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 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 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.

Keywords: Denitrification; Microbial remineralization; Nitrogen cycle; Pioneer River catchment; River runoff; Water quality.