The spatial and temporal patterns of erodibility of an intertidal flat in the East Frisian Wadden Sea, Germany
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 shear stresses 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 easily eroded due to lower level of biostabilization. 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 natural sediments.