Final Draft: Coral Bleaching Causes and Consequences

This discussion topic submitted by Whitney Long ( at 6:51 AM on 6/29/99. Additions were last made on Wednesday, May 7, 2014.

Note the extreme bleaching in this Diploria at the Grotto, San Salvador, Bahamas

Coral Reef Bleaching
Coral reef bleaching occurs when corals undergo great stress. The symbiotic algae, zooxanthallae, is expelled or its photosynthetic pigments harmed--thus the coral appears white. The corals may be weakened to the point of death. A number of environmental factors, both natural and anthropogenic, can cause bleaching. Global warming has been theorized to have caused massive global bleaching events over the past two decades. Monitoring and management may help us understand and reduce threats to corals.

I. Coral Reef Systems

A. Reef Propertie
1. Reefs are found in circumtropical shallow waters along the shores of islands and continents.
2. Reefs live in nutrient poor waters.
3. Reef substrate is composed of calcium carbonate from living and dead
scleractinian corals.
4. These corals have a symbiotic relationship with a single-celled algae known as zooxanthallae.

B. Zooxanthallae
1. Zooxanthallae live symbiotically within coral polyp tissue.
2. They assist coral in nutrient production through photosynthetic activities.
3. Corals receive coloration from the photosynthetic pigments of zooxanthallae.
4. When stressed, zooxanthallae may be expelled from coral tissue.
5. If all zooxanthallae are lost, the coral will die.

C. Importance of Reefs
1. Reef ecosystems have been compared to rain forests in terms of biodiversity and density of living organisms.
2. Total area covered by coral reefs is approximately 240,000 square miles.
3. Reefs influence the flow of water around them and have created and protect many islands surrounding them.
4. Coral reefs support a booming tourist economy on the coasts they border.
5. Reefs can provide a number of medicinal compounds.
6. Reefs serve as atmospheric carbon dioxide sinks.
7. Coral reefs act as historical climate recorders.

II. Bleaching

A. Introduction
1. Results when coral is subjected to stressor(s).
2. Lack of coloration is apparent because zooxanthallae populations decline.
or when photosynthetic pigments within the zooxanthallae decline.
3. It is hypothesized that bleaching is an adaptive process in which corals can repopulate with different, possibly more resilient, types of zooxanthallae.

B. Ecological Causes--One or a combination of stressors may induce coral bleaching.
1. Temperature--Corals live within a relatively narrow temperature margin. a. A small positive anomaly of 1-2 degrees C for 5-10 weeks in the summer
season will induce bleaching.
b. A drop in temperature accompanying upwelling episodes (-3 to -5 degrees C from 5-10 days) will induce bleaching.
2. Solar Irradiance--Both photosynthetically active radiation (PAR, 400-700nm) and ultraviolet radiation (UVR, 280-400 nm) have been implicated in bleaching.
3. Subaerial Exposure--exposure of reef flats to the atmosphere during low tides, El Nino Southern Oscillation (ENSO) related sea level drops, or tectonic uplift can potentially induce bleaching.
4. Sedimentation--Sedimentation due to logging, farming, mining, dredging, and other coastal activities blocks sunlight, reducing the photosynthetic ability of zooxanthallae. Relatively few instances have been linked to sedimentation alone.
5. Fresh Water Dilution--Rapid dilution of reef waters from precipitation and run-off causes coral bleaching events. Such events are generally confined to
nearshore areas.
6. Inorganic Nutrients--Ammonia and nitrate associated with agricultural run-off overfertilize the zooxanthallae and increase their densities by 2-3 times, known as eutrophication. The secondary effects of eutrophication such as lowering of coral resistance and greater susceptibility to disease may lead to bleaching events.
7. Xenobiotics--High concentrations of chemical contaminants such as copper, oil, and herbicides can cause the loss of zooxanthallae. Since concentrations must be high to cause bleaching, such events are generally localized.
8. Epizootics--These are the pathogens (diseases) that cause coral bleaching.

III. Global Bleaching Events

A. Discovery
1. In 1987, severe bleaching of corals in Puerto Rico led researchers to
investigate global patterns of bleaching.
2. The frequency and severity of bleaching events has increased dramatically
since the 1970's.
3. Nearly all of the world's major coral reef regions have experienced some degree of bleaching since the 1980's.
4. The coral bleaching events of the 1980's and 90's have occurred over all
geographic regions and depths.

B. Global Warming Debate
1. Evidence--Only 2 of the many stressors have possible global factors driving
changes and extremes: temperature and solar irradiance.
a. Increase in global temperatures and ENSO events change sea water
b. Ozone depletion increases the amount of UVR reaching the earth's surface.
c. The most severe bleaching events have coincided with high sea water
d. Bleaching has been reported during periods of low wind velocity, clear
skies, calm seas, and low turbidity when conditions favor localized heating
and high UVR penetration.
2. Uncertainties
a. Coincidence of high temperatures and bleaching is merely circumstantial.
b. Many unknown forces operate on these ecosystems.
c. No long-term records of seawater temperatures.
d. Mechanism of bleaching not fully understood.

C. Consequences
1. If a global warming trend increases sea temperatures, extirpation and
extinction of coral species could exceed 95% on a regional level.
2. Sea temperature rise would likely be accompanied by sea level rise. Sea level
rise may reduce light levels to potentially lethal levels.

IV. How do we deal with coral bleaching?

A. Long-term Monitoring
1. coral surveys
2. water testing for salinity, pH, depth, and temperature
3. recording climate data

B. Regional Management
1. Management techniques should bee designed to protect reefs from the many
anthropogenic threats to reefs. Bleaching is only one threat to the health of
coral reefs. Policies should manage the logging, mining, farming, dredging, and
tourist activities that harm reefs. They should also incorporate educational
programs to reduce direct human impacts to reefs.


Brown, B. and J. Ogden. 1993, January. "Coral bleaching." Scientific American. pp. 64-70.

Bunkley-Williams, L. and E.H. Williams, Jr. 1990, April. "Global assault on coral reefs." Natural History. pp. 47-54.

Center for the Study of Carbon dioxide and Global Change. 1999. The relationship between solar irradiance and coral reef bleaching events.

Davidson, O. G. 1998. The Enchanted Braid. John Wiley & Sons: New York. pp.187-193.

Hierta, E. 1994, Nov/Dec. "Rescuing the reef." National Parks. pp. 32-37.

Karlo, T.R. 1993, November 30. "Widespread coral reef bleaching as an indicator of global atmospheric warming." Environmental Earth Science.

Marine Biology Learning Center. 1999. Coral reef bleaching.

Roberts, L. 1990, Oct 12. "Warm waters, bleached corals." Science. pp. 213.

Tangley, L. 1991, April. "White Plague" Earthwatch. pp. 25-31.

Weber, P. 1993, May. "Coral reefs face threat of extinction." USA Today Magazine. pp. 62-65.

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