FACTORS INFLUENCING THE STABLE CARBON AND OXYGEN ISOTOPIC COMPOSITIONOF PORITES LUTEA CORAL SKELETONS FROM PHUKET, SOUTH THAILAND

We determined the delta(18)O and delta(13)C composition of the same fi xed growth increment in several Porites lutea coral skeletons from Phu ket, South Thailand. Skeletal growth rate and delta(18)O are inversely related. We explain this in terms of McConnaughey's kinetic isotopic disequilibria model. Annual trends in delta(18)O cannot be solely expl ained by observed variations in seawater temperature or salinity and m ay also reflect seasonal variations in calcification rate. Coral tissu e chlorophyll a content and delta(13)C of the underlying 1 mm of skele ton are positively related, suggesting that algal modification of the dissolved inorganic carbonate pool is the main control on skeletal del ta(13)C. However, in corals that bleached during a period of exception ally high seawater temperatures in the summer of 1991, delta(13)C of t he outer 1 mm of skeleton and skeletal growth rate (over 9 months up t o and including the bleaching event) are inversely related. Seasonal v ariations in delta(13)C may reflect variations in calcification rate, zooxanthellae photosynthesis or in seawater delta(13)C composition. Bl eached corals had reduced calcification over the 9-month period up to and including the bleaching event and over the event they deposited ca rbonate enriched in C-13 and O-18 compared with unaffected corals. How ever, calcification during the event was limited and insufficient mate rial was deposited to influence significantly the isotopic signature o f the larger seasonal profile samples. In profile, overall decreases i n delta(18)O and delta(13)C were observed, supporting evidence that po sitive temperature anomalies caused the bleaching event and reflecting the loss of zooxanthellae photosynthesis.