Seasonal variation in blue mussel, Mytilus edulis, retention efficiency (RE) responses to natural seston concentrations were investigated in the Lysefjord, Norway. The smallest particles shown to be effectively retained (100%) were approximately 7 μm and this was observed on only one occasion. The most common response was for RE to progressively increase from small to large particles with maximum RE at 30 to 35 μm (8 of 12 experiments). Temporal changes in the seston size distribution towards a dominance of smaller particles coincided with a decrease in the RE maxima to particles between 7 and 15 μm diameter. The RE of 1 μm and 4 μm diameter particles ranged between 14–64% and 12–86%, respectively, showing that small particles occasionally serve as a significant dietary component. The observed temporal variations in particle capture suggest a capacity of M. edulis to control particle retention mechanisms based on exogenous dietary cues that are directly or indirectly related to the ambient particle size distribution. Indirect clearance rate (CR) measurements require that all particle sizes under study are effectively retained and this has widely been assumed for particles larger than 4 μm. CR was calculated for particles ranging in size from 5 to 35 μm and the percentage reduction in CR, relative to particle sizes shown to be effectively retained, ranged from 11% to 87%. CR calculations that included all particle counts between 5 and 35 μm diameter were underestimated by a mean of 26% (0–48% range). Similar errors in diet characterization may be expected to affect particle selection and absorption efficiency measurements and the compounding of errors may become extreme when a number of physiological rates and efficiencies are integrated (e.g. scope for growth). Highlights ► Particle retention efficiency (RE) of natural seston was investigated in M. edulis. ► The smallest particles shown to be effectively retained (100%) were close to 7 μm. ► RE most often increased progressively from small to large particles. ► Maximum RE was reported at 30 to 35 μm in 8 of 12 experiments. ► Temporal changes in particle size distribution coincided with changes in RE maxima.