The Online Magazine for Sustainable Seas
September, 2000 Vol.3 No. 9
Coral Bleaching: the Whys, the Hows and What Next?
This article also appears in Tambuli No. 6, May 2000. To download print version (PDF file) of the complete Tambuli issue, click here
Corals are temperature-sensitive animals. They can only function effectively within a narrow temperature range. When the ambient (water) temperature reaches above a certain level, hard coral polyps expel their symbiotic algae in what is referred to as a bleaching event. Without the algae in their tissue, the corals become white hence, the term. Also, the microscopic algae or zooxanthellae that normally live inside the corals provide them with most of their food through photosynthesis. If the situation persists, the coral does not get enough food and eventually dies. On the other hand, if the water temperature returns to normal (below 29.5 deg. Centigrade) within about six weeks, the coral accepts the algae back and generally recovers.
Unfortunately, the warm water in 1998 persisted. The ensuing global coral bleaching and die off is unprecedented in geographic extent, depth and severity (Maniku 1999). Tropical sea surface temperatures in 1997 and 1998 have been higher than in any other time in modern history. Coral bleaching associated with high sea surface temperatures affected almost all species of corals and many other invertebrates. Coral reefs of all the countries in the Western Indian Ocean, East African region, Bahrain and the Gulf as well as the South Indian Ocean have been affected. Heavy damage to reefs more than 1,000 years old has been recorded in the South China Sea areas indicating the severity of this event. The only major reef region that seems to have been spared is the central Pacific.
The water temperature around the Indian Ocean reef systems and parts of the Philippines reached 32 deg. Centigrade in May 1998, too warm for hard coral species. Coral reefs in the Indian Ocean suffered high levels of mortality following severe coral bleaching leaving some reefs uniformly white in appearance. In some of the portions of the Indian Ocean, mortality in the upper 6-10 m has been as high as 90% while western Palawan, Philippines suffered 40% mortality.
In the Maldives, the abnormally high temperatures that persisted for too long affected the whole archipelago killing most of the branching corals such as the Acroporids and the Pocillioporids. However, approximately 50% of the brain corals and the massive corals such as Porites survived after the bleaching incident. In order to understand the implications of bleaching on the fisheries and tourism of the Maldives, great effort has been made to collect valuable information that may reveal any negative impact. The Marine Research Centre (MRC) monitored the reefs throughout selected sites in the north, central and southern part of the Maldive archipelago in May 1998. A more comprehensive survey was carried out from September to October 1998 when a number of dead corals have already been covered with algae. Very few bleached corals were identified. An abundance of herbivore (plant eaters) fish species was observed. With regard to the fisheries for the period September 1998 to February 1999, Maldives had the highest tuna catch in the past 30 years. During the same period, there has been an abundance of plankton, specifically zooplankton. This has brought in a number of filter feeders such as whales and mantas into Maldivian waters.
Reef monitoring in April 1999, almost one year after the bleaching incident, yielded encouraging observations. In a number of reefs and the monitoring sites visited by MRC scientists, recovery was evident and new recruits were found. In addition, many bleached corals that should have been resistant to bleaching in the first place, such as the massive growth forms, were recovering well from the bleaching incident. In one site close to Malé, in a 5x10 m2 area, more than 100 new recruits measuring 5-15 mm in diameter have been observed. Similar recruitment has also been observed in the northern atolls.
The Maldives is lucky. Damage on the reef was overshadowed by the positive change in the fish assemblage. This has been attributed to the unpolluted nature of the Maldivian environment. The Maldives has a fairly pristine and low impacted reef system, perhaps, the very reason for its rapid recovery. Observed recruitment shows that within a very short period of time, the reefs can return to a healthy state.
In the Philippines, Ma. Fe Divinagracia evaluated the extent and degree of coral bleaching in selected sites in Central Visayas from May to June 1999 in order to help understand coral bleaching and its impact on Philippine reefs more fully. The degree of bleaching among genera, growth forms and water depths was determined among the reefs in Apo Island, Dauin, Negros Oriental; Sumilon Island, Oslob, Cebu; Balicasag Island, Panglao, Bohol; and Pamilacan Island, Baclayon, Bohol. Divinagracia likewise described evidences on reef recolonization.
Among the sites, Apo appeared to be the most affected by the 1998 bleaching episode at 35% (0.37 sq km) and least affected was Pamilacan at 6.4% (0.16 sq km). Significant differences existed with respect to bleached cover among genera in all sites surveyed.
It is probable that along with increased water temperature, sea conditions at that time in Apo were extremely calm with exceptionally clear water such that light could have been able to penetrate farther into the water. In the absence of organic and inorganic matter in the water column, coupled with less mixing by wave action, higher than average intensities of UV (ultraviolet radiation) probably reached much deeper depths (Gleason and Wellington 1993 in Divinagracia 2000). According to Glynn (1993) , many workers have reported coral bleaching during periods of low wind velocity, calm seas and low turbidity where such favor heating of shallow waters and high solar radiation penetration. Also, there is less oxygen in the water column at higher temperatures.
In terms of coral genera and growth forms, variation in the degree of bleaching was observed among the sites. Galaxea was the most significantly affected by bleaching in Apo, Seriatopora in Sumilon, Montipora and Pocillopora in Balicasag and Pocillopora in Pamilacan. This may be related to physiological differences among species of zooxanthellae present (Goenaga et al. 1989 in Divinagracia 2000. Based on Buddmeieer and Fautin (1993) and Ware et al. (1996), bleaching is an adaptive mechanism that allows the coral to be repopulated with a different type of alga, possibly conferring greater stress resistance (Divinagracia 2000). Physiological and genetic studies showed that different strains of zooxanthellae exist both among different and within species of coral hosts and that different strains of algae show varied physiological responses to both temperature and irradiance exposure (Glynn 1993 and Brown1997 in Divinagracia 2000).
Among the different growth forms, highly affected by bleaching were the branching corals. They have been observed to be highly susceptible in various bleaching events in French Polynesia, Australia, Hawaii, Eastern Pacific, Papua New Guinea. The massive bleaching that occurred in Apo Island was probably due to the extent of surface area exposed to solar radiation.
Some zooxanthellae in reef corals are known to produce an UV absorbing pigment, S-320, in response to UV light (Glynn 1993 in Divinagracia 2000). The intensity of UV radiation diminishes with increasing depth and presumably less of this protective pigment is required at greater depths (Fitt et al. 1993 in Divinagracia 2000). Deeper corals therefore, may be more susceptible to increases in UV radiation (Goenaga et al. 1989 in Divinagracia 2000). This may explain the occurrence of bleaching at all depths that was observed in the study.
Most of the affected colonies were recolonized by small filamentous and encrusting algae. Some coral colonies had white spots all over the surface that may be a kind of coral disease (white band disease). This would require further investigation, however.
Divinagracia recommends that a monitoring site should be set up to allow for rapid and accurate recognition of bleaching events that will also enable a quick and appropriate response. Likewise, more sensitive techniques to measure temperature tolerance combining morphological, ecological and molecular genetic approaches, a range of responses and the potential for physiological and genetic adaptation are needed.
Indeed, post-bleaching is likewise critical because survivors depend on previous energy reserves (Wilkinson et al. 1999 in Divinagracia 2000 ). Survivors can only produce fewer larvae to repopulate damaged areas.
Worldwide, many coral scientists believe that this mass coral bleaching, although primarily caused by rising tropical sea temperatures, was exacerbated by other direct stresses on the reefs. In addition to high water temperatures, there are factors such as storms, pollution and other impacts due to global warming.
A dramatic warning comes from a recent study by Dr. Ove Hoegh-Guldberg, Director of Sydney University’s Coral Reef Research Institute (NEWS 1999) . Using a variety of climate change models, he predicts that the continual rise in tropical sea temperatures will lead to increased coral bleaching and calculates that events as severe as 1998 could become commonplace by 2020. He reports that corals do not have the genetic ability to acclimatize rapidly enough to rising water temperatures. According to Hoegh-Guldberg (1994), coral bleaching around the world will increase in frequency and seriousness until it occurs annually by 2030, unless global warming is reversed. Although he would not expect coral reefs to become extinct due to mass bleaching, he estimates that it could take up to 500 years for them to recover.
Researchers from the University of Georgia have also discovered that higher sea temperatures degrade the ability of the zooxanthellae to convert light into energy, thus cutting off the corals’ food supply. Likewise, other studies show that excess heat impairs the corals’ ability to reproduce. Chin predicts a severe negative impact of increased atmospheric carbon dioxide concentrations on coral reefs. An increase in dissolved carbon dioxide concentration in seawater will enhance the solubilization of calcium carbonate, decreasing the saturation state of aragonate therefore, reducing calcification. The authors predict that between preindustrial times and the middle of the next century, there will be a drop in reef calcification of 14-30% (Chin 1999).
While the problem seems too big for an individual to do anything about it, we all need to do what we can to help. If humankind would make an effort to minimize adverse anthropogenic influences, recovery of affected corals may be faster. Even small actions (like reducing carbon dioxide emissions by driving our cars less) may contribute to the solution.
In the meantime, everyone must continue to strongly support projects that protect local coral reef areas. Without effective local protection, many coral reefs will be lost in the next few decades—even without the effects of bleaching.
B. 1997. Coral bleaching: causes and consequences. Coral Reefs.