We report herein a study of the role of hydrodynamics in controlling the food supply to benthic filter feeders in the Menai Strait, a narrow channel between the island of Anglesey and north Wales, UK, which is tidally energetic with pronounced residual flow (similar to 350 to 800 m(3) s(-1)). A comparison between the cycle of chlorophyll concentrations in water above an extensive, commercially-exploited, mussel bed (Mytilus edulis L.) and the corresponding cycle over a control site showed clear evidence of the influence of horizontal tidal advection on food supply. Consumption of phytoplankton by filtration over the mussel bed reduced concentrations and resulted in a pronounced horizontal gradient (similar to 4.4 x 10(-4) mu g l(-1) m(-1)). Losses to filtration appeared to be compensated through transport of plankton-rich water into the strait by the large residual flow while advection of the gradient by the tidal current resulted in large oscillations in chlorophyll a (ch1 a) concentration, with an amplitude of similar to 50% of the mean. An analytical model of advection and consumption reproduced these features of the observed ch1 a cycle over the mussel bed. The strong tidal flow maintained a high level of turbulence, so that the water column was generally well mixed vertically. Depletion of phytoplankton in the bottom boundary layer was, therefore, not present for most of the tidal cycle but on 2 occasions, when the observed Reynolds stress was close to zero at slack water, we did observe significant depletion by up to similar to 2 mu g l(-1) at 1 m above the bed. This depletion is interpreted as the effect of mussel feeding briefly out-competing the supply of phytoplankton by vertical diffusion for the period of low turbulence. Assuming a steady state, we estimated the total supply of phytoplankton imported into the strait (similar to 9.0 t C d(-1)) and the amount consumed by filter feeders in the area of the mussel bed (similar to 4.5 t C d(-1)).