Skeletal isotopic record of a Porites coral during the 1998 mass bleachingevent

Based on skeletal oxygen and carbon isotope analyses, we found a remarkable decrease in skeletal linear extension rate of a Porites colony during the bleaching event of 1998 around the Ryukyu Islands. Water temperature in the summer of 1998 was recorded as the highest for at least last 50 years in t he region. This colony, growing in a channel of fringing reef in Ishigaki I sland, has experienced complete bleaching, as evidenced from direct observa tions of discoloration of the colony. Growth increment between the summer o f 1997 and winter of 1997-98 is about 7 mm long while the corresponding inc rement between the summer of 1998 and winter of 1998-99 is only 2 mm long. Since the surface colonization by algae and excavating sponges cannot be re cognized by visual inspection of X-radiographs, the slow growth might have been caused by the temporal perturbations in coral-algal symbiotic system, which inhibits calcification. The shift of skeletal carbon isotope ratios t oward negative during bleaching also suggests the decrease of photosyntheti c activity of symbiotic algae. Presumably, high temperature during the summ er of 1998 could have led to massive decrease in carbonate production by co ral reefs around the Ryukyu Islands through a large-scale coral bleaching e vent.