What Came First, Mud or Biostabilizers? Elucidating Interacting Effects in a Coupled Model of Mud, Saltmarsh, Microphytobenthos, and Estuarine Morphology

Abstract

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.

Benthic species as mud patrol ‐ modelled effects of bioturbators and biofilms on large‐scale estuarine mud and morphology

“…Occurrence of A. marina (AM) and C. volutator (CV) as presence‐absence in the Western Scheldt Estuary. (a) is data partly provided by Ysebaert et al (2002); (b) is the results from the Nevla‐model domain for AM and CV (more information on the model in Braat et al (2019) and Brückner et al (2020) showing similar trends in species predictions [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…To compare species predictions of the macrobenthic species with field data, we used a calibrated and optimized hydro-morphodynamic model of the lower Western Scheldt estuary (Nevla-model; for more information see Vroom et al (2015), Schrijvershof &Vroom (2016), andBrückner et al (2020)). The model was adapted from the generic vegetation simulation in Brückner et al (2020Brückner et al ( ) (bathymetry of 2008 with enhanced mud deposition on the bars. We computed the species distributions on the initial hydro-morphodynamic domain that provides values for the environmental parameters that define species growth.…”

“…The presence of mud in the bed shapes the morphology of tidal systems through the counteracting effects of sediment refinement and cohesiveness: with increasing mud content, the bed becomes first more erosive because of sediment fining; however, when the bed comprises large mud fractions, the bed becomes cohesive and erodibility is strongly reduced (Le Hir et al, 2007; Van Ledden et al, 2004). As mud settling is largely governed by the hydrodynamics, the largest mud fractions can be found in sheltered intertidal areas leading to higher bed elevations and infilling of small tidal channels (Braat et al, 2017; Brückner et al, 2020; Kleinhans et al, 2018; van de Lageweg et al, 2018). This local stabilization of the estuarine morphology leads to less dynamic channel networks with stable bars and steeper bank slopes that can be colonized by marine organisms and vegetation (Braat et al, 2017).…”

“…Many such models are one‐dimensional or cover small‐ to medium‐scale areas, simulating the effect of a single species or few key species. In recent numerical modelling studies large‐scale effects of marine vegetation led to a stabilization of the estuary shores, which limited lateral erosion of the estuary (Brückner et al, 2020; Kleinhans et al, 2018; Lokhorst et al, 2018). Possibly microphytobenthos induces a similar confinement of the estuary, whereas destabilization by macrobenthic organisms might promote erosion of the estuarine flanks facilitating lateral expansion.…”

Natural levee evolution in vegetated fluvial‐tidal environments

“…We show how this happens in fluvial-tidal vegetated environments. Levees and crevasses act as sediment conveyors (Figures 6,11,12 and Nienhuis et al 2018), while vegetation reduces the tidal prism and thus facilitate more quiet conditions required for sediment deposition (Figure 7 and Braat et al 2019;Brückner et al 2020). Even when sediment supply is too low to fill basins entirely, the combination of vegetation helping to both fill the accommodation space while also reducing the tidal prism and together with sedimentation corridors (e.g.…”