Urbanization of watersheds typically simplifies stream channel structure and reduces hydrologic exchange between the stream channel and its associated hyporheic and riparian zones. As a result, urban streams typically have reduced capacity to process and retain nutrients as compared to more pristine, forested stream ecosystems. Reduced nutrient processing coupled with increased anthropogenic nitrogen loading to urbanized watersheds leads to increased nitrogen fluxes exiting urban watersheds. One goal of this study is to evaluate the differences in nitrogen processing between urban and forested streams and the potential for urbanized streams to recover biogeochemical function.

Over 3 years we collected weekly water samples from multiple locations along five streams in the piedmont of North Carolina. Sampling sites included 2 forested, undeveloped reference streams, 2 urban streams and a 1.6 km segment of stream that is urbanized in the upper part of the catchment and then flows through 1.2 km of protected mature forest. Both urbanized streams had channel restoration projects completed during the third year of the study.

All weekly water samples were analyzed for total dissolved organic carbon (DOC), total nitrogen (TN) and dissolved inorganic nitrogen (DIN), measured as NH₄-N and NO₃-N. During the third year of the study we measured primary productivity by analyzing chlorophyll a concentration to calculate algal accrual on unglazed tiles placed in each of the streams. 

Results/Conclusions

Primary production showed no measurable differences between urban and forested streams because algal accrual rate was highly correlated with light levels. Two of the urbanized streams had intact forest cover, and the streambed experienced relatively low light levels as compared with the open-canopy urbanized stream.  DIN, TN and DOC concentrations were greater in the urban streams (mean DIN= 0.28 mg L^-1; mean TN= 0.65mg L^-1 ; mean DOC= 7.6 mg L^-1) than in the forested streams (mean DIN= 0.15mg L^-1 ; mean TN= 0.29 mg L^-1; mean DOC= 4.7 mg L^-1). The 7 points sampled along the urban stream running through mature forest showed that DIN decreased along the downward continuum, but because TN flux did not decline as sharply, the in-stream production of dissolved organic nitrogen reduces N retention.