PATTERNS OF HYDROLOGICAL EXCHANGE AND NUTRIENT TRANSFORMATION IN THE HYPORHEIC ZONE OF A GRAVEL-BOTTOM STREAM - EXAMINING TERRESTRIAL AQUATIC LINKAGES

1. The terrestrial-aquatic interface beneath a riparian corridor was i nvestigated as a region of hydrological and biological control of nutr ient flux. Subsurface flow paths were defined from the channel toward the riparian zone and also from the riparian zone toward the channel u sing tracer-injection studies. Solute transport had a rapid channel co mponent (m min-1) and a slow hyporheic flow component (m h-1, m day-1) . Subsurface flow beneath the riparian zone approximated a straight pa th entering at meanders but could also cross beneath the stream, possi bly using relic channels. 2. Dissolved oxygen (DO) concentration in th e hyporheic zone ranged from < 1.0 to 9.5 mg l-1 due to permeability v ariations in bankside sediments. DO concentration was related to the p roportion of stream water in the lateral hyporheic zone, indicating th at the channel water was the DO source. 3. The magnitude and timing of lateral water exchange was linked to previously published studies of nitrification and denitrification. Both nitrification potential and ch annel exchange decreased with distance from the channel and were absen t 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 i nversely related to channel exchange and was insignificant in channel sediments. Field amendment of acetylene plus nitrate to a flow path wi th low DO and minimal channel exchange indicated denitrification of am ended nitrate. 4. Comparison of hydraulic head to distribution of the biologically important solutes DO, ammonium, and nitrate was useful fo r interpreting previous findings and conceptualizing the riparian zone as a functioning ecotone between terrestrial and aquatic systems.