Coral Skeleton δ15N as a Proxy for Historic Nutrient Loading to a Coral Reef Ecosystem

Nitrogen (N) loading to the coastal ocean represents a major component of recent environmental change in many locations around the globe. Understanding the processes governing these changes and their associated ecological impacts requires a detailed record of changes in nutrient sources though time. Stable N isotopes (δ¹⁵N) have long been shown to be a powerful tool for evaluating N sources and cycling in the environment. Recent work has demonstrated that organic matter in calcifying coral skeletons records the N source utilized and can be preserved over long time scales, making the δ¹⁵N signature of coral intracrystalline organic matter a useful proxy for evaluating changes in N sources to coral reef ecosystems over time. Here we report a high-resolution record of coral skeleton δ¹⁵N measured in a suite of coral cores collected at Kahekili Beach Park in Maui, HI, a site of severe reef degradation and eutrophication associated with anthropogenic nutrient inputs. Results from isolating the intracrystalline organic matrix from the cores and analyzing for δ¹⁵N variability using a novel, high-sensitivity elemental analyzer-isotope ratio mass spectrometry system will be presented as a new, high-resolution proxy of nutrient loading to this important and threatened ecosystem.