Factors influencing the stable carbon and oxygen isotopic composition of Porites lutea coral skeletons from Phuket, South Thailand

We determined the delta super(18)O and delta super(13)C composition of the same fixed growth increment in several Porites lutea coral skeletons from Phuket, South Thailand. Skeletal growth rate and delta super(18)O are inversely related. We explain this in terms of McConnaughey's kinetic isotopic disequilibria model. Annual trends in delta super(18)O cannot be solely explained by observed variations in seawater temperature or salinity and may also reflect seasonal variations in calcification rate. Coral tissue chlorophyll a content and delta super(13)C of the underlying 1 mm of skeleton are positively related, suggesting that algal modification of the dissolved inorganic carbonate pool is the main control on skeletal delta super(13)C. However, in corals that bleached during a period of exceptionally high seawater temperatures in the summer of 1991, delta super(13)C of the outer 1 mm of skeleton and skeletal growth rate (over 9 months up to and including the bleaching event) are inversely related. Seasonal variations in delta super(13)C may reflect variations in calcification rate, zooxanthellae photosynthesis or in seawater delta super(13)C composition. Bleached corals had reduced calcification over the 9-month period up to and including the bleaching event and over the event they deposited carbonate enriched in super(13)C and super(18)O compared with unaffected corals. However, calcification during the event was limited and insufficient material was deposited to influence significantly the isotopic signature of the larger seasonal profile samples. In profile, overall decreases in delta super(18)O and delta super(13)C were observed, supporting evidence that positive temperature anomalies caused the bleaching event and reflecting the loss of zooxanthellae photosynthesis.