Stable Isotopic Signatures in Symbiotic Bermudian Corals: A study of nutrient and light variability on \(\delta\)\(^{15}\)N in \(\textit{Porites astreoides}\) at Bermuda

Abstract

N and C isotopes provide a non-invasive and integrative measure of modern biogeochemical cycling and can be used to study past environmental conditions. The principle objective of this study is to examine the effects that light level and nutrient level have on nutrient recycling within a symbiotic coral by analyzing \(\delta\)\(^{15}\)N and \(\delta\)\(^{13}\)C of the coral host tissues, the zooxanethellate symbionts, and the bulk mixtures (tissue and symbionts). A three-phase approach was taken in the field and culture studies. In Phase I, the \(\delta\)\(^{15}\)N of tissue, bulk, and symbionts with \(\delta\)\(^{15}\)N of macroalgae, plankton, and PON was measured at various locations on the Bermuda pedestal. The data suggest that symbiotic corals assimilate nutrients from both the environment and their symbiont pool. In Phase II, we analyzed the \(\delta\)\(^{15}\)N and \(\delta\)\(^{13}\)C of coral tissue and symbionts from incubations at three temperatures, with bleaching triggered at the highest temperature. However, an apparent failure to reach steady state compromised the interpretation of the results. In Phase III, we drew from the first two phases to analyze \(\delta\)\(^{15}\)N and \(\delta\)\(^{13}\)C data from juvenile corals that underwent four conditions in quadruplicate: (1) high light, high nutrient; (2) high light, low nutrient; (3) low light, low nutrient; (4) low light, high nutrient. Preliminary results suggest that corals in elevated nutrient conditions uptake more N from environmental sources than do corals living in lower nutrient conditions. Our results reveal the utility of stable isotopic signatures to study coral responses to eutrophication and highlight the importance of coral-symbiont N recycling efficiency.