Resource partitioning by reef corals as determined from stable isotope composition

The pattern of resource partitioning vs depth by corals collected in February 1983 from Jamaica and the Red Sea was determined from their stable carbon isotope composition. Observations were made on isolated zooxanthellae and corresponding algae-free animal tissue from eight species at four depths over a 50 m bathymetric range. Zooxanthellae 6~3C was high in shallow water and became lower as depth increased. This trend correlated significantly with the annual integrated photosynthetic rate. The trend is interpreted according to a "depletion-diffusion" hypothesis; in shallow water, at high rates of photosynthesis, metabolic CO 2 is nearly depleted and the supply of CO 2 from seawater bicarbonate is limited by diffusion. Since most of the available CO 2 is fixed, isotope fractionation is minimal. In deeper water, at lower rates of photosynthesis, metabolic CO2 is ample, and isotope fractionation is greater. Animal tissue 513C was slightly lower than corresponding zooxanthellae values in shallow water. As depth increased the difference between zooxanthellae and animal tissue 513 C increased and the latter approached the 513C of oceanic particulate organic carbon. These data suggest that carbon is translocated at all depths and that deep-water corals draw significantly on allocthonous sources of carbon.