.|  Baltimore Ecosystem Study
Urban Stream and Floodplain Restoration
There has been increasing burial, channelization, and degradation of streams due to urbanization (Elmore and Kaushal 2008), and this may contribute to inceased efficiency in transporting pollutant loads and decreased capacity for in-stream processing during storms (Shields et al. 2008, Kaushal et al. 2008). Many river miles of suburban and urban streams are now being restored in Maryland and other areas of the U.S. with ancillary objectives of improving water quality (Craig et al. 2008). Despite the billions of dollars currently invested in the over 37,000 stream restoration projects in the U.S., there are few actual measurements of the effects of stream restoration on denitrification (Bernhardt et al. 2005). Through collaborations with the U.S. Environmental Protection Agency Office of Research and Development, we have been investigating the effects of stream and floodplain restoration on denitrification in urban streams (Mayer et al. 2003, Groffman et al. 2005, Kaushal et al. 2008). These investigations include long-term monitoring in multiple streams representing varying stream restoration strategies including natural channel design versus incorporating innovative stormwater management and artificial wetland and pond creation into stream restoration designs.

Spring Branch, an urban stream in Baltimore, encased in a concrete channel prior to stream restoration. Following stream restoration, the stream and floodplain are hydrologically "reconnected" in this same reach of Spring Branch and allow stream water to interact with denitrification hot spots in the hyporheic zone.
Photo: Tammy Newcomer


Spring Branch, after restoration.
Photo: Tammy Newcomer

This research has shown that debris dams are potential “hot spots” of denitrification in urban streams and that strategies for organic matter accumulation may foster increased denitrification rates (Groffman et al. 2005). In addition this work has shown that strategies increasing hydrologic “connectivity” between streams and floodplains and increasing hydrologic residence times may increase in situ denitrification rates in restored streams measured using 15N tracer additions (Kaushal et al. 2008). Tree roots in riparian areas can increase soil organic matter and denitrification at restored sites over time after plantings (Gift et al. 2010). Additional work at the stream reach scale using 15N tracer additions has shown that there can be substantial in situ denitrification and that N uptake are related to surface water velocity across restored and unrestored sites (Klocker et al. 2009). Finally, work at the larger stream network scale shows that there can be reach scale variability in nitrogen and carbon cycling due to groundwater inputs and spatial heterogeneity of urbanized landscapes, but that there can be substantial retention of nitrogen and transformation of organic carbon in restored stream networks that span several kilometers in length (Stanko et al. Submitted). Thus, the riparian-stream interface and interactions between the stream and ground water may be an important emerging area of management of denitrification (Kaushal et al. 2008, Mayer et al. In Press). Given future increases in interactive effects of climate variability and urbanization, enhancing N sinks via both stormwater management in uplands and stream and floodplain restoration may be critical in increasing adaptive capacity in attenuating interannual pulses in N loads (Kaushal et al. 2008).

Mean in situ denitrification rates over all sites in both the restored and unrestored reaches of Minebank Run. Values are mean (standard error) of four or five wells in each reach, sampled three times between May 2003 and June 2004. Restored sites showed a higher (ANOVA, P = 0.01) overall mean denitrification rate than unrestored sites.
Kaushal et al. 2008


Relationship between hydrologic residence time and mass removal of nitrate-N in the unrestored reach (obtained using coinciding measurements of denitrification rates and groundwater flow). Potential relationship between hydrologic residence time and mass removal of nitrate-N in the restored reach (obtained from a scenario using mean denitrification rates in restored low “connected” bank during 2003 - 2004 and groundwater flow rates following denitrification measurements in 2004).
Kaushal et al. 2008

Organic carbon sources and effects on denitrification:
Organic carbon is important in regulating ecosystem function and N dynamics, and its source and abundance may be altered by urbanization. We have investigated shifts in organic carbon quantity and quality associated with urbanization and ecosystem restoration, and its potential effects on denitrification at the riparian-stream interface at 2 forested, 2 restored, and 2 degraded-urban streams at the BES LTER site (Figure 7). Stable isotopes and molar C:N ratios suggested that stream particulate organic matter (POM) was a mixture of periphyton, leaves, and grass that varied across site types. Stable-isotope signatures and lipid biomarker analyses of sediments showed that terrestrial organic carbon sources in streams varied as a result of riparian vegetation. Laboratory experiments indicated that organic carbon availability significantly increased rates of denitrification more than nitrate availability across streamflow conditions and sites (p < 0.05). Denitrification experiments with naturally occurring carbon sources showed that denitrification was significantly higher with grass clippings from home lawns and degraded-urban sites showed significantly higher denitrification rates than restored and forest sites overall (p < 0.05; Fig. 9). We found that urbanization influences organic carbon sources and quality in streams, which can have substantial downstream impacts on ecosystem services such as denitrification.

Comparison of denitrification potential rates associated with different naturally occurruing organic carbon sources at 2 forested (Pond Branch and Basiman Run), two restored (Spring Branch and Minebank Run) and two unrestored urban (Dead Run and Scotts Level Branch) streams.

Bernhardt, E.S., M.A. Palmer, J.D. Allan, and 22 others. 2005. Ecology – Synthesizing U.S. river restoration efforts. Science 308: 636-637.
Craig, L.S., M.A. Palmer, D.C. Richardson, S. Filoso, E.S. Bernhardt, B.P. Bledsoe, M.W. Doyle, P.M. Groffman, B. Hassett, S.S. Kaushal, P.M. Mayer, S.M. Smith, and P.R. Wilcock. 2008. Stream restoration strategies for reducing river nitrogen loads. Frontiers in Ecology and the Environment DOI: 10.1890/070080
Elmore, A.J. and Kaushal, S.S. 2008. Disappearing headwaters: Patterns of stream burial due to urbanization. Frontiers in Ecology and the Environment 6, doi:10.1890/070101
Gift, D., P.M. Groffman, S.S. Kaushal, P.M. Mayer, E.A. Striz. 2010. Root biomass, organic matter and denitrification potential in degraded and restored urban riparian zones. Restoration Ecology DOI: 10.1111/j.1526-100X.2008.00438.x
Groffman, P.M., A.M. Dorsey, and P.M. Mayer. 2005. Nitrogen processing within geomorphic features in urban streams. Journal of the North American Benthological Society 24: 613-625.
Kaushal, S.S., P.M. Groffman, L.E. Band, C.A. Shields, R.P. Morgan, M.A. Palmer, K.T. Belt, G. T. Fisher, C.M. Swan, and S.E.G. Findlay. Interaction between urbanization and climate variability amplifies watershed nitrate export in Maryland. Environmental Science & Technology 42, 5872–5878, 2008. 10.1021/es800264f
Kaushal, S.S., P.M. Groffman, P.M. Mayer, E. Striz, E.J. Doheny, A.J. Gold. Effects of stream restoration on denitrification in an urbanizing watershed. Ecological Applications 18: 789-804.
Klocker, C.A., S.S. Kaushal, P.M. Groffman, P.M. Mayer, and R .P. Morgan. 2009. Nitrogen uptake and denitrification in restored and unrestored streams in urban Maryland, USA. Aquatic Sciences 71: 411-424.
Mayer, P.M., E. Striz, R. Shedlock, E. Doheny, and P. Groffman. 2003. The effects of ecosystem restoration on nitrogen processing in an urban mid-Atlantic piedmont stream. Pp. 536-541 in Renard, Kenneth G., McElroy, Stephen A., Gburek, William J., Canfield, H. Evan and Scott, Russell L., eds. First Interagency Conference on Research in the Watersheds, October 27-30, 2003. U.S. Department of Agriculture, Agricultural Research Service.
Mayer, P.M., P.M. Groffman, E.A. Striz, and S.S. Kaushal. Nitrogen dynamics at the ground water and surface water interface of a degraded urban stream. Journal of Environmental Quality (In Press)
Newcomer, T.A., S.S. Kaushal, P.M. Mayer, A. Shields, E.A. Canuel, P.M. Groffman, and A.J. Gold. 2012. Influence of natural and novel organic carbon sources on denitrification in forest, urban degraded, and restored streams. Ecological Monographs 82: 449-466. doi:10.1890/12-0458.1.
Shields, C.A., L.E. Band, N.L. Law, P.M. Groffman, S.S. Kaushal, K. Savvas, and G.T. Fisher. Streamflow distribution of nitrogen export from urban-rural catchments in the Chesapeake Bay watershed. 2008. Water Resources Research 4: W09416 doi:10.1029/2007WR006360.
Stanko, G., S.S. Kaushal, P.M. Mayer, C.A. Welty, K.T. Belt, K.A. Delaney, T.A. Newcomer, and M. Grese. Longitudinal variability in streamwater chemistry and carbon and nitrogen fluxes in restored and unrestored stream networks. Journal of Environmental Monitoring (Submitted)

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.