Nitrogen isotope ratios across the Bermuda coral reef: implications for coral nitrogen sources and the coral-bound nitrogen isotope proxy

Abstract

The nitrogen (N) isotopic composition of coral tissue provides insight into N sources and cycling on reefs, and coral skeleton-bound organic matter (CS-δ15N) can extend these insights into the past. Across the Bermuda platform, we measured the δ15N of four coral species and their potential N sources, as well as an asymbiotic filter feeder as a comparative heterotroph and benthic macroalgae as a comparative autotroph. Organisms and organic N pools from the coral reefs exhibit a δ15N increase toward the Bermuda coast, likely due to anthropogenic N inputs. At all sites, the δ15N of bulk coral tissue is consistent with corals feeding dominantly on zooplankton-sized organic matter and some smaller suspended particulate N. The corals lack the trophic δ15N elevation that characterizes serpulids; this is consistent with internal recycling and retention of low-δ15N metabolic N by symbiont-bearing corals. The data are inconsistent with corals’ reliance on the dissolved inorganic N used by macroalgae at the same sites. Among coral species, two species with smaller polyps (1-2 mm) have ~1‰ lower bulk tissue δ15N than two counterparts with larger polyps (5-10 mm), perhaps due to differences in food source. Taxon-specific δ15N differences are also observed between coral tissue and skeleton-bound N, with larger differences in the two small-polyp species. In net, however, CS-δ15N mean values and spatial gradients were similar in the four species studied.