Search

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.

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.

The value of mangroves has been widely acknowledged, but mangrove forests continue to decline due to numerous anthropogenic stressors. The impact of plastic waste is however poorly known, even though the amount of plastic litter is the largest in the region where mangroves are declining the fastest: South East Asia. In this study, we examine the extent of the plastic waste problem in mangroves along the north coast of Java, Indonesia. First, we investigate how much of the forest floor is covered by plastic in the field (in number of items per m2 and in percentage of the forest floor covered by plastic), and if plastic is also buried in the upper layers of the sediment. We then experimentally investigate the effects of a range of plastic cover percentages (0%, 50% and 100%) on root growth, stress response of the tree and tree survival over a period of six weeks. Field monitoring showed that plastic was abundant, with 27 plastic items per m2 on average, covering up to 50% of the forest floor at multiple locations. Moreover, core data revealed that plastic was frequently buried in the upper layers of the sediment where it becomes immobile and can create prolonged anoxic conditions. Our experiment subsequently revealed that prolonged suffocation by plastic caused immediate pneumatophore growth and potential leaf loss. However, trees in the 50%-plastic cover treatment proved surprisingly resilient and were able to maintain their canopy over the course of the experiment, whereas trees in the 100%-plastic cover treatment had a significantly decreased leaf area index and survival by the end of the experiment. Our findings demonstrate that mangrove trees are relatively resilient to partial burial by plastic waste. However, mangrove stands are likely to deteriorate eventually if plastic continues to accumulate.

Wave flume facilities that are primarily designed for engineering studies are often complex and expensive to operate, and hence not ideal for long‐term replicated experiments as commonly used in biology. This study describes a low‐cost small wave flume that can be used for biological purposes using fresh‐ or seawater with or without sediment. The wave flume can be used as a mesocosm to study interactions between wave hydrodynamics and benthic organisms in aquatic ecosystems. The low‐costs wave maker (−1 representing shallow coastal areas with a short fetch. The system can generate both regular waves (i.e., the wave period and orbital velocity remains constant), using a wave absorber, and irregular waves (i.e., varying wave period and orbital velocity) using a fast push and slow pull motion of the wave paddle. This wave mesocosm system is particularly useful in biogeomorphology to quantify interactions between organisms, sediment, and hydrodynamics and for aquatic ecologist aiming to simulate realistic bed shear stress where short‐ and long‐term experiments (weeks–months) can be replicated.

Sandy coasts represent about one-third of the global coastline and are among the most valuable and most vulnerable areas for humans and many other species. Socio-economic development and climate change impacts, together with traditional engineering for shore protection, have pervasively resulted in coastal squeeze, thereby threatening coastal life and economic activities, and the very survival of coastal ecosystems. In the past, the responses to problems such as land loss, coastal erosion and flooding were primarily reactive, through gray engineering solutions, with little interest shown in the ecosystem processes impacted by coastal armoring. In recent decades, coastal management strategies have become more diverse, embracing traditional engineering solutions alongside ecosystem-based measures. Even so, many of these new strategies still fail to meet sustainability criteria. Inspired by Per Bruun’s “The Ten Demands for Coastal Protection” from 1972, this article attempts to consider these changes and knowledge acquired since the 1970s, in order to tentatively formulate “Ten Commandments” for the sustainability of sandy coasts in face of climate change and socio-economic development. As such, the paper offers a new vision and briefly summarizes good practices for the management of sandy coasts, particularly useful for those who, at whatever level of influence, could contribute to the long-term realization of this new vision.

Morphology and physiology are two key aspects of the adaptation of kelp to varying environments. Some of these kelp responses to co-occurring highly hydrodynamic condition and high nutrient availability are well documented, but little is known about how these factors affect frond surface shape, particularly in the central frond. In this study, morphological and physiological traits of acclimatized Saccharina latissima (Phaeophyceae) (three size classes: 44.14 ± 1.15 cm, 29.60 ± 0.75 cm, and 16.07 ± 0.45 cm) were compared after 56 days under fully controlled conditions of waves or no waves, and high or low nutrient availability (i.e., LN-NW, LN-W, HN-NW, and HN-W treatments). Waves primarily increased frond biomass, elongation rate, and carbon to nitrogen ratio (C:N ratio) and induced both a greater variety in and rougher frond surface shapes. The fastest, second-fastest, and slowest growth rates were observed in the HN-W, LN-W, and LN-NW treatments, respectively. The highest C:N ratio was observed in the LN-W treatment. Together, these results seem to suggest that the thready and spring-like shapes found in the central frond (i.e., rougher frond surface) in wave-exposed conditions can at least partly compensate for low nutrient availability by enhancing nutrient and photon acquisition, particularly in low nutrient conditions. Additionally, large individuals showed significantly larger and heavier fronds compared with other size classes, and the meristematic sections of fronds had the most variance in frond surface shapes and highest C:N ratios compared with distal and mid-sections. Together, these results indicate that frond surface shapes in the newly formed central frond of S. latissima can be regarded both as possessing high morphological and physiological plasticity that enables kelp to cope with contrasting environments.

Nearshore biogenic habitats are known to trap sediments, and may therefore also accumulate biofouled, non-buoyant microplastics. Using a current-generating field flume (TiDyFLOW), we experimentally assessed the mechanisms of microplastic trapping of two size classes, 0.5 mm and 2.5 mm particle size, by three contrasting types of biogenic habitats: 1) seagrasses, 2) macroalgae, and 3) scleractinian corals. Results showed that benthic organisms with a complex architecture and rough surface – such as hard corals – trap the highest number of microplastics in their aboveground structure. Sediment was however the major microplastic sink, accumulating 1 to 2 orders of magnitude more microplastics than the benthic structure. Microplastic accumulation in the sediment could be explained by near-bed turbulent kinetic energy (TKE), indicating that this is governed by the same hydrodynamic processes leading to sediment trapping. Thus, the most valuable biogenic habitats in terms of nursery and coastal protection services also have the highest capacity of accumulating microplastics in their sediments. A significantly larger fraction of 0.5 mm particles was trapped in the sediment compared to 2.5 mm particles, because especially the smaller microplastics are entrained into the sediment. Present observations contribute to explaining why especially microplastics smaller than 1 mm are missing in surface waters.

Coastal areas are at increasing risk from flooding and erosion due to coastal development and climate change. Seagrass meadows, like other coastal ecosystems, can stabilise sediments, indirectly reducing coastal erosion. There is evidence that seagrass can reduce both the erosion of seabeds and lateral erosion of seagrass cliffs. However, these previous studies have only considered seagrass species whose below-ground biomass form denseroot mats. This study aimed to investigate whether Enhalus acoroides, a species which does not form root mats,reduces lateral erosion rates of seagrass cliffs. To do this, samples of E. acoroides collected in sandy sediments within Xincun Bay China were transplanted into a wave flume. Lateral erosion rates were measured under exposure to waves for five cliff heights over 1 h. There was an interaction between cliff height and time on the rate of erosion, with higher erosion rates earlier in the experiment for larger cliff heights. The cores with the E. acoroides persisted longer than unvegetated cores, regardless of cliff height, but this result was not assessed statistically due to constraints in the analyses. The below-ground biomass of E. acoroides samples in this study was higher than values previously reported in the literature. Overall, these results suggest that non-dense root-mat forming seagrass species may not provide equivalent sediment stabilisation as dense root-mat-forming species, likely due to the reduced binding structure of the roots in species such as Enhalus.

Understanding how geomorphic landscape features affect physical habitat conditions is essential for predicting if such geomorphic landscape features may act as conduits facilitating invasive species expansion in coastal ecosystems. Although the effects of various abiotic and biotic factors on plant invasion in salt marshes have been well studied, little is known about the impact of tidal channels on plant invasion. Here, weexamined the effects of bottom-up determinants on the lateral expansion of Spartina alterniflora in tidal channel margins of different elevational marsh zones: the low, middle and high marsh, in the Yellow River Delta, China. Field observations and experiments showed that the microhabitats of tidal channel margins are characterized by relatively low soil salinity and high inundation, which significantly facilitated theestablishment, growth, colonization and sexual reproduction of S. alterniflora. Moreover, the facilitating effect of tidal channel margins on plant landward invasion appeared more sensitive in middle and high marshes compared with the low marshes. High propagule pressure combined with suitable physical conditions of the structural microhabitats formed within tidal channels potentially promoted the rapid expansion of S. alterniflora along the tidal channel margins. Based on these results, a conceptual model was built illustrating the lateral expansion mechanisms of the invasive plant mediated by tidal channels. These results highlight the importance of tidal channel-mediated gradients in bottom-up abiotic and biotic-dispersal factors in facilitating the expansion of coastal invasive plants. For wetlands management, our results imply that geomorphic landscape features should be incorporated into ecological management and risk assessment of invasive plants.

The benthic communities of soft-sediment intertidal ecosystems trophically underpin the migration of birds and fish. Within the East Atlantic Flyway, along the coast of West-Africa, the intertidal mudflats of Banc d’Arguin, Mauritania, host over 2 million migratory waterbirds. Despite the protected status of the Banc d’Arguin, geographical remoteness and seemingly benign human exploitation, we show that large changes have taken place in the intertidal benthic macrofauna across an interval of 28 years. We compared the results of two comparable and spatially comprehensive large-scale benthic surveys in 1986 and 2014. Over this time, the benthos changed from a diverse community to one dominated by a few species of bivalve, with a loss of polychaete worms. The change was associated with a twofold increase in the seagrass cover. Our results suggest that the intertidal habitats of Banc d’Arguin have altered markedly over the last three decades and the estimated benthic secondary production has decreased by a factor of four. These shifts in community structure and production may have contributed to declines in some benthivorous migratory shorebirds.

Climate change and rising population pressure are driving increasing risk for coastal domains worldwide, and especially for low-lying deltas. Yet, our knowledge base to manage or mitigate these challenges lags behind. Modern-day sustainable coastal design concepts (e.g. ‘Building with Nature’) demand fundamental insight into the complex interplay of hydrodynamic, morphological and ecological processes. In the past, improved understanding was mostly based on laboratory experiments or dedicated theoretical research on isolated processes. However, field observations have revealed that the coastal system behaves as a nonlinear dynamic system with emergent behaviour of unknown predictability for many processes. To cater for these new paradigms, Dutch coastal scientists are in urgent need of a world-class, natural coastline observatory to ensure continuous, long-term and highresolution monitoring of coastal processes. We therefore propose the establishment of a facility named ICON.NL, the first node in an envisioned International Coastline Observatories Network (ICON).

Seagrass meadows are vital ecosystems in coastal zones worldwide, but are also under global threat. One of themajor hurdles restricting the success of seagrass conservation and restoration is our limited understanding of ecologicalfeedback mechanisms. In these ecosystems, multiple, self-reinforcing feedbacks can undermine conservation efforts bymasking environmental impacts until the decline is precipitous, or alternatively they can inhibit seagrass recovery inspite of restoration efforts. However, no clear framework yet exists for identifying or dealing with feedbacks to improvethe management of seagrass ecosystems. Here we review the causes and consequences of multiple feedbacks betweenseagrass and biotic and/or abiotic processes. We demonstrate how feedbacks have the potential to impose or reinforceregimes of either seagrass dominance or unvegetated substrate, and how the strength and importance of these feedbacksvary across environmental gradients. Although a myriad of feedbacks have now been identified, the co-occurrence andlikely interaction among feedbacks has largely been overlooked to date due to difficulties in analysis and detection.Here we take a fundamental step forward by modelling the interactions among two distinct above- and belowgroundfeedbacks to demonstrate that interacting feedbacks are likely to be important for ecosystem resilience. On this basis, wepropose a five-step adaptive management plan to address feedback dynamics for effective conservation and restorationstrategies. The management plan provides guidance to aid in the identification and prioritisation of likely feedbacks indifferent seagrass ecosystems

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.

The interaction between macrophytes and hydrodynamic conditions is animportant feature in many aquatic ecosystems. Submerged macrophytes can formmonospecific patches that interact with the flow and alter current velocity; withinthe same vegetation patch, plants are exposed to different levels of hydrodynamicstress. Due to the high morphological variability of aquatic plants, we expect differentarchitectural and morphological traits to emerge for individuals located atdifferent positions within the same patch. In this study, we have measured the flowvelocity around a patch of Veronica anagallis-aquatica in submerged conditionsand measured the morphological traits of individuals along a gradient of exposureto flow velocity within the patch. Results show that the more exposed individualspresent smaller sizes than the sheltered ones, lower relative allocation to stems,higher allocation to roots and reduced water content in roots and stems. Theknowledge obtained helps to clarify the role of morphological adaptations to flowstress in the context of plant-flow interactions.

Seed burial (i.e. vertical seed dispersal) has become increasingly valued for its relevance for seed fate and plant recruitment. While ecosystem engineers have been generally considered as the most important drivers of seed burial, the role of physical forces, such as wind or water flow, has been largely overlooked. Using tidal habitats as a model system, and a combination of flume and mesocosm experiments, we investigated the effects of 1) currents, 2) benthic animals with different engineering activities and 3) their interplay on seed burial of a common salt marsh pioneer plant, Spartina anglica. Our results reveal that in such systems, water flow can be of equal or higher importance than ecosystem engineers for seed burial. For passive seed-burying engineers (PSE), coupling their actions with currents produced synergistic seed burial effects, whereas the interactive effects were only additive for active seed-burying engineers (ASE). This paper extends current understanding of seed burial and seed bank formation by revealing the need to incorporate physical forces into seed burial mechanisms. We provide the first empirical evidence that physical forces influence seed burial by synergistically interacting with ecosystem engineers, thus highlighting the role of biophysical interactions as important drivers for vertical seed movement.

A full-scale controlled experiment was conducted on an excavated and re-assembled coastal wetland surface, typical of floristically diverse NW 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 0.6 cm 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.

Dynamic equilibrium theory is a fruitful concept, which we use to systematically explain the tidal flat morphodynamic response to tidal currents, wind waves, sediment supply, and other sedimentological drivers. This theory stems from a simple analytical model that derives the tide- or wave-dominated tidal flat morphology by assuming that morphological equilibrium is associated with uniform bed shear stress distribution. Many studies based on observation and process-based modeling tend to agree with this analytical model. However, a uniform bed shear stress rarely exists on actual or modeled tidal flats, and the analytical model cannot handle the spatially and temporally varying bed shear stress. In the present study, we develop a model based on the dynamic equilibrium theory and its core assumption. Different from the static analytical model, our model explicitly accounts for the spatiotemporal bed shear stress variations for tidal flat dynamic prediction. To test our model and the embedded theory, we apply the model for both long-term and short-term morphological predictions. The long-term modeling is evaluated qualitatively against previous process-based modeling. The short-term modeling is evaluated quantitatively against high-resolution bed-level monitoring data obtained from a tidal flat in Netherlands. The model results show good performances in both qualitative and quantitative tests, indicating the validity of the dynamic equilibrium theory. Thus, this model provides a valuable tool to enhance our understanding of the tidal flat morphodynamics and to apply the dynamic equilibrium theory for realistic morphological predictions.

Understanding the mechanisms limiting and facilitating salt marsh vegetation initial establishment is of widespread importance due to the many valuable services salt marsh ecosystems offer. Salt marsh dynamics have been investigated by many previous studies, but the mechanisms that enable or disable salt marsh initial establishment are still understudied. Recently, the “windows of opportunity” (WoO) concept has been proposed as a framework providing an explanation for the initial establishment of biogeomorphic ecosystems and the role of physical disturbance herein. A WoO is a sufficiently long disturbance-free period following seedling dispersal, which enables successful establishment. By quantifying the occurrence of WoO, vegetation establishment pattern can be predicted. For simplicity sake and as prove of concept, the original WoO framework considers tidal inundation as the only physical disturbance to salt marsh establishment, whereas the known disturbance from tidal currents and wind waves is ignored. In this study, we incorporate hydrodynamic forcing in the WoO framework. Its spatial and temporal variability is considered explicitly in a salt marsh establishment model. We used this model to explain the observed episodic salt marsh recruitment in the Westerschelde Estuary, Netherlands. Our results reveal that this model can significantly increase the spatial prediction accuracy of salt marsh establishment compared to a model that excludes the hydrodynamic disturbance. Using the better performing model, we further illustrate how tidal flat morphology determines salt marsh establishing elevation and width via hydrodynamic force distribution. Our model thus offers a valuable tool to understand and predict bottlenecks of salt marsh restoration and consequences of changing environmental conditions due to climate change.