ULTRAVIOLET-RADIATION AND CORAL BLEACHING

EPISODES of coral bleaching resulting from dissociation of endosymbiot ic algae (zooxanthellae) from host coral tissues have occurred with in creasing frequency over the past decade on reefs throughout the tropic s1,2. These episodes have usually been attributed to increases in seaw ater temperatures3-10, but the mass bleaching events that occurred thr oughout the Caribbean during 1987 and 1990 were not readily explained by temperature alone11,12. An additional factor that may have contribu ted to these bleaching episodes is ultraviolet radiation in the 280-40 0-nm band. At many localities where bleaching occurred in 1987 and 199 0, sea conditions were described as extremely calm with exceptionally clear water13. In the absence of suspended organic and inorganic matte r in the water column, higher than average intensities of ultraviolet radiation probably reached all depths within the photic zone for sever al consecutive months. Evidence for a possible link between ultraviole t radiation and coral bleaching has not been forthcoming2. Here we rep ort results of a field experiment showing that, irrespective of high w ater temperatures, short-term (three weeks) increases in ultraviolet r adiation of a magnitude possible under calm, clear water column condit ions can readily induce bleaching in reef-building corals.