To clarify the composition of food ingested by blue mussels (Mytilus edulis) in the Menai Strait, where the largest mussel bed in the United Kingdom lies, hydrodynamic observations with organic carbon and nitrogen biochemical analysis were conducted. This strait is characterized by a narrow channel and wide tidal flats with large tidal mixing that influences the distribution of particulate organic matter (POM), which is food formussels. The maximum velocities of the tidal currents were estimated to be 0.5–1.0 m s^-1 during northeastward flow and 0.6–1.3 m s^-1 during southwestward flow. The residual current was southwestward and the water flux was estimated to be 510 m³ s^-1. Field observations indicate that the inflow of pelagic phytoplankton into the strait mainly occurred in the northern region of the strait. Resuspended benthic primary production, however, also causes higher chlorophyll-a concentration in the central region of the strait near the mussel bed and its contribution to mussel growth was estimated to be 22%–50%. The concentrations of chlorophyll-a and particulate organic carbon indicated that the POM in the strait mainly originated from detritus. Benthic microalgae is, however, an important factor to understand the total primary production in the strait. The consumption of the primary producers was estimated to be approximately 6.3 t C day^-1. In the phytoplankton community, pelagic and thycopelagic (living in both pelagic and benthic environments) species comprised 59.3%–77.2% and 12.8%–40.7% of the cells, respectively. The primary production by resuspended benthic species in the Liverpool Bay region was considerably larger than that in the Menai Strait region. As a conclusion, theMenai Strait is an area with complex hydrodynamic conditions causing horizontal advection and resuspension of both pelagic and benthic phytoplankton, and this complex would cause the most suitable condition for mussel production.