The spatial and temporal patterns of erodibility of an intertidal flat in the East Frisian Wadden Sea, Germany

Abstract

Erosion and physical and biological sediment parameters measurements were carried out at an intertidal flat in the East Frisian Wadden Sea, Germany to examine the small-scale (a meter) and large-scale (hundred of meters) spatial and temporal variation of sediment erodibility, and to identify the main processes that cause these variations. Six stations along a cross-shore transect of 1.5 km length from immediately below the salt marsh to the middle of the tidal flat were visited during several field campaigns in 2001 and 2002. These stations differ in their sediment types, tidal emersion periods and benthic macrofauna assemblages. The erodibility was determined by means of Lab and portable (in situ) EROMES erosion devices and quantified in terms of critical erosion shear stress and erosion rate. The study showed that the small and large-scale variations of sediment erodibility were mainly governed by biological factors, especially microphytobenthos (dominated by benthic diatoms). A strong spatial and temporal pattern of erodibility was observed. The sediments were more stable (i.e. higher critical erosion shearstresses and lower erosion rates) at station A (close to the salt marsh) and station F (middle tidal flat). The high stability at station A was attributed to be the results of physical process of drying and biostabilization by tube building worms. The high stability at station F was attributed mainly to be the results of biostabilization by benthic diatoms. By contrast, sediments were less stable at the site dominated by mud snail Hydrobia ulvae (station B) and this was probably due to surface tracking, pelletization of the bed material and grazing activities on benthic diatoms of the mud snails. The sediments were more stable in September 2002 compared to other sampling periods, and this attributed to be the results of biostabilization by benthic diatoms. By contrast, during June and October 2001 the sediments were easilyeroded due to lower level of bio-stabilization. The results from study of erosion potential over bedforms showed that crests of the bedforms are generally more stable than troughs. In general, crests contained more chlorophyll-a, colloidal carbohydrate and EPS than troughs. The normalized water content and wet bulk density of the crests were not significantly different from those of the troughs except at the most landward station where crests had significantly lower normalized water content and higher wet bulk density than troughs. Two different processes were identified for the difference in erodibility between crests and troughs in this study: (1) At seawards stations (B–F), the higher benthic diatom biomass on the crests increases the amount of EPS, which is likely to stabilize the sediment surface of these features. (2) At most landward station (A), where benthic diatom biomass was low, physical processes (drying, compaction) are more important for sediment stability on the crests. The measured critical erosion shear stresses fall above the abiotic non-cohesive sediment values, giving a biostabilization index of 4.2 to 11.6. Differences in critical erosion shear stress between natural and abiotic non-cohesive sediments are likely caused by the effect of biostabilization and by cohesive behaviour of naturalsediments.