Physical controls of food supply to benthic filter feeders in the Menai Strait, UK

Abstract

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 (~350 to 800 m 3 s -1 ). A com- parison 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 phytoplank- ton by filtration over the mussel bed reduced concentrations and resulted in a pronounced horizontal gradient (~4.4 × 10 -4 µg 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 (chl a) concentration, with an amplitude of ~50% of the mean. An analytical model of advection and consumption reproduced these features of the observed chl a cycle over the mussel bed. The strong tidal flow maintained a high level of turbu- lence, 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 de- pletion by up to ~2 µg 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 (~9.0 t C d -1 ) and the amount consumed by filter feeders in the area of the mussel bed (~4.5 t C d -1 ).