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

Abstract

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.

Impact of the 1998 bleaching event on δ18O records of Okinawa corals

“…These calibrations were calculated using seasonal rather than monthly SSTs and δ 18 O c values. If seasonal averages are applied to our data, the calibration slope is -0.17 ‰/°C, in agreement with the Ishigaki calibrations (Abe et al 1998, Suzuki et al 1999.Models of δ 18 O fractionation in corals predict that as SST rises, δ18 O c will be depleted and the accreted skeleton will be isotopically lighter, and indeed, previous studies have reported depletion of δ 18 O c associated with extremely high SSTs that initiated bleaching events (Porter et al 1989, Leader et al 1991, Suzuki et al 2000. Surprisingly, however, the δ 18 O c record of colony S4 reveals enrichment of 0.3 ‰ between the summers of 1997 and 1998 rather than the expected depletion.…”

“…18 O c will be depleted and the accreted skeleton will be isotopically lighter, and indeed, previous studies have reported depletion of δ 18 O c associated with extremely high SSTs that initiated bleaching events (Porter et al 1989, Leader et al 1991, Suzuki et al 2000. Surprisingly, however, the δ 18 O c record of colony S4 reveals enrichment of 0.3 ‰ between the summers of 1997 and 1998 rather than the expected depletion.…”

“…These addressed reduced calcification and linear skeletal extension rate due to uncoupling of the coral algae symbiotic system (Jokiel & Coles 1977, Goreau & Macfarlane 1990) and δ 18 O c depletion due to abnormally high SST (Carriquiry et al 1988, Porter et al 1989, Leader et al 1991, Suzuki et al 2000.…”

“…Unprecedented summer SSTs were recorded around Okinawa Island, Japan. Temperature reached a maximum of 31°C, ~3°C higher than the long-term average of maximal summer temperatures, and it was the first bleaching event reported around the Ryukyus Islands (Suzuki et al 2000).…”

“…Unprecedented summer SSTs were recorded around Okinawa Island, Japan. Temperature reached a maximum of 31°C, ~3°C higher than the long-term average of maximal summer temperatures, and it was the first bleaching event reported around the Ryukyus Islands (Suzuki et al 2000).In this study, we measured a 10 yr oxygen isotope record of an Okinawa coral, and compared its skeletal isotopic composition against local SST data. To focus our investigation on the 1998 bleaching event, additional isotopic profiles were obtained in 5 neighboring colonies for the years 1997 to 1999.…”