Toxicity of Harbour canal sediments before dredging and after off-shore disposal

Dredge material from an entrance waterway to the port of Rotterdam and sediments from the North Sea off-shore disposal site were tested for toxicity using three different sediment bioassays. The goals of the study were to evaluate if bioassays generate useful additional information to chemical based sediment quality criteria, and to investigate potential changes in toxicity of dredge material after off-shore disposal. Sediment elutriates were tested with larvae of the oyster Crassostrea gigas, whole sediments with the infaunal amphipod Bathyporeia sarsi, and sediment suspensions with the blue mussel Mytilus edulis. The contamination of the investigated dredge material was low according to the limited set of chemical parameters that was analysed. Nevertheless the three bioassays showed that some of the dredge material samples significantly affected the survival of oyster larvae and the amphipod B. sarsi, as well as the aerial survival time of mussels. Toxicity was reduced at the off-shore disposal site, probably due to the dispersal of the most polluted finer sediment fractions by strong residual currents. It is concluded that the sediment bioassays used for this study are a meaningful tool that should be incorporated in decision making frameworks for the management of dredge material.