Stable isotopes reveal limitations in C and N assimilation in the Caribbean reef corals Madracis auretenra, M. carmabi and M. formosa
Résumé
We used tissue delta N-15 and delta C-13 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 delta N-15 and delta C-13, and these had similar ranges of 5.6 and 5.2 parts per thousand, respectively, suggesting that light-driven photosynthesis is a strong factor in isotope fractionation. However, at shallow depths, delta C-13 decreased more strongly while delta N-15 remained constant, while at deeper depths delta N-15 decreased while delta C-13 remained constant. Comparisons of delta C-13 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 delta N-15 along a steep depth gradient revealed that the shallow-water coral M. auretenra was nitrogen limited throughout its entire depth range.
Domaines
Océanographie
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