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

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. Plain Language Summary The nutrient contamination of groundwater resources on tropical islands is thought to be a major contributor to declining coral reef health in the South Pacific. However, being able to clearly identify if and when changes in aquifer nutrient content occurred is difficult without long‐term records. Using the nitrogen isotopic content of organic material preserved in long‐lived coral skeletons, we show that significant groundwater nitrogen contamination occurred toward the latter part of the twentieth century on the island of Rarotonga (Cook Islands). We propose that agricultural development between 1960 and 1985 resulted in the contamination of groundwater with nitrogen. This nitrogen continued leaching into the adjacent coral reef for at least 15 years after the cessation of the agricultural boom. Key Points The nitrogen isotopic composition of coral skeleton organic material shows an increasing trend over the past century in the Cook Islands Increased groundwater nitrogen inputs from rapid agricultural development in the 1960s/70s is proposed to be responsible Coral skeletons can provide a useful history of anthropogenic land use and groundwater nutrient enrichment