A beautiful nesting brown noddy on Catto Key in Grahams Harbor, San Salvador, Bahamas. See other beautiful phenomena from the Bahamas.
|
|
Coral bleaching was first noticed in the Great Barrier reef over 60 years ago (Wells and Hanna, 1992). At first, people speculated that a certain type of disease was causing these beautiful corals to turn white, as if theyÕd been ÒbleachedÓ. It was not realized until later, when the same symptoms were observed in other types of organisms living in a symbiosis with zooxanthellae, that the actual whitening was the stress response of the corals expelling their photosynthetic algae (Davidson, 1998). Coral mortalities due to bleaching have been reported in all major reef provinces since the 1970Õs, however, the problem did not get widespread attention until the 1980Õs with the onset of prominent El Nino conditions (Glynn, 1993). El Nino is a disruptive weather pattern that affects the Pacific Ocean every 3 to 5 years, replacing normally cool water with warm water in the western Pacific Ocean (Wells and Hanna, 1992). In 1982, El Nino conditions were the strongest they had ever been, leading to catastrophic bleaching events in coral reefs in the Pacific Ocean (Glynn, 1993). Eastern Pacific mortality of corals due to bleaching ranged from 50-99% during the 1982-1983 El Nino period. Specifically, coral reef bleaching was reported at 50% in Costa Rica reefs, 80% in Panama reefs, and 95-100% in the Galapagos Island reefs (Glynn, 1993). In the last decade however, bleaching has been reported from numerous locations worldwide, many of them outside the area of influence of El Nino, suggesting bleaching due to other stress factors (Wells and Hanna, 1992).
Corals are adapted to a very narrow range of conditions, thus, even a slight variation in any of these factors can become a ÒstressÓ causing them to expel their zooxanthellae (Davidson, 1998). This exact mechanism is not fully understood, but a variety of environmental factors, both natural and anthropogenic, can cause this bleaching such as the changes in sea surface temperature, exposure to ultraviolet radiation, exposure to toxins, sedimentation, excess nutrients, salinity, aerial exposure, and disease.
Corals excel in oligotrophic waters, which are free of nutrients. This limits phytoplankton blooms that block sunlight and use up oxygen as they are decomposed by bacteria. With an increase in nutrients, eutrophication occurs, and large algal blooms and increased numbers of phytoplankton dominate, blocking light for the zooxanthellae of the corals to photosynthesize (Molles, 2008). With increasingly common anthropogenic activities such as inadequate release of human waste and deforestation, increased amounts of nutrients are running off into areas inhabiting coral reefs with damaging effects. For example, in Jakarta, Indonesia, overcrowding and improper removal of human waste has led to an increase in nutrients, specifically nitrogen and phosphorous, into the Jakarta Bay water surrounding coral reefs. This excess of nutrients has caused eutrophication in naturally clear waters, resulting in increased algal blooms. The lack of sunlight and oxygen due to these blooms has suffocated the corals causing them to lose their algal symbiants (Davidson, 1998). Increased sedimentation has a similar effect. If terrestrial matter is present in the water for prolonged periods, a sufficient amount of light is blocked, resulting coral bleaching (Davidson, 1998). Anthropogenic activities such as logging, mining, agriculture, or the dumping of cargo increases sedimentation that runs off into waters containing coral reefs. A significant example comes from excessive logging in Bacuit Bay. Studies found that as logging in this region increased by 60%, coral reef cover closest to the river that the sediment drained into, decreased by 50% (Davidson, 1998).
Introduction of toxins or harmful chemicals into the water can also cause coral bleaching. Cyanide, a harmful chemical, has been used on coral reefs in the Asia-Pacific region to facilitate the capture of fish for aquarium trade for several decades. However, recently cyanide usage has greatly increased due to the growing restaurant based demand for live reef fish (Jones & Hoegh-Guldberg, 1999). This has significantly impacted the surrounding coral reefs, resulting in a mass coral bleaching across the Philippine and Indonesian coral reefs. Elevated cyanide concentration causes corals to lose their symbiotic photosynthetic algae. Laboratory studies tested a variety of other chemicals and found the same bleaching effects (Jones & Hoegh-Guldberg, 1999). Jones and Hoegh-Guldberg of the Centre of Marine Studies, estimated that only 4.3 percent of Philippine reefs are in Òexcellent conditionÓ, due to coral bleaching induced by cyanide fishing.
Due the coralÕs narrow range of tolerance conditions, a slight increase or decrease in salinity can cause bleaching. In some areas, exposure of reef flat corals to the atmosphere at extreme low tides can also cause corals to become stressed and expel their algae due to increased solar radiation (Gleason & Wellington, 1993). Receiving too little or too much light is also a cause of coral bleaching. There are two main types of coral diseases which have been associated with bleaching. There is debate as to whether the diseases cause the corals to lose their zooxanthellae, or that corals are stressed by bleaching of another cause, and become more susceptible to disease (Glynn, 1993).
While all these factors have significant effects on the ability of corals to lose their algae, temperature change is recognized as the biggest stressor of coral bleaching. In most instances, where coral bleaching was reported, it has coincided with an increase in sea surface temperature (Wells and Hanna, 1992). Marine organisms in general, live near the top limit of their temperature tolerance, and corals are no exception. In summer months, corals live close to and within one to two degrees of their upper limit (29.5- 30 degrees C) (Wells and Hanna, 1992). Thus, an increase or decrease of just a few degrees can be fatal, resulting in bleaching, and if prolonged, death. Declines in sea surface temperatures due to cold air outbreaks or intense upwelling episodes of -3 to -5 degrees C can cause coral bleaching if the decreased temperature is maintained for 5 to 10 days (Stone, 2007). However, coral bleaching associated with temperature increases is much more common, especially in the last decade. A small positive anomaly of 1-2 degrees C could induce bleaching if held for 5 to 10 weeks. On the other hand, a large increase of 3 to 4 degrees C could induce bleaching if maintained for only a few days (Stone, 2007). For example in 1991, over half the corals on reefs in Phuket, Thailand bleached over a period where water temperatures in the Andaman sea were 2 degrees C higher than normal (Wells and Hanna, 1992). In Polynesia during this same time period, a temperature increase of only 1 degree C was enough to cause bleaching on several reefs in the Society Islands (Wells and Hanna, 1992).
There is debate as to whether global warming is causing these higher sea surface temperatures. With the continuous burning of fossil fuels in the last ten years, the rate of carbon dioxide accumulation in the atmosphere has doubled since 2002. Carbon dioxide, an important greenhouse gas, traps heat that is generated and radiates it back to the atmosphere, causing a heating of the earth which can result in an increase in temperature. Most models predict between a 1-4 degrees C additional increase in global temperature by 2100 (Molles, 2008). If these predictions are true, this would push most corals out of their temperature tolerance zone and cause severe widespread bleaching and if prolonged, mortality across many coral reefs.
With all of the negative factors affecting coral growth, it is hard to remain optimistic about the survival of corals. However, it is possible for coral reefs to bounce back after bleaching. The rate of recovery is related to severity and scale of the disturbance (Glynn, 1993). If the stress inducing the bleaching is not too severe and if it decreases in time, affected corals can usually regain their symbiotic algae within several weeks or a few months. Rapid recovery is also generally observed in areas where there is a presence of coral survivors nearby that could serve as Òseed populationsÓ to aid in recruitment of new corals (Glynn, 1993). The Great Barrier Reef serves as an example of such recovery. In 1982, El Nino conditions resulted in severe coral bleaching, but within two years coral cover increased by 36% (Glynn, 1993). However, If the stress is severe and zooxanthellae loss is prolonged, depleted coral populations may not recover, leading to coral mortality. The same 1982 El Nino event caused severe bleaching of a rare species of fire coral (Millepora) leading to their extinction. This may have been because the coral was found only in the eastern Pacific, thus, there were no Òseed populationsÓ to help repopulate the dying corals (Wells and Hanna, 1992).
In early stages of global warming, ocean temperatures might increase slowly enough for corals and zooxanthellae to adapt, but this would be a slow process involving gradual selection of different genetic makeups over time which might not be possible (Wells and Hanna, 1992). There has been speculation that bleaching could allow corals to be repopulated with different type of zooxanthellae, possibility a type with a greater stress resistance and higher temperature tolerance (Wells and Hanna, 1992). Andrew Baker, marine biologist at University of Miami, has been testing this theory. He optimistically advocates that bleaching offers opportunities for reef corals to rid themselves of suboptimal algae and acquire new partners (Baker, 2001). In his 2001 experiment, he induced bleaching in Caribbean coral species and distributed them with transplanted species. His results showed that some coral species could pick up different algal symibants, supporting the view that coral bleaching can promote rapid response to environmental change by facilitating change in algal symbiant communities that are better suited to the new environmental conditions (Baker, 2001).
Although BakerÕs research does provide some promise on the recovery of corals from bleaching, there is still a great deal unknown about how climate changes will affect coral reefs around the world. A variety of both natural and anthropogenic factors are increasingly affecting corals causing them to expel their algal symibants, representing the Òunweaving of the most basic braidÓ that Osha Gray Davidson discusses (Davidson, 19980). Extensive research on the mechanism of coral bleaching and preventative actions in reef monitoring are both needed in order to try to prevent future bleaching as well as aid in the recovery of bleached reefs.
References
Baird, A, & Bhagooli, R (2009). Coral bleaching: the rool of the host. Trends in Ecology and
Evolution. 24, 16-20.
Baker, A (2001).Ecosystems: Reef corals bleach to survive change. Nature . 411, 765-766.
Davidson, OG (1998). The Enchanted Braid: Coming to terms with nature on the coral reef.
Canada: John Wiley & Sons, Inc.
Gleason, D, & Wellington, G (1993). Ultraviolet radiation and coral bleaching. Nature. 365, 835-
837.
Glynn, P (1993).Coral reef bleaching: ecological perspectives. Coral Reefs. 12, 1-17.
Jones, R, & Hoegh-Guldberg, O (1999). Effects of cyanide on coral photosynthesis: implications
for identifying the cause of coral bleaching and for assessing the environmental effects of
cyanide fishing. Marine Ecology Progress Series. 177, 83-91.
Molles, M (2008). Ecology. Boston: McGraw Hill.
Muller-Parker, G, & D'Elia, CF (1997). Interactions between corals and their symbiotic algae.
Coral Reefs. 5, 96-113.
Stone, Richard (2007).A World Without Corals. Science. 316, 678-681.
Wells, S, & Hanna, N (1992). The Greenpeace Book Coral Reefs.Canada: Sterling Publishing.
Next Article
Previous Article
Return to Topic Menu
DOWNLOAD the Paper Posting HTML Formating HELP SHEET!
We also have a GUIDE for depositing articles, images, data, etc in your research folders.
Article complete. Click HERE to return to the Pre-Course Presentation Outline and Paper Posting Menu. Or, you can return to the course syllabus
WEATHER & EARTH SCIENCE RESOURCES |
|
OTHER ACADEMIC COURSES, STUDENT RESEARCH, OTHER STUFF
|
|
TEACHING TOOLS & OTHER STUFF
 |
It is 7:47:54 PM on Monday, November 23, 2009. Last Update: Friday, May 15, 2009
Listen to a "Voice Navigation" Intro!
(Quicktime or