Storm surge barriers and closure dams influence estuarine morphology. Minimizing consequential ecological impacts requires a thorough understanding of the morphological adaptation mechanisms and associated time scales. Both are unraveled using three decades of morphological measurements on the adaptation of the Eastern Scheldt estuary (The Netherlands) to a storm surge barrier and closure dams. Both the storm surge barrier (through a decrease in cross-sectional area) and closure dams (inducing a reduction in surface area of the estuary) contributed to a reduction in tidal prism. As a smaller tidal prism implies a smaller equilibrium volume of the channels, the channels demand sediment to adjust. Consequently, by providing sediment to the channels, the intertidal flats erode. Erosion rates decreased while the sediment demand of the channels attenuated. This attenuation in sediment demand resulted mainly from tidal prism gains, caused by intertidal flat erosion and sea level rise. Erosion rates of the intertidal flats decreased further while they flattened to adapt to the reduced tidal velocities. Furthermore, storms caused erosion events, after which the long-term adaptation pace of intertidal flats suddenly reduced. Despite decreasing erosion, sea level rise enhances the drowning of intertidal flats in sediment-scarce estuarine systems, thereby pressuring these estuarine ecosystems and raising the need for mitigation measures.

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Seagrass meadows provide valuable ecosystem services of coastal protection and chemical habitat formation that could help mitigate the impact of sea level rise and ocean acidification. However, the intensification of hydrodynamic forces caused by sea level rise, in addition to habitat degradation threaten the provision of these ecosystem services. With quantitative field measurements of the coastal protection and chemical habitat formation services of seagrass meadows, we statistically model the relationships between hydrodynamic forces, vegetation density and the provision of these ecosystem services. Utilising a high-resolution hydrodynamic model that simulates end of the century hydrodynamic conditions and three scenarios of coral reef degradation (i.e., keep up, remain or loss) we quantify how the environmental conditions within a tropical bay will change given changes to the provision of ecosystem services. Our study shows that increasing hydrodynamic forces lead to a seafloor made up of a larger grain size that is increasingly unstable and more vulnerable to erosion. The loss of a fringing reef leads to larger hydrodynamic forces entering the bay, however, the 0.87 m increase in depth due to sea-level rise reduces the bed shear stress in shallower areas, which limits the change in the ecosystem services provided by the current benthic seagrass meadow. Loss of seagrass constitutes the greatest change in a bay ecosystem, resulting in the sediment surface where seagrass existed becoming unstable and the median sediment grain size increasing by 5-7 %. The loss of seagrass also leads to the disappearance of the unique fluctuating chemical habitat, which leaves the surrounding community vulnerable to ocean acidification. A reduction or complete loss of these ecosystem services would impact the entire community assemblage while also leaving the surrounding coastline vulnerable to erosion, thus exacerbating negative effects brought about by climate change.

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Salt marshes can contribute to coastal protection, but the magnitude of the protection depends on the width of the marsh. The cross-shore width of the marsh is to a large extent determined by the delicate balance between seaward expansion and landward retreat. The influence of the magnitude of daily occurring mild weather conditions and sediment availability on the variability of salt marsh width has not been systematically assessed. This paper investigates how the magnitude of homogeneous hydrodynamic forcing, combined with sediment availability, affects the biophysical development, and more specifically retreat and expansion of salt marshes. The dynamic extent of the salt marsh is assessed by modeling online-coupled hydrodynamics, morphodynamics and vegetation growth using the numerical Delft3D-Flexible Mesh model, and a vegetation growth module. Simulated patterns around the salt marsh edge resembled field observations, as well as the simulated temporal variability of the lateral position of the salt marsh edge. In the model, the salt marsh extended seaward at low wave forcing (0.00 m; 0.05 m), and retreated landward at higher wave forcing (0.10 m; 0.15 m). With increasing physical stress, the salt marsh edge was found at lower elevations, indicating an unhealthy system with a retreating marsh edge due to vegetation mortality, whereas decreasing physical stresses result in a higher salt marsh edge, enabling expansion. This balance suggests the importance of response time of vegetation to physical stress. Yet, the salt marsh forced with higher waves was able to switch from a retreating extent retrogradational to an expansional behavior as sediment supply increased.

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Vegetation-wave interactions are critical in determining the capacity of coastal salt marshes to reduce wave energy (wave dissipation), enhance sedimentation and protect the shoreline from erosion. While vegetation-induced wave dissipation is increasingly recognized in low wave energy environments, little is known about: (i) the effect of vegetation on wave dissipation during storms when wave heights and water levels are highest; and (ii) the ability of different plant species to dissipate waves and to maintain their integrity under storm surge conditions. Experiments undertaken in one of the world's largest wave flumes allowed, for the first time, the study of vegetation-wave interactions at near-field scale, under wave heights ranging from 0.1–0.9 m (corresponding to orbital velocities of 2–91 cm s⁻¹) and water depths up to 2 m, in canopies of two typical NW European salt marsh grasses: Puccinellia maritima (Puccinellia) and Elymus athericus (Elymus). Results indicate that plant flexibility and height, as well as wave conditions and water depth, play an important role in determining how salt marsh vegetation interacts with waves. Under medium conditions (orbital velocity 42–63 cm s⁻¹), the effect of Puccinellia and Elymus on wave orbital velocities varied with water depth and wave period. Under high water levels (2 m) and long wave periods (4.1 s), within the flexible, low-growing Puccinellia canopy orbital velocity was reduced by 35% while in the more rigid, tall Elymus canopy deflection and folding of stems occurred and no significant effect on orbital velocity was found. Under low water levels (1 m) and short wave periods (2.9 s) by contrast, Elymus reduced near-bed velocity more than Puccinellia. Under high orbital velocities (≥74 cm s⁻¹), flattening of the canopy and an increase of orbital velocity was observed for both Puccinellia and Elymus. Stem folding and breakage in Elymus at a threshold orbital velocity ≥ 42 cm s⁻¹ coincided with a levelling-off in the marsh wave dissipation capacity, while Puccinellia survived even extreme wave forces without physical damage. These findings suggest a species-specific control of wave dissipation by salt marshes which can potentially inform predictions of the wave dissipation capacity of marshes and their resilience to storm surge conditions.

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A full-scale controlled experiment was conducted on an excavated and re-assembled coastal wetland surface, typical of floristically diverse northwest European saltmarsh. The experiment was undertaken with true-to-scale water depths and waves in a large wave flume, in order to assess the impact of storm surge conditions on marsh surface soils, initially with three different plant species and then when this marsh canopy had been mowed. The data presented suggests a high bio-geomorphological resilience of salt marshes to vertical sediment removal, with less than 0.6cm average vertical lowering in response to a sequence of simulated storm surge conditions. Both organic matter content and plant species exerted an important influence on both the variability and degree of soil surface stability, with surfaces covered by a flattened canopy of the salt marsh grass Puccinellia experiencing a lower and less variable elevation loss than those characterized by Elymus or Atriplex that exhibited considerable physical damage through stem folding and breakage.

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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 (ΔZ) initiates lateral marsh erosion. Marsh erosion rate was found to depend on sediment type and to increase with increasing ΔZ 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.@en

Spartina anglica is an autogenic ecosystem engineer. At a local (within-vegetation) scale, it improves plant growth by enhancing sediment accretion through attenuation of hydrodynamic energy with its shoots. This constitutes a short-range positive feedback. The vegetation also shows a long-distance negative feedback through formation of erosion troughs around the tussock, which are restricting lateral expansion. There is a growing recognition of the important role of such scale-dependent feedbacks for landscape structuring and ecosystem stability. By a series of flume studies, we provide direct experimental evidence that 1) both the local positive and the long-distance negative feedback are strongly density-dependent with clear thresholds, and 2) that both feedbacks are inherently linked via their density dependence, partly as the result of conservation laws. The observed thresholds occurred at vegetation densities commonly found in the field. We demonstrate that threshold densities are affected by external hydrodynamic forcing for the long-distance negative feedback. We expect the same to be true for the local positive feedback (sediment accretion), so that under mild hydrodynamic forcing, this local positive feedback may occur in the absence of a long-distance negative feedback (formation of erosion troughs). The density dependence of nearby positive and the long-distance negative feedback and how external conditions affect the linkage of both feedbacks are important factors shaping the vegetated salt-marsh landscape.@en

Successful management and restoration of coastal vegetation requires a quantitative process-based understanding of thresholds hampering (re-)establishment of pioneer vegetation. We expected scouring to be important in explaining the disappearance of seedlings and/or small propagules of intertidal plant species, and therefore quantified the dependence of scouring on plant traits (flexibility, size) and physical forcing by current velocity. Flume studies with unidirectional flow revealed that scouring around seedlings increased exponentially with current velocity and according to a power relationship with plant size. Basal stem diameter rather than shoot length controlled scouring volume. Flexible shoots caused far less scouring than stiff shoots, provided that the bending occurred near the sediment surface as was the case for Zostera, and not on top of a solid tussock base as we observed for Puccinellia. Therefore, shoot stiffness is likely to strongly affect the chances for initial establishment in hydrodynamically exposed areas. Plant traits such as shoot stiffness are subject to a trade-off between advantages and disadvantages, the outcome of which depends on the physical settings.

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Since the introduction of the term ecosystem engineering by Jones et al. many studies have focused on positive, facilitative interactions caused by ecosystem engineering. Much less emphasis has been placed on the role of ecosystem engineering in causing negative interactions between species. Here, we report on negative interactions between two well known ecosystem engineers occurring at the interface of salt marsh and intertidal flat (i.e. common cordgrass Spartina anglica and lugworms Arenicola marina), via modification of their joint habitat. A field survey indicated that, although both species share a common habitat, they rarely co-occur on small spatial scales (<1 m). Experiments in the field and in mesocosms reveal that establishment of small Spartina plants is inhibited in Arenicola-dominated patches because of low sediment stability induced by the lugworms. In turn, Arenicola establishment in Spartina-dominated patches is limited by high silt content, compactness and dense rooting of the sediment caused by Spartina presence. Our results show that negative interactions by modification of the environment can result in rapid mutual exclusion, particularly if adverse effects of habitat modification are strong and if both species exhibit positive feedbacks. This illustrates the potential for negative interactions via the environment to affect community composition.@en