The influence of sediment cohesiveness on bioturbation effects due to Hydrobia ulvae on the initial erosion of intertidal sediments: A study combining flume and model approaches

Laboratory experiments performed in a recirculating flume were designed to quantify the bioturbation influence of the mud snail Hydrobia ulvae, one of the most abundant deposit feeders on European intertidal mudflats. Variations in sediment moisture content that occur between bedforms in shore-normal, ridge and runnel systems of intertidal mudflats were added to the model definition. Sediment erosion thresholds, erosion rates and the microalgal pigment composition of resuspended material were quantified for different H. ulvae densities (0, 1000, 5000, 10 000 and 50 000 snails m− 2) and applied bed shear stresses. Two different sediment moisture contents were tested. In the absence of macrofauna, recorded turbidities increased up to the maximum applied value of 1.6 Pa and the addition of snails increased the resuspended mass at all bed shear stresses tested. The amount of resuspended mass depended on snail density; the amount of resuspended material was highest in sediment test beds that had the highest moisture contents (ridge-type sediments) and snail density. Resuspended and bioturbated sediment was characterised by an enrichment in phaeopigments compared to the underlying sediment. A nine-parameter model, which included sediment moisture content as a new variable, gave a reasonably good estimate of the resuspension of ridge- and runnel-type sediments for the range of snail densities. An eight-parameter version of the model was adequate for quantifying erosion rates on intertidal mudflats not characterised by a ridge/runnel geomorphology.