Nutrient uptake in a simplified stream channel: Experimental manipulation of hydraulic residence time and transient storage

Abstract

AbstractStream restoration efforts have aimed at increasing hydraulic residence time (HRT) and transient storage (TS) to enhance nutrient uptake, but there have been few controlled studies quantifying HRT and TS influences on nutrient uptake dynamics. We assessed the effects of HRT and TS on ammonium (NH4+) and phosphate (PO43−) uptake through controlled experiments in an artificial channel draining a pristine tropical stream. We experimentally dammed the channel with artificial weirs, to progressively increase HRT, and performed NH4+ and PO43− additions to estimate uptake each time a weir was added. We also ran consecutive additions of NH4+ and PO43− with no weirs, to evaluate short‐term changes in uptake metrics. Also, NH4+ was injected alone to assess potential nitrification. We observed that NH4+ and PO43− uptake rates were much greater in the very first addition, probably due to luxury uptake. The weirs increased mean HRT (from 8.5 to 12 min) and depth (from 6.5 to 8.9 cm) and decreased mean water velocity (0.40–0.28 m s−1). Surprisingly, damming decreased the relative size of transient storage zone (storage zone area/channel area, As/A from 0.72 to 0.55), indicating that greater depth increased A, but not As. Greater HRT increased uptake rates and velocities of both nutrients (p < 0.05). The NH4+ conversion to NO3− was estimated at 18% of NH4+ consumption, indicating that joint additions to measure NH4+ and NO3− uptake would not be feasible in this system. Our results suggest that increases in HRT can lead to a greater short‐term retention of nutrients, with implications for stream management and restoration initiatives.