The effects of low-head milldams on stream nitrogen processes
Date
2022
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Publisher
University of Delaware
Abstract
Stream fragmentation is prevalent throughout the US mid-Atlantic due to historic low-head milldams altering stream hydrology and biogeochemical processes. While numerous dams have breached or been purposefully removed, many remain creating slower flows and enhancing organic matter (OM) and sediment deposition above the dam. These factors promote anaerobic nutrient processing including denitrification which may alter overall nitrogen (N) removal across the watershed. We studied two streams with existing low-head milldams (Christina River in Newark, DE; drainage area of 50.7 km2 and Chiques Creek in Manheim, PA; drainage area of 127 km2, dam heights 4.0 & 2.4 m, respectively) over two years. We expected streambed sediment denitrification rates to be higher upstream of the dam compared to below and to vary seasonally, with highest rates in summer. Denitrification enzyme assays (DEA), net mineralization and nitrification, sediment particle size, and % OM, were determined from streambed sediments collected seasonally. Monthly stream grab samples were analyzed for total nitrogen (TN), nitrate (NO3-N), ammonium (NH4-N), and dissolved organic carbon. Contrary to expectations, DEA rates and nutrient concentrations did not differ above and below the dams, and highest denitrification rates occurred during autumn at Christina and winter at Chiques. Streambed sediment was dominated by sand at both sites, which has less surface area for denitrification to occur compared to smaller silt or clay particle sizes. A multilinear regression model showed OM and sediment NH4-N accounted for 33% of variability in denitrification rates. This study indicated the two milldams did not have a significant effect on streambed denitrification or stream water N concentrations, possibly due to full sediment capacity above the impoundments reducing denitrification potential. These findings will help watershed managers make informed decisions on dam removals and potential consequences for N exports.
Description
Keywords
Biogeochemical processes, Dams, Denitrification, In-stream, Nitrogen, Stream health