Intermittent streams comprise a large fraction of river networks, yet most mechanistic understanding of nitrogen dynamics derives from perennial systems. In intermittent streams, repeated transitions among wetting, flowing, and drying modify hydrologic connectivity and exchange between surface water, hyporheic sediments, and riparian zones, generating strong spatial and temporal variability in physicochemical conditions. Understanding how nitrogen responds to these connectivity shifts provides important context for predicting nutrient cycling under increasing hydrologic variability conditions.
We investigated drivers of dissolved nitrogen concentrations in surface water, hyporheic, and riparian compartments along a 5-km reach of Brushy Creek, a second-order intermittent stream in the Ozark Border region of south-central Missouri, USA. From October 2021 to June 2023, we collected monthly surface and subsurface water samples at four sites instrumented with six wells each (two hyporheic and four riparian) to characterize vertical and lateral connectivity among stream compartments. Hydrologic connectivity was quantified as the percentage of surface water contribution to well water using chloride as a conservative tracer.
Nitrogen concentrations varied across hydrologic periods and compartments, with the highest total dissolved nitrogen and nitrate concentrations observed in surface water during the drying period, followed by lower concentrations during wetting and flowing conditions. Dissolved inorganic nitrogen dominated during drying and flowing periods, whereas dissolved organic nitrogen contributed most strongly in the hyporheic zone during flowing conditions. Hydrologic connectivity was significantly associated with nitrate transformation in subsurface compartments, with higher surface water contributions corresponding to shifts in nitrate uptake and retention across hyporheic and riparian zones.