Salt marshes are among the most productive ecosystems on Earth, providing nurseries for fish species and shelter and food for endangered birds. Salt marshes also mitigate the impacts of hurricanes ...and tsunamis, and sequester large volumes of carbon in their peat soil. Understanding the mechanisms responsible for marsh stability or deterioration is therefore a key issue for society. Sea level rise is often viewed as the main driver of salt marsh deterioration. However, while salt marshes can reach equilibrium in the vertical direction, they are inherently unstable in the horizontal direction. Marsh expansion driven by sediment supply rarely matches lateral erosion by waves, creating a dynamic landscape. Recent results show that marsh collapse can occur in the absence of sea level rise if the rate at which sediment is eroded at marsh boundaries is higher than the input of sediment from nearby rivers or from the continental shelf. We propose that the horizontal dynamics and related sediment fluxes are key factors determining the survival of salt marshes. Only a complete sediment budget between salt marshes and nearby tidal flats can determine the fate of marshes at any given location, with sea level rise being only one among many external drivers. Ancient Venetians understood this dynamic very well. They manipulated the supply of sediment to the Venice lagoon, Italy, in order to control the long-term evolution of the intertidal landscape.
Shallow coastal ecosystems are threatened by marine heatwaves, but few long-term records exist to quantify these heatwaves. Here, 40-year records of measured water temperature were constructed for a ...site in a system of shallow bays with documented heatwave impacts and a nearby ocean site; available gridded sea-surface temperature datasets in the region were also examined. Water temperatures at both sites increased significantly though bay temperatures were consistently 3-4°C hotter in summer and colder in winter and were more variable overall, differences not captured in high-resolution gridded sea-surface temperature datasets. There was considerable overlap in heatwave events at the coastal bay and ocean sites. Annual heatwave exposure was similar and significantly increased at both sites while annual heatwave intensity was significantly higher at the bay site owing to the high variance of the daily temperature anomaly there. Event frequency at both sites increased at a rate of about 1 event/decade. Future simulations indicate all heatwave metrics increase, as do days above 28°C, a heat stress threshold for seagrass. Ocean temperatures on the U.S. mid-Atlantic margin have rarely exceeded this threshold, while summer bay temperatures commonly do, allowing ocean exchange with coastal bays to provide thermal relief to bay ecosystems. This will have changed by 2100, creating a thermal environment that threatens seagrass communities in these systems. Documenting such change requires development of long-term water temperature records in more shallow coastal systems.
Salt marshes are recognized as valuable resources that are threatened by climate change and human activities. Better management and planning for these ecosystems will depend on understanding which ...marshes are most vulnerable, what is driving their change, and what their future trajectory is likely to be. Both observations and models have provided inconsistent answers to these questions, likely in part because of comparisons among sites and or models that differ significantly in their characteristics and processes. Some of these differences almost certainly arise from processes that are not fully accounted for in marsh morphodynamic models. Here, we review distinguishing properties of marshes, important processes missing from many morphodynamic models, and key measurements missing from many observational studies. We then suggest some comparisons between models and observations that will provide critical tests and insights to improve our ability to forecast future change in these coastal landscapes.
Storm surge has the potential to significantly increase suspended sediment flux to microtidal marshes. However, the overall effects of storm surge on microtidal marsh deposition have not been well ...quantified, with most modeling studies focusing on regular (astronomical) tidal flooding. Here we applied the Delft3D model to a microtidal bay‐marsh complex in Hog Bay, Virginia to quantify the contributions of storm surge to marsh deposition. We validated the model using spatially distributed hydrodynamic and suspended sediment data collected from the site and ran model simulations under different storm surge conditions with/without storm‐driven water level changes. Our results show that episodic storm surge events occurred 5% of the time at our study site, but contributed 40% of marsh deposition during 2009–2020. Our simulations illustrate that while wind‐driven waves control sediment resuspension on tidal flats, marsh deposition during storms was largely determined by tidal inundation associated with storm‐driven water levels. A moderate storm surge event can double sediment flux to most marshes around the bay and deliver more sediment to the marsh interior compared to simulations that include wind waves but not storm surge variations in water levels. Simulations of bay and marsh response to different storm surge events with varying magnitude of storm surge intensity reveal that total marsh deposition around the bay increased linearly with storm surge intensity, suggesting that future changes to storm magnitude and/or frequency would have significant implications for sediment supply to marshes at our study site.
Plain Language Summary
Storm surge is a rise of water level beyond normal tidal levels caused by a storm. It can result in longer and deeper inundation on the marsh platform and potentially provide more sediment for microtidal marshes (tidal range <2 m) to gain elevation at a rate comparable to sea level rise. However, the overall importance of storm surge to sediment deposition on these marshes is not well understood. Here we used a well‐tested coastal model in a shallow microtidal bay fringed by intertidal salt marshes to run model simulations for different storm surge events with varying magnitude of storm surge intensity to determine the contributions of storm surge to marsh deposition. Our simulation results show that infrequently occurring storm surge events contributed ∼40% of marsh deposition at our study site and that total mass of marsh deposition around the bay increased linearly with the magnitude of storm surge events. Our findings have significant implications for predicting marsh change, especially if we experience a stormier future climate. Modeling studies that do not resolve storm surge effects on marsh deposition will likely underestimate rates of marsh elevation change.
Key Points
A numerical model was used to quantify the contributions of storm surge on marsh deposition in a microtidal bay‐marsh complex
Storm surge events occurred 5% of the time, but contributed 40% of marsh deposition during 2009–2020
Marshes will likely be supplied with more sediment during storms if storm magnitudes and/or frequencies increase in the future
Salt marshes can experience a significant land loss through erosion and retreat of their perimeter edges. Rates of shoreline change between 1957 and 2007 were determined for four salt marshes in a ...Virginia coastal bay using aerial photographs and the Digital Shoreline Analysis System (DSAS). High average rates of lateral erosion of 1.0–1.6 m year⁻¹were found at three marshes, while the edge of the fourth marsh, along the mainland edge of the bay, remained stable. Erosion rates were temporally consistent during the 50-year period at the three eroding sites, although there was a significant spatial variation in rates of change along the length of the edges at these sites. A simple parametric wave model and the SWAN (Simulating WAves Nearshore) spectral wave model were used to calculate incident wave energy flux along the marsh boundaries at each of the sites. Values of wave energy flux agreed fairly well between the two models but are sensitive to the manner in which wave energy flux is calculated. A stronger relationship was found between wave energy flux and volumetric erosion rates along the marsh edges than with lateral erosion rates. This is an important consideration when examining the effects of future sea level rise on marsh loss.
Near-bed wave orbital velocities and shear stresses are important parameters in many sediment-transport and hydrodynamic models of the coastal ocean, estuaries, and lakes. Simple methods for ...estimating bottom orbital velocities from surface-wave statistics such as significant wave height and peak period often are inaccurate except in very shallow water. This paper briefly reviews approaches for estimating wave-generated bottom orbital velocities from near-bed velocity data, surface-wave spectra, and surface-wave parameters; MATLAB code for each approach is provided. Aspects of this problem have been discussed elsewhere. We add to this work by providing a method for using a general form of the parametric surface-wave spectrum to estimate bottom orbital velocity from significant wave height and peak period, investigating effects of spectral shape on bottom orbital velocity, comparing methods for calculating bottom orbital velocity against values determined from near-bed velocity measurements at two sites on the US east and west coasts, and considering the optimal representation of bottom orbital velocity for calculations of near-bed processes. Bottom orbital velocities calculated using near-bed velocity data, measured wave spectra, and parametric spectra for a site on the northern California shelf and one in the mid-Atlantic Bight compare quite well and are relatively insensitive to spectral shape except when bimodal waves are present with maximum energy at the higher-frequency peak. These conditions, which are most likely to occur at times when bottom orbital velocities are small, can be identified with our method as cases where the measured wave statistics are inconsistent with Donelan's modified form of the Joint North Sea Wave Project (JONSWAP) spectrum. We define the “effective” forcing for wave-driven, near-bed processes as the product of the magnitude of forcing times its probability of occurrence, and conclude that different bottom orbital velocity statistics may be appropriate for different problems.
Accelerating losses of seagrass meadows motivate the restoration of these highly productive and beneficial ecosystems. Understanding how environmental parameters including depth and temperature ...affect restoration trajectories through time is key to conserving and restoring seagrass meadows. We used a long-term (12-year), landscape-scale experiment to test the effect of depth on eelgrass (Zostera marina) restoration success and resilience to a marine heat wave (MHW) disturbance. We found that depth was a critical determinant of seagrass restoration success, with no long-term success at sites deeper than 1.5 m below mean sea level (MSL) or shallower than – 0.8 m MSL. Seeds germinated below – 1.5 m MSL, but shoots did not persist, confirming earlier predictions from a hydrodynamic-vegetation model. Depth was also a significant predictor of seagrass resilience following MHW disturbance. Our results suggest that areas of restored seagrass that are resilient to temperature stress exist across an intermediate depth range, excluding the shallowest and deepest portions of the full habitable depth range for restored seagrass. Over the next decades, sea-level rise will likely affect both the habitable area and the resilient area, available for seagrass restoration. However, seagrass enhancement of sediment accretion may at least partially offset sea-level rise rates. As ocean temperatures warm and MHWs occur more frequently, the resilience of seagrass meadows to temperature stress will be of increasing concern. These results suggest that depth is a critical parameter that will help determine what areas are most resilient and therefore most suitable for conservation and restoration.
Both submerged aquatic vegetation and salt marsh are important coastal ecosystems known for their effectiveness in sediment trapping and carbon burial. However, their proximity can lead to ...competition for limited sediment resources, potentially compromising their capacity to facilitate sufficient sediment deposition to withstand sea‐level rise. Here we applied the Delft3D flow and sediment transport model in a shallow coastal bay to assess how seasonal variations in seagrass density and storms modulate sediment retention and connectivity between subtidal flats and intertidal marsh. Results show that submerged seagrass meadows acted as temporary storage for fine sediment and altered the timing of sediment transport to an adjacent marsh through seasonal growth cycles. When seagrass occupied subtidal flats, sediment flux to the marsh was controlled by seasonal seagrass density variations, with peak fluxes during winter senescence. In contrast, wave‐induced sediment resuspension on the flats was the major contributor for marsh sediment input in the simulation with no seagrass, with peak storm‐driven fluxes in summer/autumn. Overall, seagrass meadows significantly increased annual sediment accumulation on the vegetated subtidal flats by tenfold, while reducing annual marsh sediment flux and edge erosion by 20%. Seagrass meadows and marsh were both able to maintain a sediment deposition rate comparable to the rapid sea‐level rise at the study site despite the absence of terrestrial sediment sources. Our findings highlight the strong seasonal control subtidal aquatic vegetation has in the vertical and horizontal dynamics of tidal flat‐marsh systems and provide insights that can inform wetland restoration and management strategies in similar systems.
Plain Language Summary
Submerged aquatic vegetation and salt marsh are valuable coastal ecosystems known for their effectiveness in sediment trapping. However, their proximity can lead to competition for limited sediment resources, compromising their capacity to vertically accrete and withstand sea‐level rise. Here we used a well‐tested coastal model in a shallow bay hosting the world’s largest seagrass restoration site to examine the seasonal sediment dynamics between a submerged seagrass meadow and an adjacent intertidal marsh. We find that seagrass changed the timing of sediment transport to the marsh through seasonal growth cycles and acted as a sediment source for the marsh during low‐density periods in winter. On an annual scale, the seagrass meadow significantly increased sediment accumulation on the vegetated subtidal flats by tenfold, while concurrently reducing marsh sediment supply and edge erosion by 20%. Both ecosystems coped well with rapid sea‐level rise at the site, even in the absence of terrestrial sediment sources.
Key Points
Sediment retention in seagrass meadows increased annual deposition on vegetated subtidal flats tenfold compared to bare‐flat conditions
Seagrass meadows were a sediment source for the marsh during winter senescence while limiting transport to the marsh during growing seasons
Seagrass and marsh accumulated sediment at rates comparable to sea‐level rise at the site despite the lack of terrestrial sediment sources
Oyster reef restoration in shallow estuarine environments has been thought to have the potential to provide shoreline protection as well as oyster habitat. This study was designed to address the ...question of how effective oyster reefs are at attenuating wave energy in shallow coastal bays. Measurements were made of waves on both sides of four restored intertidal oyster reefs and at a control site with no reef; mean water depths ranged from 0.9 to 1.3 m. The reefs differed in composition and position relative to the shoreline, but all had reef crest elevations between 0.3 and 0.5 m below mean sea level. Differences in wave heights between the exposed/sheltered sides and upwind/downwind sides of the reefs were used to quantify the effects of the reefs on waves under varying tidal and wind conditions. All four reefs were able to reduce wave heights by an average of 30–50% for water depths of 0.5–1.0 m (bracketing the heights of reef crests) and 0–20% for water depths of 1.0–1.5 m (reef crests > 0.25 m below the water surface). For water depths greater than 1.5 m, there was < 10% change in wave heights. In contrast, there was no average decrease in wave height from the more seaward (exposed) to the more landward wave gauge at the control site regardless of water depth. Based on our results, we conclude that fringing oyster reefs can reduce the wave energy reaching the shoreline of marshes with edge elevations close to mean sea level. However, reefs like those in our study have little effect on waves during deeper water conditions, which allow for the largest waves, and are therefore less likely to offer protection to marshes characterized by high edge scarps and marsh surface elevations well above mean sea level.
The movement of salt marshes into uplands and marsh submergence as sea level rises is well documented; however, predicting how coastal marshes will respond to rising sea levels is constrained by a ...lack of process-based understanding of how various marsh zones adjust to changes in sea level. To assess the way in which salt marsh zones differ in their elevation response to sea-level change, and to evaluate how potential hydrologic drivers influence the response, surface elevation tables, marker horizons, and shallow rod surface elevation tables were installed in a Virginia salt marsh in three zones that differed in elevation and vegetation type. Decadal rates of elevation change, surface accretion, and shallow subsidence or expansion were examined in the context of hydrologic drivers that included local sea-level rise, flooding frequency, hurricane storm surge, and precipitation. Surface elevation increases were fastest in the low-elevation zone, intermediate in the middle-elevation zone, and slowest in the high-elevation zone. These rates are similar to (low and middle marsh) or less than (high marsh) local rates of sea-level rise. Root zone expansion, presumably due to root growth and organic matter accumulation, varied among the three salt marsh zones and accounted for 37%, but probably more, of the increase in marsh surface elevation. We infer that, during marsh transgression, soil-forming processes shift from biogenic (high marsh) to minerogenic (low marsh) in response, either directly or indirectly, to changing hydrologic drivers.
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