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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).

Contains fulltext : 199582.pdf (Publisher’s version ) (Open Access) SER Europe Conference 2018E Conference 2018, 9-13 September 2018, Reykjavik Iceland.

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.

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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.

Contains fulltext : 151373.pdf (Publisher’s version ) (Open Access) 38 p.

One of the most frequently quoted ecosystem services of seagrass meadows is their value for coastal protection. Many studies emphasize the role of above-ground shoots in attenuating waves, enhancing sedimentation and preventing erosion. This raises the question if short-leaved, low density (grazed) seagrass meadows with most of their biomass in belowground tissues can also stabilize sediments. We examined this by combining manipulative field experiments and wave measurements along a typical tropical reef flat where green turtles intensively graze upon the seagrass canopy. We experimentally manipulated wave energy and grazing intensity along a transect perpendicular to the beach, and compared sediment bed level change between vegetated and experimentally created bare plots at three distances from the beach. Our experiments showed that i) even the short-leaved, low-biomass and heavily-grazed seagrass vegetation reduced wave-induced sediment erosion up to threefold, and ii) that erosion was a function of location along the vegetated reef flat. Where other studies stress the importance of the seagrass canopy for shoreline protection, our study on open, low-biomass and heavily grazed seagrass beds strongly suggests that belowground biomass also has a major effect on the immobilization of sediment. These results imply that, compared to shallow unvegetated nearshore reef flats, the presence of a short, low-biomass seagrass meadow maintains a higher bed level, attenuating waves before reaching the beach and hence lowering beach erosion rates. We propose that the sole use of aboveground biomass as a proxy for valuing coastal protection services should be reconsidered.

In low-nutrient, macrophyte-dominated coastal zones, benthic ammonium (NH4+) uptake may be influenced by the structural properties of plant canopies via their effect on near-bed hydrodynamics. Using a dual-tracer (uranine and 15NH4+) method that does not require enclosures, we examined how this process affects nutrient uptake rates within a tidally dominated, patchy Caulerpa prolifera–Cymodocea nodosa landscape. NH4+ uptake was determined by calculating tissue 15N excesses and correcting for 15N enrichment as derived from uranine concentration. Vertical hydrodynamic profiles were measured in the downstream flow direction from outside to inside of the C. nodosa bed by using an array of acoustic Doppler velocimeters. The transition from a C. prolifera to a C. nodosa bed included a change in both benthic canopy properties (short and dense to tall and sparse) and sediment topography (0.2-m increase in water column depth) that resulted in an increase in longitudinal advection and turbulent diffusivity within the C. nodosa canopy between 0.5 and 1.5 m from the leading edge. Vertical differences in canopy water exchange appeared to explain variations in uptake between biotic functional groups; however, no clear differences in longitudinal uptake were found. Using in situ labeling, this study demonstrated for the first time the role of hydrodynamics in structuring NH4+ uptake within an undisturbed, patchy macrophyte landscape.

The long-term (10100 years) evolution of tidal channels is generally considered to interact with the bio-geomorphic evolution of the surrounding intertidal platform. Here we studied how the geometric properties of tidal channels (channel drainage density and channel width) change as (1) vegetation establishes on an initially bare intertidal platform and (2) sediment accretion on the intertidal platform leads to a reduction in the tidal prism (i.e. water volume that during a tidal cycle floods to and drains back from the intertidal platform). Based on a time series of aerial photographs and digital elevation models, we derived the channel geometric properties at different time steps during the evolution from an initially low-elevated bare tidal flat towards a high-elevated vegetated marsh. We found that vegetation establishment causes a marked increase in channel drainage density. This is explained as the friction exerted by patches of pioneer vegetation concentrates the flow in between the vegetation patches and promotes there the erosion of channels. Once vegetation has established, continued sediment accretion and tidal prism reduction do not result in significant further changes in channel drainage density and in channel widths. We hypothesize that this is explained by a partitioning of the tidal flow between concentrated channel flow, as long as the vegetation is not submerged, and more homogeneous sheet flow as the vegetation is deeply submerged. Hence, a reduction of the tidal prism due to sediment accretion on the intertidal platform, reduces especially the volume of sheet flow (which does not affect channel geometry), while the concentrated channel flow (i.e. the landscape forming volume of water) is not much affected by the tidal prism reduction.

Mangroves grow in the intertidal parts of sheltered tropical coastlines, facilitating coastal stabilization and wave attenuation. Mangroves are widely threatened nowadays, although past studies have indicated their contribution to coastal safety. Most of these studies were based on numerical modeling however and a proper database with field observations is lacking yet. This paper presents part of the results of an extensive field campaign in a mangrove area in Trang Province, Thailand. The study area covers the outer border of an estuarine mangrove creek catchment. Data have been collected on elevation, vegetation, water levels, flow directions and flow velocities throughout this study area. Due to the tough conditions in the field, developing a suitable method for data collection and processing has been a major challenge in this study. Analysis of the hydrodynamic data uncovers the change of flow directions and velocities throughout a mangrove creek catchment over one tidal cycle. In the initial stages of flooding and the final stages of ebbing, creeks supply water to the lower elevated parts of the mangroves. In between these stages, the entire forest bordering the estuary is flooded and flow directions are perpendicular to the forest fringe. Flow velocities within the creeks are still substantially higher than those within the forest, as the creeks also supply water to the back mangroves. These insights in flow routing are promising for the future analysis of sediment input and distribution in mangroves.

The salt marsh grass Spartina anglica is an important habitat-modifying ecosystem engineering agent that facilitates large-scale salt marsh formation by enhancing sediment accretion. It dominates many European tidal environments and is invasive in many other parts of the world. We question (1) to what extent the ecosystem engineering ability of patchy Spartina vegetation depends on large-scale abiotic processes, and (2) whether tussock shape provides an indicator for future lateral salt marsh development. Analysing the topography of 83 individual tussocks in contrasting environments revealed that there are 6 clearly distinguishable tussock shapes, and that the classical example of a sediment-accumulating dome-shaped tussock only occurs under a limited set of abiotic conditions. The outcome of habitat modification by S. anglica is shown to be conditional, depending on large-scale morphodynamics and sediment grain size. Resulting tussock shape provides a clear indication for the long-term development of the pioneer zone. Understanding of the conditional outcome of ecosystem engineering is highly relevant in this era of climate change and ongoing anthropogenic influences on coastal ecosystems.

Feedback between vegetation growth, water flow, and landform is important for the biogeomorphic evolution of many landscapes, such as tidal marshes, alluvial rivers, and hillslopes. While experimental studies often focus on flow reduction within static homogeneous vegetation, we concentrate on flow acceleration around and between dynamically growing vegetation patches that colonize an initially bare landscape, with specific application to Spartina anglica, a pioneer of intertidal flats. Spartina patches were placed in a large-scale flow facility of 16 × 26 m, simulating the growth of two vegetation patches by increasing the patch diameter (D = 13 m) and decreasing the interpatch distance (d = 2.30 m). We quantified that the amount of flow acceleration next to vegetation patches, and the distance from the patch where maximum flow acceleration occurs, increases with increasing patch size. In between the patches, the accelerated flow pattern started to interact as soon as D/d ≥ 0.430.67. As the patches grew further, the flow acceleration increased until D/d ≥ 6.6710, from which the flow acceleration between the patches was suppressed, and the two patches started to act as one. These findings are in accordance with theory on flow around and between nonpermeable structures; however, the threshold D/d values found here for permeable vegetation patches are higher than those for nonpermeable structures. The reported flow interactions with dynamic vegetation patches will be essential to further understanding of the larger-scale biogeomorphic evolution of landscapes formed by flowing water, such as tidal flats, floodplain rivers, and hillslopes.

1. Populations of marine megaherbivores including green turtle (Chelonia mydas) have declined dramatically at a global scale as a result of overharvesting and habitat loss. This decline can be expected to also affect the tolerance of seagrass systems to coastal eutrophication. Until now, however, simultaneous effects of topdown control by megaherbivore grazing and bottomup control by nutrient input have not been tested experimentally.2. We therefore investigated the interacting effects of nutrient (N and P) addition and mimicked green turtle grazing on seagrass and epiphyte productivity, seagrass biomass and nutrient contents in exclosures at a pristine seagrass site in the Indo-Pacific region (Kalimantan, Indonesia).3. Grazing almost doubled leaf biomass production rates, while nutrient addition (N+P, slow-release granules) did not have an effect on these rates. Rhizome biomass was, however, strongly reduced by nutrient addition. In contrast to phosphorus, tissue nitrogen contents increased after nutrient addition, showing that nitrogen was not limiting primary productivity. Epiphyte growth was, however, strongly correlated with high water column P concentrations, indicating an indirect negative effect of eutrophication when turtle grazing would be absent. We calculated that green turtle leaf grazing leads to substantial exports of N and P, at rates of at least 8% of the standing stock per day equalling the daily seagrass production, up to 13 (N) and 1.4 (P) mg m(-2) day(-1).4. Synthesis. By combining our quantified effects with literature data, we propose a conceptual model of seagrass functioning under megaherbivore leaf grazing and eutrophication. In tropical seagrass systems with high green turtle grazing pressure, grazing alleviates the negative effects of eutrophication by the stimulation of seagrass production and concomitant nutrient uptake, the increased export of nutrients and the indirect prevention of low below-ground biomass. Similar to the role of terrestrial megaherbivores, these strong top-down controls show the pivotal role of green turtles in current coastal systems, which is lacking in systems where their numbers have greatly declined. These marine megaherbivores do not only drive structure and functioning of their foraging grounds but also increase the tolerance of seagrass ecosystems to eutrophication.

1. Macrophytes in running waters experience an often dynamic and harsh environment. To avoid breakage, plants have to reduce the experienced drag force. However, by reducing leaf area, photosynthetic production is less. Aquatic plants therefore have to find a balance between reducing drag and maintaining photosynthetic capacity. 2. In the experiments in this study, we assessed to what extent different morphological strategies (emergent vs. submerged) were able to minimize drag while maximizing leaf area. 3. From our measurements, it is clear that with increasing water velocities, emergent plant species have a drag value three to four times higher than submerged species. 4. To test the versatility of leaves, leaves were removed and their effect on drag and bending was investigated. Almost 60% of the drag is contributed by the leaves, and stems bend less when leaves are removed. 5. Because high submerged leaf area increases not only plant drag but also photosynthetic yield, a trade-off between both parameters was investigated in the function of stream velocity. Emerged species had a more favourable trade-off at low stream velocities. However, with increasing stream velocity, submerged species could reduce their drag more in comparison with blunt objects. Within these submerged species, a clear distinction was seen between those (Potamogeton natans) concentrating their leaf area on or just beneath the water surface (Stuckenia pectinata) and those with more or less evenly distributed biomass (Callitriche platycarpa and Ranunculus penicillatus). 6. These results indicate that aquatic plants with an emergent strategy are able to take better advantage of zones with reduced hydraulic forces than submerged plants. Additionally, this plant occurrence will be determined by the relationship between total plant drag and root strength.

We assessed the uptake of inorganic and organic nitrogen compounds by several macrophytes, epiphytes and phytoplankton in a seagrass meadow. Using double labeled (13C and 15N) substrates of differential complexity, the net transfers from the dissolved nitrogen and carbon pools to phytoplankton, heterotrophic bacteria, seagrasses, and a macroalga were quantified. The phytoplankton compartment was the largest nitrogen sink, but seagrasses dominated carbon fixation. There was a general preference for ammonium over nitrate and urea. Specific uptake rates of individual amino acids were inversely proportional to the complexity of their chemical structure. The bacterial contribution to carbon and nitrogen uptake from amino acids increased for more complex amino acids, as indicated by biomarker-specific measurements (polar lipid-derived fatty acids and D-alanine). Algae-derived dissolved organic nitrogen was taken up by all primary producers, but algae-derived dissolved organic carbon was almost exclusively used by the planktonic compartment. By contrast, a similar composed pool of bacteria-derived dissolved organic matter was not utilized in significant quantities by any of the primary producers. DON should be considered in studies of nutrient dynamics in seagrass meadows, and the role of the planktonic microbial community in seagrass meadows should be investigated in more detail.

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Coastal sediments in sheltered temperate locations are strongly modified by ecosystem engineering species such as marsh plants, seagrass, and algae as well as by epibenthic and endobenthic invertebrates. These ecosystem engineers are shaping the coastal sea and landscape, control particulate and dissolved material fluxes between the land and sea, and between the benthos and the passing water or air. Above all, habitat engineering exerts facilitating and inhibiting effects on biodiversity. Despite a strongly growing interest in the functional role of ecosystem engineering over the recent years, compared to food web analyses, the conceptual understanding of engineering-mediated species interactions is still in its infancy. In the present paper, we provide a concise overview on current insights and propose two hypotheses on the general mechanisms by which ecosystem engineering may affect biodiversity in coastal sediments. We hypothesise that autogenic and allogenic ecosystem engineers have inverse effects on epibenthic and endobenthic biodiversity in coastal sediments. The primarily autogenic structures of the epibenthos achieve high diversity at the expense of endobenthos, whilst allogenic sediment reworking by infauna may facilitate other infauna and inhibits epibenthos. On a larger scale, these antagonistic processes generate patchiness and habitat diversity. Due to such interaction, anthropogenic influences can strongly modify the engineering community by removing autogenic ecosystem engineers through coastal engineering or bottom trawling. Another source of anthropogenic influences comes from introducing invasive engineers, from which the impact is often hard to predict. We hypothesise that the local biodiversity effects of invasive ecosystem engineers will depend on the engineering strength of the invasive species, with engineering strength defined as the number of habitats it can invade and the extent of modification. At a larger scale of an entire shore, biodiversity need not be decreased by invasive engineers and may even increase. On a global scale, invasive engineers may cause shore biota to converge, especially visually due to the presence of epibenthic structures.

The hypotheses that (1) different seagrass morphologies may facilitate different nutrient uptake rates under similar hydrodynamic forcing and (2) this effect on nutrient uptake rates is spatially explicit, with the highest uptake rates at edges of patches, where currents and turbulence are highest, were examined under unidirectional flow conditions. Homogeneous patches (2 m long) of two seagrass species (Cymodocea nodosa and Zostera noltii) with contrasting shoot size and density were placed in a race track flume. 15NH4+ uptake and hydrodynamic properties along a gradient from outside to inside the patch were measured at a range of current velocities (0.05 to 0.3 m s-1). For each velocity we also determined the height and bending of the canopy. Water velocity affected the ammonium uptake rate of both species. The almost double uptake rates of C. nodosa shoots, compared to those of Z. noltii, were mainly attributed to a twofold difference in the within-canopy water flow (Qc, m3 s-1). Spatial patterns in canopy water flow were highly correlated with spatial patterns in NH4+ uptake, thereby explaining the 20% higher uptake rates at the leading edge of both canopies. The correlation between spatial patterns in canopy water flow and ammonium uptake rates underlines the role of canopy and patch configuration in determining the functioning of seagrass landscapes and their associated ecosystem services, such as nitrogen assimilation.