Abstract
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.
Highlights
The “urban stream syndrome” provides a framework for evaluating changes associated with urbanization [1,2,3,4,5], including physical habitat modifications, hydrological alterations, and elevated nutrient loads occurring in catchments across the globe [6,7]
Nitrogen dynamics in restored urban rivers including nitrogen cycling [10,11,12] which can be compounded by elevated nutrient loads from gutters and storm drains [2,13,14]
This study sought to determine whether river restoration activities could influence N dynamics of degraded rivers in London
Summary
The “urban stream syndrome” provides a framework for evaluating changes associated with urbanization [1,2,3,4,5], including physical habitat modifications, hydrological alterations, and elevated nutrient loads occurring in catchments across the globe [6,7]. ‘Urban karsts’, encompassing a complex, predominantly hidden, network of buried headwaters streams, sewers, and potable water pipes can further modify hydrological processes, reducing water infiltration and inhibiting nutrient storage capacity [12,15]. Together these factors can play a major role in influencing nitrogen dynamics in urbanised river ecosystems
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