Large-scale experimental exclusion of lugworms (Arenicola marina) from 400 m2 intertidal fine sand revealed significant effects of their bioturbation and bioirrigation on sediment characteristics, benthic infauna composition, and the dominant mineralization and benthic-pelagic exchange processes in the sediment. Absence of lugworms resulted in sediment clogging with organic-rich fine particles, an eightfold decrease in sediment permeability, low oxygen penetration depths, and accumulation of reduced mineralization products in the pore water. The sand flat inhabited by lugworms had low fine-particle and chlorophyll contents and low sulfide and nutrient concentrations in the pore water. The effects were not limited to the vicinity of lugworm burrows but extended throughout the entire sediment down to ,20-cm depth. Sediments with the lugworm shared the characteristic of low-organic, advection-driven permeable sand rather than of muddy, diffusion-dominated sediments in the absence of lugworms. Areal oxygen uptake rates depended strongly on hydrodynamic conditions: under calm conditions, sedimentary oxygen uptake was slightly higher at the exclusion site. Experimental flushing using controlled hydrodynamic conditions showed that oxygen uptake at the lugworm site would be higher during more dynamic conditions (e.g., storms) due to significantly deeper oxygen penetration permitted by higher sediment permeability. Our results indicate an interactive effect of bioturbating organisms and hydrodynamics on water-sediment exchange processes and highlight the importance of benthic infauna for sedimentary processes even in physically dominated systems.