Lagrangian tracking measurements revealed the temporal dynamics of nitrogen and phosphorus spiralling in a large Japanese river

The in-stream processing of nutrients plays an important role in the fluvial nutrient transport from lands to the ocean, but few empirical studies have addressed the temporal dynamics of net nutrient removal rates in large river systems. The present study applied the recently developed Lagrangian approach to the sixth-order Fuji River to identify the seasonal dynamics of in-stream uptake rates for inorganic nitrogen and phosphorus by tracking a specific water parcel along the river course. The Lagrangian measurements revealed temporal variability of nutrient uptake metrics, but the overall pattern showed a net positive uptake of ammonium (NH4) and a net release of nitrite (NO2) and nitrate (NO3) from the riverbed from May through December. The net uptake rates of phosphate (PO4) showed both positive and negative values depending on the sampling periods. Environmental correlates of the net areal uptake rates (U-net) suggested that biological transformation processes such as assimilation for organic matter synthesis and utilization for energy in oxidation-reduction reactions, as well as abiotic sorption processes, influenced the nutrient removal in the Fuji River. The present study highlights that physical factors (temperature and light), reaction substrates and epilithic organisms may responsible for nutrient dynamics in large river ecosystems.