Abstract
Stream restoration is a popular approach for managing nitrogen in degraded, flashy urban streams. Here, we investigated the long-term effects of geomorphic stream restoration on riparian and in-stream N transport and transformation in an urban stream in the Chesapeake Bay watershed. We examined relationships between hydrology, chemistry, and biology using a Before/After-Control/Impact (BACI) study design to determine how flashiness and N concentrations and flux changed after the restoration. We examined two independent surface water and groundwater data sets collected from 2002–2012 at our study sites in the Minebank Run watershed, modeled N flux, and compared our data to similar long-term data from the Baltimore Ecosystem Study LTER (BES) that served as reference sites. Restoration was completed during 2004 and 2005. Afterward, the monthly flashiness index, based on mean monthly discharge, decreased over time from 2002 and 2008. Groundwater nitrate (NO3−) concentrations trended slightly downward over time after the restoration at the restored site while dissolved organic carbon (DOC) concentrations trended upward whereas no trends were observed at the control site. Comparisons of NO3− concentrations with Cl− concentrations and specific conductance in both groundwater and surface water suggested that N reductions over time at the restored sites were not due to dilution. Similar patterns at BES sites suggested that declining NO3− was a function of restoration and watershed management, not larger regional factors such as decreased atmospheric inputs. DOC and NO3− were negatively related before and after restoration suggesting C limitation of N transformation. Long-term trends in surface water NO3− based on USGS data showed downward trends after restoration at both the restored and control sites while specific conductance showed no trend, suggesting that load reductions were not responsible for NO3− patterns. Modeled NO3− flux decreased post restoration in both the short and long-terms. Groundwater NO3− concentrations varied among stream features suggesting that some engineered features may be functionally better at creating optimal conditions for N removal. However, some engineered features eroded and failed post restoration thereby reducing efficacy of the restoration to reduce flashiness and NO3− flux. N management via stream restoration will be most effective where flashiness can be reduced, and DOC made available for denitrifiers. Stream restoration may be an important component of holistic watershed management including stormwater management and nutrient source control.
UAV Monitoring of Stream Restorations
