The adaptive capacity of ecosystems, or their ability to function despite altered environmental conditions, is crucial for resilience to climate change. However, the role of landscape complexity or species traits on adaptive capacity remains unclear. Here, we combine field experiments and morphodynamic modelling to investigate how ecosystem complexity shapes the adaptive capacity of intertidal salt marshes. We focus on the importance of tidal channel network complexity for sediment accumulation, allowing vertical accretion to keep pace with sea-level rise. The model showed that landscape-scale ecosystem complexity, more than species traits, explained higher sediment accumulation rates, despite complexity arising from these traits. Landscape complexity, reflected in creek network morphology, also improved resilience to rising water levels. Comparing model outcomes with real-world tidal networks confirmed that flow concentration, sediment transport and deposition increase with drainage complexity. These findings emphasize that natural pattern development and persistence are crucial to preserve resilience to climate change.@en

Tidal flats are shrinking in extent globally. The dynamics of the response of estuarine tidal flats to global environmental changes remain unclear. Tidal-flat morphology is shaped by the interplay among wave and tidal forces, river discharge and sediment supply, and preservation of tidal flats requires a balance between erosional and depositional processes be maintained. Here we assess tidal-flat morphodynamic changes of 40 globally distributed estuaries with contrasting tidal amplitudes between 1986 and 2011 from analyses of 4,939 satellite images. We consider both vegetated and unvegetated intertidal areas. From comparisons with remote-sensing-derived turbidity estimates, we identify a critical turbidity threshold indicative of a minimum sediment supply along with the hydrodynamic forces, which is necessary to maintain tidal flats. Tidal flats in intertidal areas in estuaries with low turbidity face retreat, with the critical turbidity threshold increasing with increasing tidal amplitudes. By contrast, estuaries with high turbidity tend to exhibit laterally or vertically expanding tidal flats. However, despite estuaries with limited tidal ranges having relatively low turbidity thresholds, environmental or anthropogenic alterations can still adversely affect the morphology of tidal flats. Our findings demonstrate the need to consider sediment supply in integrated estuarine management strategies to maintain the ecological integrity and flood defence function of tidal flats.

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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.

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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.

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Tidal flats provide valuable ecosystem services such as flood protection and carbon sequestration. Erosion and accretion processes govern the ecogeomorphic evolution of intertidal ecosystems (marshes and bare flats) and, hence, substantially affect their valuable ecosystem services. To understand the intertidal ecosystem development, high-frequency bed-level change data are thus needed. However, such datasets are scarce due to the lack of suitable methods that do not involve excessive labour and/or costly instruments. By applying newly developed surface elevation dynamics (SED) sensors, we obtained unique high-resolution daily bed-level change datasets in the period 2013-2017 from 10 marsh-mudflat sites situated in the Netherlands, Belgium, and the United Kingdom in contrasting physical and biological settings. At each site, multiple sensors were deployed for 9-20 months to ensure sufficient spatial and temporal coverage of highly variable bed-level change processes. The bed-level change data are provided with synchronized hydrodynamic data, i.e. water level, wave height, tidal current velocity, medium sediment grain size (D₅₀), and chlorophyll a level at four sites. This dataset has revealed diverse spatial morphodynamics patterns over daily to seasonal scales, which are valuable to theoretical and model development. On the daily scale, this dataset is particularly instructive, as it includes a number of storm events, the response to which can be detected in the bed-level change observations. Such data are rare but useful to study tidal flat response to highly energetic conditions. The dataset is available from 4TU.ResearchData (https://doi.org/10.4121/12693254.v4; Hu et al., 2020), which is expected to expand with additional SED sensor data from ongoing and planned surveys.

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Globally, erosion of muddy tropical coasts that are dominated by aquaculture ponds, is an increasing problem. Restoration of mangrove greenbelts may counteract such erosion, by restoring the sediment balance. Hence, we aim to unravel the processes controlling natural mangrove regeneration in both “landward” (i.e., into aquaculture ponds) and seaward direction, using the fast eroding coastline of Demak (Indonesia) as case study. Firstly, we investigated which physical and chemical factors drive landward mangrove expansion by relating them to the presence/absence of mangrove seedlings in abandoned aquaculture ponds. Secondly, we investigated which physical parameters control seaward mangrove expansion by relating them to expansion and retreat at the sea-side of mature mangrove stands. Landward mangrove expansion into abandoned aquaculture ponds was positively related to both emergence time (%) and sediment stability (i.e., shear strength), which are in turn both associated to bed level elevation and pond drainage. Surprisingly, there was no effect of soil chemistry. Seaward expansion of existing mangrove stands was strongly associated to foreshore morphology. Mangroves only expanded in the presence of an elevated mudflat, whereas the absence of a mudflat in combination with a concave (hollow) profile was associated with mangrove retreat. Our findings suggest that restoration of a mangrove greenbelt can be stimulated landward by improving drainage of abandoned aquaculture ponds. This enhances sediment stability and allows ponds to accrete. Seaward expansion can be induced by restoring foreshore morphology. Present results are discussed in the context of large-scale applications.

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Understanding the spatial structure of microphytobenthos (MPB) on intertidal flats is necessary to gain insight in the benthic community structure and ecosystem processes. The increasing availability of high resolution satellite sensors provides the opportunity to better understand spatial patterns of MPB on various (meter to km) scales. We tested how MPB patch size (indicated by the range derived from a semi-variogram) and degree of patchiness (indicated by the sill) vary as function of seasons, salinity, tidal flat type (muddy fringing versus sandy mid-channel tidal flats) or ecotopes (defined by hydrodynamics, silt content and elevation), in the Westerschelde estuary, the Netherlands. We used Sentinel-2 imagery (2016–2019) with 10 m spatial resolution to derive (omnidirectional) semi-variogram parameters from the NDVI (used as indicator for MPB biomass) and evaluated (seasonality in) patchiness of MPB in the different categories. We demonstrated that MPB patch size (the range) remains constant from winter to summer, while the sill increased from winter to summer. The location of patches on tidal flats was variable throughout the year and shows a remarkable similarity with seasonality in the spatial heterogeneity of the silt content on tidal flats. The patch size and degree of patchiness is higher on relatively sandy mid-channel tidal flats than on relatively silt rich fringing tidal flats. This implies that spatial patterning of MPB biomass on the meso-scale is likely closely linked to abiotic conditions and that spreading processes or grazing activity play a minor role. We observed visually that some areas with a relatively high MPB biomass (‘patches’) remain visible throughout the year, while other patches were only present during a particular season.

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Microphytobenthos forms an important part of the diet of macrofauna (macrozoobenthos) in many intertidal ecosystems. It is unclear, however, whether the dependence of macrofauna on microphytobenthos varies spatially within and among tidal systems. We aim (1) to assess the spatial variability in the importance of microphytobenthos in the diet of macrofauna (i.e., between and within two tidal basins and as function of elevation), (2) to quantify grazing pressure of the macrofaunal community on different potential food sources (microphytobenthos, phytoplankton and terrestrial organic material) for several sites in two tidal basins and (3) to compare microphytobenthic production and summer/autumn grazing of the total macrofaunal community and grazing pressure per feeding type, with potential microphytobenthic production estimated from rates in early spring, when grazing was low. Using a natural stable isotope approach, we identified microphytobenthos as a more important food source for macrofauna than phytoplankton and terrestrial organic material. Microphytobenthos dependency differed between tidal basins for the genera Bathyporeia (sand digger shrimp), Macoma (Baltic tellin), and Peringia (mudsnail) and for sampled individuals of all genera combined, and did not vary as function of elevation. We showed that macrofaunal grazing on microphytobenthos is quantitatively important and, in some cases, approached microphytobenthic production rates in early spring. No positive relation between microphytobenthic production in early spring and macrofaunal grazing in summer/autumn was observed. This suggests that the studied consumer-resource interactions are coupled on a larger spatial scale (i.e., mesoscale, ≈10 to 100 km), rather than the fine (mm to m) scale.@en

Salt marshes are in danger of degradation due to human impact and climate change. A thorough understanding of mechanisms controlling sedimentation and erosion in salt marshes is essential for their conservation and restoration. To understand short‐term dynamics of sediment availability and deposition around marsh edges, two contrasting marshes, Rattekaai and Sint Annaland, were studied in the Oosterschelde (southwest Netherlands). Suspended sediment concentration (SSC) was measured by siphon samplers along four transects perpendicular to the marsh edge in each marsh, during nine flood tides between March and December 2013. Each transect was comprised of four sampling sites (−10 m and −1 m on the mudflat and +1 m and +10 m on the marsh plateau, relative to the marsh edge). Sediment deposition was measured along the transects on the marsh, at +1 m and +10 m from the marsh edge, over seven c. 14‐day intervals during the same 10‐month period. Two types of sediment traps were used, one measuring gross sediment deposition (TTD – tube trap deposition) and one measuring net sediment deposition (FTD – filter trap deposition). Wave loggers were deployed 10 m away from the marsh edge on the mudflat at each marsh. The results showed that both SSC and sediment deposition varied greatly through space, both between the two marshes and within each marsh along the marsh edge. The SSC and gross sediment deposition were much higher at Rattekaai than at Sint Annaland. SSC was significantly correlated with wind speed during sampling. Sediment deposition rates (TTD and FTD) and retention ratio (FTD/TTD) were significantly correlated with cumulative wave energy during the measurement period. A conceptual model of local sediment dynamics is proposed to explain the sediment dynamics around the marsh edge. This study highlights the importance of incorporating local sediment dynamics when evaluating marsh vulnerability and stability@en

Salt marshes are valuable ecosystems that provide important ecosystem services. Given the global scale of marsh loss due to climate change and coastal squeeze, there is a pressing need to identify the critical extrinsic (wind exposure and foreshore morphology) and intrinsic factors (soil and vegetation properties) affecting the erosion of salt marsh edges. In this study, we quantified rates of cliff lateral retreat (i.e., the eroding edge of a salt marsh plateau) using a time series of aerial photographs taken over four salt marsh sites in the Westerschelde estuary, the Netherlands. In addition, we experimentally quantified the erodibility of sediment cores collected from the marsh edge of these four marshes using wave tanks. Our results revealed the following: (i) at the large scale, wind exposure and the presence of pioneer vegetation in front of the cliff were the key factors governing cliff retreat rates; (ii) at the intermediate scale, foreshore morphology was partially related to cliff retreat; (iii) at the local scale, the erodibility of the sediment itself at the marsh edge played a large role in determining the cliff retreat rate; and (iv) at the mesocosm scale, cliff erodibility was determined by soil properties and belowground root biomass. Thus, both extrinsic and intrinsic factors determined the fate of the salt marsh but at different scales. Our study highlights the importance of understanding the scale dependence of the factors driving the evolution of salt marsh landscapes.

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Thorough understanding of the potential for threshold dynamics and catastrophic shifts to occur in natural systems is of great importance for ecosystem conservation and restoration. However, verifying the presence of alternative stable states, one of the theoretical explanations for sudden shifts in natural systems, has proven to be a major challenge. We examine processes on local and landscape scales in salt-marsh pioneer zones, to assess the presence of alternative stable states in this system. To that end, we investigated the presence of typical characteristics of alternative stable states: bimodality and threshold dynamics. We also studied whether vegetation patches remained stable over long time periods. Analysis of false-color aerial photographs revealed clear bimodality in plant biomass distribution. By transplanting Spartina anglica plants of three different biomass classes on three geographically different marshes, we showed that a biomass threshold limits the establishment of Spartina patches, potentially explaining their patchy distribution. The presence of bimodality and biomass thresholds points to the presence of alternative stable states and the potential for sudden shifts, at small, within-patch scales and on short time scales. However, overlay analysis of aerial photographs from a salt marsh in The Netherlands, covering a time span of 22 years, revealed that there was little long-term stability of patches, as vegetation cover in this area is slowly increasing. Our results suggest that the concept of alternative stable states is applicable to the salt-marsh pioneer vegetation on small spatio-temporal scales. However, the concept does not apply to long-term dynamics of decades or centuries of heterogeneous salt-marsh pioneer zones, as landscape-scale processes may determine the large-scale dynamics of salt marshes. Hence, our results provide the interesting perspective that threshold dynamics may occur in systems with, on the long term, only a single stable state.@en