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We used tissue δ 15 N and δ 13 C to elucidate nitrogen and carbon utilization in 3 branching coral species of the genus Madracis. Coral branches were sampled between 5 and 47 m water depth, and tissue was sub-sampled from the tips and sides of branches. Photosynthetically active radiation (PAR) ranged between 1 and 71% of surface PAR. There was a good correlation between δ 15 N and δ 13 C, and these had similar ranges of 5.6 and 5.2‰, respectively, suggesting that light-driven photo- synthesis is a strong factor in isotope fractionation. However, at shallow depths, δ 13 C decreased more strongly while δ 15 N remained constant, while at deeper depths δ 15 N decreased while δ 13 C remained constant. Comparisons of δ 13 C in polyps and zooxanthellae revealed that prey capture at the tips of branches was more pronounced than at the corresponding sideward-facing positions and that corals at deeper depths did not increase their food uptake to compensate for a decrease in photosynthetic carbon assimilation. This indicates that foraging in branching Madracis depends on current regimes and prey encounter rather than selective food capture. The systematic analysis of tissue δ 15 N along a steep depth gradient revealed that the shallow-water coral M. auretenra was nitrogen limited throughout its entire depth range.

Stable isotopes reveal limitations in C and N assimilation in the Caribbean reef corals Madracis auretenra, M. carmabi and M. formosa