Resource Partitioning by Reef Corals as Determined from Stable Isotope Composition II. ~l5N 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 N 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 30to 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 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. RESOURCE UTILIZATION by scleractinian reef corals is profoundly affected by endosymbiotic dinoflagellates (zooxanthellae). Although coral polyps feed on particulate organic carbon and nitrogen (Lewis and Price 1975, Lewis 1976, 1977, Clayton and Lasker 1982), their phototrophic endosymbionts take up and assimilate inorganic carbon (Muscatine and Cernichiari 1969, Schmitz and Kremer 1977, Crossland et al. 1980, Black and Burris 1983) and nitrogen (Franzisket 1974, Crossland and Barnes 1977, D'Elia and Webb 1977, Muscatine and D'Elia 1978, Webb and Wiebe 1978, Muscatine et al. 1979, Muscatine 1980a, Burris 1983, D'Elia et al. 1983, I Research was supported by grants from the National Science Foundation (OCE-85105l8) to L.M. and the Department of Energy (EY-763-03-0034) to I.R.K. Manuscript accepted 15 August 1993. 2 Department of Biology, University of California, Los Angeles, California 90024. 3 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90024. Wafar et al. 1985, Summons et al. 1986, Anderson and Burris 1987, Rahav et al. 1989) from the environment and from host catabolism. Organic carbon and nitrogen is translocated from algae to host (Muscatine 1980b, Falkowski et al. 1984) and possibly from host to algae (e.g., see Cook 1983, Steen 1986). The symbiotic algae also enable the retention and recycling of carbon and nitrogen atoms within the coral. These features confer an apparent selective advantage on coral animals in oligotrophic environments. There is little information, however, on how depth and light attenuation might influence these potential fluxes and consequently the selective advantage of the symbiosis to the partners. In previous studies, Davies (1984), McCloskey and Muscatine (1984), and Muscatine et al. (1984) noted that photosynthetic rates by zooxanthellae in shallow-water corals are high and that carbon translocated from algae could meet the daily carbon demand of the animal for respiration and