Surely, many divers and snorkelers have argued that to swim among the plants and animals in a tropical coral reef is one of life’s most pleasant experiences. Those with a scientific bent are easily drawn to the diversity of fishes and other sea life or the play of the tide between the coral columns. Most first-time visitors, however, are simply overwhelmed by the color of the seascape. Against the backdrop of azure, other colors (reds, yellows, greens, and purples) stand out on the bodies of the fishes, crustaceans, corals, and other forms of life.
Bleached staghorn coral (Acropora cervicornis) in the Caribbean Sea--Stephen Frink/Corbis
In most reef ecosystems, however, some of the corals are sickly white, and the fish and other organisms that inhabit them are absent.
This phenomenon, called coral bleaching, has natural, as well as anthropogenic (human-caused), origins. Before this can be explained, it is important to understand how corals obtain their color. Corals are transparent animals related to jellyfish. Like jellyfish, they have a mobile form called a medusa and a sessile (sedentary) form called a polyp. The structures commonly known as coral are actually large colonies of coral polyps and the intricate calcium carbonate skeletons they secrete.
Sea fan (genus Gorgonia), a branching variety of coral, in the Belize Barrier Reef--© Herbert Schwarte/FPG International
The brilliant color of the reef comes from the algae from the genus
Zooxanthella that live symbiotically within the tissues of the coral. Zooxanthellae provide the coral with food and oxygen through photosynthesis. In return, the algae are sheltered from many of the vagaries of the changing ocean environment and they have better access to the coral’s waste products, which serve as raw materials for their growth.
Bleaching occurs when coral polyps are separated from their algal symbionts in response to disease or serious environmental stress; however, it is sometimes observed when the algae lose their pigment. With the algae removed, coral polyps and their skeletons appear starkly white. Some examples of the stressors capable of causing coral bleaching include changes in seawater chemistry resulting from pollution or ocean acidification, sedimentation, and exposure to the air during low tide. Some stressors impair the process of photosynthesis, which results in a loss of nutrients for the coral, or cause zooxanthellae to manufacture versions of helpful compounds that are harmful to the coral. In addition, some zooxanthellae might grow too quickly or divide too rapidly within the coral polyp. The end result of these activities is the breakdown of the symbiosis between the algae and the coral breaks down. If the stress is mild and does not last too long, zooxanthellae will recolonize the coral, and the coral colony will recover. On the other hand, if the bleaching lasts longer than a few months, the coral will starve and perish.
Most incidences of coral bleaching involve extended changes in seawater temperature. In general, tropical corals and zooxanthallae occur in seawater between 16 and 30 °C (about 61 to 86 °F), and the temperature tolerances of one species may differ greatly from those of another. Studies have shown that temperature increases of 1 to 2 °C (1.8 to 3.6 °F) above a coral’s upper tolerance limit for a period of 5–10 weeks during the warmer months of the year are enough to induce bleaching, and such heat stress appears to affect the zooxanthellae first. Warm seawater prompts zooxanthellae to manufacture forms of oxygen and other chemical products that are toxic to the coral, and these toxins build up in the coral’s tissues. Many scientists think that the coral can detect this buildup and jettison the algae. Furthermore, heat stress also increases the susceptibility of the zooxanthellae, as well as the coral polyps, to disease and exacerbates problems caused by other stressors.
Cold water, too, can be an enemy. Some corals have been shown to bleach when seawater temperatures drop 3 to 5 °C (5.4 to 9 °F) below their lower tolerance limit for 5–10 days. During episodes of cold-water stress, photosynthesis slows or shuts down completely, which may also lead to a buildup of toxins in the tissues of both zooxanthellae and the coral.
Temperature stress can be caused by seasonal changes occurring in the oceans, or it can be caused by major disruptions in normal climate patterns. The amount of heat energy available to marine ecosystems, even those in tropical and subtropical latitudes, changes with the time of year.
A diver exploring a coral reef in the Maldives--A. Witte/C. Mahaney—Stone/Getty Images
The oceans receive more heat energy during the warmest months of the year than they do during the coldest months. Slight alterations to the paths of warm and cold ocean currents result, and some coral colonies could be bathed in water whose temperature is either too warm or too cold. The bleaching that follows such events is often temporary and limited.
On the other hand, bleaching episodes produced by large-scale climate disruptions—such as those caused by El Niño, La Niña, and climate change brought on by global warming—last longer, are more severe, and their influence on seawater temperature can extend to marine ecosystems across the globe. These forces often push seawater temperatures beyond the tolerance limits of many corals and zooxanthelae for weeks and months at a time, and thus have the potential to kill the coral colonies occurring over wide areas. El Niño brings unusually warm sea-surface conditions to the tropical Pacific Ocean that may last several months. Along with its counterpart, La Niña (which delivers cooler-than-average sea-surface conditions to the region), El Niño can influence prevailing seasonal climatic patterns beyond the Pacific basin and cause mass bleaching events in areas as far flung as the Caribbean Sea and the western Indian Ocean.
In the aftermath of the unusually powerful El Niño of 1997–98, scientists studying Australia’s Great Barrier Reef estimated that more than 60 percent suffered from some sort of bleaching and that nearly 90 of the corals were killed in some areas. Scientists also note that general increases in seawater temperatures caused by global warming (1 °C [1 °F] by the year 2050) will have the effect of reducing the coral’s upper margin of temperature tolerance. Consequently, they fear that coral colonies will bleach more frequently and more completely in the coming decades, result in greater incidences of coral death.
In terms of biological diversity, coral reefs in the oceans are comparable to tropical rainforests on land. They contain 25 percent of all marine species, and the reef itself, which is largely made up of vast living coral colonies, provides habitat to multitudes of fishes, crustaceans, and other marine life. So, when coral death occurs, the impact is felt in the various species that eat coral, as well as those that rely on other species that live within coral colonies.
Juvenile fish taking shelter in a large brain coral, Cahuita, Costa Rica--© Brandon Liddell
Under these circumstances, many reef-specialized fishes and other species can go extinct locally, and the nature of the reef ecosystem can change as other, more-generalized organisms move in. Scientists studying the aftermath of the 1997–98 El Niño report that marine communities that thrived before the onset of that severe bleaching event show little signs of recovery even years afterward.
Although the coral bleaching occurs naturally, the continued release of heat-trapping carbon dioxide and other greenhouse gases from human activities appears to be exacerbating the problem, because some of the heat trapped by the atmosphere is transferred to the oceans. Since heat stress has been blamed for most of the coral bleaching cases around the world, we humans should do whatever we can to prevent this heat transfer from occurring.
The most obvious way to do this is to reduce the amount of greenhouse gases we release from our industries, homes, and automobiles. While we wait for our leaders to come up with laws that truly confront the problem of global warming, all of us should do what we can to conserve energy and find alternatives to greenhouse-gas-producing fossil fuels.
