Influence of relict milldams on riparian sediment biogeochemistry

PurposeRiparian zones are important modifiers of nutrient flux between terrestrial and aquatic ecosystems. 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.MethodsWe 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 (Mehlich-3 extractable 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.ResultsElement concentrations were not clearly correlated with grain size or organic matter content and, although generally higher, were not significantly more concentrated in upstream riparian sediments when similar (shallow, variably saturated) depths were compared. Pronounced differences were observed: upstream of milldams, sediment concentrations of Ca and Mg were highest in variably saturated shallow sediments, while Fe and Mn were highest in deeper, continuously saturated, low-oxygen sediments. Additionally, data was significantly different by milldam site, a result of differences in land-use histories (e.g., road salt and fertilizer application/runoff) and dominant bedrock geology.ConclusionOverall, results highlight the combined importance of milldams (and associated influences on groundwater hydrology and sediment redox conditions) and external drivers (other land-use legacies and bedrock geology) in influencing spatial patterns of bioavailable elements in riparian sediments.

Automated summary

Riparian zones play a crucial role in regulating nutrient flux between terrestrial and aquatic ecosystems. Dams, however, can significantly alter the characteristics of riparian zones, trapping sediment and creating low-oxygen conditions, thereby affecting sediment biogeochemistry in ways that are not yet fully understood.