The effect of bedforms (crest and trough systems) on sediment erodibility on a back-barrier tidal flat of the East Frisian Wadden Sea, Germany

Abstract

The erosion potential over bedforms in a tidal flat of the East Frisian Wadden Sea was studied by conducting erosion and physical and biological sediment property measurements on the crests and troughs of bedforms. Five stations along a cross-shore transect of 1.5 km length from immediately below the salt marsh to the mid tide-level of the tidal flat were visited during two field campaigns in June and September 2002. Measurements of sediment erodibility were made on both crests and troughs using an EROMES erosion device and quantified in terms of critical erosion shear stress and erosion rate. Surface sediment scrape samples (upper 1 mm layer) were taken from crests and troughs to determine various physical and biological properties of the sediment. The results show that crests are generally more stable (i.e. higher critical erosion shear stresses and lower erosion rates) than troughs. In general, crests contained more chlorophyll a, colloidal carbohydrate, and EPS (extracellular polymeric substance) than troughs. Median grain-size, water content and wet bulk density of the crests showed no statistically significant difference from those of the troughs with the exception at the most landward station immediately below the salt marsh margin, where crests had significantly lower water content and higher wet bulk density than troughs.Two different processes were identified for the difference in erodibility between crests and troughs: (1) At stations with emersion times less than 6 h, the higher benthic diatom biomass (measured as chlorophyll a concentration) on the crests increases the amount of EPS, which is likely to stabilize the sediment surface of these features; (2) in a saltmarsh transition area (most landward station), physical processes such as surface drying and compaction seem to enhance in a synergistic way the sediment stability on the crests.

Spatial Self‐Organization on Intertidal Mudflats through Biophysical Stress Divergence

“…1). Spatial patterning on muddy intertidal flats has been observed in a number of studies (Blanchard et al 2000;de Brouwer et al 2000;Gouleau et al 2000;Whitehouse et al 2000;Lanuru et al 2007). They range from banded patterns aligned parallel to the flow direction (ridge-runnel systems) in areas where tidal currents are high Blanchard et al 2000;Le Hir et al 2000;Whitehouse et al 2000) to round-shaped patterns where water currents are lower (de Brouwer et al 2000).…”

A review on substances and processes relevant for optical remote sensing of extremely turbid marine areas, with a focus on the Wadden Sea

“…The crests of the Xats generally show larger benthic growth rates and higher stability than troughs, due to excretion products of benthic organisms (Lanuru et al 2007;De Jonge 1992). This is supposed to be due to the extended emersion times of the crests and, therefore, more eVective irradiance (De Jonge 1992;Colijn and Dijkema 1981) or higher temperatures (Rasmussen et al 1983).…”

“…At Xats emerging for long periods, high salinities and pHs have a negative inXuence on primary production (Rasmussen et al 1983). Such Xats are stabilised by physical processes such as drying and compaction (Lanuru et al 2007). Also, the protection of Xats from waves is important for their stability and Chl-a content, since storms might destroy the surface layer of benthic organisms.…”

“…In tidal Xats with low mud content (<20% by weight), mainly sand eroded with an erosion rate six-to tenfold higher than at places with a high mud content, where mud and sand both eroded (Houwing 1999). Benthic diatoms stabilise the mudXat (Lanuru et al 2007;Austen et al 1999) with their extracellular polymeric substances (excretion products of algae) (correlation Chl-a content-erosion threshold: r = 0.99) (Austen et al 1999). Wind can damage the Wlm of benthic diatoms on a tidal Xat, making the Xat more vulnerable to erosion and causing resuspension of the benthic cells (Colijn and Dijkema 1981; Hommersom et al 2009).…”

Top-down control inhibits spatial self-organization of a patterned landscape

“…In this ecosystem, a spatial pattern of elevated hummocks alternating with water-filled hollows develops each spring (Blanchard et al 2000, Gouleau et al 2000, Whitehouse et al 2000, Lanuru et al 2007), due to a strong interaction between diatom growth and geomorphological processes ( Fig. 1A; Weerman et al 2010).…”