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Solar Radiation As a Cause of Coral Bleaching - Summary
The link between solar radiation and coral reef bleaching goes back nearly a century to when MacMunn (1903) postulated that ultraviolet radiation could be potentially damaging to corals.  It wasn't until half a century later, however, that scientists began to confirm this suspicion via laboratory and field studies (Catala-Stucki, 1959; Siebeck, 1988; Gleason and Wellington, 1995).

Many investigators of the solar irradiance-coral bleaching link have studied the phenomenon in the field by transplanting reef corals from deep to shallow waters.  Gleason and Wellington (1993), for example, transplanted samples of the reef-building Montastrea annularis from a depth of 24 meters to depths of 18 and 12 meters.  Using sheets of acrylic plastic to block out ultraviolet radiation on some of the coral samples, they found that the shielded corals experienced less bleaching than the unshielded corals, and that the unshielded corals at the 12-meter depth had significantly lower amounts of zooxanthellae and chlorophyll per square centimeter than all other treatment and control groups.  Likewise, Hoegh-Guldberg and Smith (1989) reported bleaching in the corals Stylophora pistillata and Seriatopora hystrix when they were moved from a depth of 6 meters to 1.2 meters; and Vareschi and Fricke (1986) obtained similar results when moving Plerogyra sinuosa from a depth of 25 meters to 5 meters.  On the other hand, Glynn (1996) notes that artificially reduced light levels have also been observed to cause coral bleaching.

Laboratory studies have provided additional evidence for a link between intense solar irradiance and coral reef bleaching; but identifying a specific wavelength or range of wavelengths as the cause of the phenomenon has been a difficult task.  Fitt and Warner (1995), for example, reported that the most significant decline in symbiont photosynthesis in Montastrea annularis occurred when it was exposed to ultraviolet and blue light; but other studies have reported coral bleaching to be most severe at shorter ultraviolet wavelengths (Droller et al., 1994; Gleason and Wellington, 1995), while still others have found it to be most strongly expressed at longer photosynthetically-active wavelengths (Lesser and Shick, 1989; Lesser et al., 1990; Brown et al., 1994a).

As additional studies continue to provide evidence for a solar-induced mechanism of coral reef bleaching (Brown et al., 1994b; Williams et al., 1997; Lyons et al., 1998), some are providing evidence for a solar radiation-temperature 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 claims - in one of the strongest attempts yet made to portray global warming as the cause of bleaching in corals - that "coral bleaching occurs when the photosynthetic symbionts of corals (zooxanthellae) become increasingly vulnerable to damage by light at higher than normal temperatures."  However, as indicated in our summaries of other causes of coral bleaching, the story is considerably more complicated, more so, in fact, than what almost everyone has assumed.

Brown, B.E., Le Tissier, M.D.A. and Dunne, R.P.  1994a.  Tissue retraction in the scleractinian coral Coeloseris mayeri, its effect upon coral pigmentation, and preliminary implications for heat balance.  Marine Ecology Progress Series 105: 209-218.

Brown, B.E., Dunne, R.P., Scoffin, T.P. and Le Tissier, M.D.A.  1994b.  Solar damage in intertidal corals.  Marine Ecology Progress Series 105: 219-230.

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.

Catala-Stucki, R.  1959.  Fluorescence effects from corals irradiated with ultra-violet rays.  Nature 183: 949.

Droller, J.H., Faucon, M., Maritorena, S. and Martin, P.M.V.  1994.  A survey of environmental physico-chemical parameters during a minor coral mass bleaching event in Tahiti in 1993.  Australian Journal of Marine and Freshwater Research 45: 1149-1156.

Fitt, W.K. and Warner, M.E.  1995.  Bleaching patterns of four species of Caribbean reef corals.  Biological Bulletin 189: 298-307.

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

Gleason, D.F. and Wellington, G.M.  1995.  Variation in UVB sensitivity of planula larvae of the coral Agaricia agaricites along a depth gradient.  Marine Biology 123: 693-703.

Glynn, P.W.  1996.  Coral reef bleaching: facts, hypotheses and implications.  Global Change Biology 2: 495-509.

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.

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.

Lesser, M.P. and Shick, J.M.  1989.  Effects of irradiance and ultraviolet radiation on photoadaptation in the zooxanthellae of Aiptasia pallida: Primary production, photoinhibition, and enzymatic defense against oxygen toxicity.  Marine Biology 102: 243-255.

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.

Lyons, M.M., Aas, P., Pakulski, J.D., Van Waasbergen, L., Miller, R.V., Mitchell, D.L. and Jeffrey, W.H.  1998.  DNA damage induced by ultraviolet radiation in coral-reef microbial communities.  Marine Biology 130: 537-543.

MacMunn, C.A.  1903.  On the pigments of certain corals, with a note on the pigment of an asteroid. In: The fauna and geography of the Maldive and Laccadive Archipelagoes.  Gardiner, J.S., ed.  Cambridge, UK: Cambridge University Press, Vol. 1, pp. 184-190.

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.

Siebek, O.  1988.  Experimental investigation of UV tolerance in hermatypic corals (Scleractinian).  Marine Ecology Progress Series 43: 95-103.

Vareschi, E. and Fricke, H.  1986.  Light responses of a scleractinian coral (Plerogyra sinuosa).  Marine Biology 90: 395-402.

Williams, D.E., Hallock, P., Talge, H.K., Harney, J.N. and McRae, G.  1997.  Responses of Amphistegina gibbosa populations in the Florida Keys (U.S.A.) to a multi-year stress event (1991-1996).  Journal of Foraminiferal Research 27: 264-269.