RELATIONSHIP BETWEEN LIGHT AND THE DELTA-N-15 OF CORAL TISSUE - EXAMPLES FROM JAMAICA AND ZANZIBAR

Nitrogen isotope values from coral tissue collected over depth-light g radients are reported from Jamaica and Zanzibar. The Jamaica suite con sists of multiple specimens of three coral species (Montastrea annular is, Porites astreoides, and Agaricia agaricites) sampled at increasing depths. For each species, combined tissue/zooxanthellae delta(15)N de creases significantly with decreasing availability of photosynthetical ly active radiation. The Zanzibar sample suite was collected from thre e coral colonies (all Porites lobata). Multiple samples, occupying dif ferent depths and Light regimes, were collected from each coral corall um. The Zanzibar suite shows a significant decrease in delta(15)N over the relatively small depth range represented by each coral colony. To gether, these two sample suites suggest that light is an influence on the nitrogen isotopic composition in corals containing symbiotic zooxa nthellae. We propose that in high-irradiance conditions, uptake and as similation of dissolved inorganic nitrogen (DIN) by symbiotic algae an d(or) the host strongly depletes the coral internal DIN pool, leading to reduced fractionation relative to reef DIN. In lower light conditio ns, less dissolved nitrogen is assimilated and fractionation increases . The autotrophic portion of the diet is thought to be sufficiently ab undant in nitrogen and isotopically depleted to result in lower delta( 15)N of host tissues under lower Light conditions.