Salt Marsh Development Research Articles

Hybrid engineering incorporating salt marsh terraces into sea wall repair maintains their defence function and creates new habitats

Sea walls are widely-used engineered structures designed to protect low lying land from flooding. Rising sea levels and coastal erosion threatens sea wall integrity and requires maintenance and repair. As an alternative to conventional repairs, a hybrid engineering design incorporating sediment-filled terraces to allow the development of salt marsh has been trialled to protect sea walls in the Colne-Blackwater Estuary complex, Essex. U.K. Over a 10 year period, three different trajectories of development were measured. Salt marsh halophytes colonised 7 out of 15 terraces, with % plant cover ranging between 5% and 170%. Halophyte species richness on terraces ranged from 1 to 8 species (average = 1.7 per 4 m2), compared to existing salt marsh adjacent to the repairs (max. species richness 9, average 6.85), with positive significant relationships between % halophyte cover and halophyte richness and between % halophyte cover and sediment water content (19–51% on terraces). Organic carbon content was significantly lower on the terraces compared to existing salt marsh (7–19 % AFDW on terraces, 17–24% in marsh sediments). A sediment shear strength of 30 kPa was optimal for % plant cover on terraces. The height of the terraces relative to tidal inundation was a key determinant of successful halophyte colonisation. A second trajectory (3 of 15 terraces) resulted in development of macro- and microalgal mats, up to 100 % cover. Both these trajectories resulted in up to 25 cm of vertical sediment accretion. Shell banks formed on 2 terraces. Seven terraces were subject to erosion, associated with desiccation and absence, or loss, of halophyte cover. Eroding terraces showed both surface lowering (20–30 cm) and massive sediment failure (blocks >50 cm deep falling away). After a decade, protection of the sea wall was still provided by 14 of the 15 terraces, with only one having to be rebuilt. Utilising a nature-based approach incorporating sea wall terraces resulted in the formation of three types of linear habitat (salt marsh, microbial mats, shell banks) providing some ecosystem services that would not have existed if a conventional hard-engineered repair had been used.

Temporal and spatial characteristics of vegetation coverage and their influencing factors in the Sugan Lake wetland on the northern margin of the Qinghai–Tibet Plateau

Fractional vegetation cover (FVC) is an important indicator of the development or reversal of salt marsh due to its absence in arid desert areas. Many studies have emphasized the differences in factors that affect fractional vegetation cover (FVC) in different temporal and spatial scales. However, few studies have reported on the temporal and spatial variations in vegetation coverage and their response to climate and also on the hydrological environment in inland salt marsh wetlands. Accordingly, based on a wetland community survey, different types of data, such as meteorological data, hydrological data, and Landsat remote sensing image data, recorded during the period from 1990 to 2020 were collected. The characteristics of the spatial and temporal distribution of vegetation coverage in the Sugan Lake wetland over the past 30 years were analyzed using a binary pixel model. Furthermore, a quantitative analysis on the response of vegetation coverage to hydrological and meteorological factors was undertaken. The results of the present study showed that the dimidiate pixel model had a high simulation accuracy in retrieving the vegetation coverage in inland salt marsh wetlands. The vegetation coverage of the Sugan Lake wetland increased with each year from 1990 to 2020, and its annual average was 19.34%. The spatial distribution of vegetation coverage was patchy and decreased from the center to the edge of the wetland. Within the same period, the vegetation coverage showed an increasing trend in Quan-shui and He-hong areas and a decreasing trend in the Shan-hong area. Vegetation coverage was mainly affected by various factors, such as precipitation, lake area, surface runoff, groundwater depth, and residential density, respectively. It was significantly positively correlated with precipitation (R2 = 0.56, P < 0.01), lake area (R2 = 0.50, P < 0.01), air temperature (R2 = 0.46, P < 0.01), and river system density (R2 = 0.52, P < 0.01) and negatively correlated with groundwater depth (R2 =-0.57, P < 0.01) and residential density (R2 = −0.38, P < 0.05). Implicit in these findings are complex mechanisms of change in vegetation coverage that help prevent the degradation of vegetation in fragile ecosystems.

Using salt marshes for coastal protection: Effective but hard to get where needed most

Abstract Salt marshes fronting coastal structures, such as seawalls and dikes, may offer important ecosystem‐based coastal defence by reducing the wave loading and run‐up levels during storms. We question (i) how the long‐term salt marsh development in the Dutch Wadden Sea relates to the tidal‐flat foreshore bathymetry and (ii) how the wave run‐up onto dikes, which enhances the risk of dike failure, depends on foreshore bathymetry, the presence/absence of marshes, marsh vegetation properties, tidal range and wind exposure. We analysed 15 years of vegetation and bathymetry maps along the entire Dutch Wadden Sea coast, in combination with detailed process‐based measurements at five locations during 3 years, to understand where salt marshes naturally form and what features determine their contribution to coastal protection. The horizontal extent of marshes along the dikes remained relatively stable over the past decade. The presence of marshes was associated with higher elevations of adjacent tidal flats (above ~0.5 m NAP), while landward‐directed marsh retreat was associated with surface erosion of the fronting tidal flats. Wave run‐up during storms was lower at sites with wider marshes and higher foreshore elevations. This was attributed to the marsh attenuation effect, which led to a reduction in wave heights at the dike toe. As the tidal range varies across the Dutch Wadden Sea, areas to the East with generally higher water levels experienced higher wave run‐up. Synthesis and applications. We found that (i) marshes, where present, effectively protected the dikes from wave loading and (ii) the sites where marshes typically do not develop spontaneously were the most vulnerable to high wave run‐up. This catch‐22 problem implies that increasing reliance on nature‐based coastal defences along soft‐bottom coasts may require human interventions to stimulate marsh formation at the locations where it is most needed. Alternatively, ‘hard engineering’ solutions may remain necessary where implementing nature‐based solutions are either too costly, unachievable, or at the expense of other ecological values, such as causing the loss of mudflats that are important for migratory birds.

Innovative tidal control successfully promotes saltmarsh restoration

The reduction of saltmarsh habitat at a global scale has seen a concomitant loss of associated ecosystem services. As such, there is a need and a push for habitat rehabilitation. This study examined an innovative saltmarsh restoration project in Australia which sought to address the threats of mangrove encroachment and sea level rise. The project was implemented in 2017, using automated hydraulic control gates, termed “SmartGates,” to lower the tidal regime over one site, effectively reversing sea level rise at a local level. Measured indicators of saltmarsh cover, number of species, seedling counts, and saltmarsh assemblages all showed significant positive development over time, with trends varying based on saltmarsh zone. The saltmarsh, predominantly Sarcocornia quinqueflora, developed from remnant supralittoral (previously high) marsh which remained at 45% cover to achieve over 15% coverage across the cleared habitat after 3 years. Slower development in the low marsh (<5%) compared to other zones contrasts with other saltmarsh restoration studies which may be due to the unique nature of the restoration method or the nature of Australian saltmarsh species which favor higher elevations and drier conditions. The development of saltmarsh at the treatment site was found to track toward that at comparison sites over time, becoming similar to some comparison sites by the studies end. This study highlights the usefulness of the novel restoration method used and of the measured indicators for assessing saltmarsh development. This innovative tidal control method could play an important role in the future of saltmarsh restoration worldwide.

Salt marsh fragmentation in a mesotidal estuary: Implications for medium to long-term management

During the last decades many salt marshes worldwide have suffered important losses in their extent and associated ecosystem services. The salt marshes of San Vicente de la Barquera estuary (N Spain) are a clear example of this, with a drastic reduction in vegetation surface over the last 60 years. This paper provides insights into the main factors controlling salt marsh functioning in sheltered estuarine areas. Regional and local factors have been disaggregated to identify the main drivers controlling the functioning of the salt marsh to develop appropriate management measures according to the evolution of the system. These factors have been studied in their spatial context through detailed maps of change in vegetation cover combined with topographic data obtained from UAV and RTK-DGPS surveys. The results demonstrate that in this estuary the salt marsh area is declining following a fragmentation process. No clear pattern of vegetation loss/gain with elevation has been identified. However, the results point to increased hydrodynamic stress in the area, with stronger currents inside the estuary. This is probably the major factor responsible for the decline of the salt marshes in the San Vicente de la Barquera estuary. Furthermore, several human interventions during the 20th century (local drivers) have also probably contributed to a lower resilience against SLR (regional driver). This work demonstrates that both natural and human drivers of change need to be considered when characterizing the evolution of salt marshes, wherever efficient management strategies need to be designed.

Advances in biophysical feedbacks and the resulting stable states in tidal flat systems

<p indent="0mm">As an integral part of the coastal wetland system, tidal flats play an important role in maintaining the health of coastal ecosystems, resisting natural coastal disasters, and blue carbon sequestration. Tidal flats can provide suitable habitats for a variety of organisms; simultaneously, such organisms mediate hydrodynamic and sediment processes, hence affecting the geomorphic evolution and stability of tidal flat systems. In recent years, tidal flat systems have faced the comprehensive impacts of rapid human activities, frequent storm surges, and reduced sediment availabilities, which have aggravated tidal flat erosion. Severe erosion is accompanied by a critical biomass loss, especially under complex biological-physical interactions. Tidal flat degradation is often amplified, threatening the stability of tidal flat systems. The ultimate goal of ecological protection and restoration of tidal flat systems is to establish a self-sustaining method with less artificial assistance. To date, only a few ecological restoration projects in China have considered the integration of biological and physical processes in tidal flats. The most important reason is the lack of understanding of the basic laws of natural construction and operation of the systems. Therefore, understanding and giving way to the self-organization process driven by the biophysical interactions in tidal flat systems have become an important scientific and technical necessity in the ecological protection and restoration of tidal flats. First, we summarized the biophysical effects of salt marsh vegetation and benthic microalgae on the hydrodynamic and sediment movement process of tidal flats. Salt marsh plants and benthic microalgae change the hydrodynamic and sediment processes through wave dissipation and sediment stabilization. Conversely, the changing physical conditions affect the growth and development of salt marsh and benthic microalgae. Second, we qualitatively analyzed the bistability and catastrophic shift of a tidal flat system driven by the salt marsh-sediment and benthic microalgae-sediment feedback, both containing some similarities and differences. Specifically, driven by biophysical feedbacks, both the salt marsh- and benthic microalgae-sediment systems have alternative stable states; with the increased bottom shear stress, both systems shift from a state characterized by high biomass and sediment deposition to that with low biomass and sediment deposition; the bistable range of the salt marsh-sediment system is wider, and the system has a greater hysteresis, indicating that once degraded, it is more difficult for the salt marsh system to be restored to its original state. Third, we discussed the self-organized bio-geomorphic characteristics. The scale-dependent feedback between organisms and physical environments results in spatial patterns. The self-organized patterns have been widely found in a tidal flat ecosystem, for example, circular salt marsh clusters, salt marsh cliffs, and patterned mudflats characterized by the microalgae-covered hummocks alternating with water-filled hollows. Self-organized patterns can improve the primary productivity and sediment deposition of tidal flats, thereby enhancing the stability of ecosystems, which is of great significance for the ecosystem function of tidal flats. Finally, this paper proposed the critical scientific problems that still need to be solved in the future, such as the continuous field observation at the system level, the regulation of biological interspecific interactions under physical conditions, and the quantitative simulation of catastrophic shift thresholds in the tidal flat bio-geomorphic system.

A Minimalist Model of Salt-Marsh Vegetation Dynamics Driven by Species Competition and Dispersal

We present a new bidimensional, spatially-explicit ecological model describing the dynamics of halophytic vegetation in tidal saline wetlands. Existing vegetation models employ relatively simple deterministic or stochastic mechanisms, and are driven by local environmental conditions. In the proposed model, in contrast, vegetation dynamics depend not only on the marsh local habitat, but also on spatially-explicit mechanisms of dispersal and competition among multiple interacting species. The role of habitat quality, here determined by the local elevation relative to the mean sea level as a proxy for environmental conditions, is mathematically modeled by a logistic function that represents the fundamental (theoretical) niche of each halophytic species. Hence, the model does not artificially impose any constraints to the ability of a species to colonize elevated areas where it is usually not observed: such limitations naturally arise through competition with fitter species across marsh topographic gradients. We qualitatively test our model against field data based on a suitable assemblage of focus species, and perform a sensitivity analysis aimed at determining how dynamic equilibria in vegetation distributions are affected by changes in model input parameters. Results indicate that the model is robust and can predict realistic vegetation distributions and species-richness patterns. More importantly, the model is also able to effectively reproduce the outcomes of classical ecological experiments, wherein a species is transplanted to an area outside its realized niche. A direct comparison shows that previous models not accounting for dispersal and interspecific competitions are unable to reproduce such dynamics. Our model can be easily integrated into virtually any existing morphodynamic model, thereby strengthening our ability to simulate the coupled biotic and abiotic evolution of salt marshes under changing climate forcings.

Modeling the Dynamics of Salt Marsh Development in Coastal Land Reclamation

AbstractThe valuable ecosystem services of salt marshes are spurring marsh restoration projects around the world. However, it is difficult to determine the final vegetated area based on physical drivers. Herein, we use a 3D fully coupled vegetation‐hydrodynamic‐morphological modeling system to simulate the final vegetation cover and the timescale to reach it under various forcing conditions. Marsh development in our simulations can be divided in three distinctive phases: A preparation phase characterized by sediment accumulation in the absence of vegetation, an encroachment phase in which the vegetated area grows, and an adjustment phase in which the vegetated area remains relatively constant while marsh accretes vertically to compensate for sea level rise. Sediment concentration, settling velocity, sea level rise, and tidal range each comparably affect equilibrium coverage and timescale in different ways. Our simulations show that the Unvegetated‐Vegetated Ratio also relates to sediment budget in marsh development under most conditions.

Image-based machine learning for monitoring the dynamics of the largest salt marsh in the Yangtze River Delta

The extreme decline in fluvial sediment discharge and rapid increase in sea level have increased salt marsh vulnerability in some of the world’s mega-delta. However, limited research has addressed both the vertical accretion and horizontal/lateral progradation of salt marshes induced by anthropogenic activities in recent decades. Here, a machine learning-based method for retrieving remote sensing images of the salt marsh along the Eastern Chongming Wetland (ECW), the largest wetland in the Yangtze River Delta, was used to monitor salt marsh dynamics between 2002 and 2019. The results demonstrate that salt marshes have experienced significant expansion, including seaward progradation and accretion with ranges of −18.5–60.6 m/yr and 0.103–0.178 m/yr, respectively. Nevertheless, the bare mudflat areas adjoining the salt marshes have remained almost unchanged, while their progradation and accretion have also shown similar trends with the ranges of −13.3–103.7 m/yr, and 0.066–0.256 m/yr, respectively. Although there was a 70% reduction in fluvial sediment supply in the Yangtze River Delta after the Three Gorges Dam (TGD) began operating in 2003, it is less understood if the constant local suspended sediment concentration (SSC) of the estuary could be responsible for supporting enough sediment to enable salt marsh and mudflat expansions. Meanwhile, the results showed that the seaward expansion of the mudflats provided suitable space for the salt marsh to trap vast amounts of sediment and gradually occupy the adjoining mudflat area. The mudflat progradation further provided a larger space for the growth of salt marsh vegetation and promoted salt marsh expansion. Moreover, the accretion of the ECW indicates the high resilience of these salt marshes to sea-level rise (SLR). The present work highlights the external factors and internal driving forces of the salt marsh evolution process, providing information that can be used by communities and coastal managers to conserve and restore the salt marshes in the future.

Secular diachronic analysis of coastal marshes and lagoons evolution: Study case of the Po river delta (Italy)

Coastal lagoons and salt marshes are rapidly changing under the influence of sea level rise and human induced changes. Within this context, the proposed study describes the evolution of the lagoons of the Po Delta (Italy) and the historical transformations of the salt marshes using historical maps and aerial data from 1892 to 2018. The methodology applied provides a crucial quantification of coastal lagoon and salt marsh evolution.Image analysis shows that most of the lagoons were formed between 1892 and 1934, while the most recent are developed between 1978 and 1988. Lagoons reached their actual shape and dimensions by 1955. Lagoons present different morphological characteristics with the lagoon of Caleri having a higher morphological diversity due to the presence of salt marshes and a complex hydraulic network. Since 1988, fringing salt marshes were the dominant morphology, then marsh morphology switched to a fringing, isolated and channelized morphology.Over the last 120 years, the Po Delta lagoons have experienced a high rate of erosion and environmental degradation. Our analysis suggests that three main phases are present in coastal lagoon marsh evolution that explain the development. In the last years, those lagoons experienced a progressive reduction of fringing and isolated salt marshes. The first one (1892–1934), characterized by high fluvial sediment input and a fast seaward progression of the river mouths, corresponds to the maximum salt marsh development. The second phase (1934–1978), characterized by a negative sediment budget and human activity induced alterations (subsidence), presents a small increase of the lagoon extension associated with a reduction of the salt marsh. Finally, the third phase (after 1978), characterized by a low sedimentary budget and high human control, is characterized by a stabilization of the lagoon extension and a drastic reduction of the salt marshes.

Are saltmarshes younger than mangrove swamps?

Temperate saltmarshes and tropical mangrove swamps (mangals) are marine‐influenced, productive ecosystems that enhance nutrient transfers between land and sea and facilitate colonization of lineages between terrestrial and marine habitats. Mangals have existed since the late Cretaceous, but the time of origin of saltmarshes is less clear. On the basis of phylogenetic and fossil evidence for plants and molluscs specialized to these ecosystems, I propose that saltmarsh vegetation of angiosperms began during the latest Eocene to Early Oligocene (35–30 Ma), at least 34 m.y. after the origin of mangals. The plants that colonized saltmarshes then and later have mainly temperate origins, contrasting with the tropical‐forest origins of mangroves. Unlike the plants, the few saltmarsh‐specialized molluscs are derived from tropical lineages and reflect recent colonizations. The development of saltmarshes during the Neogene enhanced near shore productivity along temperate and Arctic coastlines.

Locality of Topographic Ground Truth Data for Salt Marsh Lidar DEM Elevation Bias Mitigation

Lidar-derived digital elevation models (DEMs) are crucial for modeling salt marsh evolution, forecasting inundation depth, frequency and duration, and simulating sea level rise (SLR). Advances in lidar acquisition and data processing techniques have led to increased accuracy, however in densely vegetated coastal salt marsh areas, lidar-derived DEMs are generally unreliable without adjustment. In this paper, we investigate the need for local topographic ground truth data to train Random Forest (RF) DEM adjustment models for two similar Northern Gulf of Mexico salt marshes. Two GNSS-RTK field surveys were conducted to acquire ground truth topographic elevations near St. Marks, Florida (n&#x003D;377) and Pascagoula, Mississippi (n&#x003D;610). These elevations, along with Sentinel-2A MSI reflectance values and lidar DEM elevations, were used to validate and train local and combined RF salt marsh DEM adjustment models. The local RF models achieved MAE values of 0.054 m and 0.045 m in the leave-one-out cross-validation for St. Marks and Pascagoula, respectively. Elevation bias predictions using remote RF models were far worse and those using the combined RF model were marginally worse. Using the local RF predictions to mitigate the bias in the lidar DEMs improved their accuracy by 69.1&#x0025; for St. Marks and 90.9&#x0025; for Pascagoula. The DEM elevation was identified as the most important predictor. This evidence suggests that local ground truth data are necessary for mitigating bias in salt marsh lidar DEMs, although it remains to be seen if increasing the data set size and incorporating additional hydrologic predictor variables could narrow the accuracy gap.

Intertwined Eco‐Morphodynamic Evolution of Salt Marshes and Emerging Tidal Channel Networks

AbstractThe formation and development of tidal channels and salt marshes are controlled by complex interactions between hydrodynamics, sediment transport, and vegetation dynamics. Tidal channels affect and, at the same time, are affected by the growth of salt marshes fringing them. The coupled evolution of these morphological units, mediated by vegetation growth, is thus a key ingredient for simulating the behavior of tidal environments. Considering these two factors, we developed a mathematical model to investigate the eco‐morphodynamic evolution of intertidal areas fringing a main channel and of the tidal creeks cutting through them. Model results indicate that vegetation promotes the development of channel networks, leading to more complex channel structures and higher drainage efficiency. Vegetation encroachment influences sediment deposition patterns by trapping sediment in the seaward and middle intertidal areas, while reducing the amount of sediment delivered to landward areas. In the presence of sea level rise, this deficit of sediment enhances the landward‐decreasing trend of the intertidal platform and leads to more isolated vegetation patches. Overall, sea level rise restricts the extension of salt marshes and consequently reduces the effect of vegetation on channel network form and function.

Flows, Transport, and Effective Drag in Intertidal Salt Marsh Creeks

AbstractIntertidal creeks (channel width &lt;5 m) weave through salt marshes, delivering water, nutrients, and sediments into the marsh interior and affecting spatial heterogeneity in plant and animal distributions. Despite their global prevalence, creek connectivity, and the mechanisms controlling cross‐marsh hydrodynamics, remain poorly resolved. In this study, we measured flow and total suspended solids transport in three intertidal creeks within a confined drainage basin in a Georgia, USA salt marsh. We discovered that the effective drag is 3–12 times greater than bed drag, reaching levels similar to those observed in coral reefs. Furthermore, the drag between tidal flood and ebb phases differs, indicating an asymmetric drag. Analyses of along‐channel momentum reveal that pressure gradient m/s2 and friction m/s2 dominate creek momentum balance. Divergence in tidal and suspended solids transport between adjacent creeks revealed contrasting tidal transport asymmetries (i.e., flood or ebb dominated) within this confined basin. We discuss how these differences may alter the eco‐geospatial evolution of salt marshes and their response to sea‐level rise.

Seasonality and Characterization Mapping of Restored Tidal Marsh by NDVI Imageries Coupling UAVs and Multispectral Camera

Salt marsh evolution is strongly affected by tidal processes and ecology, which regulate sediment accretion and erosional rates. A balance between marsh erosion and deposition in a restored tidal wetland is crucial for analyzing restoration strategies to adopt in a natural context. Here, we present an integrated approach monitoring salt marsh seasonal changes over several months in a microtidal restored salt marsh of the Paul S. Sarbanes Ecosystem Restoration Project at Poplar Island (MD, USA). The project is undertaken at a restoration site where sediment dredged from the shipping channels in the upper Chesapeake Bay is being used to restore a tidal marsh habitat in mid-Chesapeake Bay. We flew an Unmanned Aerial Vehicle (UAV) with an RGB and a multispectral camera to obtain a high-resolution map of the planimetric position of vegetation and to monitor the health of the marsh vegetation in diverse seasons. Due to its extension of 400 m by 400 m, a total of four flight plans were necessary to cover the entire marsh flying at a 40 m altitude obtaining a 2 cm Ground Sample Distance (GSD). This technique provides reliable results at a very low cost, enabling an accurate assessment of the marsh platforms to be conducted over time, due to both the very high spatial resolution and the precise georeferencing of the images for the comparisons. Our results show seasonal variability in the two dominant species colonizing the low marsh, Spartina alterniflora, and high marsh, Sporobolus pumilus. While the lower marshes showed a higher variability along seasons, the up-land vegetation showed persistent green foliage during cold seasons. Detecting salt marsh evolution and seasonality coupled with field measurements can help to improve the accuracy of hydrodynamic and sediment transport models. Understanding the drivers of salt marsh evolution is vital for informing restoration practices and designs, in order to improve coastal resilience, and develop and coastal management strategies.

Salt marsh construction as a nature-based solution in an estuarine social-ecological system

Constructed salt marshes as a Nature-Based Solution for coastal defense offer additional benefits over conventional engineering, but project realization is often hampered by practical and governmental obstacles. We assessed the execution of a local-scale salt marsh construction project as a Nature-Based Solution (NBS) with respect to the regional-scale Social-Ecological System (SES) in an explicitly linked NBS-SES framework. A local municipality came up with various plans to develop its waterfront but these proved unrealizable without wider stakeholder participation. Crucial for success was that the local initiative was turned into an NBS integrating livability, biodiversity and flood safety, and that it was linked to the governance systems and actors in a regional SES. The chosen NBS consists of a city beach and two salt marshes, a salt marsh park that is open to the public and a pioneer salt marsh that is only accessible for research. The pioneer salt marsh was constructed by raising the seabed to around mean high tide with sand obtained from a capital dredging project. It was used as a large-scale natural experiment in salt marsh construction. To test the effect of enrichment with silt and clay on initial salt marsh development, six hectare-scale compartments were created in which mud was mixed with sand in the top 1.0 m of the bed to three mud contents of on average 8%, 25% and 48%. Heavy machinery was needed to mix mud through the upper meter of the sandy bed. Mixing mud was softening the sediment causing the machines to sink into the 48% mud enriched bed. To test whether seeding with a pioneer plant species accelerates salt marsh development, fragments of Salicornia procumbens plants were seeded in half of three compartments. Field observations between November 2018 to September 2020 showed that seeding of Salicornia plant fragments resulted in significant differences in vegetation cover and species richness in the first growing season. Mud content showed significant positive effects on vegetation cover and species richness in the two monitored growing seasons, where the compartments with on average 7–9% mud had the lowest vegetation cover and species numbers. When constructing a salt marsh by raising sand and mixing mud, a mud content of 25% is practically feasible and results in high vegetation cover and species richness.

Estimating Waves and Currents at the Saltmarsh Edge Using Acoustic Doppler Velocimeter Data

Waves, currents, and related sediment transport are important factors driving the development of saltmarshes. Separating the effects of waves and currents accurately from simple field observations is a technical challenge in shallow water environments with limited inundation such as saltmarshes. In this paper, the estimation method of both wave and current information was studied mainly through the data obtained by a solely used field ADV (Acoustic Doppler Velocimeter, Vector). Phase and pressure inversion wave methods were both used to estimate wave parameters, which were then compared to synchronous observation by wave loggers at the front of a saltmarsh. Our results show that ADV is able to achieve simultaneous observations of tidal currents and waves independently. The pressure inversion wave method (r2 ∼ 0.996) is more effective than the phase method (r2 ∼ 0.876) in estimating wave energy when comparing with wave logger results. However, the former is more restricted by submergence depth and duration in field, while the latter provides cut-off frequencies for the pressure inversion wave method. Both methods can be combined to best estimate wave parameters from field ADV data. Further, hydrodynamic observation on a Scirpus mariqueter patch at the front of a saltmarsh was used as an application to indicate the importance of obtaining both wave and current information from field data. The patch was found to mainly reduce the advection of tidal currents, but it slightly increases wave energy. Therefore, it is necessary to consider the different effects of plants on tidal currents and waves simultaneously in field observations. This study confirms that ADV and associated analysis can detect waves and currents at reliable accuracy at the marsh edge, which is vital in assessing the long-term resilience of marshes to sea level rise and increased storm severity.

Primary succession in an Atlantic salt marsh: From intertidal flats to mid‐marsh platform in 35 years

Abstract Although salt marsh is a classic example of primary succession, the underlying mechanisms and their time‐scales are poorly understood. As salt marsh succession depends on sediment accretion, the amelioration of abiotic conditions associated with increasing elevation suggests potential explanatory roles for facilitation, competition and the stress‐gradient hypothesis. We present a 35‐year longitudinal study of salt marsh development from intertidal flat to a mid‐marsh platform at Odiel Marshes in south‐western Iberia. Using permanent plots, this work chronicles changes in elevation and marsh morphology, their evolving effects on sediment redox potential and salinity and the colonisation and changing patterns of dominance of halophytic species. Sporadically colonising clumps of the low‐marsh species Spartina maritima trapped sediment to form raised tussocks, which increased in elevation and area. Reduced tidal inundation and locally improved drainage promoted higher redox potentials and allowed colonisation by a sequence of species less tolerant of reducing conditions: Sarcocornia perennis, its hybrid with high‐marsh S. fruticosa, and Atriplex portulacoides. Unlike its centrifugally colonising predecessors, A. portulacoides invaded from the tussock edges. Transplant experiments designed to investigate its late establishment on tussocks showed that seedling survival depended on elevational differences as small as 4 cm. After increasing in elevation by c. 1 m (c. 29 mm/year), coalescence of the tussocks formed a marsh platform at a level corresponding to mean high tides. This supports a theoretical punctuated transition from ‘submergence marsh’ to ‘emergence marsh’, previously postulated for this tidal elevation. Synthesis. The unexpected rapidity of this primary succession highlights the central role of facilitation. Vertical sediment accretion, locally engineered by colonising species, progressively alleviates abiotic stress and allows colonisation by species that are less tolerant of chemically reducing conditions but are ultimately better competitors.

High-Resolution Monitoring of Tidal Systems Using UAV: A Case Study on Poplar Island, MD (USA)

Tidal processes regulating sediment accretion rates and vegetated platform erosion in tidal systems strongly affect salt marsh evolution. A balance between erosion and deposition in a restored salt marsh is crucial for analyzing restoration strategies to be adopted within a natural context. Marsh morphology is also coupled with tidal mudflats and channel networks and this makes micro-tidal systems crucial for a detailed assessment of restoration interventions. Here, we present a methodological approach for monitoring channel morphodynamics and vegetation variations over a time frame of six years in a low tidal energy salt marsh of the Paul S. Sarbanes Ecosystem Restoration Project at Poplar Island (Maryland, USA). The project is a restoration site where sediment dredged from the shipping channels in the upper Chesapeake Bay is used to restore a tidal marsh habitat in mid-Chesapeake Bay. Aerial surveys with an Unmanned Aerial Vehicle (UAV) have been performed for the high-resolution mapping of a small tidal system. Flight missions were planned to obtain a Ground Sample Distance (GSD) of 2 cm. Structure-from-Motion (SfM) and Multi-View-Stereo (MVS) algorithms have been used to reconstruct the 3D geometry of the site. The mapping of channel morphology and an elevation assessment on the mudflat were performed using orthomosaics, Digital Terrain Models (DTMs) and GNSS survey. The results highlight that the workflow adopted in this pilot work is suitable to assess the geomorphological evolution over time in a micro-tidal system. However, issues were encountered for salt marsh due to the presence of dense vegetation. The UAV-based photogrammetry approach with GNSS RTK ground surveys can hence be replicated in similar sites all over the world to evaluate restoration interventions and to develop new strategies for a better management of existing shorelines.

Onset of runaway fragmentation of salt marshes

Summary Salt marshes are valuable but vulnerable coastal ecosystems that adapt to relative sea level rise (RSLR) by accumulating organic matter and inorganic sediment. The natural limit of these processes defines a threshold rate of RSLR beyond which marshes drown, resulting in ponding and conversion to open waters. We develop a simplified formulation for sediment transport across marshes to show that pond formation leads to runaway marsh fragmentation, a process characterized by a self-similar hierarchy of pond sizes with power-law distributions. We find the threshold for marsh fragmentation scales primarily with tidal range and that sediment supply is only relevant where tides are sufficient to transport sediment to the marsh interior. Thus the RSLR threshold is controlled by organic accretion in microtidal marshes regardless of the suspended sediment concentration at the marsh edge. This explains the observed fragmentation of microtidal marshes and suggests a tipping point for widespread marsh loss.