Hyporheic denitrification in urban streams
 
Peter M. Groffman and Ann Myers Dorsey
 
Hyporheic zones are areas where the water in a stream 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. Denitrification is an anaerobic microbial process that converts nitrate (NO3-), a cause of eutrophication in salt waters, into nitrogen (N) gas. Hyporheic zones can be important sites for denitrification if they contain enough organic matter and support enough microbial activity (that consumes oxygen) to allow for anaerobic conditions to develop.
 
We measured denitrification in hyporheic features in four streams in and around Baltimore. Sediments from pools, riffles, gravel bars (vegetated and non-vegetated) and debris dams were sampled in streams in forested reference (Pond Branch) and suburban (Baisman Run, Gwynns Falls at Glyndon, Mine Bank Run) watersheds. Mine Bank Run is a highly degraded (extensive stream channel incision) stream that is undergoing extensive restoration. Samples were taken from both restored and degraded sections of this stream. Denitrification, and a series of related variables (organic matter content, potential net mineralizaton and nitrification, respiration) were measured in a 10-day laboratory incubation of unamended sediments and denitrificatoin potential was measured in soil slurries amended with nitrate and carbon and incubated under anaerobic conditions.
 
Denitrification potential was highest in organic debris dams and mucky gravel bars (Figure 1) - features with high organic matter content. Organic debris dams in suburban streams had higher denitrification than debris dams in the forested reference streams, likely due to the higher NO3- content of the suburban streams. This result suggests that hyporheic denitrification increases in response to high NO3- levels and that this process may be an important “sink” for this ion in urban or suburban streams. It is important to note however that high denitrifying features (e.g. organic debris dams) are rare in urban streams with high storm flows (e.g. Mine Bank Run).
 
Keywords: streams watershed nitrate
 

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