FINE-GRAINED SEDIMENT DEPOSITION ON MUSSEL BEDS IN THE OOSTERSCHELDE (THE NETHERLANDS), DETERMINED FROM ECHOSOUNDINGS, RADIOISOTOPES AND BIODEPOSITION FIELD EXPERIMENTS

Sedimentation rates of 5-20 cm year(-1) were calculated from Cs-137+Cs -134 activity profiles and echosoundings in a mussel cultivation area in the western part of the Oosterschelde. Sedimentation rates based on Pb-210 activity profiles are far too low, mainly due to the depositio n of 'aged' sediment, i.e, sediment eroded elsewhere inside the basin and not fully enriched with Pb-210 at th, moment of deposition. Also, specific adsorption to the finest sediment components, mixing, winnowi ng, and mobilization of the caesium have affected the lead and caesium profiles. Field experiments on mussel plots showed that mussels depos it 5-10 cm of fine-grained sediment during the summer. This biodeposit ion about equals the settling from flocculation in the water column. M icro-fabric microscope studies revealed that pellets do not dominate t he sediments, indicating that some of the pellets are broken down or r esuspended.