Fine-grained sediment deposition on mussel beds in the oosterschelde (The Netherlands), determined from echosoundings, radio-isotopes and biodeposition field experiments

Sedimentation rates of 5–20 cm year−1 were calculated from 137Cs+134Cs activity profiles and echosoundings in a mussel cultivation area in the western part of the Oosterschelde. Sedimentation rates based on 210Pb activity profiles are far too low, mainly due to the deposition of ‘aged’ sediment, i.e. sediment eroded elsewhere inside the basin and not fully enriched with 210Pb at the moment of deposition. Also, specific adsorption to the finest sediment components, mixing, winnowing, and mobilization of the caesium have affected the lead and caesium profiles. Field experiments on mussel plots showed that mussels deposit 5–10 cm of fine-grained sediment during the summer. This biodeposition about equals the settling from flocculation in the water column. Micro-fabric microscope studies revealed that pellets do not dominate the sediments, indicating that some of the pellets are broken down or resuspended.