Geography of Coral Reefs

Katarina Samurović


With a dazzling display of colors and shapes seen in their animal and algal life, the coral reefs are a synonym for ocean biodiversity.

For decades, we have taken this marine life for granted, backed by the idea that these ecosystems will be around forever. However, in recent decades, the world has witnessed massive coral sicknesses and die-offs. The most damaging condition is called coral bleaching.

There are debates about the potential of these events to wipe out coral reefs as we know them. The truth is that currently, it is unknown if the corals will be able to survive the anthropogenic “Ocean 2.0” – a significantly warmer ocean due to climate change, more acidic, and commonly laden with harmful chemicals.

To be able to help these unique ecosystems – or at least to form an opinion about the unfolding events – we need to know more about the coral reefs and look into their biological IDs. 

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A satellite imagery of shades of blue showing coral reefs in Fiji.
Coral reefs on the north shore of Vanua Levu, Fiji’s second largest island. Satellite imagery: NASA, Landsat 8, May 10, 2015.

How are coral reefs formed? How do they grow? Where are they distributed? And lastly – what will happen to coral reefs in the future? 

What are coral reefs? What are the origins of coral reefs?

Coral reefs are ridges or hummocks formed in the warm, shallow parts of the ocean. They are created by tiny organisms called coral polyps (phylum: Cnidaria, class: Anthozoa, if you’re interested in taxonomy) and the large exoskeletons they build over time. These skeletons can be external or internal and have a leathery, horn-like, or stonelike texture – and the stonelike are probably the most famous ones.

We can think of coral hard structures as “buildings” in which polyps live. Interestingly, individual polyps on their own are mostly colorless, but still, the entire reef structure is usually warmly colorful, which is a consequence of a relationship between the polyps and their endosymbiotic microscopic algae – the zooxanthellae.

A photograph taken underwater of a coral with blue fish around it on the ocean floor.
Lettuce coral off the Cape Range National Park along the Ningaloo Coast in Western Australia. The blue fish are cerulean damselfish. Photo: Curt Storlazzi, USGS, public domain.

The Zooxanthellae provide corals with up to 90% of their nutrients through photosynthesis. In return, they get carbon dioxide and nutrients from the polyps and better access to sunshine due to the heightof the corals. 

However, the polyps are not the sole reef creators. Various organisms are also a part of their structure and the basic living community – above all, the calcareous algae, or stony seaweeds. A notable example is a group of Coralline algae – vividly colored red algae characterized by hardened vegetative tissue or thallus. 

Other coral reef contributors include mollusks, echinoderms, and protozoans.

Further on, as the growing reef structure accumulates sediment, sand, mud, and other algae. Plants are also able to take hold and use the coral surface to grow on it – from the colonies of microscopic blue-green algae to seagrasses.

That’s the basis of coral reef building. Species from many taxonomic groups – marine worms, sea slugs, larger anemones, crustaceans, cephalopods, echinoderms, and of course, fish – use the reefs as their primary habitat.

In fact, many of these are specialized reef dwellers that have evolved in tune with their habitat. That is the explanation for their amazing color display. 

Their diversity is the reason that coral reefs are dubbed “the rainforests of the sea.” It is estimated that the coral reefs offer sanctuary and habitat to about 25 percent of sea life, yet, they constitute only 0.2% of the sea floor.

Types of corals and their distribution

The distribution of corals and their structures largely depends on the marine environmental conditions – specifically, the average and minimum water temperature and sun exposure. 

The reef-building corals – mainly the stony corals of the order Scleractinia – thrive in shallow and well-lit waters. They prefer normal water salinity, with an annual maximum temperature exceeding 72 °F (22 °C ) but still below 82 °F (28 °C). The reef-building activity cannot happen if the minimum winter temperatures drop below 59 °F (15 °C). 

As for the depth, on average, they commonly create reefs at a depth between the low-water mark and 36 feet (11 m) but can cope with depths of up to between 130 and 180 feet (40 to 55 m). 

Because of their thermophilic requirements, 90 percent of coral reefs are found in the Indo-West Pacific region. The rest are found in the East Pacific and East and West Atlantic, all between 30° N and 30° S. 

Satellite imagery of the Great Barrier Reef showing brown landscapes and deep blue colored oceans.
Satellite imagery of the Great Barrier Reef off the northeast coast of Australia. Satellite imagery: NASA’s Terra satellite, August 8, 2004, public domain.

Also, there are three main coral reef formations, depending on their position and size –

  • Fringing reefs are found in shallow coastal waters in proximity to the shorelines.
  • Barrier reefs are large, wall-like structures found off-coast, off-coast; the Australian Great Barrier reef is indeed great since it is the largest coral reef in the world. 
  • Atolls are circular formations that usually evolve from fringing reefs when the island they once surrounded sinks into the sea; a circular lagoon appears in its place and is surrounded by a coral ring.

Besides coral reefs, there are other types of coral communities.

Branching corals form thickets and coppices that merge into banks, not reefs. They thrive in cooler and deeper waters, with minimum winter temperatures winter minimum temperatures between 4 and 15 °C (39 and 59 °F) and at depths of about 60 to 200 meters (200 to 650 feet).

A picture taken at 42 meters deep showing red and pink corals with a striped red and grey fish.
Corals at 42 meters below sea level on Farnsworth Bank in Southern California. Leptogorgia chilensis, Stylaster californicus, and a painted greenling (Oxylebius pictus) are pictured. Photo: NOAA Southwest Fisheries Science Center, public domain.

Consequently, these coral banks are located at the peripheries of the eastern Atlantic shelf – from Cape Verde islands in the South and Norway in the north, from the Gulf of Mexico and the Bahamas in the west, and the Niger river delta in the east of the Atlantic. Also, coral Reefs appear of the coast of New Zealand – at the Chatham Rise and Campbell Plateau, and in the northwest Pacific near Japan. 

The third group of modern corals, consisting of only a few genera, don’t form elaborate structures but are solitary and adapted to even colder and deeper seas. They are found in cold waters with temperatures of 36 to 43 °F (2 to 6 °C) – the abyssal plains, as well as the cool shelves around Falkland Islands, Patagonia, and even Antarctica.

As you can notice from the figures above, reef and bank-forming corals are completely temperature dependent and form only within constrained temperature niches. Accordingly, abnormal temperature shifts influenced by climate change have the potential to wreak havoc on these ecosystems.

Are Coral Reefs in Danger? And How?

Sometimes there are biological factors that damage coral reefs – specifically the organism that feed on them.

The first coral reef “crisis” that was widely repored on happened in the 1960s. The population of crown-of-thorns starfish (Acanthaster planci) that feeds on the soft, living coral tissue suddenly exploded, leading to extensive damage to the living corals on many reefs of the Southwest Pacific.

Later on, however, it was noticed that the infestation was time-limited. After it passed its peak, the coral reefs regenerated rapidly in the late 1970s

However, the phenomenon known as coral bleaching leaves much broader and harder-to-fix consequences. 

What is Coral Bleaching?

Coral bleaching is a joint stress response of coral polyps. It is usually thought of as a coral die-off; however, in reality, it is slightly more complicated. 

The coral polyps don’t simply die – instead, when stressed, they expel the symbiotic photosynthetic algae from their bodies. Since it’s the algae that give color to the coral community, their disappearance leaves the coral polyps transparent, consequently causing the “bleached” look.

Although most bleached corals are bright white due to the calcium carbonate skeleton background, some can be in pastel hues – pinkish, yellow, or bluish – due to their specific proteins.

Deprived of their nutrient source, the bleached corals become weak and are in danger of perishing. However, there is a possibility of recovery, although it’s usually a slow one.

A map showing sea surface temperature anomalies with higher temperatures in red.
A map of the sea surface temperature (SST) anomalies off the northeast coast of Australia on March 14, 2022 – many areas were more than 2°C (3.6°F) warmer than normal. The higher temperatures led to a mass bleaching event in the Great Barrier Reef. Map: NASA, public domain.

The main proposed mechanism behind bleaching events is the ocean water temperature rise. As the temperature increases above the coral’s comfort zone, the zooxanthellae start to produce reactive oxygen species (ROS) such as peroxides and superoxides.

These are toxic to the polyps; the symbiotic relationship that was once symbiotic becomes amensalistic and damaging to the polyps. In an effort to protect themselves, polyps then systematically expel the algae, causing the bleaching.

Other coral bleaching triggers have been proposed. They include pollution by a variety of chemical compounds (including herbicides and fertilizers), cyanide fishing, disease, sedimentation, ultraviolet radiation, extreme low-tide events, and viral and other diseases.

The Short History of Coral Bleaching Events

Coral bleaching has probably been around as long as the coral reefs themselves. The first known large bleaching event occurred during the Late Devonian Mass Extinction – it took down the Devonian coral reefs, the largest reef systems in Earth’s history.

However, the problem with coral bleaching is that has become so much more common in the past 40 or so years due to the human influence on the world seas and the climate. During this time, both the quantity and the intensity of bleaching events have increased. 

A photo of a white coral taken underwater.
Photograph of a reef off Islamorada, Florida, where colonies of “blade fire coral” that “bleached,” or lost their symbiotic algae. Due to the extremely warm ocean temperatures during the summer of 2014, both hard and soft corals lost their symbiotic algae—all throughout the coral reefs of the Florida Keys. Photo: Kelsey Roberts, USGS, public domain.

El Niño events and coral bleaching

That increase was first noticed in the 1980s. After extensive bleachings in 1982-83, the first global bleaching events were recorded in 1998 and then again in 2010; all had an El Niño temperature rise background.

The area that was hit the heaviest was Southeast Asia. Some corals that managed to recover from the 1998 event were affected again in 2010.

Most damaging coral bleaching event

The longest and the most damaging bleaching effect on record happened between 2014 and 2017. Lasting for entire three years, it has touched many coral reefs around the world, including the reefs of the Caribbean, Hawaii, but most of all, the Great Barrier Reef – thus, it is often called the Great Barrier Reef mass bleaching event. This particular bleaching was so destructive because of its successive nature. Many corals were hit again just as soon as they started to recover.

The current state of coral reefs 

Today, corals remained highly threatened. Their sensitivity to sea warming makes them one of the ecosystems that are the most vulnerable to climate change, plus, all of the bleaching risk factors listed above are still very real.

An underwater photo showing brightly colored corals and fish.
A healthy coral reef with a variety of fish swimming across it in the Tumon Bay Marine Preserve off Tumon, Guam. The reef has a high coral cover and is made up of both hard and soft coral species. Photo: Curt Storlazzi, USGS, public domain.

However, bleaching is not the only threat the reefs face. Overfishing is the greatest local threat to their future because the removal of certain fish species – especially the herbivorous ones – disrupts the reef’s delicate ecological balance.

In 2020, a new report on the state of coral reefs was published by UNEP – the first of its kind in 13 years. Unsurprisingly, it found that since 2010, there has been a steady decrease in hard coral cover, with the greatest losses in “South Asia, Australia, the Pacific, East Asia, the Western Indian Ocean, The Gulf and Gulf of Oman.”

At the same time, the amount of coralline algae has increased by 20 percent. Where corals bleach, algae take over. Unfortunately, this decreases the overall biodiversity as many organisms associated with corals disappear with them.

UNEP’s projections estimate that with the current climate change trajectory, by 2034, bleaching events will occur every year. That means that the corals won’t have time for recovery as they get damaged year after year, presumably until their gone – unless they adapt to warmer temperatures. Also, by the end of the century, all reefs worldwide will be affected by bleaching.

All is not lost, though. 

As said priorly, a bleaching event doesn’t mean the death of the coral. They can recover gradually; also, the community composition of the reef will likely not be the same anymore. Still, the Great Barrier Reef community has managed to recover quite a bit from the worrying 2020 bleaching event

Scientists hope that the same will be true for the latest Great Barrier Reef event of March 2022. This time, it happened during the La Nina event, responsible for cooler weather in general. Despite that, 91% of the Reef was impacted by bleaching. It was the first recorded La Nina bleaching event. 

Another interesting twist is that despite all the pressures, the amount of coral on global coral reef cover has increased by 2 percent.

The populations of corals that are less vulnerable to climate change impacts are extremely important for coral reef survival in the future. Scientists have identified these pockets of resilient coral communities near the coasts of Australia, India, Indonesia, Kenya, Malaysia, and Tanzania.

To safeguard the future of the “rainforests of the sea,” it is vital to protect these pockets while working on the main endangering factors – global warming, reducing carbon dioxide emissions, and halting overfishing.

Read next: History of Artificial Coral Reefs



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About the author
Katarina Samurović
Katarina Samurović is an environmental analyst and a freelance science writer. She has a special interest in biodiversity, ecoclimatology, biogeography, trees, and insects.