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.

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