MEPS 412:103-112 (2010)  -  DOI: https://doi.org/10.3354/meps08674

ABSTRACT: We used tissue δ¹⁵N and δ¹³C 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 δ¹⁵N and δ¹³C, and these had similar ranges of 5.6 and 5.2‰, respectively, suggesting that light-driven photosynthesis is a strong factor in isotope fractionation. However, at shallow depths, δ¹³C decreased more strongly while δ¹⁵N remained constant, while at deeper depths δ¹⁵N decreased while δ¹³C remained constant. Comparisons of δ¹³C 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 δ¹⁵N along a steep depth gradient revealed that the shallow-water coral M. auretenra was nitrogen limited throughout its entire depth range.

KEY WORDS: Stable isotopes · Nitrogen limitation · Carbon · Heterotrophy · Scleractinian coral