Coral skeletal δ15N records provide a historical perspective on coastal coral reefs eutrophication

Abstract

Human-driven eutrophication is thought to be a major threat to coastal coral reefs worldwide. Yet, we only have a limited understanding of the causes and the consequences of this problem, preventing the application of adequate mitigation policies. Indeed, we lack crucial environmental baselines to identify the sources of nutrients and understand how these nutrients might have changed the coral reef ecosystem over time. The natural abundance of nitrogen isotopes in coral skeleton-bound organic matter (CS - δ15N) may be a promising tool to identify environmental nitrogen (N) sources and to track their changes over time. Until now, such studies have been prevented by the lack of adequate methodology, mainly due to the low N content in coral skeleton. A recent study provided a sensitive and precise method for analyzing CS- δ15N (Wang et al., 2015), paving the way for a new field of investigation. To further explore the ability of the CS - δ15N to track human-driven eutrophication we have generated two annually-resolved CS - δ15N records from cores collected in historically eutrophic places: Guam USA, (56 years) and Hong Kong SAR (150 years). These two cases revealed that the CS - δ15N recorded significant changes in the N sources on decadal to centennial scales, in response to increasing human activities. The two records showed that sewage-derived N has been present in the environment for more than 50 years. Although these records come from hand-picked locations, they suggest that some coastal reefs worldwide could have been under eutrophication stress for half a century.