Linking isotopic signatures of nitrogen in nearshore coral skeletons with sources in catchment runoff

We use a multi-tracer approach to identify catchment sources of nitrogen (N) in the skeletons of nearshore Porites corals within the Great Barrier Reef. We measured 815N, 813C and C:N ratios of particulate organic matter (POM) sampled from the Pioneer River catchment and identified five distinct end-members: (1) marine planktonic and algal-dominated matter with higher 815N values from the river mouth and coastal waters; (2) estuarine planktonic and algal matter with lower 815N values associated with estuarine mixing; (3) lower river freshwater phytoplankton and algal-dominated matter in stratified reservoirs adjacent to catchment weirs, with the 15Nenriched source likely caused by microbial remineralization and denitrification; (4) upper river low 815N terrigenous soil matter eroded from cane fields bordering waterways; and (5) terrestrial plant detrital matter in forest streams, representing a low 815N fixed atmospheric nitrogen source. The 815N values of adjacent, nearshore Porites coral skeletons is reflective of POM composition in coastal waters, with 15N-enriched values reflective of transformed N during flood pulses from the Pioneer River.

Automated summary

A multi-tracer approach was used to identify nitrogen sources in the skeletons of nearshore Porites corals within the Great Barrier Reef. Different end-members from marine plankton, estuarine matter, freshwater phytoplankton, terrigenous soil, and terrestrial plant detritus were identified based on isotopic values. The nitrogen composition in coastal waters, including 15N-enriched values during flood pulses from the Pioneer River, was reflected in the adjacent nearshore Porites coral skeletons.