What was done
As a result of field observations following a strong ENSO event in 1999 that caused corals to bleach in winter after an unusual spate of cold weather, the authors collected small branches of the scleractinian coral Montipora digitata from the southern reef flat at Heron Island, Australia, and established them in experimental aquaria, where they conducted a number of experiments designed to determine the nature of this somewhat surprising phenomenon.

What was learned
In the words of the authors, "responses to cold temperature stress of M. digitata appeared similar to that observed in corals exposed to warmer than normal temperatures, suggesting a common mechanism."  Specifically, they observed "decreased photosynthetic efficiency, loss of symbiotic dinoflagellates and changes in photosynthetic pigment concentrations."  Also, they found that "corals in higher light regimes were more susceptible to cold temperature stress," and that "severe cold stress resulted in photodamage, bleaching and increased mortality."

What it means
The take-home message of this study is well described by the authors of another study of cold-induced coral bleaching (Meehan and Ostrander, 1997): "in many cases the absolute water temperature (whether warm or cool) may not be the cause of a bleaching event; instead the rapidity of the temperature fluctuation in conjunction with a movement outside the normal temperature range to which corals are locally adapted may precipitate bleaching," which is nearly identical to the earlier conclusion of Kobluk and Lysenko (1994): "in many cases it may not be the absolute water temperature (whether warm or cool) in itself that precipitates a bleaching reaction, but rather the rapidity of temperature change in combination with the deviation from normal temperatures to which corals in a region are adapted."

References
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.

Meehan, W.J. and Ostrander, G.K.  1997.  Coral bleaching: a potential biomarker of environmental stress.  Journal of Toxicology and Environmental Health 50: 529-552.