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Temperature As a Cause of Coral Bleaching - Summary
One of the most frequently cited causes of coral bleaching is anomalously high water temperature (Linden, 1998).  The origin of this attribution can be traced to the strong El Niņo event of 1982-83, in which widespread bleaching was reported in corals exposed to unusually high surface water temperatures (Glynn, 1988).  Since that time, a number of other such observations have been made (Cook et al., 1990; Glynn 1991; Montgomery and Strong, 1994; Brown et al., 1996); and several laboratory studies have demonstrated that elevated seawater temperatures can indeed induce bleaching in corals (Hoegh-Guldberg and Smith, 1989; Jokiel and Coles, 1990; Glynn and D'Croz, 1990).  However, just as anomalously high seawater temperatures have been found to be correlated with coral reef bleaching events, so have anomalously low seawater temperatures been identified with this phenomenon (Walker et al., 1982; Coles and Fadlallah, 1990; Muscatine et al., 1991; Gates et al., 1992); and these observations suggest that the crucial link between temperature and coral reef bleaching may not reside in the absolute temperature of the water surrounding the corals, but in the rapidity with which the temperature either rises above or falls below the temperature regime to which the corals are normally adapted.

As an example of this phenomenon, Winter et al. (1998) studied relationships between coral bleaching and nine different temperature indices, concluding that although "prolonged heat stress may be an important precondition for bleaching to occur," sharp temperature changes act as the "immediate trigger."  In a related study, Jones (1997) reported coral bleaching on a portion of Australia's Great Barrier Reef just after average daily sea water temperature rose by 2.5°C over the brief period of eight-days; but Kobluk and Lysenko (1994) observed severe coral bleaching following an 18-hour decline of 3°C in seawater temperature.  And because the corals they studied had experienced massive bleaching two years earlier as a result of an anomalous 4°C increase in temperature, the authors concluded that coral bleaching is more a function of corals not being able to adapt to the rapidity of a temperature change than it is of the absolute magnitude or sign of the change, i.e., heating or cooling.

Further evidence that high or low seawater temperatures per se are not the critical factors in producing coral bleaching is provided by Podesta and Glynn (1997), who examined a number of temperature-related indices of surface waters in the vicinity of Panama over the period 1970-1994.  Their analysis revealed that for the two years of highest maximum monthly sea surface temperature, 1972 and 1983, coral bleaching was only reported in 1983, while 1972 produced no bleaching whatsoever, in spite of the fact that water temperatures that year were just as high as they were in 1983.

Additional studies provide evidence for a temperature-solar radiation stress synergism (Gleason and Wellington, 1993; Rowan et al., 1997; Jones et al., 1998).  There have been a number of situations, for example, in which corals underwent bleaching when changes in both of these parameters combined to produce particularly stressful conditions (Lesser et al., 1990; Glynn et al., 1992; Brown et al., 1995), such as during periods of low wind velocity and calm seas, which favor the intense heating of shallow waters and concurrent strong penetration of solar radiation.

This two-parameter interaction has much to recommend it as a primary cause of coral bleaching.  It is, in fact, the mechanism favored by Hoegh-Guldberg (1999), who notes that "coral bleaching occurs when the photosynthetic symbionts of corals (zooxanthellae) become increasingly vulnerable to damage by light at higher than normal temperatures."  Hence, as indicated by these several studies, as well as those reported in our summaries of other causes of coral bleaching, the story is considerably more complicated than that portrayed by the simple claim that massive coral bleaching is due to gradual global warming.

References
Brown, B.E., Le Tissier, M.D.A. and Bythell, J.C.  1995.  Mechanisms of bleaching deduced from histological studies of reef corals sampled during a natural bleaching event.  Marine Biology 122: 655-653.

Brown, B.E., Dunne, R.P. and Chansang, H.  1996.  Coral bleaching relative to elevated seawater temperature in the Andaman Sea (Indian Ocean) over the last 50 years.  Coral Reefs 15: 151-152.

Coles, S.L. and Fadlallah, Y.H.  1990.  Reef coral survival and mortality at low temperatures in the Arabian Gulf: New species-specific lower temperature limits.  Coral Reefs 9: 231-237.

Cook, C.B., Logan, A., Ward, J., Luckhurst, B., and Berg, C.J., Jr.  1990.  Elevated temperatures and bleaching on a high latitude coral reef: The 1988 Bermuda event.  Coral Reefs 9: 45-49.

Gates, R.D., Baghdasarian, G. and Muscatine, L.  1992.  Temperature stress causes host cell detachment in symbiotic cnidarians: Implication for coral bleaching.  Biological Bulletin 182: 324-332.

Gleason, D.F. and Wellington, G.M.  1993.  Ultraviolet radiation and coral bleaching.  Nature 365: 836-838.

Glynn, P.W.  1988.  El Niņo-Southern Oscillation 1982-83: Nearshore population, community, and ecosystem responses.  Annual Review of Ecology and Systematics 19: 309-345.

Glynn, P.W.  1991.  Coral bleaching in the 1980s and possible connections with global warming trends.  Ecology and Evolution 6: 175-179.

Glynn, P.W. and D'Croz, L.  1990.  Experimental evidence for high temperature stress as the cause of El-Niņo-coincident coral mortality.  Coral Reefs 8: 181-190.

Glynn, P.W., Imai, R., Sakai, K., Nakano, Y. and Yamazato, K.  1992.  Experimental responses of Okinawan (Ryukyu Islands, Japan) reef corals to high sea temperature and UV radiation.  Proceedings of the 7th International Coral Reef Symposium 1: 27-37.

Hoegh-Guldberg, O.  1999.  Climate change, coral bleaching and the future of the world's coral reefs.  Marine and Freshwater Research 50: 839-866.

Hoegh-Guldberg, O. and Smith, G.J.  1989.  The effect of sudden changes in temperature, light and salinity on the population density and export of zooxanthellae from the reef corals Stylophora pistillata Esper. and Seriatopora hystrix Dana.  Journal of Experimental Marine Biology and Ecology 129: 279-303.

Jokiel, P.L. and Coles, S.L.  1990.  Response of Hawaiian and other Indo-Pacific reef corals to elevated sea temperatures.  Coral Reefs 8: 155-162.

Jones, R.J.  1997.  Changes in zooxanthellar densities and chlorophyll concentrations in corals during and after a bleaching event.  Marine Ecology Progress Series 158: 51-59.

Jones, R.J., Hoegh-Guldberg, O., Larkum, A.W.D. and Schreiber, U.  1998.  Temperature-induced bleaching of corals begins with impairment of the CO2 fixation mechanism in zooxanthellae.  Plant, Cell and Environment 21: 1219-1230.

Kobluk, D.R. and Lysenko, M.A.  1994.  "Ring" bleaching in Southern Caribbean Agaricia agaricites during rapid water cooling.  Bulletin of Marine Science 54: 142-150.

Lesser, M.P., Stochaj, W.R., Tapley, D.W. and Shick, J.M.  1990.  Bleaching in coral reef anthozoans: Effects of irradiance, ultraviolet radiation, and temperature on the activities of protective enzymes against active oxygen.  Coral Reefs 8: 225-232.

Linden, O.  1998.  Coral mortality in the tropics: Massive causes and effects.  Ambio 27: 588.

Montgomery, R.S. and Strong, A.E.  1994.  Coral bleaching threatens oceans life.  EOS 75: 145-147.

Muscatine, L., Grossman, D. and Doino, J.  1991.  Release of symbiotic algae by tropical sea anemones and corals after cold shock.  Marine Ecology Progress Series 77: 233-243.

Podesta, G.P. and Glynn, P.W.  1997.  Sea surface temperature variability in Panama and Galapagos: Extreme temperatures causing coral bleaching.  Journal of Geophysical Research 102: 15,749-15,759.

Rowan, R., Knowlton, N., Baker, A. and Jara, J.  1997.  Landscape ecology of algal symbionts creates variation in episodes of coral bleaching.  Nature 388: 265-269.

Walker, N.D., Roberts, H.H., Rouse, L.J., Jr. and Huh, O.K.  1982.  Thermal history of reef-associated environments during a record cold-air outbreak event.  Coral Reefs 1: 83-87.

Winter, A., Appeldoorn, R.S., Bruckner, A., Williams, E.H., Jr. and Goenaga, C.  1998.  Sea surface temperatures and coral reef bleaching off La Parguera, Puerto Rico (northeast Caribbean Sea).  Coral Reefs 17: 377-382.