Food depletion in mussel cultivation has been rarely studied and seldom demonstrated. In this study, concentrations of phytoplankton in and around a blue mussel Mytilus galloprovincialis raft culture unit in the Ría de Vigo were measured during a 2 wk study period in July 2004. Flow direction and current speed were measured using an Aanderaa current meter and fine-scale Acoustic Doppler Velcimeter probes at different positions in the raft. Flow speeds were reduced compared to outside the raft, but a clear tidal signal and significant flow velocities could still be observed inside the raft. At the upstream corners of the raft, a zone of high turbulence but reduced advection was observed. Concentrations of chlorophyll a (chl a) were measured on 3 different spatial scales. On a macro-scale, fluorescence profiles were taken inside and outside the raft on several occasions and there was depletion of chlorophyll inside the raft corresponding to ~80% of the outside concentration, whereas there was no depletion below the ropes. On a meso-scale, from just upstream to just downstream of the raft, fluorescence profiles, as well as water samples, at several depths revealed similar depletion, however, with larger depletion of size-fractionated chl a >2 µm. On a micro-scale, water was sampled within 20 cm of the ropes using siphon mimics. In the middle of the raft, concentration profiles towards the mussel ropes could be observed at 2 depths, whereas less clear profiles were observed on the turbulent upstream corner. The present study documents food depletion in a mussel culture and emphasizes the importance of physical forcing and phytoplankton composition for food availability.