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

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 δ 15N 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. © Inter-Research 2010, www.int-res.com.