Vocabulary:

• coral bleaching—a process in which coral reefs turn white
• diversity—variety, or having more than one kind
• ecosystem—a community of living things together with their physical environment
• habitat—the place where a plant or animal lives
• invertebrates—animals that do not have a backbone, such as corals, snails, oysters, or jellyfish
• marine—found in or produced by the ocean
• pollution—anything harmful or poisonous that is put into an environment
• resilience—an ecosystem’s ability to resist or recover from changing conditions (adjective: resilient)
• runoff—water that is not absorbed by soil but instead runs off of land into a body of water
• sediment—very small pieces of broken-down rocks

Coral reefs are complex ecosystems found in tropical and sub-tropical waters. They are built from colonies of coral polyps, tiny animals related to sea anemones. The reefs provide food and habitat for thousands of species of fish and invertebrates as well as other marine life such as sea turtles. People use coral reefs for fishing, snorkeling, and diving. In addition to being valuable recreational and economic resources, coral reefs protect coasts by acting as a natural buffer against heavy waves.

However, coral reefs are very sensitive to environmental changes. Most reef corals have a symbiotic (mutually beneficial) relationship with a type of algae called zooxanthellae, which live within the coral polyps. The algae use waste products from the corals as nutrients and, in turn, provide the corals with food. If the corals are stressed because of changes in their environment, they expel the algae, which leaves the corals looking white. This is known as coral bleaching. Without the algae, the corals may die.

In 1998, there was a mass bleaching event that led to the death of 16 percent of the world’s reefs. The bleaching event was linked to the El Niño Southern Oscillation, which brought unusually warm waters to coral reefs. Large-scale bleaching events are usually caused by higher water temperatures; localized bleaching can be caused by other factors as well, such as changes in water chemistry, increased sunshine, sedimentation, and disease.

Coral reefs are threatened by both climate change (which is raising ocean levels, temperatures, and acidity) and human activities. For example, agricultural runoff can lead to increased levels of nutrients in the water; excess nutrients can cause an overgrowth of algae or a starfish infestation—both of which kill coral. Land development can increase sedimentation, which blocks essential sunlight. Toxic chemical pollutants (such as mercury and pesticides) can affect ecosystem health. So can the clearing of mangroves. The trees filter sediment and other pollutants from the shore and are nurseries for coral reef fish. Tourism, a key source of income in many areas, can damage reefs from physical contact. Overfishing, particularly of algae-eating species, can also be harmful, as can destructive fishing practices.

Many coastal communities rely on coral reefs as sources of food and income. Their challenge is how to continue to use reefs as resources while reducing the impacts on them. Because of the changing climate, mass bleaching events are predicted to increase in both frequency and severity in the future. A healthy reef ecosystem with a diversity of marine life is stronger and more likely to recover from disturbances. Protective measures, such as establishing managed marine areas and minimizing destructive human activities, can help improve the resiliency of coral reefs.

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