Effect of suspended bottom material on growth and energetics in Mytilus edulis

The influence of suspended, natural silt (0 to 20 mg l-1) in addition to unicellular algal cells (Phaeodactylum tricornutum) (o to 20.000 cells ml-1) on clearance, growth and energetics in Mytilus edulis has been studied. Clearance increased by 32 to 43% by the addition of 5 mg silt l-1 as compared to clearance in a pure algal suspension. Ingestion and growth rate increased with algal concentration, and growth rate was further increased by 30 to 70% by the addition of 5 mg silt l-1. A growth rate comparable to maximum natural growth rates was reached only at the highest algal concentration in the presence of 5 mg siltl-1. Assimilation efficiency of P. tricornutum decreased from 77% at 5,000 cells ml-1 to 52% at 20,000 cells ml-1. In the experiments with silt added, some 20 to 30% of the assimilated organic matter originated from the suspended bottom material. Net growth efficiency increased with growth rate at a decelerating rate, approaching a maximum of about 70%. It is concluded that suspended bottom material, which is always present in M. edulis' natural habitats, serves as an additional food source, and that M. edulis depends on suspended bottom material to exploit fully its clearance potential, and to reach the maximum growth rates observed in nature.