Seawater velocity, meat content, food availability, and algae toxins were measured in a commercial long-line (200 × 15 m) mussel farm (Mytilus edulis [L]) in Lysefjorden, in southern Norway. The mean current speed decreased rapidly within the farm area during the 4 days current measurements. The current speed 30 m inside the farm was reduced to less than 30% compared with the current speed outside the farm. The reduction was a consequence of friction from the mussels and farm structures. More than 50% of the incoming phytoplankton biomass (chlorophyll a) was depleted within the first 30 m in the mid section of the farm. After this decrease the chlorophyll a concentrations stabilized at approximately 0.6 mg m⁻³ throughout the farm. The reduction in current speed led to food depletion and lower meat content within the farm. The concentration of diarrhetic shellfish toxins (DST) was inversely related to the meat content of the mussels. This relationship can be used to optimize monitoring programs for shellfish toxins. The range of DST in mussels varied from 0.40 to 1.60 mg kg⁻¹ steamed meat within the farm. It is suggested that depuration of DST was faster in areas with high food availability.