Relationship between light and the δ15N of coral tissue: Examples from Jamaica and Zanzibar

Nitrogen isotope values from coral tissue collected over depth–light gradients are reported from Jamaica and Zanzibar. The Jamaica suite consists of multiple specimens of three coral species (Montastrea annularis, Porites astreoides, and Agaricia agaricites) sampled at increasing depths. For each species, combined tissue/zooxanthellae δ15N decreases significantly with decreasing availability of photosynthetically active radiation. The Zanzibar sample suite was collected from three coral colonies (all Porites lobata). Multiple samples, occupying different depths and light regimes, were collected from each coral corallum. The Zanzibar suite shows a significant decrease in δ15N over the relatively small depth range represented by each coral colony. Together, these two sample suites suggest that light is an influence on the nitrogen isotopic composition in corals containing symbiotic zooxanthellae. We propose that in high‐irradiance conditions, uptake and assimilation of dissolved inorganic nitrogen (DIN) by symbiotic algae and(or) the host strongly depletes the coral internal DIN pool, leading to reduced fractionation relative to reef DIN. In lower light conditions, less dissolved nitrogen is assimilated and fractionation increases. The autotrophic portion of the diet is thought to be sufficiently abundant in nitrogen and isotopically depleted to result in lower δ15N of host tissues under lower light conditions.