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Understanding the sensitivity of tidal flats to environmental changes is challenging. Currently, most studies rely on process-based models to systematically explain the morphodynamic evolution of tidal flats. In this study, we proposed an alternative empirical approach to explore tidal flat dynamics using statistical indices based on long-term time series of daily surface elevation development. Surface elevation dynamic (SED) indices focus on the magnitude and period of surface elevation changes, while morphodynamic signature (MDS) indices relate sediment dynamics to environmental drivers. The statistical analyses were applied to an intervention site in the Netherlands to determine the effect of recently constructed groynes on the tidal flat. Using these analyses, we were able to (1) detect a reduction in the daily SED and (2) determine that the changes in the daily SED were predominantly caused by the reduction in wave impact between the groynes rather than the reduction in tidal currents. Overall, the presented results showed that the combination of novel statistical indices provides new insights into the trajectories of tidal flats, ecosystem functioning, and sensitivity to physical drivers (wind and tides). Finally, we suggested how the SED and MDS indices may help to explore the future trajectories and climate resilience of intertidal habitats.

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.

Storm surge barriers (SSBs) protect the hinterland of estuaries against flooding, while in open state the tidal dynamics are maintained to some extent. Even when tidal dynamics are maintained, tidal conditions are inevitably affected by the confinement of the flow. As a result, intertidal flats – providing important ecosystem services – face losses through erosion. In this work, we integrated research on (1) morphological consequences of the Eastern Scheldt SSB (The Netherlands, 1987) and (2) intertidal flat nourishments mitigating these negative eco-morphological consequences. Through decades of data, we show that the SSB induced persistent erosion. We have demonstrated that sand nourishments on tidal flats can effectively mitigate SSB-induced erosion and ecological consequences. We have found that net lowering of tidal flats in systems as the Eastern Scheldt will proceed, also given accelerated sea level rise. Even though tidal flat nourishments are an effective mitigation strategy, we urge to steer first on minimizing irreversible hydromorphological changes caused by future SSB projects (e.g., considered for Galveston Bay). Through monitoring data on intertidal flat sand nourishments in this system, we identify the potential of these mitigation measures and provide recommendations for future SSB deployments and mitigation.

Vegetated coastal ecosystems such as saltmarshes, mangroves and seagrass beds are increasingly promoted as sustainable storm and flood defence solutions by reducing wave energy. Yet, there is still intense debate on the ability of root mats to mitigate erosion, with some studies arguing that the direct contribution of roots in preventing sediment erosion is minor, while others consider them of major importance. Here, we hypothesized that the contrasting findings on the role of seagrass root mats in preventing erosion may stem from differences in sediment type. To test this idea, we investigated how root mats of seagrass that thrives in both sandy and muddy sediments mitigate wave-induced cliff erosion using Zostera marina in manipulative flume experiments. Results demonstrate that roots are very effective in reducing cliff erosion rates in sandy sediments. Cliff erosion rates were reduced up to 70% in sandy sediment with high seagrass root biomass. In contrast, cliff erosion rates in cohesive muddy sediments were low and unaffected by seagrass roots. This highlights the important role of seagrass roots in erosion mitigation, which has been overlooked compared to the role of canopies, which has received more attention. We suggest that management strategies should be developed to enhance the stabilization of sandy sediment, such as (1) using species with high belowground biomass, (2) using fast-growing pioneer species and (3) applying temporary stabilising measures.

The implementation of climate change mitigation strategies based on the conservation and restoration of Blue Carbon ecosystems requires a deep understanding of themagnitude and variability in organic carbon (Corg) storage across and within these ecosystems. This study explored the variability in soil Corg stocks and burial rates across and within intertidal estuarine habitats of the Atlantic European coast and its relation to biotic and abiotic drivers. A total of 136 soil cores were collected across saltmarshes located at different tidal zones (high marsh, N = 45; low marsh, N = 30), seagrass meadows (N=17) and tidal flats (N=44), and from the inner to the outer sections of five estuaries characterized by different basin land uses. Soil Corg stocks were higher in high-marsh communities (65±3 Mg ha-1) than in low-marsh communities (38 ±3 Mg ha-1), seagrass meadows (40 ± 5 Mg ha-1) and unvegetated tidal flats (46± 3Mg ha-1) whereas Corg burial rates also tended to be higher in highmarshes (62±13 g m-2 y-1) compared to low marshes (43 ± 15 g m-2 y-1) and tidal flats (35 ± 9 g m-2 y-1). Soil Corg stocks and burial rates decreased from inner to outer estuarine sections in most estuaries reflecting the decrease in the river influence towards the estuary mouth. Higher soil Corg stocks were related to higher content of silt and clay and higher proportion of forest and natural land within the river basin, pointing at new opportunities for protecting coastal natural carbon sinks based on the conservation and restoration of upland ecosystems. Our study contributes to the global inventory of Blue Carbon by adding data from unexplored regions and habitats in Europe, and by identifying drivers of variability across and within estuaries. The authors acknowledge the financial support fromthe Government of Cantabria through the “Fénix Programme”. IM was supported by a Juan de la Cierva Incorporación postdoctoral fellowship of the Spanish Ministry of Science, Innovation and Universities (JC2020-045917-I).OS was supported by I+D+i projects RYC2019-027073-I and PIE HOLOCENO 20213AT014 funded by the Spanish Ministry of Science, Innovation and Universities (MCIN/AEI/10.13039/501100011033) and FEDER. The IAEA is grateful for the support provided to its Marine Environment Laboratories by the Government of the Principality of Monaco. PM was supported by the Australian Research Council. J.G.O. acknowledges the financial support of the Spanish Ministry of Science, Innovation and Universities, through the “Maria de Maeztu” program for Units of Excellence (CEX2019-000940-M) and the Generalitat de Catalunya (MERS; 2017 SGR–1588). This research was carried out with the contribution of the LIFE Programme of the European Union to the Project ADAPTA BLUES (ref. LIFE18 CCA/ES/001160). This document reflects only the authors' view and the Agency/Commission is not responsible for any use that may be made of the information it contains.

Identify critical factors driving seedling establishment is essential for saltmarsh restoration. Recent studies highlight drainage can facilitate seedling establishment for different saltmarsh species, yet the underling mechanism remains largely unknown. By a manipulative mesocosm experiment with contrasting sediment drainage treatments, we investigated the impacts of drainage on Spartina anglica seedlings survival, growth and stability. Surprisingly, neither S. anglica seedling survival nor seedling growth were observed to be directly promoted by well drained treatment. However, the critical erosion depth (maximum erosion tolerance) of S. anglica seedlings was found to positively correlate with seedling root length. Measurements on the sediment properties revealed that drainage increased the critical shear strength of sediments with lower water content and higher dry bulk density. These findings indicate that, by dewatering, drainage aids to sediment consolidation and critical shear strength, and thereby facilitate seedling establishment in tempering sediment dynamics. The latter is supported by a field study, in which we found that the magnitude of the sediment dynamics on two tidal flats were positively related to water content of sediment. Overall, present study suggests that drainage could be a key factor that determines seedling stability during the critical windows of opportunity for seedling establishment, and that it could be integrated into future designs for saltmarsh restoration to enhance establishment success.

Tidal marshes are increasingly valued for their nature-based shoreline protection function, as they reduce waves, currents and erosion. The effectiveness of this function depends on the ability of tidal marsh plants to grow and survive under pressure from waves and currents. However, how this varies with species-dependent plant traits is poorly understood. We performed a field transplantation experiment to quantify species-specific growth responses to different levels of hydrodynamic exposure and tidal inundation for 3 NW European marsh species: Schoenoplectus tabernaemontani, Bolboschoenus maritimus and Phragmites australis. In this order, these species showed increasing shoot stiffness, length and biomass, which are traits that increase hydrodynamic drag forces experienced by plants. Increased exposure to tidal inundation and hydrodynamics reduced the growth of all 3 species, but species with lower biomass and shorter, thinner and more flexible shoots could better cope with higher hydrodynamic exposure and tidal inundation. Furthermore, transplants of S. tabernaemontani (i.e. the species with the lowest shoot stiffness, length and biomass that survived under all tested conditions) developed smaller, thinner and more flexible shoots in response to higher hydrodynamic exposure and inundation. Hence our study indicates that similar inter- and intra-specific plant traits drive plant growth in response to hydrodynamics and inundation. This suggests that the spatial distribution of species typically observed in tidal marshes results not only from species-specific tolerance to tidal inundation gradients but also from hydrodynamic gradients. Allowing enough space for development of species zonation may be important to increase the efficiency of nature-based shoreline protection by tidal marshes.

Tidal marshes often present cyclic dynamics, with alternating phases of expansion and lateral retreat. Marsh seedling establishment is a primary condition for marsh colonization and expansion. Successful establishment is often episodic due to bio-physical thresholds, as expressed in the ‘Windows of Opportunity’ concept. Identifying critical factors and understanding how they enable or hamper seedling establishment, is highly relevant for coastal management and restoration schemes. This study quantified the effects of critical factors in tidal marsh establishment processes. Firstly, by exposing marsh seedlings to different disturbances in terms of sedimentation regime and wave exposure, this study found that these disturbances affect seedling establishment both directly and via bio-morphological adjustments (Chapter 2 and 3). Secondly, channel related drainage relief was found to facilitate seedling survival, especially in the early phase of marsh establishment in muddy systems (Chapter 4). Thirdly, by using marsh seedlings with different disturbance-free periods in both laboratory and field experiments, this study confirmed that a longer disturbance-free period can strongly enhance seedling survival as well as their resistance to later extreme disturbances (Chapter 2 and 4). The erosion of tidal marshes often leads to the development of a cliff at the boundary between tidal flat and marsh. The formation of a cliff shifts marsh development from a phase of lateral expansion to a retreating phase, as a result of the interdependent feedbacks between marsh vegetation and external forcing. This thesis investigated the tipping point conditions of cliff formation at marsh edges with different marsh species. The clonal growth of vegetation at marsh edges was found to respond to different sediment types and to affect the cliff formation processes. Species-specific clonal growth strategies in step length have important effects on initiating cliffs, lateral retreat and thus landscape dynamics at marsh edges (Chapter 5). In summary, this study provides mechanistic insight into the primary expansion (i.e., seedling establishment) of tidal marshes under a variety of critical forcing factors, as well as insight in the factors contributing to cliff formation and the onset of lateral retreat. It highlights the variability in relevant species-specific responses, as well as the importance of location-dependent factors. These results add quantitative support to models predicting the critical transitions in marsh dynamics, both marsh expansion and marsh retreat. The findings of this study are instructive for marsh restoration and nature based coastal defense schemes. While they contribute to understanding the overall stability of marsh ecosystems in the face of global change, they also highlight the need for tailor-made solutions for the application of nature-based solutions at particular locations.