Coral Skeleton \u03b415N as a Tracer of Historic Nutrient Loading to a Coral Reef in Maui, Hawaii

Joseph Murray,Adina Paytan,Sara Peek,Nancy G Prouty

doi:10.1038/s41598-019-42013-3

Abstract

Excess nutrient loading to nearshore environments has been linked to declining water quality and ecosystem health. Macro-algal blooms, eutrophication, and reduction in coral cover have been observed in West Maui, Hawaii, and linked to nutrient inputs from coastal submarine groundwater seeps. Here, we present a forty-year record of nitrogen isotopes (δ15N) of intra-crystalline coral skeletal organic matter in three coral cores collected at this site and evaluate the record in terms of changes in nitrogen sources. Our results show a dramatic increase in coral δ15N values after 1995, corresponding with the implementation of biological nutrient removal at the nearby Lahaina Wastewater Reclamation Facility (LWRF). High δ15N values are known to be strongly indicative of denitrification and sewage effluent, corroborating a previously suggested link between local wastewater injection and degradation of the reef environment. This record demonstrates the power of coral skeletal δ15N as a tool for evaluating nutrient dynamics within coral reef environments.

Highlights

Δ15N measurements are commonly used to distinguish between different N sources[20]
There is no instrumental record of water quality or N content available for this site prior to the early 1990’s so it is hard to determine if the N loading and associated coral decline can be categorically attributed to the Lahaina Wastewater Reclamation Facility (LWRF) treated sewage injection
We use δ15N of intra-crystalline coral skeletal organic matter in Porites lobata coral cores collected from three locations, representing sites adjacent to seep discharge and far from the direct seep influence, along the fringing reef at Kahekili Beach Park (KBP) to construct a record of δ15N in the water at each of the three sites

Summary

Introduction

Δ15N measurements are commonly used to distinguish between different N sources[20]. dissolved inorganic nitrogen (DIN) derived from sewage typically has δ15N values ranging from +7‰ to +38‰, with values as high as 74‰ occurring at our study site[20,21]. We use δ15N of intra-crystalline coral skeletal organic matter in Porites lobata coral cores collected from three locations, representing sites adjacent to seep discharge and far from the direct seep influence, along the fringing reef at KBP to construct a record of δ15N in the water at each of the three sites.

Results

Conclusion