CROSS-CONTINENTAL SHELF TRENDS IN DELTA-C-13 IN CORAL ON THE GREAT-BARRIER-REEF

We studied scleractinian corals from the central region of the Great Barrier Reef, Australia, to determine the degree to which they utilize terrigenous carbon as an ultimate food source. DeltaC-13 was analyzed in both the tissue and zooxanthellae of Porites lobata and Acropora formosa. Both tissue and zooxanthellae of P. lobata have deltaC-13 values which increase linearly with distance from shore from -16 to -11 parts per thousand. A similar relationship was found for tissue and zooxanthellae from A. formosa, although the variance was higher. Most of the variance observed (72 to 76 %) was explained by cross-shelf differences. The correlation between values for tissue and zooxanthellae in both species was highly significant and strongly linear, e.g. 0.926 in P. lobata. The slopes of all relationships observed were found to be not significantly different for the 2 species, but the deltaC-13 values for A. formosa were consistently less than for P lobata, by 1 to 2 parts per thousand. When coral tissue and zooxanthellae were analyzed as homogenates together, the same general cross-shelf trend was found, although the variance was higher, indicating that a crude extract may still be used to indicate general trends. This study implies that inshore corals derive much of their nutrients from terrigenous sources, and that a terrigenous influence on diet is measurable out to the edge of the continental shelf, ca 110 km offshore. Previous data derived from POC (particulate organic carbon) in sediments have implied that the limit of the terrigenous influence was 10 to 20 km. Judging from differences between the 2 species examined, P lobata is less dependent upon autotrophy and more dependent on exogenous carbon sources than A. formosa.