Use of nitrogen stable isotope ratio of periphyton for monitoring nitrogen sources in a river system

In order to confirm the usefulness of the N stable isotope ratio of periphyton (mainly composed of attached algae) as an indicator for monitoring the N sources in river watersheds, we measured the isotope ratio of periphyton along the Chikuma River. In the river, both the concentrations of dissolved total nitrogen (DTN) and the δ¹⁵N values of periphyton increased downstream. Specific nitrogen loading rates (SNLR) calculated from administrative data also showed an increase downstream from 7 to 11 kg N ha⁻¹ yr⁻¹, with the increasing contribution by sewage and livestock waste from 6 to 40% to total N loading. There are significant positive relationships between the DTN concentration and the SNLR (r² = 0.54, P<0.05), and the δ¹⁵N values of periphyton and the SNLR (r² = 0.78, P<0.05). The increase in DTN concentration reflected the increase in input of N loading. The increase in δ¹⁵N of periphyton might reflect the increase in relative contribution by sewage and livestock waste down the river, especially the increase in sewage. The present study indicates the usefulness of the N stable isotope ratio of periphyton as an indicator for monitoring N sources in a river system.