As we know, there are three biotic ecosystems that are very important in coastal areas, namely coral reefs, seagrass beds and mangroves. The global conditions of these three ecosystems at this time have a tendency to experience degradation continuously, so that their very important ecological functions are declining. Damage to the coastal area with its ecosystem is caused by natural and human factors, but human factors are the most dominant cause of damage.

In the coral reef ecosystem, there is a relatively new type of damage, namely in the form of coral bleaching. Coral bleaching is a color change in coral tissue from its natural brownish or greenish color, to a pale white color. Coral bleaching can lead to death in corals.

The coral bleaching events that have occurred have been observed by researchers from the Indonesian Institute of Sciences (LIPI) between March and May 1983, where massive coral bleaching occurred with extensive mortality rates ranging from the Sunda Strait (West Java), the Thousand Islands (Jakarta ), to the Karimunjawa Islands (Central Java). Then there was another global coral bleaching event in 1998, where more than 55 countries experienced high levels of coral bleaching and death. As many as 90% of corals died from bleaching in 1998 in West Sumatra and the Gili Islands, Lombok. There are also many corals in other parts of Indonesia that are affected by bleaching.

Causative factor

The loss of symbiotic algae called zooxanthellae, which lives a lot on coral tissue, or the loss of color pigments that give color to corals, can cause bleaching of corals. Without the zooxanthellae, corals cannot survive longer.

As is known, most of the corals are small animals (called polyps), which live in colonies and form reefs. They get their food in two ways: first, by using their tentacles to catch plankton, and secondly, through small algae (called zooxanthellae) that live in coral tissue. Zooxanthellae are found in large quantities in each polyp, they live in symbiosis, give color to polyps, energy from photosynthesis and about 90% of carbon polyps. Zooxanthellae also receives important nutrients from corals, and provides as much as 95% of photosynthesis (energy and nutrition) to coral.

It can be said, the main cause of coral bleaching is the result of human activities, both directly and indirectly. Human activities that are direct causes include sedimentation, pollution, and fishing with explosives. Whereas the indirect, what makes this bleaching happen globally is global warming that causes climate change.

That climate change is the main cause of coral reef bleaching events at this time. Climate change that causes an increase in sea surface temperature and the level of the sun's ultraviolet light, will eliminate symbiotic algae (zooxanthellae) which is a place for coral polyps to depend on for food. Bleaching conditions that are too long (more than 10 weeks) can cause the death of these coral polyps. Corals can live within a temperature tolerance range ranging from 20 to 30oC. Critical temperatures that can cause bleached corals depend on the coral adaptation to the average sea water temperature in the area where it lives. Corals tend to turn white when the temperature rises sharply in a short time, or the temperature rises slowly over a long period of time.

The combined effect of high sea surface temperatures and high levels of sunlight (on ultraviolet long waves) can speed up the bleaching process by defeating the natural mechanism of coral to protect itself from excessive sunlight. The results of a recent study by a research team led by Professor Don Levitan, Chair of the Florida State University Department of Biological Sciences, said that a process where high water temperature or UV light would be able to push the reef to the point where it would lose a symbiotic algae partner that gives color pigments to coral, and also affect coral fertility in the long run. On average, coral takes 3-4 years to recover from bleaching. "Even if we can improve what causes these corals, even then it will be difficult for coral populations to recover, because living corals cannot possibly produce enough offspring to restore coral reef populations," he said.

While other natural disorders that can cause coral bleaching are sudden changes in salinity, long-term lack of light, and disease.