce ingested prey to its components. See also: Marine Microbes Drive the Aquatic Food Web.
Corals and Their Symbiotic Zooxanthellae
Many warm water corals have symbiotic algae that live inside them. (Symbiotic creatures are organisms that help one another.) These algae, known as Zooxanthellae (or dinoflagellates), are single-celled phytoplankton that live in the tissues of marine animals. It’s these algae that are responsible for the green, brown, and reddish colors that corals have. As the algae photosynthesize, they transfer some of the food they make from the sun’s energy to their hosts, and in return the host coral gives the algae shelter and nutrients. This mutually beneficial relationship is the reason that corals are able to achieve the enormous growth required to build reefs.
The point is, when rapid growth occurs, a marine animal produces waste compounds of carbon, phosphorus and nitrogen, that must either be expelled or else they will poison the growth process. Because corals are very simple organisms, they have no specialized structures for such waste removal. Fortunately, the zooxanthellae living inside the coral tissue have the answer. They utilize the coral’s waste products as the raw materials for their photosynthesis.
For a long time, this happy utilization of waste products was thought to be the algae’s only benefit to the coral. However, scientists now know that a major portion of the energy photosynthesized by the algae is passed on to the coral. In particular, zooxanthellae supply the coral with glucose, glycerol and amino acids. From these nutrients, the coral makes proteins, fats, and carbohydrates, as well as calcium carbonate. The relationship between the zooxanthellae and coral polyp enables an extremely efficient recycling of nutrients in tropical waters. Indeed, up to 90 percent of the algae’s photosynthetically produced nutrients are transferred to the host coral tissue. 5
To recap, corals receive two major benefits from their resident zooxanthellae: (a) Their metabolic ‘waste’ is used up (therefore removed). (b) They are given the lion’s share of the energy produced by the algae. 6
How Do Coral Reefs Grow?
Once the coral polyps have anchored themselves to their rock supports, they secrete a form of calcium carbonate (aragonite) from the underside of their skin. This forms a protective skeleton to safeguard the coral animals from predators. In addition, it also creates a base upon which new coral polyps can settle, thus adding to the baby reef. As the corals grow their reef structure, they reinforce it with additional crystals, to help it cope with currents and wave action, as well as attacks by worms, molluscs, and parrotfish.
In general, the density and strength of the reef skeleton varies significantly, according to the coral species. Some corals are very solid and are able to resist major wave disturbance, while others are lighter and are only found in sheltered coastal waters.
Individual polyps within a reef are usually very small – about 1-2 cm in diameter. The largest specimens, which can exceed 5 inches in diameter, occur in mushroom corals. However, corals are colonial and a colony can grow as big as a car. Reefs are made up of numerous colonies, and are therefore bigger still. The world’s largest coral reef is the Great Barrier Reef, which runs for 2,300 km (1,400 miles) off the coast of Queensland, Australia.
The metabolic process of growing the coral skeleton consumes a large amount of the polyp’s energy. When the polyp dies, its protective skeleton remains, and another coral will grow on top of the old one. A single coral reef contains millions of polyps.
Different corals grow at different speeds depending on water temperature, salinity, wave action, and the supply of food. The largest corals tend to be the slowest growers, adding only about 5-25 millimeters (1 inch) annually to their length. Some species (e.g. staghorn corals) grow much faster – up to 20 centimeters (8 inches) each year.
How Do Corals Reproduce?
Corals reproduce asexually by budding or fragmentation. Budding occurs when new clonal polyps bud off from parent polyps to form a new colony. This takes place when the parent polyp reaches a certain size and divides: a process that continues throughout the animal’s life. Occasionally, during a storm or when a boat runs aground on the reef, an entire colony (not just an individual polyp) branches off to start a new colony. This is known as fragmentation. 7
Most reproduction by corals takes place during mass spawning. About three-quarters of all reef-building corals produce male and/or female reproductive cells. Most release very large numbers of eggs and sperm into the water over a wide area in a mass event known as ‘broadcast spawning.’
The two sets of cells combine to form planktonic larvae (planulae), which eventually – as we saw earlier – settle on a hard substrate like a rock. The time between planulae formation and settlement on bedrock is fraught with danger from predators, therefore massive numbers of planulae are produced, in order to offset the threats. 7
On many reefs, mass spawning is a synchronized event, when all the species release their eggs and sperm at the same time. Such a release must be carefully timed, and scientists believe it happens in response to a series of environmental triggers. The entire process is believed to be dependent on water temperature and its rate of change. The initial cue for action is thought to be lunar, with the actual release timed to coincide with sunset. 8
Where Are Reef Building Corals Found?
Reef-building corals cannot tolerate water temperatures below 18°C. They grow fastest in warm water between 21°C and 29°C (70–85°F), although some can tolerate temperatures as high as 40°C for short periods. Most corals require very salty water (from 32 to 42 parts of salt per thousand), so they rarely thrive close to river mouths or in coastal waters with excessive runoff. They also prefer clear, shallow water, with plenty of sunlight in which their resident algae can photosynthesize.
The corals’ need for bright sunlit water also explains why most species are restricted to the euphotic zone, the ocean layer where light penetrates – usually down to a depth of about 70 meters (235 feet). That said, corals have been discovered at far greater depths. Lophelia coral, a species of cold-water coral found throughout the North Atlantic Ocean, was discovered in 1,250 feet of water off the coast of North Carolina.
What Are the 4 Different Types of Coral Reefs?
Scientists commonly classify coral reefs into four types: fringe reefs, barrier reefs, atolls, and patch reefs.
Most coral reefs are known as “fringe reefs” because they fringe the coastline around islands and larger land masses. Usually, they are separated from the shoreline by narrow, shallow lagoons. They are the most common type of reef in the ocean. The Ningaloo Reef, stretching 260 km (160 miles) along the western coast of Australia is the largest fringing reef in the world. Unlike Australia’s Great Barrier Reef which must be accessed by boat, it is possible to swim out to the Ningaloo reef.
Barrier reefs also develop parallel to the coastline but are separated from it by a deeper, wider lagoon, or body of water. If they reach the water’s surface, they form a natural “barrier” to marine navigation. The Great Barrier Reef in Australia is the largest and most famous reef of this type, in the world.
Sometimes a fringing reef surrounds a volcanic island, formed after a volcanic eruption from the sea bed. Over millions of years, the volcanic island gradually sinks due to tectonic processes, but the reef continues to grow, both upwards and outwards. After millions of years, the island becomes completely submerged leaving only the surrounding ring of coral reefs visible. This is called a coral atoll. Wave action heaps sand and other debris on top of the corals, creating a strip of land.
Coral atolls are far more prevalent in the Indian and Pacific oceans than in the Caribbean due to the relative abundance of submarine volcanoes. Atolls include the Marshall, Cook, and Caroline Islands in the Pacific and the Maldives, Chagos, Seychelles, and Cocos Islands in the Indian.
Patch reefs are small, isolated reefs that grow up from the open bottom of the island platform or continental shelf. They are usually found within the lagoon behind the barrier or atoll, in 10-20 feet of water. Patch reefs are common in waters off the Bahamas, Bermuda, the Caribbean islands and the Pacific Islands.
Patch reefs are often classified as part of larger reef formations, so they are rarely recorded as individual reef systems. Because they occur in the sheltered waters of a lagoon, they provide excellent nurseries for young fish, well away from the more dangerous predators patrolling the edges of fringing and barrier reefs.
When Did Coral Reefs First Appear?
Geological data shows that the ancestors of modern coral reef ecosystems first appeared about 535 million years ago. 9 Most of today’s established reefs are between 5,000 and 10,000 years old. Individual coral colonies frequently live decades to centuries, and some deep-water colonies are more than 4000 years old. The Great Barrier Reef, for example, began growing about 20,000 years ago. Fortunately, corals exhibit annual rings, just as trees do. These rings can tell us the age of the reef and also what conditions (e.g. water temperature) were like thousands of years ago.
Effect of Climate Change on Coral Reefs
All the most dangerous threats to warm water coral reefs come from humans. The biggest is man-made climate change, caused by excessive emissions of greenhouse gases (GHGs) which ramp up the natural greenhouse effect. The main GHG is carbon dioxide, which is emitted largely from the burning of fossil fuels like coal, oil and gas, and from deforestation. The CO2 gathers in the troposphere, where it captures heat escaping from Earth’s surface and re-radiates it downwards. Around 93 percent of this heat is absorbed into the ocean.
The most damaging effects of ocean warming on coral reefs, are (a) coral bleaching and (b) damage to coral skeletons from ocean acidification. Rising temperatures cause corals to expel their resident algae which leads directly to coral bleaching, while ocean acidification makes it more difficult for corals to build their calcium carbonate skeletons. And those that are built are weaker.
Ocean deoxygenation may also have negative impacts on corals, since shallow tropical waters, and their coral reefs, are particularly vulnerable to extreme low O2 (hypoxia) events. However, little research has been done on this subject, so we don’t know what constitutes lethal oxygen thresholds within coral reefs, or the role these thresholds might play in determining what future reefs will look like.
Bleaching of Coral Reefs
“Coral bleaching” happens when corals lose their symbiotic algae, the zooxanthellae. Without their algae, the living coral tissues are almost transparent, allowing the white skeleton to become visible, hence the name “coral bleaching”. Several kinds of stress factors can cause coral bleaching, including: water that is too hot or too cold, water with too much or too little light, or too little salt. Right now, the biggest cause of coral bleaching is rising temperatures caused by global warming. In particular, we are seeing more frequent marine heatwaves – sudden and intense bouts of ocean warming – that have had catastrophic effects (including bleaching) on corals in Australian waters. Any temperature more than 1°C (2°F) above the normal seasonal maximum can result in bleaching.
Bleached corals do not always die, but if temperatures are very hot, or too warm (or cold) for an extended period, corals typically die from starvation or disease. Back in 1998, four-fifths of the corals in the Indian Ocean bleached and one fifth perished. In 2005, the U.S. lost half its Caribbean coral reefs due to a massive bleaching event. Satellite data confirmed that thermal stress was greater than the previous 20 years combined.
Recent Coral Bleaching Due to Global Warming
A new study shows that the past five years (2014, 2015, 2016, 2018 and 2019) are the five warmest years ever recorded in the ocean, and the past 10 years are also the 10 warmest years on record. The amount of heat entering the oceans is equivalent to every man, woman and child on the planet, running 100 microwave ovens 24/7. 10
According to Professor John Abraham at the University of St Thomas, in Minnesota, US, one of the study team members, “The oceans are really what tells you how fast the Earth is warming. Using the oceans, we see a continued, uninterrupted and accelerating warming rate of Planet Earth. This is dire news.”
Another study, which analyzed the health of 100 coral reefs around the world, shows that the time interval between severe episodes of coral bleaching, has declined by a factor of five. 11
In all, from 2014 to 2017, mass bleaching affected more than 75 percent of global reefs; with nearly 30 percent mortality. This extended period of bleaching, during which iconic reefs in Australia and in the North-western Hawaiian Islands experienced their worst bleaching ever, was the most destructive period on record. 12
In 2016, for instance, heat stress affected 51 percent of coral reefs around the world. The first mass bleaching of the northern and far-northern Great Barrier Reef, in which 85 percent of corals were bleached, ended up killing nearly 30 percent of the reef’s shallow water corals. Other mass bleaching events occurred throughout the western half of the Indian Ocean. In the Seychelles, for example, 69 percent to 99 percent of all corals were bleached and 50 percent perished. 13
In March 2020, the Great Barrier Reef Marine Park Authority said its latest aerial surveys have detected widespread bleaching, including extensive patches of severe damage. What is striking, is that for the first time the bleaching extended to the southern half of the reef. Here, the water should be cooler, but not this year. Measurements taken in February 2020 revealed the highest sea-surface temperatures across the Great Barrier Reef since monitoring began 120 years ago.
Regional weather systems – such as the El Nino-Southern Oscillation (ENSO) in the tropical Pacific Ocean, and the Indian Ocean Dipole (IOD) between East Africa and SE Asia – account for much of the climate variability in terms of rainfall, temperatures and storms. These variations can be a key influence on the health of coastal ecosystems, involving seagrasses, mangroves and coral reefs.
When carbon dioxide (CO2) enters the ocean, due to global warming, it causes important changes in ocean chemistry. For example, chemical reactions take place that reduce seawater pH, as well as levels of carbonate ions, and calcium carbonate minerals. These chemical effects are known collectively as “ocean acidification” (OA). Calcium carbonate minerals are essential materials in the building of skeletons and shells of many marine organisms, including corals. At the same time, corals depend on a specific pH balance to extract calcium from the seawater, a need which is particularly important for young polyps, as they settle onto rock and start building their skeletons. Studies have shown a 50-70 percent decline in larvae settlement in waters with lower pH levels. 14
In particular, ocean acidification impedes the thickening process used by corals to strengthen the structure of their skeletons, thus decreasing the structures’ density and leaving them more vulnerable to breaking. If OA continues at current rates, scientists forecast that by 2085 ocean conditions will be so acidic that corals around the globe will start to dissolve. 15 16
IPCC Special Report on Oceans
In their recent Special Report on the Ocean and Cryosphere in a Changing Climate (2019), the IPCC emphasized that warm-water coral reefs are particularly vulnerable to global warming, since they suffer significant losses whenever water temperatures remain 1-2°C above the normal seasonal range, for any length of time. The report also noted, that after mass coral mortalities due to bleaching, reef recovery usually takes at least 10–15 years to complete. 17
Other Threats to Coral Reefs
Corals and coral reefs are also badly affected by local and global pollution.
Local pollution includes nitrogen and phosphorous run-off from agricultural and domestic fertilizer use, as well as unregulated discharges of human sewage and animal waste. An excess of this organic material can cause a bloom of algae that block the sunlight and deplete the oxygen, that corals need to survive. These nutrients can also support the growth of both bacteria and fungi, that are toxic to corals.
The release of toxic substances from local sources can also seriously undermine the health of corals. Such substances include: heavy metals (e.g. mercury, lead), organic chemicals (e.g. PCBs, oxybenzone and dioxin), pesticides, sunscreens, and runoff from mine workings and landfills.
Microplastics are the latest global worry, along with marine debris, such as discarded fishing lines and nets (also called marine debris). Microplastics and other plastic products are ingested by corals, fish, sea turtles, and other reef creatures, causing many of them to die from a blocked digestive tract. Fishing ropes and nets can snag on corals entangling and killing all sorts of marine animals.
Overfishing is another global threat to coral health. When certain fish are overfished it can alter the food-web with cascading effects. The loss of reef sharks, for example, allows other ‘mid-species’ to overeat the grazing fish that keep corals clean of algal overgrowth.
Effect of Sunscreen Chemicals on Corals
A number of separate studies have shown that sunscreen products (typically containing zinc or titanium blocking agents) cause rapid and complete bleaching of warm water corals, even at low concentrations. An estimated 10 percent of reefs could be affected. Researchers have estimated that in tropical countries alone, 16,000-25,000 tons of sunscreen will be applied of which at least 25 percent will be washed off in the sea. Even if the wearer doesn’t enter the water, a significant proportion of sunscreen chemicals will find their way to the ocean through the sewer system. Some US coastal cities have sewer systems that send runoff from personal care products into the ocean.
The most damaging chemical in sunscreen is oxybenzone, which has been found to have several serious effects on coral reefs.
In 2016, a study conducted by a team of international scientists found that Oxybenzone, or BP-3 – a common chemical in sunscreen products – is highly toxic to juvenile corals and other marine life. It removes nutrients from the coral, aggravating the bleaching process, while also causing hormone and DNA damage, and sometimes death, to both young and mature coral organisms. Oxybenzone is also found in more than 3,500 skin care products worldwide. The compound has been identified as entering the ocean both through wastewater drains and directly from sunscreen wearers. 18
- “The effectiveness of coral reefs for coastal hazard risk reduction and adaptation.” Ferrario, F. et al. Nat Commun 5, 3794 (2014).
- “Corals & Coral Reefs “. Smithsonian
- “How fast are the oceans warming?” Lijing Cheng, John Abraham, Zeke Hausfather, Kevin E. Trenberth. Science 11 Jan 2019: Vol. 363, Issue 6423, pp. 128-129.
- IPCC Special Report on Oceans. Summary for Policymakers. FAQ Chapter 5.1: How is life in the sea affected by climate change?
- “Corals/Zooxanthellae.” NOAA
- “Corals of Oman.” Steve Coles.
- “An Introduction to Marine Ecology.” Third Edition: R. S. K. Barnes R. N. Hughes. 1999. Print ISBN:9780865428348
- Veron, J. E. N. (2000). Corals of the World. Vol. 1–3. Australian Institute of Marine Science and CRR, Queensland, Australia.
- Pratt, B.R.; Spincer, B.R.; Wood, R.A.; Zhuravlev, A.Yu. (2001). “12: Ecology and Evolution of Cambrian Reefs” (PDF). Ecology of the Cambrian Radiation. Columbia University Press. p. 259
- “Record-Setting Ocean Warmth Continued in 2019.” Cheng, L. et al. Adv. Atmos. Sci. 37, 137–142 (2020).
- “Spatial and temporal patterns of mass bleaching of corals in the Anthropocene.” Terry P. Hughes, et al; Science 05 Jan 2018: Vol. 359, Issue 6371, pp. 80-83.
- State of the Climate Report 2017. UN/WMO
- “Fast Facts on Coral Reefs.” NOAA.
- “Ocean acidification compromises recruitment success of the threatened Caribbean coral Acropora palmata.” Rebecca Albright, Benjamin Mason, Margaret Miller, and Chris Langdon. PNAS November 23, 2010 107 (47) 20400-20404
- “Ocean acidification affects coral growth by reducing skeletal density.” Nathaniel R. Mollica, et al; PNAS February 20, 2018 115 (8) 1754-1759
- For solutions to the socio-economic impacts of ocean acidification, see: “Ocean acidification impacts on coral reefs: From sciences to solutions.” Denis Allemand, David Osborn. Regional Studies in Marine Science. Volume 28, April 2019, 100558.
- IPCC Special Report on Oceans. Summary for Policymakers. FAQ Chapter 5.1: How is life in the sea affected by climate change?
- “Toxicopathological Effects of the Sunscreen UV Filter, Oxybenzone (Benzophenone-3), on Coral Planulae and Cultured Primary Cells and Its Environmental Contamination in Hawaii and the U.S. Virgin Islands.” Downs, C.A., et al. Arch Environ Contam Toxicol 70, 265–288 (2016).