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

ABSTRACT

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 δ15N, δ13C and CN 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 δ15N values from the river mouth and coastal waters; (2) estuarine planktonic and algal matter with lower δ15N values associated with estuarine mixing; (3) lower river freshwater phytoplankton and algal-dominated matter in stratified reservoirs adjacent to catchment weirs, with the 15N-enriched source likely caused by microbial remineralization and denitrification; (4) upper river low δ15N terrigenous soil matter eroded from cane fields bordering waterways; and (5) terrestrial plant detrital matter in forest streams, representing a low δ15N fixed atmospheric nitrogen source. The δ15N 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.