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 B arrier 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 fo rmosa. Both tissue and zooxanthellae of P. lobata have deltaC-13 value s which increase linearly with distance from shore from -16 to -11 par ts per thousand. A similar relationship was found for tissue and zooxa nthellae from A. formosa, although the variance was higher. Most of th e variance observed (72 to 76 %) was explained by cross-shelf differen ces. The correlation between values for tissue and zooxanthellae in bo th species was highly significant and strongly linear, e.g. 0.926 in P . lobata. The slopes of all relationships observed were found to be no t 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 par ts per thousand. When coral tissue and zooxanthellae were analyzed as homogenates together, the same general cross-shelf trend was found, al though the variance was higher, indicating that a crude extract may st ill be used to indicate general trends. This study implies that inshor e 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 li mit of the terrigenous influence was 10 to 20 km. Judging from differe nces between the 2 species examined, P lobata is less dependent upon a utotrophy and more dependent on exogenous carbon sources than A. formo sa.