Skeletal isotopic record of a Porites coral during the 1998 mass bleaching event

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 the region. This colony, growing in a channel of fringing reef in Ishigaki Island, has experienced complete bleaching, as evidenced from direct observations of discoloration of the colony. Growth increment between the summer of 1997 and winter of 1997-98 is about 7 mm long while the corresponding increment 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 recognized 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 toward negative during bleaching also suggests the decrease of photosynthetic activity of symbiotic algae. Presumably, high temperature during the summer of 1998 could have led to massive decrease in carbonate production by coral reefs around the Ryukyu Islands through a large-scale coral bleaching event.