Coral skeletal δ15N reveals isotopic traces of an agricultural revolution

This study introduces a new method of tracing the history of nutrient loading in coastal oceans via δ15N analysis of organic nitrogen preserved in the skeleton of the massive Porites coral. Four coral cores were collected in Bali, Indonesia, from reefs exposed to high levels of fertilizers in agricultural run-off, from lagoonal corals impacted by sewage, and from a reef located 30 km offshore. Skeletal δ15N in the agriculturally exposed coral declined from 10.7 ± 0.4‰ in 1970–1971, when synthetic fertilizers (−0.8‰ ± 0.2‰) were introduced to Bali, to a depleted “anthropogenic” baseline of 3.5‰ ± 0.4% in the mid-1990s. δ15N values were negatively correlated with rainfall, suggesting that marine δ15N lowers during flood-bourn influxes of waste fertilizers. Reef cores exposed to untreated sewage in terrestrial discharge were enriched (7.8 and 7.3 ± 0.4‰), while the offshore core reflected background oceanic signals (6.2 ± 0.4‰). δ15N, N concentration, and C:N systematics indicate that the N isotopic composition of skeletal organic matter was generally well preserved over 30 years. We suggest that skeletal organic δ15N can serve as a recorder of past nitrogen sources. In Bali, this tracer suggests that the intensification of Western style agricultural practices since 1970 are contributing to the degradation of coastal coral reefs.