ABSTRACT:

Dam removals are on the increase across the US with Pennsylvania currently leading the nation. While most dam removals are driven by aquatic habitat and public safety considerations, we know little about how dam removals impact water quality and riparian zone processes. Dam removals decrease the stream base level, which results in dewatering of the riparian zone. We hypothesized that this dewatering of the riparian zone would increase nitrification and decrease denitrification, and thus result in nitrogen (N) leakage from riparian zones. This hypothesis was tested for a 1.5 m high milldam removal. Stream, soil water, and groundwater N concentrations were monitored over two years. Soil N concentrations and process rates and δ15N values were also determined. Denitrification rates and soil δ15N values in riparian sediments decreased supporting our hypothesis but no significant changes in nitrification were observed. While surficial soil water nitrate-N concentrations were high (median 4.5 mgN L-1), riparian groundwater nitrate-N values were low (median 0.09 mgN L-1), indicating that nitrate-N leakage was minimal. We attribute the low groundwater nitrate-N to denitrification losses at the lower, more dynamic, groundwater interface and/or dissimilatory nitrate reduction to ammonium (DNRA). Stream water nitrate-N concentrations were high (median 7.6 mgN L-1) and contrary to our dam-removal hypothesis displayed a watershed-wide decline that was attributed to regional hydrologic changes. This study provided important first insights on how dam removals could affect N cycle processes in riparian zones and its implications for water quality and watershed management.

ABSTRACT:

Spatiotemporal patterns of salt marsh lateral change vary along the Oregon coast, reflecting complex drivers of morphodynamics. To identify potential drivers of expansion/contraction, marsh edge position and area were measured from aerial imagery (~10 y resolution over ~80 y) in five Oregon estuaries with variable morphologies, fluvial sediment supplies, and relative sea level variation. In addition to highlighting the combined importance of these forcings, results suggest that intensive timber harvest in the mid-20th century coincident with increased precipitation during the wet phase of the Pacific Decadal Oscillation caused marsh expansion in all estuaries. More recently, rates of expansion decreased, sometimes giving way to net contraction. Although the exact reasons remain unclear, reduced timber harvest and improved logging methods are likely culprits. If these trends persist, continued salt marsh contraction is expected into the future along the Oregon coast especially under accelerated sea level rise.

ABSTRACT:

Large storms can erode, transport, and deposit substantial amounts of particulate ni-trogen (PN) in the fluvial network. The fate of this input and its consequence for water quality is poorly understood. This study investigated the transformation and leaching of PN using a 56-day incubation experiment with five PN sources: forest floor humus, upland mineral A hori-zon, stream bank, storm deposits, and stream bed. Experiments were subjected to two moisture regimes: continuously moist and dry-wet cycles. Sediment and porewater samples were collected through the incubation and analyzed for N and C species, and quantification of nitrifying and denitrifying genes (amoA, nirS, nirK). C and N rich watershed sources experienced decomposi-tion, mineralization, and nitrification and released large amounts of dissolved N, but the amount of N released varied by PN source and moisture regime. Drying and rewetting stimulated nitri-fication and suppressed denitrification in most PN sources. Storm deposits released large amounts of porewater N regardless of the moisture conditions, indicating that they can readily act as N sources under a variety of conditions. The inputs, processing, and leaching of large storm-driven PN inputs become increasingly important as the frequency and intensity of large storms is predicted to increase with global climate change.

ABSTRACT:

Riparian zones are key ecotones that buffer aquatic ecosystems through removal of nitrogen (N) via processes such as denitrification. How dams alter riparian N cycling and buffering capacity is however poorly understood. Here we hypothesize that elevated groundwater and anoxia due to the back-up of stream water above milldams may enhance denitrification. We assessed denitrification rates (using denitrification enzyme assays) and potential controlling factors in riparian sediments at various depths upstream and downstream of two relict US mid-Atlantic milldams. Denitrification was generally low and not different between upstream and downstream, although was greater per river km upstream considering deeper and wider geometries. Further, denitrification typically occurred in hydrodynamically variable, surface sediments where nitrate-N and organic matter were most concentrated. At depths below 1 m, both denitrification and nitrate-N decreased while ammonium-N concentrations substantially increased, indicating suppression of ammonium consumption or dissimilatory nitrate reduction to ammonium. These results suggest that denitrification occurs where dynamic groundwater levels result in higher rates of nitrification and mineralization, while another N process that produces ammonium-N competes with denitrification for limited nitrate-N at deeper, more stagnant depths. Additionally, nitrate-N-rich runoff from agricultural areas increases denitrification rates, while Na-rich runoff due to road salt application limits denitrification, highlighting the importance of synergistic interactions between land-use legacies. Ultimately, while it is unclear whether relict milldams are sources of N, limited denitrification rates indicate that they are not always effective sinks; thus, milldam removal – especially accompanied by removal of ammonium-N rich sediment terraces – may improve riparian N buffering.

ABSTRACT:

How milldams alter riparian hydrologic and groundwater mixing regimes is not well understood. Understanding the effects of milldams and their legacies on riparian hydrology is key to assessing riparian pollution buffering potential and for making appropriate watershed management decisions. We examined the spatiotemporal effects of milldams on groundwater gradients, flow directions, and mixing regime for two dammed sites on Chiques Creek, Pennsylvania (2.4 m tall milldam), and Christina River, Delaware (4 m tall dam), USA. Riparian groundwater levels were recorded every 30 minutes for multiple wells and transects. Groundwater mixing regime was characterized using 30-minute specific conductivity data and selected chemical tracers measured monthly for about two years. Three distinct regimes were identified for riparian groundwaters – wet, dry, and storm. Riparian groundwater gradients above the dam were low but were typically from the riparian zone to the stream. These flow directions were reversed (stream to riparian) during dry periods due to riparian evapotranspiration losses and during peak stream flows. Longitudinal (parallel to the stream) riparian flow gradients and directions also varied across the hydrologic regimes. Groundwater mixing varied spatially and temporally between storms and seasons. Near-stream groundwater was poorly flushed or mixed during storms whereas that in the adjacent swales revealed greater mixing. This differential groundwater behavior was attributed to milldam legacies that include: berm and swale topography that influenced the routing of surface waters, varying riparian legacy sediment depths and hydraulic conductivities, evapotranspiration losses from riparian vegetation, and runoff input from adjoining roads.

ABSTRACT:

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.

ABSTRACT:

The compounding effects of anthropogenic legacies for environmental pollution are significant, but not well understood. Here, we show that centennial-scale legacies of milldams and decadal-scale legacies of road salt salinization interact in unexpected ways to produce hot spots of nitrogen (N) in riparian zones. Riparian groundwater and stream water concentrations upstream of two mid-Atlantic (Pennsylvania and Delaware) milldams, 2.4 and 4 m tall, were sampled over a two year period. Clay and silt rich legacy sediments with low hydraulic conductivity, stagnant and poorly-mixed hydrologic conditions, and persistent hypoxia in riparian sediments upstream of milldams produced a unique biogeochemical gradient with nitrate removal via denitrification at the upland riparian edge and ammonium-N accumulation in near-stream sediments and groundwaters. Riparian groundwater ammonium-N concentrations upstream of the milldams ranged from 0.006 to 30.6 mg L-1 while soil-bound values were 0.11- 456 mg kg-1. We attribute the elevated ammonium concentrations to ammonification with suppression of nitrification and/or dissimilatory nitrate reduction to ammonium (DNRA). Sodium inputs to riparian groundwater (25-1504 mg L-1) from road salts may further enhance DNRA and ammonium production and displace sorbed soil ammonium-N into groundwaters. This study suggests that legacies of milldams and road salts may undercut the N buffering capacity of riparian zones and need to be considered in riparian buffer assessments, watershed management plans, and dam removal decisions. Given the widespread existence of dams and other barriers and the ubiquitous use of road salt, the potential for this synergistic N pollution is significant.

ABSTRACT:

Stream, floodplain, and wetland restorations have enhanced water quality and to some extent ecological function; however, soil health is prioritized infrequently in restoration planning or monitoring. Buried, historic, hydric soils – common across U.S. mid-Atlantic valley bottoms beneath legacy sediments – are not included in most floodplain restoration designs, though they may retain favorable biogeochemical characteristics and host legacy microbial communities that could support ecosystem recovery if exhumed and preserved. To assess the efficacy of including historic hydric soils in floodplain restoration, we characterized pre-Colonial wetland soils buried below legacy sediments and now exposed along incised streambanks across the mid-Atlantic. We compared carbon (C) and nitrogen (N) contents; C:N ratios; nitrate-N and ammonium-N concentrations; denitrification rates; functional genes for denitrification (nosZ) and nitrification (amoA for AoA+AoB); and phospholipid fatty acid (PLFA) biomasses of historic wetland soils with contemporary wetland soils before and after a one-year incubation in a recently restored floodplain. Compared to modern wetland soils, historic hydric soils that are now buried by legacy sediment are less nutrient-rich, have fewer functional genes for and lower rates of denitrification, and possess significantly less microbial biomass. Following the one-year incubation, many of these concentrations, rates, and gene counts increased in historic soils, though incubated modern soils showed greater improvements. Ultimately, our results suggest that while inclusion of historic, hydric soils and their legacy microbiomes is valuable in floodplain restoration, the recovery of historic, hydric soils is slow and attainment of restoration goals, such as increased denitrification, may require multiple years.

ABSTRACT:

Blue carbon ecosystems buffer climate change via sediment carbon capture up to two orders of magnitude faster than terrestrial ecosystems on a per-area basis, gaining elevation and mitigating sea level rise in the process. Carbon sequestration and accretion estimates share a common methodology, whereby dry masses are converted to volume using self-packing densities. However, our analysis of >23,300 tidal marsh data points from the Coastal Carbon Atlas shows that these methods overestimate organic carbon contribution to long-term sequestration and accretion because they incorporate both dissolved and mineral-associated organic matter. Dissolved and mineral-associated organic matter in surficial uncompacted (0-25 cm) sediments is 36% greater than deeper compacted sediments, suggesting that some of the carbon thought to be sequestered is lost, most likely through porewater flushing, sediment autocompaction, and decomposition, and does not contribute to long-term carbon storage. Neither dissolved nor mineral-associated organic matter contribute to sediment volume, thus the volumetric budgets underlying estimates of organic matter contribution to predicted marsh resilience are inflated by up to 380% in the top 25 cm. Combined, we demonstrate that traditional methods, which are often applied across blue carbon ecosystems, overestimate organic matter contributions to tidal marsh carbon stocks and accretion.