Mangrove forests are increasingly valued as wave-attenuating buffers in coastal flood defence strategies. However, as mangroves are vulnerable to wave-induced erosion, this raises the question, how can the stability of these protective mangrove forests be promoted? To address this question, we investigate how mangrove dynamics in a microtidal system can be related to different types of foreshores. We used remote sensing to investigate mangrove fringe stability over multiple years in relation to intertidal mudflat width (i.e., emerged at low tide) and the presence stability of cheniers, which are sand bodies on top of muddy foreshores that are characteristic for eroding coastlines. In addition, we investigated local and short-term foreshore effects by measuring wave propagation across two cross-shore transects, one with a mudflat and chenier and one with a deeper tidal flat foreshore. The satellite images (Sentinel-2) revealed that mangrove dynamics over multiple years and seasons were related to chenier presence and stability. Without a chenier, a mudflat width of 110 m (95%CI: 76–183 m) was required to make mangrove expansion more likely than mangrove retreat. When a stable chenier was present offshore for two years or more, a mudflat width of only 16 m (95%CI: 0–43 m) was enough to flip chances in favor of mangrove expansion. However, mangrove expansion remained heavily influenced by seasonal changes, and was highly event driven, succeeding only once in several years. Finally, although mudflat width was a direct driver of mangrove expansion, and could be targeted as such in coastal management, our field measurements demonstrated that cheniers also have an indirect effect on mangrove expansion. These sand banks significantly reduce wave height offshore, thereby likely creating favorable conditions for mudflat accretion landward, and thus mangrove habitat expansion. This makes stabilization - and possibly also the temporary creation - of cheniers an interesting target for mangrove conservation and restoration.

With the capacity to reduce wave energy and trap sediment, Scirpus mariqueter has become an important native species of annual grass for ecology restoration at the Yangtze Estuary in eastern China. Due to seasonal variances of biophysical characteristics, S. mariqueter usually bends and breaks in winter, resulting in flattened stems that may reduce its wave attenuation capacity. To investigate the effects of vegetation flattening on wave attenuation, a set of flume experiments were conducted for flattened and standing vegetation under different wave conditions. The model vegetation was designed to represent the wilted S. mariqueter collected in winter with dynamic similarity. Results showed that the wave damping coefficient for flattened vegetation (β_F) was 33.6%-72.4% of that for standing vegetation (β_S) with the same vegetation length. Both β_F and β_S increased with wave height but decreased with water depth. A wave attenuation indicator (WAI) was defined to generate empirical formulas for β_S and β_F as well as their ratio β_F/β_S. The empirical formulas were then applied to modify the existing standing vegetation-based wave attenuation model for flattened vegetation and performed successfully. Understanding the wave attenuation characteristics of flattened vegetation is essential for the management of ecological restoration and coastal protection.

The establishment of young organisms in harsh environments often requires a window of opportunity (WoO). That is, a short time window in which environmental conditions drop long enough below the hostile average level, giving the organism time to develop tolerance and transition into stable existence. It has been suggested that this kind of establishment dynamics is a noise-induced transition between two alternate states. Understanding how temporal variability (i.e. noise) in environmental conditions affects establishment of organisms is therefore key, yet not well understood or included explicitly in the WoO framework. In this paper, we develop a coherent theoretical framework for understanding when the WoO open or close based on simple dichotomous environmental variation. We reveal that understanding of the intrinsic timescales of both the developing organism and the environment is fundamental to predict if organisms can or cannot establish. These insights have allowed us to develop statistical laws for predicting establishment probabilities based on the period and variance of the fluctuations in naturally variable environments. Based on this framework, we now get a clear understanding of how changes in the timing and magnitude of climate variability or management can mediate establishment chances.

Tidal flats are biogeomorphic landscapes, shaped by physical forces and interaction with benthic biota. We used a metabolic approach to assess the overarching effect of bioturbators on tidal landscapes. The benthic bivalve common cockle (Cerastoderma edule) was used as model organism. The effect of C. edule on sediment resuspension was approximated as a function of the overall population metabolic rate per unit of area. We combined i) laboratory observations on how C. edule affect sediment resuspension along gradients of bioturbation activity, sediment cohesiveness and hydrodynamic force with ii) spatial data on the natural distribution of intertidal C. edule populations. This allowed us to build an integrated model of the C. edule effect on sediment resuspension along the tidal gradient. Owing to the temperature dependence of metabolic rate, the model also accounted for seasonal variation in bioturbators activity. Laboratory experiments indicated that sediment resuspension is positively related to the metabolic rate of the C. edule population especially in cohesive sediments. Based on this observation, we predicted a clear spatial and seasonal pattern in the relative importance of C. edule contribution to sediment resuspension along a tidal transect. At lower elevations, our model indicates that hydrodynamics overrules biotic effects; at higher elevations, inter-tidal hydrodynamics should be too low to suspend bioturbated sediments. The influence of C. edule on sediment resuspension is expected to be maximal at the intermediate elevation of a mudflat, owing to the combination of moderate hydrodynamic stress and high bioturbator activity. Also, bio-mediated sediment resuspension is predicted to be particularly high in the warm season. Research into metabolic dependency of bio-mediated sediment resuspension may help to place phenomenological observations in the broader framework of metabolic theories in ecology and to formulate general expectations on the coastal ecosystem functioning.

The intensity of major storm events generated within the Atlantic Basin is projected to rise with the warming of the oceans, which is likely to exacerbate coastal erosion. Nature-based flood defence has been proposed as a sustainable and effective solution to protect coastlines. However, the ability of natural ecosystems to withstand major storms like tropical hurricanes has yet to be thoroughly tested. Seagrass meadows both stabilise sediment and attenuate waves, providing effective coastal protection services for sandy beaches. To examine the tolerance of Caribbean seagrass meadows to extreme storm events, and to investigate the extent of protection they deliver to beaches, we employed a combination of field surveys, biomechanical measurements and wave modelling simulations. Field surveys of seagrass meadows before and after a direct hit by the category 5 Hurricane Irma documented that established seagrass meadows of Thalassia testudinum remained unaltered after the extreme storm event. The flexible leaves and thalli of seagrass and calcifying macroalgae inhabiting the meadows were shown to sustain the wave forces that they are likely to experience during hurricanes. In addition, the seagrass canopy and the complex biogeomorphic landscape built by the seagrass meadows combine to significantly dissipate extreme wave forces, ensuring that erosion is minimised within sandy beach foreshores. The persistence of the Caribbean seagrass meadows and their coastal protection services during extreme storm events ensures that a stable coastal ecosystem and beach foreshore is maintained in tropical regions.

A better understanding of how tidal-flat reclamation changes the flood hazard is critical for climate-proofing coastal flood defense design of heavily urbanized areas. Since the 1950s, large-scale reclamation has been performed along the Shanghai coast, China, to fulfill the land demands of city expansion. We now show that the loss of tidal flats may have resulted in harmful impacts of coastal storm flooding. Using the foreshore profiles measured before and after reclamation (i.e., wide vs. narrow tidal flat), we determined the long-term changes in flood risk using a numerical model that combines extreme tidal level and wave overtopping analysis. Results show that wide tidal flats in front of a seawall provide efficient wave damping even during extreme water levels. Reclamation of these tidal flats substantially increased wave heights and correspondingly reduced the return period of a specific storm. As a result, estimates of overtopping are aggravated by more than 80% for the varying return periods examined. It is concluded that the disasters of coastal flooding after the 1997 tidal-flat reclamation in Hangzhou Bay, China are a consequence of both anthropogenic and natural activities. Moreover, our model calculations provide an equation describing the equivalent dike height needed to compensate for the loss of every km tidal flat of a certain elevation, and vice versa. For example, for every km of tidal flat ranging from high marsh to bare tidal flat that is being regained, the dike can be lowered by 0.84 m–0.67 m, when designing for a 1 in 200 years storm event. Overall, we suggest that wide tidal flats are ideally restored in front of dikes, and that when tidal areas are reclaimed, the seawall height is raised as part of the intertidal reclamation procedure. Using such an equivalent protection standard is relevant to designing hybrid flood defense system worldwide.

Seagrasses provide an important ecosystem service by creating a stable erosion-resistant seabed that contributes to effective coastal protection. Variable morphologies and life-history strategies, however, are likely to impact the sediment stabilization capacity of different seagrass species. We question how opportunistic invasive species and increasing grazing by megaherbivores may alter sediment stabilization services provided by established seagrass meadows, using the Caribbean as a case study. Utilizing two portable field-flumes that simulate unidirectional and oscillatory flow regimes, we compared the sediment stabilization capacity of natural seagrass meadows in situ under current- and wave-dominated regimes. Monospecific patches of a native (Thalassia testudinum) and an invasive (Halophila stipulacea) seagrass species were compared, along with the effect of three levels of megaherbivore grazing on T. testudinum: ungrazed, lightly grazed and intensively grazed. For both hydrodynamic regimes, the long-leaved, dense meadows of the climax species, T. testudinum provided the highest stabilization. However, the loss of above-ground biomass by intensive grazing reduced the capacity of the native seagrass to stabilize the surface sediment. Caribbean seagrass meadows are presently threatened by the rapid spread of the invasive opportunistic seagrass, H. stipulacea. The dense meadows of H. stipulacea were found to accumulate fine sediment, and thereby, appear to be effective in reducing bottom shear stress during calm periods. This fine sediment within the invasive meadows, however, is easily resuspended by hydrodynamic forces, and the low below-ground biomass of H. stipulacea make it susceptible to uprooting during storm events, potentially leaving large regions vulnerable to erosion. Overall, this present study highlights that intensive megaherbivore grazing and opportunistic invasive species threaten the coastal protection services provided by mildly grazed native species. Synthesis. Seagrass meadows of dense, long-leaved species stabilize the sediment surface and maintain the seabed integrity, thereby contributing to coastal protection. These services are threatened by intensive megaherbivore grazing, which reduces the stability of the surface sediment, and opportunistic invasive species, which are susceptible to uprooting in storms and thereby can leave the seabed vulnerable to erosion.

Shallow tropical bays in the Caribbean, like Orient Bay and Galion Bay in Saint Martin, are often sheltered by coral reefs. In the relatively calm environment behind the reefs, seagrass meadows grow. Together, these ecosystems provide valuable ecosystem services like coastal protection, biodiversity hotspots, nursery grounds for animals and enhancing tourism and fisheries. However, sea-level rise imperils these ecosystems and the services they provide because of changing hydrodynamic conditions, with potential effects on the interdependencies between these ecosystems. By means of a hydrodynamic model that accounts for the interaction with vegetation (Delft3D Flexible Mesh), the impact of sea-level rise (0.87 m in 2100) is investigated for three scenarios of future reef development (i.e. keep-up, give-up and catch-up). If coral reefs cannot keep up with sea-level rise, the wave height and flow velocity increase significantly within associated bays, with the wave height doubling locally in case of eroding reefs in our model simulations. Since the presence of seagrass strongly depends on the hydrodynamic conditions, the response of seagrass to the future hydrodynamic conditions is projected using a habitat suitability model that is based on a logistic regression. The spatial character of the bays determines the response of seagrass. In Orient Bay, which is deeper and partly exposed to higher waves, the seagrass will likely migrate from the deeper parts to shallow areas that become suitable for seagrass because of the surf zone moving landward. In contrast, the conditions for seagrass worsen in Galion Bay for the catch-up and give-up scenario; due to the shallowness of this bay, the seagrass cannot escape to more suitable areas, resulting in significant seagrass loss. It is shown that healthy coastal ecosystems are able to limit the change in hydrodynamic conditions due to sea-level rise. Therefore, preserving these ecosystems is key for ensuring the resilience of shallow tropical bays to sea-level rise and maintaining their ecosystem services.

Global change amplifies coastal flood risks and motivates a paradigm shift towards nature-based coastal defence, where engineered structures are supplemented with coastal wetlands such as saltmarshes. Although experiments and models indicate that such natural defences can attenuate storm waves, there is still limited field evidence on how much they add safety to engineered structures during severe storms. Using well-documented historic data from the 1717 and 1953 flood disasters in Northwest Europe, we show that saltmarshes can reduce both the chance and impact of the breaching of engineered defences. Historic lessons also reveal a key but unrecognized natural flood defence mechanism: saltmarshes lower flood magnitude by confining breach size when engineered defences have failed, which is shown to be highly effective even with long-term sea level rise. These findings provide new insights into the mechanisms and benefits of nature-based mitigation of flood hazards, and should stimulate the development of novel safety designs that smartly harness different natural coastal defence functions.

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.

Spatial patterns formed through the process of self-organization are found in nature across a variety of ecosystems. Pattern formation may reduce the costs of competition while maximizing the benefits of group living, and thus promote ecosystem persistence. This leads to the prediction that self-organizing to obtain locally intermediate densities will be the optimal solution to balance costs and benefits. However, despite much evidence documenting pattern formation in natural ecosystems, there is limited empirical evidence of how these patterns both influence and are influenced by tradeoffs between costs and benefits. Using mussels as a model system, we coupled field observations in mussel-culture plots with manipulative laboratory experiments to address the following hypotheses: 1) labyrinthine spatial patterns, characteristically found at intermediate to high patch densities, are the most persistent over time; this is because labyrinthine patterns 2) result in adequately heavy patches that can maximize resistance to dislodgement while 3) increasing water turbulence with spacing, which will maximize food delivery processes. In the field, we observed that labyrinthine ‘stripes’ patterns are indeed the most persistent over time, confirming our first hypothesis. Furthermore, with laboratory experiments, we found the ‘stripes’ pattern to be highly resistant to dislodgement, confirming the second hypothesis. Finally, with regards to the third hypothesis, we found positive effects of this pattern on local turbulence. These results suggest that the mechanisms of intraspecific facilitation not only depend on initial organism densities, but may also be influenced by spatial patterning. We hence recommend taking into account spatial patterns to maximize productivity and persistence in shellfish-cultivation practices and to increase the restoration success of ecosystems with self-organizing properties.

Bed level dynamics at the interface of the salt marsh and tidal flat have been highlighted as a key factor connecting the long-term biogeomorphological development of the marsh to large-scale physical forcing. Hence, we aim to obtain insight into the factors confining the location of the marsh edge (i.e., boundary between tidal flat and salt marsh). A unique data set was collected, containing measurements of daily bed level changes (i.e., integrative result of physical forcing and sediment properties) at six intertidal transects in the North Sea area. Moreover, various biophysical parameters were measured, such as sediment characteristics, waves, inundation time, and chlorophyll-a levels. The data show that both bed level change and waves decreased from the lower intertidal flat toward the marsh edge and further diminished inside the marsh. However, no direct general relation was found between waves and bed level change. Bed level change inside the marsh was always small, regardless of wave energy. By combining the data sets, we demonstrate that the location of the lower marsh edge is restricted by two interacting factors: inundation time and bed level change. For vegetation establishment to withstand longer inundation stress, which slows down plant growth, more stable bed levels are required so that plants are not heavily disturbed. Conversely, to withstand more dynamic bed levels that disturbs plant growth, lower inundation stress is needed, so that plants grow fast enough to recover from the stress. The results suggest that bed level change is important in determining the position of the marsh edge.

Seedling establishment is an important process relevant for the restoration of salt marsh within the framework of sustainable coastal defense schemes. Recent studies have increasingly highlighted how the short-term (i.e., the day-to-day) sediment dynamics can form major bottlenecks for seedling establishment. Until recently, studies on quantifying the threshold values of such short-term sediment dynamics for marsh seedlings remain rare. As accretion/erosion trends and dynamics may differ greatly under global change, we study the effects of short-term sediment disturbance-regimes on seedling establishment of two globally distributed foundation species: Spartina alterniflora and Spartina anglica. Seedlings with different disturbance-free periods were exposed to a set of different accretion/erosion-regimes in the laboratory. Seedling survival appeared to be much more sensitive to erosion than accretion, seedlings with short disturbance-free periods were more sensitive than seedlings with longer ones, and S. alterniflora was more sensitive than S. anglica. Seedlings were less sensitive to gradual changes in sediment height (accretion/erosion) than to abrupt changes where time for morphological adjustment is lacking. Critical erosion depth (the maximum erosion that seedlings are able to withstand) was shown to mainly depend on sedimentation history. Our results confirm that the establishment of Spartina seedlings requires a flooding disturbance-free “window of opportunity” and that sediment disturbances affect their survival both directly and via morphological adjustment. These results provide fundamental insights into seedling establishment that can be used for designing engineering measures to create suitable conditions and enable marsh creation/restoration for nature goals or as part of coastal defense schemes under global change.