Comparison of the erosion shear stress of oxic and anoxic sediments in the East Frisian Wadden Sea

Abstract In June 1996, large anoxic areas appeared on the intertidal sediment surface of the Fast Frisian Wadden Sea. These conditions caused large-scale mortality of benthic flora and fauna, and may thus have produced widespread changes in sediment stability. In this study potential differences in the erodibility of largely comparable oxic and anoxic sediment surfaces were investigated. At sites on the Baltrum back-barrier tidal flats, anoxic sediment samples showed systematically lower erosion shear stresses than their oxic counterparts. This observation, however, could not be assigned to a disappearance of sediment-stabilising microorganisms since anoxic surfaces did not always show significantly lower chlorophyll-a concentrations. On the other hand, the observed differences in erosion shear stress are well within the range of variability observed on oxic sediment surfaces for a larger data set incorporating the Sylt-Romo tidal flats of the North Frisian Wadden Sea. Thus, a slight decrease in sediment stability at sites with anoxic surfaces is evident from the data for the East Frisian Wadden Sea but the morphodynamic effect of this phenomenon does not appear to be significant.