How do inorganic nitrogen processing pathways change quantitatively at daily, seasonal, and multiannual scales in a large agricultural stream?

Abstract. Large agricultural streams receive excessive inputs of nitrogen. However, quantifying the role these in nitrogen processing remains limited because continuous direct measurements interacting and highly time-varying pathways larger rivers are very complex. Therefore, we employed a monitoring-driven modelling approach with high-frequency situ data river water quality model Water Quality Analysis Simulation Program (WASP) 7.5.2 27.4 km reach sixth-order stream called Lower Bode (central Germany) for 5-year period (2014–2018). Paired sensor (15 min interval) discharge, nitrate, dissolved oxygen, chlorophyll at upstream downstream stations were used as boundaries setting constraints. The WASP simulated 15 intervals oxygen Nash–Sutcliffe efficiency values higher than 0.9 calibration validation, enabling calculation gross net inorganic uptake pathway rates on daily, seasonal, multiannual scale. Results showed daily rate ranged from −17.4 to 553.9 mgNm-2d-1. highest could almost 30 % total input loading, which occurred extreme low flow summer 2018. growing season (spring summer) accounted 91 average annual measured period. In spring, both DIN dominated by phytoplankton pathway. summer, benthic algae assimilation Conversely, became source negative autumn winter, mainly release surpassed processes. Over 5 years, 124.1 56.8 mgNm-2d-1, only 2.7 1.2 loadings Bode, respectively. decreased through denitrification. Our study highlights value combining obtain reliable budget instream facilitates our ability manage aquatic systems. This provides methodology that can be applied any large quantify dynamics complete understanding cycling.