Nitrogen fluxes between the ocean and a river basin using stable isotope analysis

Total nitrogen (TN), consisting of total particulate nitrogen (TPN) and total dissolved nitrogen (TDN) pools, may be transported to the ocean not only via river channels but also from across the entire river basin via groundwater and migratory animals. Since marine derived nitrogen (MDN) is hypothesized to largely be transported in particulate form, it is necessary to investigate the contribution of particulate MDN in ground surface soils to the total MDN at the river ecosystem scale. In this study, we investigated TN export from an entire river basin to the ocean, and also estimated the contribution of pink (Oncorhynchus gorbuscha) and chum salmon (O. keta) to total oceanic nitrogen input across a river basin. The maximum potential contribution of MDN entering the river basin from the ocean as salmon was 23.8% relative to the total amount of TN exported from the river basin. The contribution of MDN from the ocean to particulate nitrogen in river basin soils was estimated to be 22.9% with standard deviation (SD) of 3.6% using stable isotope analysis (SIA) of nitrogen (δ¹⁵N).