Nitrogen uptake and denitrification in restored and unrestored streams in urban Maryland, USA

• Published in: Aquatic sciences 2009-12, Vol.71 (4), p.411-424
• Main Authors: Klocker, Carolyn A., Kaushal, Sujay S., Groffman, Peter M., Mayer, Paul M., Morgan, Raymond P.
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Aquatic sciences; Base flow; Biological and medical sciences; Biomedical and Life Sciences; Creeks & streams; Denitrification; Ecology; Environmental restoration; Floodplains; Fresh water ecosystems; Freshwater; Freshwater & Marine Ecology; Fundamental and applied biological sciences. Psychology; General aspects; Life Sciences; Marine & Freshwater Sciences; Nitrate content; Nitrates; Nitrogen; Oceanography; Research Article; Sediments; Storm drains; Stormwater; Stormwater management; Streams; Surface water; Synecology
• ISSN: 1015-1621; 1420-9055
• DOI: 10.1007/s00027-009-0118-y

There is growing interest in rates of nitrate uptake and denitrification in restored streams to better understand the effects of restoration on nitrogen processing. This study quantified nitrate uptake in two restored and two unrestored streams in Baltimore, Maryland, USA using nitrate additions, denitrification enzyme assays, and a 15 N isotope tracer addition in one of the urban restored streams, Minebank Run. Restoration included either incorporation of stormwater ponds below a storm drain and catch basins to attenuate flow or hydrologic “reconnection” of a stream channel to its floodplain. Stream restoration was conducted for restoring aging sanitary and bridge infrastructure and introducing some stormwater management in watersheds developed prior to current regulations. Denitrification potential in sediments was variable across streams, whereas nitrate uptake length appeared to be significantly correlated to surface water velocity, which was low in the restored streams during summer baseflow conditions. Uptake length of NO 3 − –N in Minebank Run estimated by 15 N tracer addition was 556 m. Whole stream denitrification rates in Minebank Run were 153 mg NO 3 − –N m −2  day −1 , and approximately 40% of the daily load of nitrate was estimated to be removed via denitrification over a distance of 220.5 m in a stream reach designed to be hydrologically “connected” to its floodplain. Increased hydrologic residence time in Minebank Run during baseflow likely influenced rates of whole stream denitrification, suggesting that hydrologic residence time may be a key factor influencing N uptake and denitrification. Restoration approaches that increase hydrologic “connectivity” with hyporheic sediments and increase hydrologic residence time may be useful for stimulating denitrification. More work is necessary, however, to examine changes in denitrification rates in restored streams across different seasons, variable N loads, and in response to the “flashy” hydrologic flow conditions during storms common in urban streams.