Metal contamination in zebra mussels (Dreissena polymorpha) along the St. Lawrence River

In order to evaluate the use of zebra mussels as biomonitors for metal bioavailability in the St. Lawrence River, we tested the hypothesis that the concentrations of 11 metals in zebra mussels vary significantly between sites along the river and that the season of collection and body size affect metal bioaccumulation. Mussels were collected at 14 sites during June 1996 and at monthly intervals at one site. Specimens were grouped in three size classes and their soft tissue was analyzed for As, Ca, Cd, Cr, Hg, Mn, Ni, Pb, Se, and Zn. Significant size effects were found for Ca, Cd, Cr, Cu, Ni and Zn. Spatial and seasonal variations in bioconcentration were significant for all metals. Spatial patterns in contamination that corresponded to known point sources of pollution or hydrology of the river were identified by principal component analysis. Seasonal variations can be attributed to the reproductive cycle of mussels and hydrological variability of the river. In comparison with values reported for zebra mussels in other contaminated sites in North America and Europe, levels of metal in the St. Lawrence River are low or intermediate. Our results show that when controlled for size and seasonal effects, zebra mussels represent a useful biomonitor for metal availability in the river and may offer an interesting alternative to native mussels and fish for such a role. Local contamination by some toxic metals is still a cause for concern in the St. Lawrence River.