Effects of relic low-head dams on stream denitrification potential: seasonality and biogeochemical controls.

The majority of dams in the contiguous United States are small, low-head dams that are no longer operational but can influence the water quality of contemporary stream ecosystems. Potential effects of low-head dams on stream nitrogen removal (denitrification) have been rarely quantified, and yet they can be an important part of the decision-making process of removing low-head dams. Here, we provide novel empirical data on potential denitrification rates and their biogeochemical controls above and below two mid-Atlantic low-head dams over a 2-year period. Our results show that low-head dams did not increase streambed potential denitrification in comparison to dam-free sections in the same rivers. In our study sites, potential denitrification above low-head dams was generally low (15.7 ± 3.5 µg N [kg sediment]−1 h−1) despite recurring events of water hypoxia (< 50% dissolved oxygen saturation) and high NO3− and DOC concentrations. Overall, we observed higher potential denitrification during winter samplings (9.2 and 50.1 µg N [kg sediment]−1 h−1 on average) and significant effects of sediment surface area and organic matter content on potential denitrification rates above the dams. Results from this study suggest limited effects of relic low-head dams on nitrogen removal and transport in stream ecosystems, and can contribute to the decision-making process of removing low-head dams. [ABSTRACT FROM AUTHOR]