Influence of urban river restoration on nitrogen dynamics at the sediment-water interface

摘要

River restoration projects focused on altering flow regimes through use of in-channel structures can facilitate ecosystem services, such as promoting nitrogen (N) storage to reduce eutrophication. In this study we use small flux chambers to examine ammonium (NH4⁺) and nitrate (NO3^-) cycling across the sediment-water interface. Paired restored and unrestored study sites in 5 urban tributaries of the River Thames in Greater London were used to examine N dynamics following physical disturbances (0–3 min exposures) and subsequent biogeochemical activity (3–10 min exposures). Average ambient NH4⁺ concentrations were significantly different amongst all sites and ranged from 28.0 to 731.7 μg L^-1, with the highest concentrations measured at restored sites. Average NO3^- concentrations ranged from 9.6 to 26.4 mg L^-1, but did not significantly differ between restored and unrestored sites. Average NH4⁺ fluxes at restored sites ranged from -8.9 to 5.0 μg N m^-2 sec^-1, however restoration did not significantly influence NH4⁺ uptake or regeneration (i.e., a measure of release to surface water) between 0–3 minutes and 3–10 minutes. Further, average NO3^- fluxes amongst sites responded significantly between 0–3 minutes ranging from -33.6 to 97.7 μg N m^-2 sec^-1. Neither NH4⁺ nor NO3^- fluxes correlated to sediment chlorophyll-a, total organic matter, or grain size. We attributed variations in overall N fluxes to N-specific sediment storage capacity, biogeochemical transformations, potential legacy effects associated with urban pollution, and variations in river-specific restoration actions.