Benthic organisms are known to modify sediment properties and influence the flux of materials to the water column. In this study, the density-dependent effects of bioturbation by the estuarine clam, Scrobicularia plana, on the biogeochemical properties and erodibility of the sediment were assessed using laboratory annular flumes. Sediments with different mud contents (~98% and 90%) were collected undisturbed from two different sites of the Tagus estuary. S. plana were added to the sediment at increasing densities (0, 57, 115 and 229 ind m–2) and left to bioturbate the sediment before the experimental erosion runs. All erosion runs were carried out following a stepwise increase of current velocities (U) up to 0.25 m s–1. Sediment chlorophyll a, pheopigments and extracellular polymeric substance fractions were measured at the time of sampling in the field and in the end of the erosion runs. Increasing densities of S. plana in the muddier sediment raised eroded mass, while the sediment with less mud behaved as non-cohesive and registered a decrease in eroded mass possibly due to a switch in the feeding behaviour of S. plana.