Dammed for 200 years: Biogeochemical hotspots in riparian zones due to milldams and their evolution following dam removal
Abstract
Human activities have significantly altered natural ecosystems and landscapes worldwide and the imprints of these activities can persist for many years. In the eastern US, starting in the 1700s, thousands of milldams were constructed every few miles along creeks and tributaries. There are now active efforts to remove many of these dams creating marked effects on terrestrial-aquatic interactions. Milldams obstructed water movement, created millponds, and resulted in the accumulation of legacy sediments, dramatically altering stream and riparian ecosystems. At many locations, valley-spanning milldams resulted in new, taller and wider riparian terraces with fine-grained sediments and buried organic horizons. We propose that this unique three-dimensional mosaic of hydrologic, fluvial, and soil conditions results in hotspots of redox and biogeochemical activity in riparian zones upstream and downstream of the dams. The biogeochemical hotspots vary considerably for conditions before and after dam removal. The gradients of reducing and oxidizing environments in riparian soils strongly modify nitrogen forms and concentrations across the riparian zone. Furthermore, redox modification of nitrogen (N) processes (e.g., denitrification) by milldams also alters (enrichment or depletion) the stable isotope concentrations of nitrogen (15N) in soils. We hypothesize that N isotopic imprints could evolve following removal of the dam creating a legacy effect of dams in riparian soils. We evaluate these hypotheses using data from multiple sites in the mid-Atlantic with existing and recently removed milldams. Initial observations support the expected nitrate-N decrease in reducing riparian soil conditions (likely due to denitrification) upstream of milldams, but also reveal unexpected near-stream hotspots for ammonium-N, with ammonium-N concentrations orders of magnitude greater than nitrate-N in groundwaters and soils. Preliminary data also exhibit sharp changes in 15N for riparian soils following dam removal and riparian soil drainage. Understanding the spatial and temporal variability of N processes in milldam-affected riparian zones will not only help us better manage these ecosystems, but also provide important insights into how legacies of anthropogenic activities persist through time.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFM.H52B..01I