Tracking the Dumping and Bed Load Transport of Dredged Sediment in the St. Lawrence Estuarine Transition Zone and Assessing Their Impacts on Macrobenthos in Atlantic Sturgeon Habitat
Pierre Nellis, Simon Senneville, Jean Munro, Georges Drapeau, Daniel Hatin, Gaston Desrosiers, and François J. Saucier
Abstract.—The short- and long-term impacts of the annual disposal of dredged sediment within Atlantic sturgeon Acipenser oxyrinchus habitat in the St. Lawrence estuary were assessed by simulating sediment transport over 10 years and by sampling sediment and macrobenthos on impacted and control radials. The model applied bed load transport functions embedded in a baroclinic hydrodynamic model. The path predicted by the model was validated at its half-way point (5 years) using multibeam sonar images of the seabed as well as qualitative and quantitative sediment analyses. Sediment took more than 1 year to completely clear the disposal site, indicating continuous sand drift along the predicted path. In the first 2 years, the sediment path increased rapidly in length (10 km), bypassing a field of sand dunes and following the deepest depressions (10 m) in the downstream direction. In the middle of its 10-year course (ca 12 km), the sediment track crossed the most important core area used by early juvenile Atlantic sturgeon. Afterward, sediment progression slowed and accumulated in a 15-mdeep channel between islands in the middle of the estuarine transition zone. The impact stations showed a significant reduction in the average biomass of tubificids, the most important food item of juvenile Atlantic sturgeon in the St. Lawrence. Conversely, the number and abundance of epi- and suprabenthic taxa increased. Model and field results suggest that sand drift generated from disposal operations reduces benthic productivity along the modeled path, including critical habitats of early juvenile Atlantic sturgeon. Sand accumulation from dredged spoil that drift in dead current areas are also likely to reduce juvenile sturgeon habitat quality.