Zooming in and out: Scale dependence of extrinsic and intrinsic factors affecting salt marsh erosion

Abstract

Salt marshes are valuable ecosystems that provide important ecosystem services. Given the global scale of marsh loss due to climate change and coastal squeeze, there is a pressing need to identify the critical extrinsic (wind exposure and foreshore morphology) and intrinsic factors (soil and vegetation properties) affecting the erosion of salt marsh edges. In this study, we quantified rates of cliff lateral retreat (i.e., the eroding edge of a salt marsh plateau) using a time series of aerial photographs taken over four salt marsh sites in the Westerschelde estuary, the Netherlands. In addition, we experimentally quantified the erodibility of sediment cores collected from the marsh edge of these four marshes using wave tanks. Our results revealed the following: (i) at the large scale, wind exposure and the presence of pioneer vegetation in front of the cliff were the key factors governing cliff retreat rates; (ii) at the intermediate scale, foreshore morphology was partially related to cliff retreat; (iii) at the local scale, the erodibility of the sediment itself at the marsh edge played a large role in determining the cliff retreat rate; and (iv) at the mesocosm scale, cliff erodibility was determined by soil properties and belowground root biomass. Thus, both extrinsic and intrinsic factors determined the fate of the salt marsh but at different scales. Our study highlights the importance of understanding the scale dependence of the factors driving the evolution of salt marsh landscapes.

Role of eelgrass on bed‐load transport and sediment resuspension under oscillatory flow

“…( a ) Diagram of the hydraulic wave flume and sensors. The wave flume is a further elaboration of the wave mesocosms used by La Nafie et al () and the wave tanks used by Wang et al () and (Lo et al ), and ( b ) diagram of the top view of the wave flume. The total length of the flume is 3.5 m, 0.6 m wide, and 0.8 m high.…”

Quantifying Bed Level Change at the Transition of Tidal Flat and Salt Marsh: Can We Understand the Lateral Location of the Marsh Edge?

“…The largest seasonally aggregated bed level change was observed at the locations at the northern shores of the Westerschelde (Figure 3), which are both located perpendicular to the prevailing southwesterly winds (Wang et al, 2017) and hence designated as wind exposed by Callaghan et al (2010). Location 4S.…”

“…Over annual to decadal time scales, marsh edge is able to extend seaward and retreat landward (Allen, 2000;Cox et al, 2003;Francalanci et al, 2013;Van der Wal et al, 2008;Wang et al, 2017), affecting the width of the marsh. A narrower marsh width decreases its capacity for wave attenuation (e.g., Vuik et al, 2016), making in-depth understanding of the dynamics driving marsh width key to being able to use marshes for coastal defense (Bouma et al, 2014).…”

“…The switch between these two phases is determined by two key processes: initiation of marsh erosion (i.e., start of retreat) and start of expansion by seedling establishment (Bouma et al, ) and growth of clonal shoots (Silinski et al, ). Whereas the rate of marsh erosion is determined by frequent flooding and wave events (Fagherazzi, ; Wang et al, ), the onset of erosion may originate from a discontinuity by having a stable marsh platform level adjacent to a dynamic tidal flat, where the bed level can vary greatly over time (Bouma et al, ). The process of seedling establishment at the tidal flat is mainly affected by the physical parameters: relative elevation in the tidal frame, which determines the inundation frequency and period (Balke et al, ); and bed level change, which possibly lead to uprooting and/or burying seedlings (Hu, van Belzen, et al, ).…”

“…The switch between these two phases is determined by two key processes: initiation of marsh erosion (i.e., start of retreat) and start of expansion by seedling establishment and growth of clonal shoots (Silinski et al, 2016). Whereas the rate of marsh erosion is determined by frequent flooding and wave events (Fagherazzi, 2014;Wang et al, 2017), the onset of erosion may originate WILLEMSEN ET AL. 2509…”