The hydroelectric dam of Marckolsheim (Northeast of France) is one of numerous structures located along the 1200 km of the Rhine River, where sediments have been accumulated for several decades. For the maintenance and the safety of the dam, dredging by pu mping/dilution with direct discharge in the Rhine was organized in 2013 by the French firm EDF (owner of the dam) in accordance with local regulations. Dredging can generate the remobilisation of sediments that are contaminated by non - degradable compounds such as metals, and subsequently lead to alterations of the aquatic ecosystem. In order to assess the environmental impact of the dredging project, a long - term monitoring study using both chemical and biological tools was performed at two sampling sites l ocated upstream and downstream from the discharge area. Environmental monitoring began three years before the project to acquire data on the natural evolution of metal contamination of the Rhine during full annual hydrological cycles (seasons, floods, etc. ). Thus, DGT (or Diffusive Gradient in Thin films) - labile, dissolved and total concentrations of Cd, Co, Cr, Cu, Ni, Mn, Pb and Zn were measured monthly in river water as well as metal concentrations in suspended matter samples from October 2010 to July 20 13 at both sites. In parallel, size - calibrated zebra mussels ( Dreissena polymorpha ), which are filter - feeders that have the advantage of accumulating metals from both dissolved and particulate fractions, were caged and transplanted at both sites to assess metal bioavailability. During the long - term monitoring, low temporal variations in metal contamination were observed in the Rhine and transplanted mussels. DGT – labile concentrations showed that speciation of some metals in the water column was modified d uring the dredging project in 2013 (4 weeks). In particular, Co and Mn were remobilised under more labile chemical forms that are potentially more available for biota. Besides, dredging led to significant accumulations of Pb, Cr and Mn in caged mussels exp osed to the discharge. Nevertheless, the mussels recovered their initial state with respect to their basal levels of contamination two weeks after the end of the dredging , due to their ability to excrete metals. This suggests a transient impact of the disc harge on the se organisms . Furthermore, chemical analyses indicated high Cu and Zn concentrations in suspended sediments when compared to ones measured in the Rhine during the long - term monitoring. Nevertheless, no significant increase of Cu and Zn levels w as measured in transplanted mussels probably because these essential metals are well regulated by aquatic organisms . These results highlight the interest of coupling chemical and biological approaches for a better assessment of environmental risk