The origin of variations in the isotopic record of scleractinian corals .2. Carbon

This study examines the relationship between the delta(13)C 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 21/2 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 delta(13)C determined. Despite substantial high-frequency variations, a strong seasonal cycle was evident in the skeletal delta(13)C records of all the corals throughout the experimental period. The skeletal delta(13)C and delta(18)O values varied approximately in phase, and showed a weak, but statistically significant positive relationship with each other. The delta(13)C Of the coral skeletons, when corrected for changes in the delta(13)C of dissolved inorganic carbon (DIG), 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 fret (1978) that increases in photosynthesis act to isotopically deplete the delta(13)C of the coral skeleton, we note that the inverse association between delta(13)C and P/R arises because of a slight positive association between delta(13)C 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 delta(13)C of the DIC which are translated into the delta(13)C Of the food chain, (2) changes from heterotrophy to autotrophy, and (3) changes in the partitioning of delta(13)C between the zooxanthellae and the coral tissue. Based on previous studies which we have carried out we believe that changes in the skeletal delta(13)C are not related to sexual reproduction or growth rate. Contrary to previous work we were unable to measure any significant differences in the skeletal delta(13)C between the fast growing tops of the coral and the slower growing sides.