Influence of Relict Milldams on Riparian Sediment Biogeochemistry

Purpose: Riparian zones perform important ecosystem services including acting as sediment and nutrient sinks. However, dams alter riparian zones – trapping fine-grained, organic matter-rich sediment and creating poorly mixed, low oxygen conditions – thereby affecting sediment biogeochemistry in poorly understood ways.
Methods: We characterized the impact of two relict US mid-Atlantic milldams (one from a primarily agricultural watershed and one from a mixed land use/urban watershed) on spatial patterns of bioavailable element concentrations (P, K, Ca, Mg, Mn, Zn, Cu, Fe, B, S, and Na) in sediments upstream and downstream of milldams, with depth, and along transects running parallel and perpendicular to the stream.
Results: Bioavailable element concentrations were not clearly correlated with grain size or organic matter content and, although generally higher, were not significantly more concentrated in riparian sediments above milldams when similar (shallow) depths were compared. However, when considering the deeper, wider terraces created by milldams, per area element masses were higher upstream. Upstream of milldams, sediment concentrations of Ca and Mg were highest in variably saturated surface sediments, while Fe and Mn were highest in deeper, continuously saturated, low-oxygen sediments. Principal component analysis revealed that data cluster most by milldam site, indicating the importance of other land-use histories, including high sediment Na concentrations linked to road salt runoff and high element concentrations reflective of fertilizer runoff, in controlling riparian biogeochemistry.
Conclusion: Overall, results highlight the combined importance of milldams (and associated influences on groundwater hydrology and sediment redox conditions) and other land-use legacies in influencing spatial patterns of bioavailable elements in riparian sediments.