What is Coral Bleaching

Coral reefs are unique and beautiful ecosystems. People flock from all over the world to snorkel and dive amongst the diverse sea life inhabiting the reefs. However, coral reefs provide more than just an enjoyable experience for tourists, they provide important habitat for many organisms that reside in the ocean. Today, there are many threats that jeopardize the health of coral reefs; one of these threats is coral bleaching.

To understand coral bleaching, one must first understand the composition of coral reefs. Coral reefs are made up of a collection of communities containing organisms such as sponges, starfish, and mollusks. The main components of coral reefs, however, are the corals themselves. Corals are tiny animals that live in colonies called polyps. Polyps secrete a hard compound called calcium carbonate, also known as limestone. The calcium carbonate provides protection for the corals. Over time, the calcium carbonate builds up creating the base of the coral reef. The base of the reef grows as old corals die and new ones build upon them. Since calcium carbonate is secreted from the base of the polyp, the living community remains on the surface of the reef.

Corals have formed an important relationship with a type of alga called zooxanthellae. This relationship is known as a symbiotic relationship, where both organisms benefit. The zooxanthellae live within the polyps where they are protected from predators, but still have access to light. They need light so they can produce food through photosynthesis. In return for their protection, the zooxanthellae provide corals with food. Another important aspect of the relationship is that zooxanthellae are responsible for the bright colors often associated with coral reefs.

Coral bleaching occurs when the corals lose their zooxanthellae and the white calcium carbonate is exposed. Once the zooxanthellae are gone, the corals have little chance for survival. Unless the zooxanthellae come back within a couple of weeks, the corals will die. When the corals die, the habitat they provide dies along with them. The loss of this coral reef habitat will result in the decline of innumerable marine invertebrates and fish that depend on it for survival.

There are many hypotheses as to why coral bleaching is occurring however, the most recognized hypothesis is the rise in sea surface temperature. The optimal temperature for most coral reefs is between 23 and 25 C. According the National Oceanic and Atmospheric Administration, coral bleaching events worldwide have been attributed to sea surface temperatures rising and staying as little as 1C higher than the usual average monthly maximum during the hottest months of the year. In addition to temperature, stresses from hurricanes, pollution, solar radiation, salinity and bacteria have all been associated with coral bleaching.


Coral Reef Protection (n.d). Retrieved June 13, 2009 from the U.S. Environmental Protection Agency: http://www.epa.gov/OWOW/oceans/coral/.

Coral Bleaching (n.d.). Retrieved June 13, 2009 from the National Oceanic and Atmospheric Administration’s Coral Health and Monitoring Program: http://www.coral.noaa.gov/cleo/coral_bleaching.shtml.