ABSTRACT: The influence of the macroalgal mats of Enteromorpha intestinalis on near-bed current velocities and sediment dynamics was quantified by placing relatively undisturbed cored sediments in annular flumes. Density-dependent relationshipswere established for E. intestinalis densities; these ranged from 10 to 60% cover when air-exposed (biomass of 4 to 40 g ash-free dry wt m^-2) and in comparison with bare sediment (0% cover). There was a significant increase in frictiondrag with increasing E. intestinalis biomass and percent cover, ranging from a mean 18% reduction in current velocities at 10% cover to 56% reduction at 60% cover. The net result of a reduction in currents (depth-averaged between 1 to 12 cm abovethe bed) and the physical protection of the bed by Enteromorpha was a marked reduction in sediment erosion of 60% at 10% cover to 90% at 60% cover. The presence of E. intestinalis also significantly enhanced sediment deposition measured as aflux from the water column to the bed. At 60% cover the deposition rate was 48% higher than on bare sediment during the first 30 min of slack water (<0.05 m s^-1). The results show that E. intestinalis has a marked influence on waterflow over the bed and the flux of particulate material across the sediment-water interface.