The impact of low-head dams on riverine and riparian nitrogen dynamics

Dams number more than two million in the United States. Interrupting river connectivity of downstream and upstream reaches, these impoundments modify key ecological processes by affecting sediment transport, faunal migration, and nutrient fluxes. Currently, the impact of low-head dams on nitrogen cycling in rivers and their associated riparian areas is unknown. By understanding nitrogen dynamics along this lateral gradient, we can identify spatial features along rivers that serve as potential processors and/or removers of nitrogen. To assess nitrogen processing in channel and riparian sediments in impounded versus riverine reaches, we focused our study on low-head dams on the Olentangy River in Central Ohio. Nitrogen transformations were measured by potential denitrification enzyme activity and short-term nitrification assays; nitrogen retention was measured through soil and sediment extractions. Preliminary results indicate that denitrification ranged from approximately 0.3 – 12 mgN/kg-sediment/hr in impounded sites and from 0.09 – 4.3 mgN/kg-sediment/hr in riverine sites. Nitrification ranged from 0.006 – 1.7 mgN/kg-sediment/hr in impounded sites and 0.009 – 0.4 mgN/kg-sediment/hr in riverine sites. Sediment water saturation and percentage carbon were not strongly associated with nitrogen processing in both in-stream and riparian areas. The high variability of our results indicate that other factors such as water depth, water velocity, temperature and/or sediment redox may explain the variability of nitrogen dynamics amongst sites.