What was done
The authors of this review article analyze the evidence for attributing bleaching in corals to a number of environmental stresses.

What was learned
The authors' review of the literature led them to conclude that coral bleaching "appears to be a generalized response caused by many different stressors or combination of stressors," including "temperature fluctuations, increased solar radiation, salinity fluctuations, increased sedimentation, and exposure to toxicants."

Concerning temperature fluctuations, they indicate that "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."  As for solar radiation, it was suggested that different coral species have different levels of resistance to ultraviolet light, and that prolonged exposure - primarily due to an increase in water clarity - may induce coral bleaching in corals that "cannot increase concentrations of UV-blocking pigments quickly enough to adapt to such sudden changes."

Both low and high levels of salinity were suggested as causes of coral bleaching, with low salinity levels generally originating from sea water dilution caused by high precipitation events or storm runoff, and high salinity levels produced by the operation of desalination plants.

What it means
This study demonstrates that many factors may act separately or in concert to induce coral bleaching.  The actual mechanisms by which environmental stresses cause coral bleaching, however, remain "poorly understood."