The Science of Coral Reefs

This article is also available at Inspire EdVentures.

What Are Coral Reefs?

Coral reefs cover less than two percent of the world’s oceans, but host a huge variety of life. An estimated one out of every four ocean species depend on coral reefs at some stage of their lives. For their importance and biodiversity, reefs are sometimes compared to oases in a desert, and are known as the “rainforests of the sea.” 

Though many coral species thrive in deep, cold water, reefs are most common in the warm, shallow, sunlit waters of the tropics. This is because many reef-building corals depend on zooxanthellae, a symbiotic algae that produces energy through photosynthesis. In the best conditions, coral reefs can survive for thousands of years and grow large enough to be seen from space.

An image of the Great Barrier Reef, Australia, taken by the Copernicus Sentinel-2A satellite. Contains modified Copernicus Sentinel data, 2017. Image by the Copernicus Program.

Many kinds of corals may be present on or near a reef, but the structure of a reef is composed of stony coral, Scleractinia. (Learn more about corals here!) Stony corals have hard exoskeletons made of aragonite, a form of calcium carbonate. As stony coral colonies expand and reproduce, they build on top of the hard skeletons of former coral colonies. Over time, these layers of calcium carbonate, combined with sand, rock and other substrate from the surrounding sea floor, grow to form the backbone of the coral reef.

Coral reefs grow slowly, but continuously. Stony coral colonies typically grow at a rate of one to six inches (2.5 to 15 cm) per year in optimal conditions. Colonies, which are made up of thousands of individual polyps, may thrive for hundreds or even thousands of years themselves; the oldest known coral colonies are an estimated 4,000 years old! Because reefs grow by slowly forming layers on top of layers, they can be dated with a similar method to tree rings, and scientists can study these layers for clues to ocean and atmospheric conditions in the reef’s past.

A variety of corals found on the Belize Barrier Reef. Image by Inspire EdVentures.

There are four main types of coral reefs: fringing reefs, barrier reefs, patch reefs, and atolls. Fringing reefs are found close to or even attached to a land mass, such as an island or continental coast. These reefs have a “flat” facing the land mass and a steeper “slope” on the side facing the open water. Fringing reefs are the most common kind of reef. 

Barrier reefs are similar to fringing reefs, but are found farther from the coast and are separated from the neighboring land mass by a stretch of deep water. Barrier reefs can grow to awesome sizes; Australia’s Great Barrier Reef, the largest reef in the world, stretches nearly 1,500 miles (2,400 km). 

Patch reefs, also known as platform reefs, are small chunks of reef that often form near other, larger reefs, such as between a barrier reef and the nearby coast or within the lagoon of an atoll. Patch reefs are typically small and isolated, and grow along sea beds or shallow rocky outcroppings.

Atolls are circular reefs with a patch of water, or lagoon, in the center. Atolls begin as barrier reefs surrounding volcanic islands. Over time, as the volcano becomes dormant, the island sinks and disappears, but the reef remains. Atolls may be closed ring shapes or U-shaped with an open side. Over time, buildup of sand and other sediment can create a new island, known as a cay (also spelled caye, key, or quay).

Baa Atoll, an atoll in the Maldives. Image by Frédéric Ducarme.

What Is the Belize Barrier Reef?

The Belize Barrier Reef is the largest reef in the Western and Northern Hemispheres and the second-largest reef in the world. The reef system is composed of a series of barrier reefs, fringe reefs, patch reefs, and atolls - in fact, they’re the only coral atolls found in the Western Hemisphere. This complex of reefs stretches for nearly 190 miles (300 km) off the coast of Belize in the Caribbean Sea. 

The Belize Barrier Reef also contains over 450 small islands, or cayes. In this way, the reef is not only a home to marine life, but also to terrestrial plants and animals. These islands and coastal areas include unique wetland and mangrove ecosystems, while the shallow coastal waters provide nursery grounds for terrestrial and marine species alike. 

The Great Blue Hole, a famous feature of the Belize Barrier Reef. The Great Blue Hole is a result of the limestone bedrock underlying the reef, a kind of topography known as karst. Image by the U.S. Geological Survey.

The Belize Barrier Reef is a home to over 500 species of fish, several dozen species of stony coral and a variety of marine mammals and reptiles. These include threatened and vulnerable species such as the American manatee (Trichechus manatus), the green turtle (Chelonia mydas), and the American crocodile (Crocodylus acutus). The reef’s many islands and coastal areas provide habitat and nesting grounds for birds from the osprey (Pandion haliaetus) to the red-footed booby (Sula sula), and have even been known to support large terrestrial predators such as jaguars (Panthera onca). 

In 1996, to protect this irreplaceable reef system and the species that depend on it, the Belize Barrier Reef was declared a UNESCO World Heritage Site. Around 12% of the total reef system is currently under protection of UNESCO, which manages the reef and prevents harm to it from tourism, overfishing and mining for oil and gas.

An aerial view of the Belize Barrier Reef. The reef system stretches through a variety of marine habitats, from shallow coasts to deeper ocean waters. Image by Inspire EdVentures.

What Are the Threats to Coral Reefs?

Most threats to coral reefs are caused by humans. Overfishing of reef species and damage to reefs from fishing equipment and can upset the delicate balance of reef ecosystems, while terrestrial pollution and runoff causes damage to corals and the reef’s inhabitants. One example of this is an event known as “smothering,” in which sediment runoff, often from erosion due to agriculture or deforestation near the coast, settles over a reef and prevents the corals from accessing the sunlight they require for photosynthesis. Reefs may also be threatened by natural disasters, such as hurricanes, or by imbalances in their own ecosystems, such as loss of predators or the arrival of invasive species. 

Invasive lionfish caught at South Water Caye, Belize. Lionfish are not native to the Caribbean region, and their presence can greatly damage reef ecosystems. Image by Inspire EdVentures.

Perhaps the most well-known threat to coral reefs is the phenomenon of coral bleaching. Coral bleaching occurs when ocean conditions such as temperature or acidity become inhospitable to coral species, resulting in the corals expelling the symbiotic zooxanthellae from their tissues. (Learn more about the relationship between coral and zooxanthellae in the Science of Coral article.) Because these zooxanthellae often provide the corals’ coloration, the coral colonies left behind appear to have been “bleached” white. 

Corals do not usually die immediately from bleaching events; if the harsh conditions that lead to the bleaching event are reversed, coral colonies and the reefs as a whole can recover. However, severe or repeated bleaching events can prevent this recovery and lead to entire coral colonies - and sometimes entire sections of reefs - dying from starvation. Without the corals marine species depend on for food and habitat, reef ecosystems begin to collapse.

Bleached coral at Alifu Dhaalu Atoll, Maldives, in 2006. Image by Bruno de Giusti.

Large-scale coral bleaching was first observed in the 1980s, and quickly became associated with rising ocean temperatures and increasing levels of dissolved carbon dioxide in ocean water. During the 1980s and ‘90s, the corals of the Caribbean Sea fared better than those of the Indian and Pacific Oceans, where bleaching events were often more severe and recovery limited. However, in 1998, a mass bleaching event of unprecedented scale occurred in the Belize Barrier Reef. Cores extracted from the reef showed that the temperature spike leading to this event - a maximum water temperature of 31.5 degrees Celsius (88.7 degrees Fahrenheit) - was the most severe seen by the reef in the past 3,000 years. 

Though high temperatures and ocean acidification are not the sole causes of coral bleaching, they are the factors most associated with frequent, large-scale bleaching events, and are tied directly to human-driven climate change. In the decades since coral bleaching first came to the public eye, more awareness has been drawn to the plight of coral reefs and their importance for the world’s oceans.

A partially bleached reef at Molasses Reef, Florida, in 2014. Image by Matt Kieffer.

Learn more about coral reefs and how you can protect these critical ecosystems with Inspire EdVentures and the Tobacco Caye Marine Station at Coral Reefs Live!

References and Further Reading

The Belize Barrier Reef at UNESCO

Corals and Coral Reefs at the Smithsonian

Aronson, R., Precht, W., Macintyre, I. et al. Coral bleach-out in Belize. Nature 405, 36 (2000). https://doi.org/10.1038/35011132

Britannica, The Editors of Encyclopaedia. "Belize Barrier Reef". Encyclopedia Britannica, https://www.britannica.com/place/Belize-Barrier-Reef.

Britannica, The Editors of Encyclopaedia. "coral reef". Encyclopedia Britannica, https://www.britannica.com/science/coral-reef.