Growth conditions and eco‐engineering effects of vegetation on local conditions in coastal environments have been extensively studied. However, interactions between salt marsh settling, growth, and mortality as a function of hydromorphology and eco‐engineering lack sufficient understanding to forecast morphological development of dynamic systems. We predict salt marsh establishment with an ecomorphodynamic model that accounts for literature‐based seasonal settling and life‐stage‐dependent growth and mortality of a generic salt marsh species. The model was coupled to a calibrated hydromorphodynamic model of an intertidal bar and, on a coarser grid, to the entire Western Scheldt estuary. To quantify the importance of eco‐engineering effects we compared the dynamic model results to a static model approach. The ecomorphodynamic model reproduces spatial pattern, cover, and growth trends over 15 years. The modeled vegetation cover emerges from the combination of a positive and a new negative eco‐engineering effect: vegetation reduces tidal flow strength facilitating plant survival while the developing salt marsh increases the hydroperiod, which limits large‐scale marsh expansion. The reproduced spatial gradient in vegetation density by our model is strongly correlated to their life‐stages, which underlines the importance of age‐dependence when modeling vegetation and for predictions of the stability of the marsh. Upscaling of the model to the entire estuary on a coarser grid gives implications for grid size‐dependent modeling of hydrodynamics and vegetation. In comparison with static model results, the eco‐engineering effects reduce vegetation cover, showing the importance of vegetation dynamics for predictions of salt marsh growth.

Mud accretion and establishment of biostabilizers, such as microphytobenthos and saltmarsh vegetation, govern the development of estuarine morphology. Mud facilitates saltmarsh survival and microphytobenthos growth, which in turn promotes sedimentation and reduces mud erosion. Consequently, an increasing extent and thickness of mud cover might lead to a stabilization of large‐scale estuarine morphology. To disentangle the interactions between saltmarsh establishment, microphytobenthos colonization, and mud layer formation, we use our novel eco‐morphodynamic model applied to the Western Scheldt estuary. Our model shows that presence of dynamic saltmarsh vegetation and microphytobenthos enhances predictions of mud location in the computations compared to field data. Saltmarsh establishment is partly determined by the antecedent mud content in the bed, resulting in varying emerging vegetation coverage between model experiments of a generic saltmarsh and a saltmarsh species that requires prior mud for establishment. In contrast to microphytobenthos enhancing seasonal mud accretion during their growth period, saltmarshes promote largest accretion when lower biomass and high water levels are present. Interestingly, thick long‐term mud is enhanced despite the biostabilizers seasonal growth. The combination of saltmarsh and microphytobenthos leads to expanding saltmarsh cover and mud area. Generally, mud layer thickness is governed by the ratio of hydroperiod and maximum flow velocity that is mediated by the biostabilizers. On estuary scale, the presence of intertidal vegetation leads to increased mud volumes in the intertidal. Mud layers are enhanced in extent by a mud‐dependent species and in thickness by a generic species. Thus, local biostabilization alters large‐scale morphology controlling long‐term estuarine development.

Tidal marshes play an important role in climate change mitigation through natural coastal protection. The effectiveness of the natural coastal defense by tidal marshes is closely related to their channel network which is in turn greatly influenced by their vegetation cover and shape. Previous research suggests a dual effect of vegetation on marsh topography; stabilizing sediment on the one hand versus promoting erosion and channel incision on the other hand. This study links these effects to different vegetation species,Salicornia procumbens,Spartina anglica, andPuccinellia maritima(further referred to asSalicornia,Spartina, andPuccinellia), by means of a coupled bio‐hydromorphodynamic modeling study. Single species, species‐assemblages, and species shifts were studied, incorporating both species‐specific physical plant properties and spatiotemporal growth strategies. The results indicate the influence of vegetation on the marsh topography to be highly species‐dependent, but also of a very complex nature. Both the presence ofSpartinaandPuccinelliaresulted in significant channel development, whereasSalicorniadid not induce topographic change. The combination of several species promoted or reduced channel development depending on the included species. Species‐shifts linked with climatic changes resulted in increased erosion of the existing channel network potentially reducing the protective capacity of the marsh.

Summary: Sediment‐stabilizing and ‐destabilizing organisms, i.e. microphytobenthos (biofilms) and macrozoobenthos (bioturbators), affect the erodibility of muddy sediments, potentially altering large‐scale estuarine morphology. Using a novel eco‐morphodynamic model of an idealized estuary, we investigate eco‐engineering effects of microphytobenthos and two macrozoobenthic bioturbators. Local mud erodibility is based on species pattern predicted through hydrodynamics, soil mud content, competition and grazing.Mud resuspension and export is enhanced under bioturbation and prevented under biostabilization through respective exposure and protection of the supra‐ and intertidal. Bioturbation decreases mud thickness and bed elevations, which increases net mud fluxes. Microphytobenthos reduces erosion, leading to a local mud increase of intertidal sediments.In multi‐species scenarios, an effective mud‐prone bioturbator strongly alters morphology, exceeding that of a more abundant sand‐prone moderate species, showing that morphological change depends on species traits as opposed to abundance. Altering their habitat, the effective mud‐prone bioturbator facilitates expansion of the sand‐prone moderate bioturbator. Grazing and species competition favor species distributions of dominant bioturbators. Consequently, eco‐engineering affects habitat conditions while species interactions determine species dominance.Our results show that eco‐engineering species determine the mud content of the estuary, which suggests large effects on the morphology of estuaries with aggravating habitat degradation.

Abundant research has shown that macrobenthic species are able to increase sediment erodibility through bioturbation. So far, however, this has been at the level of individual species. Consequently, we lack understanding on how such species effects act on the level of bioturbator communities. We assessed the isolated and combined effects of three behaviorally contrasting macrobenthic species, i.e., Corophium volutator, Hediste diversicolor, and Limecola balthica, at varying densities on the critical bed shear stress for sediment resuspension (τ_cr). Overall, the effect of a single species on sediment erodibility could be described by a power function, indicating a relatively large effect of small bioturbator densities which diminishes toward higher individual density. In contrast to previous studies, our results could not be generalized between species using total metabolic rate, indicating that metabolic rate may be only suitable to integrate bioturbation effects within and between closely related species; highly contrasting species require consideration of species-specific bioturbation strategies. Experiments at the benthic community level revealed that the ability of a benthic community to reduce τ_cr is mainly determined by the species that has the largest individual effect in reducing τ_cr, as opposed to the species that is dominant in terms of metabolic rate. Hence, to predict and accurately model the net effect of bioturbator communities on the evolution of tidal flats and estuaries, identification of the key bioturbating species with largest effects on τ_cr and their spatial distribution is imperative. Metabolic laws may be used to describe their actual activity.