The Origin of Variations in the Isotopic Record of Scleractinian Corals: II. Carbon

This study examines the relationship between the δ13C of the skeleton of a zooxanthellate coral (Montastraea annularis) growing on the Florida Reef Tract and environmental variables (insolation and temperature), physiological variables (growth rate, respiration, calcification, and photosynthesis). Colonies of this species were grown in the field for a 2.5 year study period, during which the rates of photosynthesis, respiration, and calcification were measured on fifteen separate occasions, spaced approximately equally throughout the study period. The corals were stained with alizarin-red S within seven days after each set of physiological measurements. At the end of the period the corals were sacrificed and their skeletal extension, density, and skeletal δ13C determined. Despite substantial high-frequency variations, a strong seasonal cycle was evident in the skeletal δ13C records of all the corals throughout the experimental period. The skeletal δ13C and δ18O values varied approximately in phase, and showed a weak, but statistically significant positive relationship with each other. The δ13C of the coral skeletons, when corrected for changes in the δ13C of dissolved inorganic carbon (DIC), exhibited an inverse correlation with P/R, a finding opposite to what was expected based on current models of isotopic fractionation in coral skeletons. Although such findings tend to support the model of Erez (1978) that increases in photosynthesis act to isotopically deplete the δ13C of the coral skeleton, we note that the inverse association between δ13C and P/R arises because of a slight positive association between δ13C and respiration. We therefore believe that the association may be a result of seasonal variation in some parameters of the system which was not constrained in our study. Alternatives include (1) variations in the δ13C of the DIC which are translated into the δ13C of the food chain, (2) changes from heterotrophy to autotrophy, and (3) changes in the partitioning of δ13C between the zooxanthellae and the coral tissue. Based on previous studies which we have carried out we believe that changes in the skeletal δ13C are not related to sexual reproduction or growth rate. Contrary to previous work we were unable to measure any significant differences in the skeletal δ13C between the fast growing tops of the coral and the slower growing sides