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Feedbacks between sediment and plants in the submersed aquatic vegetation (SAV) beds of the Susquehanna Flats help modulate sediment delivery into the upper Chesapeake Bay from its major tributary (Susquehanna River). Recent modeling work has shown that SAV can steer the river plume, directly controlling sediment transport and fate. However, transport mechanisms likely differ between flood and non‐flood conditions and depend on event timing. An opportunity to evaluate these insights in the field occurred with several flood events in summer 2018. Sediment and biomass samples were collected after the flood in the large, continuous main SAV bed of the Flats and in smaller patches to the west of the main bed to evaluate the effect of SAV bed size (width). Sediment characteristics (grain size, organic content) and deposition rates were compared to previous observations during non‐flood conditions in the main bed in 2014–2015 and in the smaller patches in 2019. Overall, sediment deposition rates could be predicted by an empirical model including sediment load, vegetation biomass, and SAV bed width, reflecting the role of vegetation in steering the Susquehanna River plume and modulating sediment input to the upper Chesapeake Bay. Understanding spatiotemporal patterns of sedimentation is essential for realistic estimates of sediment and nutrient (especially nitrogen and carbon) storage in freshwater SAV systems, which have not received as much attention as their saltwater counterparts. [ABSTRACT FROM AUTHOR]

Shallow coastal muddy sediments are important for global nutrient cycling and carbon storage and provide a habitat for diverse communities of organisms. Bottom shear stress from physical disturbances such as storms and hurricanes resuspend coastal muds, disrupting their cohesive structure. How biological vs. physical processes affect reconsolidation of physically disturbed natural muds, however, is poorly understood. We compare the recovery of sediment properties related to compaction and cohesion of muds following physical disturbance. We hypothesized that recovery of cohesion would take longer than compaction and that changes in surface and subsurface cohesion would occur on different timescales. We collected muddy sediment cores and resuspended the top 5 cm in the lab to simulate storm disturbance. At several timepoints following resuspension (1–30 d), we measured sediment properties providing metrics of compaction (sediment height, porosity, grain size distribution, sound speed) and of surface and subsurface cohesion (erodibility, fracture behavior, exopolymeric substances [EPS]). Compaction and cohesion increased rapidly over the first 1–2 d after resuspension and continued to slowly increase for 2 weeks. Between 2 and 4 weeks, however, subsurface cohesion increased and turbidity decreased, consistent with sediment restabilization. In contrast, subsurface compaction decreased and eroded mass increased, suggesting destabilization. We attribute this apparent destabilization to delayed activity of small‐bodied infauna. Surprisingly, EPS concentration did not explain cohesion changes. These results highlight the importance of including biological parameters when predicting the recovery of sediment structure following a physical disturbance. This work has implications for understanding coastal sediment transport dynamics in frequently disturbed sediments. [ABSTRACT FROM AUTHOR]

Methane (CH4) produced in anoxic sediments plays a significant role in the carbon economy of many lakes and reservoirs. CH4 released from sediments first crosses the bottom boundary layer (BBL), the layer of water overlying the lakebed where currents are slowed by friction with the sediments below. Physical and biogeochemical conditions in the BBL, which can fluctuate hourly to daily with basin‐wide internal waves (seiches), likely influence CH4 transport from sediments into the hypolimnion. In this study, we estimated CH4 fluxes across the BBL of a eutrophic lake using a novel in situ flux gradient approach adapted from marine applications. For 2–6 h periods throughout the spring and summer, we estimated CH4 fluxes across the BBL using simultaneous measurements of CH4 concentrations, turbulent mixing, and thermal stratification. Sub‐daily variation in CH4 fluxes was high, and CH4 fluxes sometimes changed several‐fold within hours. These rapid shifts in BBL fluxes were likely influenced by fluctuations in seiche‐driven variations in the intensity of BBL turbulent mixing. Fluxes increased from spring to summer, concurrent with the development of lake stratification, and fueled an accumulation of CH4 below the thermocline. Throughout the summer, CH4 flux across the BBL exceeded CH4 accumulation below the thermocline, suggesting significant methanotrophy in the hypolimnion, consistent with incubation‐based oxidation rates. Our results are the first to demonstrate sub‐daily and seasonal variability in the timing and magnitude of CH4 fluxes within a lake BBL, and highlight a need to quantify such variability in other lentic systems. [ABSTRACT FROM AUTHOR]

Seagrass meadows are important carbon sinks in the global coastal carbon cycle yet are also among the most rapidly declining marine habitats. Their ability to sequester carbon depends on flow–sediment–vegetation interactions that facilitate net deposition, as well as high rates of primary production. However, the effects of seasonal and episodic variations in seagrass density on net sediment and carbon accumulation have not been well quantified. Understanding these dynamics provides insight into how carbon accumulation in seagrass meadows responds to disturbance events and climate change. Here, we apply a spatially resolved sediment transport model that includes coupling of seagrass effects on flow, waves, and sediment resuspension in a seagrass meadow to quantify seasonal rates of sediment and carbon accumulation in the meadow. Our results show that organic carbon accumulation rates were largely determined by sediment accumulation and that they both changed non‐linearly as a function of seagrass shoot density. While seagrass meadows effectively trapped sediment at meadow edges during spring–summer growth seasons, during winter senescence low‐density meadows (< 160 shoots m−2) were erosional with rates sensitive to density. Small variations in winter densities resulted in large changes in annual sediment and carbon accumulation in the meadow; meadow‐scale (hundreds of square meters) summer seagrass dieback due to marine heatwaves can result in annual erosion and carbon loss. Our findings highlight the strong temporal and spatial variability in sediment accumulation within seagrass meadows and the implications for annual sediment carbon burial rates and the resilience of seagrass carbon stocks under future climate change. [ABSTRACT FROM AUTHOR]

Sedimentary denitrification accounts for more than half of the global marine fixed nitrogen loss, but this removal is often restricted by nitrate diffusive supply in sediments. The results of this study showed that foraminiferal nitrate transport may supply nitrate below the nitrate penetration depth and promote nitrogen removal in sediments in contact with oxic overlying water. Six dominant foraminiferal species in the Yellow Sea sediments accumulated high concentrations of intracellular nitrate (25–68 mM), which is 1000 times higher than the maximum pore water nitrate concentration. Notably, this is the first time that Protelphidium tuberculatum, the most dominant species at almost all stations, is reported to have elevated intracellular nitrate. P. tuberculatum contributes 45% to 92% of the foraminiferal intracellular nitrate pools and plays an important role in foraminiferal nitrate transport. The potential foraminiferal denitrification rates below the nitrate penetration depth induced by foraminiferal nitrate transport ranged from 4 to 117 μmol N m−2 d−1 and contributed 3% to 81% of sedimentary denitrification. The intensity of foraminiferal nitrate transport showed a negative correlation with the nitrate penetration depth and positively correlated with the chlorophyll a (Chl a) and chloroplastic pigment equivalent concentrations, which indicate that shallower redox depth (nitrate penetration depth) and increased food supply (Chl a, chloroplastic pigment equivalent) may promote foraminiferal nitrate transport. The trophic oxygen model was modified by adding the pore water nitrate to explain foraminiferal nitrate transport in various marine environments, and this modified trophic oxygen model predicts the observed high foraminiferal nitrate transport intensity. [ABSTRACT FROM AUTHOR]

Coastal vegetation is widely attributed to stabilize sediment. While most studies focused on how canopy causes flow reduction and thereby affects sediment dynamics, the role of roots and rhizomes on stabilizing the surface sediment has been less well studied. This study aims to quantify interactions between above‐ and belowground biomass of eelgrass (i.e., living Zostera marina plants and mimics) with surface sediment erosion (i.e., bed load and suspended load), under different hydrodynamic forcing that was created using a wave flume. Belowground biomass played an important role preventing bed‐load erosion, by roughly halving the amount of sediment transported after being exposed to maximal orbital velocities of 27 cm s−1, with and without canopy. Surprisingly, for suspended sediment transport, we found opposite effects. In the presence of eelgrass, the critical erosion threshold started at lower velocities than on bare sediment, including sand and mud treatments. Moreover, in muddy systems, such resuspension reduced the light level below the minimum requirement of Z. marina. This surprising result for sediment resuspension was ascribed to a too small eelgrass patch for reducing waves but rather showing enhanced turbulence and scouring at meadow edges. Overall, we conclude that the conservation of the existent eelgrass meadows with developed roots and rhizomes is important for the sediment stabilization and the meadow scale should be taken into account to decrease sediment resuspension. [ABSTRACT FROM AUTHOR]

Wind‐driven turbidity plumes frequently occur in the western arm of Lake Superior and may represent a significant cross‐shelf transport mechanism for sediment, nutrient, and biota. Here, we characterize a plume that formed in late April 2016 using observations from in situ sensors and remote sensing imagery, and estimate the volume of cross‐shelf transport using both the observations and an idealized analytical model of plume formation. The steady‐state, barotropic model is used to determine a relationship between the intensity and duration of a wind event and the volume of water transported from nearshore to offshore during the event. The model transport is the result of nearshore flow in the direction of the wind and a pressure‐gradient‐driven counter flow in the deeper offshore waters, consistent with observations. The volume of offshore transport associated with the 2016 plume is estimated by both methods to have been on the order of 1010 m3. Analysis of similar events from 2008 to 2016 shows a strong relationship between specific wind impulse and plume volume. Differences in the intensity and duration of individual events as well as ice cover, which prevents plume formation, lead to interannual variability of offshore transport ranging over an order of magnitude and illustrates how wind‐driven processes may contribute to interannual variability of ecosystem functioning. [ABSTRACT FROM AUTHOR]

Our study aims to enhance process understanding of the long-term (decadal and longer) cyclic marsh dynamics by identifying the mechanisms that translate large-scale physical forcing in the system into vegetation change, in particular ( i) the initiation of lateral erosion on an expanding marsh, and ( ii) the control of seedling establishment in front of an eroding marsh-cliff. Short-term sediment dynamics (i.e., seasonal and shorter changes in sediment elevation) at the mudflat causes variation in mudflat elevation over time ( δzTF). The resulting difference in elevation between the tidal flat and adjacent marsh (Δ Z) initiates lateral marsh erosion. Marsh erosion rate was found to depend on sediment type and to increase with increasing Δ Z and hydrodynamic exposure. Laboratory and field experiments revealed that seedling establishment was negatively impacted by an increasing δzTF. As the amplitude of δzTF increases towards the channel, expanding marshes become more prone to lateral erosion the further they extend on a tidal flat, and the chance for seedlings to establish increases with the distance that marsh has eroded back towards the land. This process-based understanding, showing the role of sediment dynamics as explanatory factor for marsh cyclicity, is important for protecting and restoring valuable marsh ecosystems. Overall, our experiments emphasize the need for understanding the connections between neighbouring ecosystems such as mudflat and salt marsh. [ABSTRACT FROM AUTHOR]

Oxygen availability restricts groundfish to the oxygenated, shallow margins of Saanich Inlet, an intermittently anoxic fjord in British Columbia, Canada. New and previously reported 210Pb measurements in sediment cores compared with flux data from sediment traps indicate major focusing of sediments from the oxygenated margins to the anoxic basin seafloor. We present environmental and experimental evidence that groundfish activity in the margins is the major contributor to this focusing. Fine particles resuspended by groundfish are advected offshore by weak bottom currents, eventually settling in the anoxic basin. Transmittance and sediment trap data from the water column show that this transport process maintains an intermediate nepheloid layer (INL) in the center of the Inlet. This INL is located above the redox interface and is unrelated to water density shifts in the water column. We propose that this INL is shaped by the distribution of groundfish (as resuspension sources) along the slope and hence by oxygen availability to these fish. We support this conclusion with a conceptual model of the resuspension and offshore transport of sediment. This fish-induced transport mechanism for sediments is likely to enhance organic matter decomposition in oxygenated sediments and its sequestration in anoxic seafloors. [ABSTRACT FROM AUTHOR]

We studied the impact of the removal of a 3-m dam on sediment and nutrient export. In the year after dam removal, sediment and nitrogen (N) export increased by an order of magnitude over the previous 2-yr mean. Longitudinal surface water samples were collected, sediments were cored, and the channel was surveyed during different seasons to determine the mechanisms driving sediment and nutrient dynamics in the recovering system. The majority of sediment transport occurred in pools and in the lowest 50 m of the 620-m restored reach. Phosphate export occurred primarily during large storms, with the restored reach acting as a phosphate sink during most flow conditions. The majority of surface water N originated from areas within the sediment wedge that had high extractable N concentrations (average NH4-N - 50 µg g-1 and NO3-N = 38 µg g-1) and dried out on a seasonal basis. Near the former dam site, year-round water saturation apparently inhibited nitrification and export of N as nitrate. This wetland area was the only portion of the restored reach that was an ammonium sink. After dam removal, N leaching from sediments occurred in autumn 2003 and again during the subsequent autumn, suggesting that N leaching from reservoir sediments is largely a seasonal process that may affect downstream aquatic ecosystems for a number of years. [ABSTRACT FROM AUTHOR]

Sedimentation poses a significant threat to stream ecosystems throughout the world, and increases in bedload can be especially detrimental to benthic communities. To examine how increased bedload affects algal- and detrital-based stream communities, we manipulated sediment (via daily sediment addition to 0.25-m² areas of stream bottom) and top-down effects of macro-consumers (fishes and crayfish, via electric exclusion) in two factorial experiments, one using tiles and one using leaf packs as sampling substrates. Sediment addition had significant effects on both algal-and detrital-based stream benthic communities, most notably via the alteration of macroconsumer-mediated biotic interactions; these effects largely were due to sediment transport across the stream bottom, rather than sediment deposition. In the tile experiment, macroconsumers reduced total insect biomass and biomass of several dominant insect taxa under ambient sediment conditions. Sediment addition eliminated all macroconsumer effects except their reduction of chironomid biomass. In the leaf pack experiment, sediment addition eliminated macroconsumer effects on fungal accumulation rates; in general, however, leaf packs were not as affected by sediment addition as tiles. Direct effects of sediment addition were minimal in both experiments: algal composition was altered on tiles, while dipteran predator biomass tended to decline in leaf packs. These experiments demonstrate that small, environmentally realistic increases in bedload affect benthic communities, primarily via the alteration of macroconsumer effects. Although indirect effects of sedimentation have been examined less frequently than direct biotic responses, this study demonstrates the importance of sediment regime in determining the outcome of fish-and crayfish-mediated species interactions. [ABSTRACT FROM AUTHOR]

Presents a study concerning global estimates of enhanced solute transport in marine sediments. Data and methods; Enhanced oxygen uptake at the seafloor; Patterns in enhanced transport.

The patterns of underwater acoustic signals, called soundscapes, were characterized at the river segment scale. A hydrophone was mounted onto the frame of an inflatable multihull raft and held just below the water's surface while floating for 5-24 km to continuously record acoustic signals along the longitudinal axis of the main channel (thalweg) of five hydro-geomorphologically different river segments in Italy, Switzerland, and the United States of America. The river segments could be clearly distinguished based on sound pressure level (SPL), sound variability, and the spatial organization of the acoustic signal (10 octave bands: 0.0315-16 kHz). The spatial soundscape diversity between river segments was most likely related to the organization of turbulence along the segment length (distance between rapids) and to sound source (hydraulic turbulence or local sediment transport). Hydraulically and morphologically heterogeneous segments revealed more complex soundscapes than more homogeneous ones. Higher flow levels resulted in higher SPLs over most frequency bands. The acoustic variability of single octave bands increased from base to intermediate flow, while it decreased from intermediate to bankfull flow. The pulsating sound produced by breaking and reforming turbulent waves on flow obstacles was associated with SPL peaks and high acoustic variability in midrange frequency bands (0.063-0.5 kHz), whereas high-frequency SPLs (1-16 kHz) were related to particle collisions during streambed sediment transport. Underwater soundscapes provide an independent measure to quantify habitat heterogeneity and spatial habitat organization that can be applied to river corridors at the segment scale. [ABSTRACT FROM AUTHOR]