Abstract. The structure of momentum and concentration boundary layers developing over a bed of Potamocorbula amurensis clam mimics was studied. Laser Doppler velocimetry (LDV) and laser-induced fluorescence (LIF) probes were used to quantify velocity and concentration profiles in a laboratory flume containing 3969 model clams. Model clams incorporated passive roughness, active siphon pumping, and the ability to filter a phytoplankton surrogate from the flow. Measurements were made for two crossflow velocities, four clam pumping rates, and two siphon heights. Simultaneous use of LDV and LIF probes permited direct calculation of scalar flux of phytoplankton to the bed. Results show that clam pumping rates have a pronounced effect on a wide range of turbulent quantities in the boundary layer. In particular, the vertical turbulent flux of scalar mass to the bed was approximately proportional to the rate of clam pumping.