Salt Marsh Establishment and Eco‐Engineering Effects in Dynamic Estuaries Determined by Species Growth and Mortality

Abstract

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.

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

“…The existence of a geomorphologic window of opportunity is thus based on the occurrence of seasonal disturbances such as winter storms and might help elucidate observed differences in interannual saltmarsh growth in dynamic coastal environments. However, we expect that as soon as saltmarsh establishment takes place vegetation potentially relies less on mud for survival due to its eco‐engineering capabilities (Brückner et al., 2019) but still constitutes an important habitat for mud to settle. We show that the feedback loop between mud sedimentation and species growth leads to different emerging species abundances that are controlled by the preceding geomorphic setting and lead to species‐specific mud layer formation.…”

“…To investigate the effect of saltmarshes on mud accumulation and bed accretion, we tested both a generic saltmarsh species as in Brückner et al. (2019) and a mud‐dependent species that only colonizes cells with a mud fraction in the top layer larger than 40% that is similar to the critical mud fraction for cohesion (Van Ledden et al, 2004). At each coupling time step, the results of the Delft3D model are fed into the dynamic vegetation model to calculate the new vegetation parameters.…”

“…The dynamic vegetation model includes colonization, growth, aging, and mortality rules based on literature (Brückner et al., 2019). Colonization is defined at the beginning of each year while vegetation growth is defined by a seasonal increase in biomass, based on height and stem diameter, throughout the ecological year.…”

A Convolution Method to Assess Subgrid‐Scale Interactions Between Flow and Patchy Vegetation in Biogeomorphic Models

“…With such a grid resolution, SRM is representative of today's state-ofthe-art two-dimensional tidal-marsh biogeomorphic models (e.g. Schwarz et al, 2014;Belliard et al, 2015;Best et al, 2018;Mariotti, 2018;Sandi et al, 2018;Brückner et al, 2019;Zhang et al, 2019;Bij de Vaate et al, 2020), which by definition are unable to represent fine-scale interactions between flow and small vegetation patches (order of m 2 ) and hence their biogeomorphic impacts at the landscape scale (order of km 2 ). 2.…”

On the Impact of Salt Marsh Pioneer Species‐Assemblages on the Emergence of Intertidal Channel Networks

“…Thus, we incorporate three vegetation species in our model through different physical parameters (equation (2)) and separate stress tolerance of juvenile and adult plants (further referred to as spatial variation in growth). Furthermore, the model includes seasonal variation in growth (Brückner et al, ). Seasonal variation is defined from literature by species‐specific colonization and growth periods and reduced plant height during winter (Figure b)…”