Nitrogen and phosphorus dynamics in a large river estimated by an in situ Lagrangian tracking approach

1. Streams and rivers transport and process land-derived nutrients to coastal ecosystems, but few empirical studies quantify in-stream processing of nutrients in large rivers due to methodological constraints. 2. Here we suggest a novel Lagrangian method to estimate the net nutrient dynamics of a large river (sixth-order Fuji River, discharge 20-48 m(3)/s) by concurrently tracking a specific water parcel along the river course and measuring the longitudinal profiles of inorganic nutrient (nitrogen and phosphorus) concentrations. 3. Our method revealed exceedingly high net nutrient removal rates (U-net) for both NH4 and PO4. Moreover, net release of both NO2 and NO3 were observed, likely from benthic nitrification, indicating the considerable contribution of biotic and abiotic in-stream processing to fluvial nutrient dynamics in a large river. 4. In the Fuji River, net uptake velocity (v(f-net)) and net uptake length (S-net) were lower and longer, respectively, for NH4 and PO4 than would be expected from high net nutrient removal (U-net), because of the high nutrient concentration and fast water velocity. 5. The results suggest that in high-relief mountainous areas, anthropogenic nutrients loaded in large rivers are influential on distant downstream ecosystems despite high nutrient processing rates in river ecosystems.