|Processing of nitrogen in urban streams|
An important change resulting from urbanization is an increase in impervious surface cover, which affects the hydrology of urban areas. Impervious surfaces increase surface runoff and flood peak discharges and decrease infiltration and groundwater recharge. The engineering of stream channels with channels and storm drainage systems compound the effects of impervious surface cover by directing surface runoff directly into streams. These increased flows have dramatic effects on stream channel geomorphology widening and deepening channels and eroding stream banks (Figure 1). High flows and peaks can also lead to stream scouring, reducing the presence of large woody debris in urban streams. Large woody debris are important to the formation and maintenance of organic debris dams which obstruct stream flow and function as "hot spots" of nutrient cycling in streams. The systematic degradation of urban streams has come to be referred to as "urban stream syndrome" (Walsh et al. 2005).
One approach to analysis of stream N sink and source dynamics is to consider different stream features, characterize their sink or source potential and then quantify their importance in the overall flow of water. While most water in a stream flows in its main, open channel, stream water often diverges from the main channel into sediments below the stream, gravel bars next to the stream or organic debris dams in the middle of the stream. Analysis of C and N dynamics in these features is useful for characterizing their potential to produce and consume inorganic N. An ecosystem-scale assessment requires coupling this analysis with hydrologic information on the amount of water that passes through the different features. Consideration of the natural and anthropogenic factors that influence the genesis and maintenance of different features, e.g., flow regime, riparian vegetation, geologic substrate) is an additional necessary component of these studies.
We have also found that the high salt levels in urban streams may have a negative effect on denitrification (Hale and Groffman 2006). In laboratory microcosms, DEA in debris dam material from a forested reference stream was increased by nitrate additions. However, chloride additions constrained the response of DEA to nitrate additions in material from the forested stream, but had no effect on DEA in material from streams with a history of high chloride levels. Chloride additions changed the sign of net N mineralization from negative (consumption of inorganic N) to positive in debris dam material from the forested reference stream, but had no effect on net mineralization in material from streams with a history of exposure to chloride. Understanding the factors regulating the maintenance and N cycling activity of organic debris, and incorporating them into urban stream management plans, could have important effects on N dynamics in suburban watersheds.
Groffman, P. M., A. M. Dorsey, and P. M. Mayer. 2005. N processing within geomorphic structures in urban streams. Pages 613-625.
Hale, R. L. and P. M. Groffman. 2006. Chloride effects on nitrogen dynamics in forested and suburban stream debris dams. Journal of Environmental Quality 35:2425-2432.
Harrison, M. D., P. M. Groffman, P. M. Mayer, and S. S. Kaushal. 2012. Microbial biomass and activity in geomorphic features in forested and urban restored and degraded streams. Ecological Engineering 38:1-10.
Walsh, C. J., A. H. Roy, J. W. Feminella, P. D. Cottingham, P. M. Groffman, and R. P. Morgan. 2005. The urban stream syndrome: current knowledge and the search for a cure. Journal of the North American Benthological Society 24:706-723.
|This research was supported by funding from the NSF Long-term Ecological Research (LTER) Program. This material is based upon work supported by the National Science Foundation under Grant No. 1027188. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.|