Patterns of hydrological exchange and nutrient transformation in the hyporheic zone of a gravel‐bottom stream: examining terrestrial—aquatic linkages

SUMMARY The terrestrial‐aquatic interface beneath a riparian corridor was investigated as a region of hydrological and biological control of nutrient flux. Subsurface flow paths were defined from the channel toward the riparian zone and also from the riparian zone toward the channel using tracer‐injection studies. Solute transport had a rapid channel component (m min −1 ) and a slow hyporheic flow component (mh −1 , m day −1 ). Subsurface flow beneath the riparian zone approximated a straight path entering at meanders but could also cross beneath the stream, possibly using relic channels. Dissolved oxygen (DO) concentration in the hyporheic zone ranged from <1.0 to 9.5mgl −1 due to permeability variations in bankside sediments. DO concentration was related to the proportion of stream water in the lateral hyporheic zone, indicating that the channel water was the DO source. The magnitude and riming of lateral water exchange was linked to previously published studies of nitrification and denitrificarion. Both nitrification potential and channel exchange decreased with distance from the channel and were absent at sites lacking effective exchange, due to low DO. Field amendment of ammonium to an aerobic flow path indicated nitrification potential under natural hydrological conditions. Denitrification potential was inversely related to channel exchange and was insignificant in channel sediments. Field amendment of acetylene plus nitrate to a flow path with low DO and minimal channel exchange indicated denitrificarion of amended nitrate. Comparison of hydraulic head to distribution of the biologically important solutes DO, ammonium, and nitrate was useful for interpreting previous findings and conceptualizing the riparian zone as a functioning ecotone between terrestrial and aquatic systems.