Nitrogen Isotopic Records of Terrestrial Pollution Encoded in Floridian and Bahamian Gorgonian Corals

摘要

Stable nitrogen isotope (δ~(15)N) analysis has proven an effective "fingerprint" of sewage contamination in coral reef environments; however, short-term variability in nitrogen cycling and isotopic fractionation may obscure long-term trends. Here, we examine δ~(15)N signatures in the organic endoskeletons of long-lived (20-40 years) gorgonian corals. Specimens were collected from relatively pristine reefs off Green Turtle Cay, Bahamas, and from reefs off southeast Florida heavily impacted by multiple sources of anthropogenic nitrogen. The δ~(15)N of the most recently grown skeleton (branch tips) ranged from +2 to +3 ‰ at Green Turtle Cay, and +4.5 to +10 ‰ off Florida. These values closely match the δ~(15)N of macroalgae collected from the same locations, indicating that gorgonian corals are isotopically similar to primary producers, and therefore suitable for assessing sources of dissolved inorganic nitrogen. Differences in the δ~(15)N between younger and older skeleton indicated an overall decline of -0.34 ± 0.06 ‰ (1 s.e) over the last 20-40 years at Green Turtle Cay, reflecting a possible increase in nitrogen fixation and/or atmospheric deposition of anthropogenic nitrogen. Off southeast Florida, there was an overall increase in δ~(15)N over the same time period, reflecting increasing wastewater discharges from the rapidly growing population. These results highlight the usefulness of δ~(15)N recorded in gorgonians and other long-lived organisms in assessing spatiotemporal patterns of nitrogen sources to coastal marine environments.