Biological Mediation of the Settling Velocity of Bed Material Eroded from an Intertidal Mudflat, the Danish Wadden Sea

The equivalent settling diameter of the eroded material from erosion experiments in situ has been examined for almost two years on a temperate, intertidal mudflat. The erosion experiments were carried by use of a portable EROMES erosion equipment. After each erosion experiment the eroded material was allowed to settle for some minutes and the settling velocity distribution and hereby equivalent settling diameters were calculated using Stoke’s Law. The settling velocities were strongly dependent on the fecal pellet content of the material showing larger settling diameters with increasing contents of fecal pellets whereas velocities were not dependent on applied bed shear stress, primary grain size, dry bulk density, organic content or chlorophyll a content of the bed material. The pellets at the investigated mudflat were produced by the snail Hydrobia ulvae which lives in densities up to about 300 000 indiv. m 2 at the site. The average settling diameter for eroded material without fecal pellets was about 20 m whereas it was about 80 m for highly pelletized material. The pellets are eroded quite easily, but will probably not escape from the tidal basin due to the increase in settling velocities and associated increase in settling lag and scour lag. Therefore, it is argued that although H. ulvae will tend to destabilize the surface of the mudflat, the net effect with respect to the fine-grained sediment budget is probably positive due to the increase in settling velocity caused by the pelletisation by the snail. The temperature dependent ingestion by H. ulvae causes a clear seasonality in the pellet content and hereby settling velocity of the material eroded at the mudflat. This may partly explain earlier observations of seasonal variations in erosion and deposition on the mudflat.