Nitrate Removal in Forested Intermittent Streams of Southern New England.

Intermittent streams, which flow continuously for extended periods in spring and fall, can be important elements of stream networks in certain physiographic/climate conditions, but the extent, structure and function of these intermittent streams are poorly understood. We hypothesize that intermittent streams bordered by riparian forest have long retention times, shallow depths and interaction with labile carbon, creating conditions favorable for substantial nitrogen (N) removal. We examined six forested intermittent streams in glacial till deposits of Rhode Island. On each stream, we estimated the hydroperiod (extent of time with continuous flow) using an array of temperature probes. Over two years, we conducted slug tests with bromide and nitrate-N to characterize N removal, flow rates and velocities of each stream. We also characterized the structural elements (e.g., pools, riffles, runs) within each reach. Denitrification enzyme activity (DEA) of sediments in these features was measured. Additionally, we conducted constant rate injections of bromide and nitrate-N to characterize the transient storage and nitrate-N spiraling of each reach. We found evidence of water in the stream channel on 80 to 95% of the days from winter to mid-summer. Flow rates ranged from 0.5 to 16.7 L s-1 across all seasons. We observed nitrate removal capacities ranging from 0 to 5.5 kg N per month over a 100 m reach of stream, with each site displaying elevated rates in at least one slug test. Pools were found to be hotspots of nitrate removal with the highest DEA (mean = 1299 mg N kg-1 h-1). Shallower depths (< 5m) and lower flow rates (<5L s-1) yielded higher nitrate removal rates; these factors may be more controlling than season or temperature. To maintain the nitrate removal potential of intermittent streams, preserving the woody vegetation of the riparian area will be critical as it creates opportunities for hyporheic exchange.