Coral Skeletons Record Increasing Agriculture‐Related Groundwater Nitrogen Inputs to a South Pacific Reef Over the Past Century

Abstract

Anthropogenic activity on tropical islands has been linked with nitrogen (N) contamination of groundwater and subsequent coral reef health decline. However, identifying the temporal patterns of groundwater N contamination has proven difficult because of an absence of long‐term records. Here we use δ15N in coral skeleton organic material (CS‐δ15N) to reconstruct historical patterns of groundwater N discharge to a coral reef system at Rarotonga in the Cook Islands in the South Pacific. Analysis of coral skeletal material dating back to 1880 CE clearly shows that the δ15N of N available in the reef environment around Rarotonga increased between 1980 and 2000. We propose that rapid agricultural development in the Cook Islands between 1960 and 1985 increased aquifer N concentrations leading to the elevated δ15N of groundwater NO3−. The discharge of this groundwater N appears to have continued for at least 15 years after the cessation of the agricultural boom. This has important implications for the management of groundwater contamination on low‐lying tropical islands.

Submarine Groundwater Discharge and Stream Baseflow Sustain Pesticide and Nutrient Fluxes in Faga'alu Bay, American Samoa

“…There is mounting evidence that populated Pacific islands struggle with water quality problems that affect human as well as ecosystem health (Mosley and Aalbersberg, 2003;Bolabola, 2007;Erler et al, 2018). Typically, population density is highest along coastlines, where centralized sanitary systems serve only a small fraction of the population and decentralized agriculture leaves room for unregulated practices, resulting in nutrient, pathogen, and pesticide pollution.…”

Has Nitrogen Supply to Coral Reefs in the South Pacific Ocean Changed Over the Past 50 Thousand Years?

“…Nevertheless, the weight of our evidence points to groundwater discharge as a driver of low CS‐δ 15 N in the fossil corals. Notably, the inferred decrease in fossil coral δ 15 N as a result of groundwater discharge is in direct contrast to the modern situation in the Pacific Islands where groundwater is a source of N with a high δ 15 N to coastal systems (Erler et al, ; Prouty et al, ). For instance, on the nearby island of Rarotonga, groundwater contains high δ 15 N‐NO 3 − (~8.5–25‰), the discharge of which is recorded in fringing reef corals (Erler et al, ).…”

“…First, the idea that subsurface oceanic N supply increases under warm wet conditions is counterintuitive given that terrestrial runoff and groundwater discharge are highest under such conditions. Second, Erler et al () present a high‐resolution Ba/Ca record from one of the modern Rarotonga corals that is significantly and positively correlated ( r = 0.23) with CS‐δ 15 N. This implies that groundwater discharge, which is a known source of Ba, is also responsible for the increases in CS‐δ 15 N. Accepting this logic, such increases in CS‐δ 15 N in the modern coral are indicative of enhanced aquifer N inputs probably resulting from colonization by the Polynesians over 1600 years B.P. This colonization was known to have resulted in major changes to flora and fauna in the Cook Islands (Steadman & Kirch, ), so it is not too difficult to conceive that it was also accompanied by changes in aquifer N dynamics.…”

“…Modern coral skeletal δ 18 O data for the L3R core is from Linsley et al (2006). The modern CS-δ 15 N (‰) data are from Erler et al (2018).…”

Coral skeletons reveal the history of nitrogen cycling in the coastal Great Barrier Reef

“…δ 15 N, where δ 15 N = [( 15 N/ 14 N) sample /( 15 N/ 14 N) air ] − 1) of N trapped in the organic skeletal matrix of massive reef-building corals (hereafter called CS-δ 15 N) to understand how N cycling in the coastal GBR has responded to anthropogenic nutrient discharge since European settlement. The proxy-based approach relies on the fact that the δ 15 N of marine N, which is consumed by corals and recorded in their skeletons [38][39][40] , is an integrated signal of changes in N sources and availability. Greater N availability as a result of anthropogenic activity should have changed CS-δ 15 N to match the δ 15 N of terrestrial N impacted by fertiliser application and land-clearing.…”