Nitrogen isotopic composition of organic matter from a 168 year-old coral skeleton: implications for coastal nutrient cycling in the Great Barrier Reef Lagoon

Ongoing human activities are known to affect nitrogen cycling on coral reefs, but the full history of anthropogenic impact is unclear due to a lack of continuous records. We have used the nitrogen isotopic composition of skeleton-bound organic matter (CS-δ¹⁵N) in a coastal Porites coral from Magnetic Island in the Great Barrier Reef as a proxy for N cycle changes over a 168 yr period (1820–1987 AD). The Magnetic Island inshore reef environment is considered to be relatively degraded by terrestrial runoff; given prior CS-δ¹⁵N studies from other regions, there was an expectation of both secular change and oscillations in CS-δ¹⁵N since European settlement of the mainland in the mid 1800s. Surprisingly, CS-δ¹⁵N varied by less than 1.5‰ despite significant land use change on the adjacent mainland over the 168-yr measurement period. After 1930, CS-δ¹⁵N may have responded to changes in local river runoff, but the effect was weak. We propose that natural buffering against riverine nitrogen load in this region between 1820 and 1987 is responsible for the observed stability in CS-δ¹⁵N. In addition to coral derived skeletalδ¹⁵N, we also report, for the first time, δ¹⁵N measurements of non-coral derived organic N occluded within the coral skeleton, which appear to record significant changes in the nature of terrestrial N inputs. In the context of previous CS-δ¹⁵N records, most of which yield CS-δ¹⁵N changes of at least 5‰, the Magnetic Island coral suggests that the inherent down-core variability of the CS-δ¹⁵N proxy is less than 2‰ for Porites.