Marine Ecosystems: Coral Reefs and Mangroves

Coral reefs are underwater structures built by colonies of tiny animals (coral polyps) that live in partnership with algae, and mangroves are salt-tolerant trees and shrubs that grow in the intertidal zone of tropical and subtropical coasts; both are highly productive ecosystems that protect coasts and support fisheries and biodiversity. In brief: coral reefs cover less than 1% of the ocean floor but are home to about a quarter of all marine species. Mangroves shelter young fish, trap sediments and store large amounts of carbon, but they are being lost to coastal development and shrimp farming. Both are threatened by climate change, pollution and overuse.

These ecosystems link land and sea and provide food, protection and income for hundreds of millions of people. This guide explains how reefs and mangroves are built and the conditions they need, their benefits, the threats and conservation, compares the two ecosystems in a table, works through calculations, outlines the history of their study and corrects common mistakes.

How do coral reefs form and work?

Reef-building corals are animals that secrete a hard skeleton of calcium carbonate. Inside their tissues live microscopic algae called zooxanthellae, which use sunlight to photosynthesize and give the coral up to 90% of its food energy, while the coral provides shelter and nutrients. This symbiosis allows corals to thrive in clear, nutrient-poor tropical water. Over thousands of years, new corals grow on the skeletons of old ones and build the reef.

Reef-building corals need warm water (typically 23–29 °C), clear, shallow water (generally less than about 50 m deep, so that enough light reaches the algae), salinity of about 32–42 parts per thousand, hard surfaces to settle on and low sediment and nutrient levels. For this reason they are found mostly between about 30° N and 30° S, on the east coasts of continents, where warm currents flow, and not near large river mouths. Charles Darwin proposed in 1842 that the three main types are stages in the history of a volcanic island: fringing reefs grow next to the shore, barrier reefs are separated from land by a lagoon (the Great Barrier Reef, which stretches about 2,300 km along the coast of Australia, is the largest) and atolls are rings of coral around a lagoon after the island has subsided, as in the Maldives.

Reefs are among the most diverse ecosystems, with thousands of species of fish, mollusks, crustaceans and other organisms. About a quarter of marine species depend on them at some stage, and they support the fisheries and tourism that provide food and income to hundreds of millions of people. They also act as natural breakwaters, reducing wave energy by a large share and protecting coasts and islands.

How do mangroves work?

Mangroves are about 80 species of trees and shrubs that grow along tropical and subtropical coasts, in estuaries and sheltered shores, mainly between 30° N and 30° S. They are adapted to a hard environment of salt water, shifting mud and low oxygen:

• Prop (stilt) roots and breathing roots (pneumatophores) that anchor the tree in soft mud and take in oxygen from the air.
• Salt management: some species filter salt at the roots, and others excrete it through glands on their leaves.
• Viviparous seeds: the seedlings (propagules) germinate while still on the tree and drop into the water, ready to root.

The world has about 14–15 million hectares of mangroves, most in Asia, with Indonesia holding the largest area, followed by Brazil, Nigeria and Mexico. They provide nursery habitats for fish, shrimp and crabs, coastal protection by absorbing wave and storm surge energy, sediment trapping that builds up land and improves water quality, wood and other products for local people and carbon storage. Mangroves store carbon in their biomass and especially in their waterlogged soils, called “blue carbon,” and per hectare, they can hold several times more carbon than most terrestrial forests. Salt marshes and seagrass meadows are similar blue carbon ecosystems.

What are the threats?

For coral reefs: marine heat waves cause coral bleaching, in which stressed corals expel their algae and turn white. If the heat lasts, the coral starves and dies. Mass bleaching events occurred worldwide in 1998, 2010, 2014–2017 and 2023–2024, and on the Great Barrier Reef in 2016, 2017, 2020, 2022 and 2024. Ocean acidification, caused by the absorption of carbon dioxide, makes it harder for corals to build skeletons. Other threats are pollution and runoff of nutrients and sediments, overfishing and destructive fishing (blast and cyanide fishing), coastal development, physical damage by anchors and divers, disease and outbreaks of the crown-of-thorns starfish. Scientists estimate that a large share of the world’s reefs has been lost or degraded since the 1950s, and that, without strong cuts in emissions, most reefs could be lost at warming of 1.5 to 2 °C or more.

For mangroves: clearing for shrimp and fish ponds, agriculture such as rice and oil palm, urban and tourism development, logging for timber and charcoal, pollution and changes in river flow. A significant share of the global mangrove area has been lost in the last decades, estimates range from a fifth to more than a third since the 1970s, and the rate of loss has slowed in recent years. Sea-level rise and intense storms also threaten them if they cannot migrate inland.

How are they protected?

• Marine protected areas and no-take zones: as in the Great Barrier Reef Marine Park and in Belize’s barrier reef reserve system.
• Reducing local stress: better wastewater treatment, farming practices that reduce runoff, fishing regulations and controls on coastal development.
• Mangrove protection and restoration: legal protection, community management and replanting, ideally at sites with suitable tides and soil. Replanting in unsuitable places, such as mudflats, often fails.
• Coral restoration: growing and transplanting coral fragments and studying heat-tolerant corals, which can help at a local scale but cannot replace emission cuts.
• Climate action: limiting warming is essential for the future of reefs.
• Sustainable livelihoods: ecotourism, sustainable aquaculture and payments for the carbon stored by mangroves.

Reefs and mangroves compared

Worked example: heat stress, area and carbon

The values are hypothetical or approximate and illustrate the calculations.

1. Degree heating weeks: NOAA’s coral bleaching alerts use degree heating weeks (DHW), which add the amount by which sea temperature exceeds the local maximum monthly mean by at least 1 °C, summed over a 12-week period, in °C-weeks. If the temperature is 2 °C above the threshold for 4 weeks, DHW = 2 × 4 = 8 °C-weeks. Values of about 4 or more indicate significant bleaching and about 8 or more indicate widespread bleaching and mortality.
2. Reef area: reefs cover about 250,000 km², while the ocean covers about 361,000,000 km². The share is 250,000 ÷ 361,000,000 = 0.07%, less than 0.1% of the ocean’s area, although they support about 25% of marine species.
3. Mangrove area: 14.7 million hectares is 147,000 km². If 20% is lost in 40 years, the loss is 0.20 × 14.7 = 2.9 million hectares, or an average of 73,500 hectares a year.
4. Carbon in mangroves: if a hectare of mangrove holds 400 tonnes of carbon in its biomass and soil, then 10,000 hectares hold 4,000,000 tonnes. Converting to CO₂ (× 3.67), this is 14,680,000 tonnes of CO₂, which would be released if the area were cleared and the carbon were oxidized.
5. Wave protection: if a reef reduces wave energy by 90% before it reaches the coast, only 10% remains. Wave energy is proportional to the height squared, so the wave height is reduced to √0.10 = 0.32, so a 2 m wave is cut to about 0.63 m.
6. Value of protection: if a coast with a reef and mangroves avoids flood damage of 8 million a year, over 30 years the value is 240 million, compared with the cost of protection and restoration of 15 million, a benefit-cost ratio of 16.

How did the study of these ecosystems develop?

Charles Darwin’s theory of atoll formation (1842) was based on observations during the voyage of the Beagle, and was confirmed by drilling at Eniwetok Atoll in the 1950s, which reached volcanic rock beneath the coral. The discovery of the symbiosis of corals and algae in the late 19th century, and the study of reef ecology on the Great Barrier Reef from the 1920s, expanded understanding. Mass coral bleaching drew global attention after the 1982–83 and 1997–98 El Niño events. Mangroves were long seen as wasteland, and their value as a nursery habitat and a coastal defense was recognized in the late 20th century, especially after the 2004 Indian Ocean tsunami, when areas with intact mangroves and reefs often suffered less damage. In the 2000s, the idea of blue carbon led to more interest in protecting and restoring coastal vegetation.

Which mistakes are common?

Mistake 1: Thinking that corals are plants or rocks

Corals are animals that build limestone skeletons, and they depend on symbiotic algae for most of their food energy.

Mistake 2: Assuming that bleached coral is always dead

Bleached corals can recover if the stress ends quickly enough, but prolonged or repeated heat leads to widespread death.

Mistake 3: Seeing mangroves as useless mud and swamp

They protect coasts, support fisheries, store carbon and filter water, so clearing them often costs more than it gains.

Mistake 4: Believing that planting mangroves anywhere restores the ecosystem

Success depends on the right location, tidal conditions, species and community support. Many projects fail when seedlings are planted on unsuitable sites.