Under optimal conditions, bivalves tend to filter the ambient water at a maximum rate but under suboptimal environmental conditions, including low or very high algal concentrations, the filtration rate is reduced. The upper algal concentration at which the blue mussel, Mytilus edulis, exploits its filtration capacity over an extended period of time was identified by stepwise raising the algal (Rhodomonas salina) concentration in steady-state experiments above the threshold for continuous high filtration rate. The duration time before incipient saturation reduction decreased with increasing algal concentration, and the threshold concentration for incipient saturation reduction of filtration activity was found to be between about 5,000 and 8,000 cells mL −1 , equivalent to 6.3 and 10.0 µg chl a L −1 , respectively. Reduced filtration rate was related to total number of algal cells ingested previous to incipient saturation and found to be 11.4 ± 1.7 × 10 6 cells. Video-microscope recordings of pseudofaeces production revealed that the trigger threshold concentration for formation of pseudofaeces was about 12,000 cells mL −1 . Faeces produced by saturated mussels consisted of closely packed undigested algal cells, indicating severe overloading of the digestive system caused by high algal concentrations which mussels are not evolutionary adapted to cope with.