Resource Partitioning by Reef Corals as Determined from Stable Isotope Composition II. 15N of Zooxanthellae and Animal Tissue versus Depth

The pattern of resource partitioning versus depth for corals collected in February, 1983, from Jamaica was investigated by analyzing their stable nitrogen isotope composition. Observations were made on isolated zooxanthellae and corresponding algae-free animal tissue from nine species of symbiotic corals at four depths over a 50-m bathymetric range, and from a nonsymbiotic coral at 1 m. 5 15N values versus depth ranged from +3.54 to -2.15 %0 for zooxanthellae and from +4.71 to +0.23 %0 for animal tissue. In those species that occurred over a 30- to 50-m depth range, both animal tissue and zooxanthellae tended to be depleted in 15N as depth increased to 30 m. In a few species animal tissue was enriched in 15N from 30 to 50 m. Depletion of 15N in zooxanthellae with increasing depth may be the result of depth-dependent differences in their nitrogen-specific growth rates. Animal tissue was consistently more depleted in 15N than for the nonsymbiotic coral Tubastrea coccinea (Ellis) at the same depth, but it was still slightly more enriched in 15N than corresponding zooxanthellae in 16 of 25 paired samples. The latter trend was not correlated with depth. A comparison of 5 13C and 5 15 N for zooxanthellae and animal tissue over 50 m revealed a tendency toward depletion of heavy isotopes as depth increases. Increased carbon fixation appears to be accompanied by decreased nitrogen fractionation