This study was designed to elucidate the processes controlling the supply of phytoplankton to the commercial mussel beds in the Menai Strait which is a major centre for mussel aquaculture in the UK, accounting for approximately 50 % of the total production of Mytilus edulis in 2005. The Menai Strait is a shallow (mean water depth ~ 10 m), energetic regime dominated by tidal currents of order ~ 1 m s-¹• In consequence the water column remains well mixed and there is a tidally driven net transport through the Strait. Spatial and temporal surveys reveal a pronounced longitudinal gradient in Chlorophyll concentration (Chl) over the mussel beds as well as large semi-diurnal oscillations in Chl which have been shown to result from the advection of the gradient by the residual and tidal currents. In the vertical, the depletion of Chl near the bed is limited by the strong mixing and is observed only to occur around periods of slack water. Downstream (in the residual tidal advection) from the mussel beds, Chl also shows a strong oscillation over the spring-neap cycle due to the change in relative strength of mussel filtration to residual tidal advection. Observations show the range of Chl to be~ 1.7 µg 1-¹ over a fortnight. Results on a seasonal time scale show a consistency between different years, and with previous longer term measurements of Chl in the Strait. The PHYBIO model is a simplified 1-D model of
the ecosystem which includes tidal advection, diffusion, mussel feeding and phytoplankton production. Simulations with this model reproduce the main qualitative and quantitative features of Chl variations observed in the Strait and validate the hypothesis that these oscillations and gradients are due to the interaction of the hydrodynamics and the mussel feeding. On the longer time
scale of the spring-neap cycle, the patterns predicted by the PHYBIO simulation are in good agreement with the observations. The modelling effort has been expanded to a 2-D hydrodynamic model, which uses a passive tracer to simulate the distribution of Chl in the Menai Strait. Using a semi-implicit source/sink term, the local production by phytoplankton and the localised consumption by the commercial mussel lays have been simulated. Results show a promising agreement with the outcomes from observations and the 1-D PHYBIO model. Calculations of Chlorophyll consumption and mussel production are compliant with annual production numbers from the shellfishery.