Spatial Distribution of Total Cadmium, Copper, and Zinc in the Zebra Mussel ( Dreissena polymorpha) Along the Upper St. Lawrence River

Abstract

The zebra mussel ( Dreissena polymorpha) was utilized to assess the spatial distribution of three trace metals, cadmium, copper, and zinc, in the upper St. Lawrence River and to test the hypothesis that outflow from Lake Ontario influenced levels of these metals in near-shore biota. Zebra mussels, collected from twelve sites along the southern shore, were analyzed for total cadmium, copper, and zinc in their soft tissues. Total cadmium and copper concentrations were elevated at all sites compared to relatively uncontaminated waters and were highest at sites near the outflow of Lake Ontario and an industrial area farthest downstream. Total zinc concentrations approached levels found at uncontaminated sites. Concentrations were rarely related to animal size at any site. However, body burdens (metal content per individual) showed strong positive relationships with size. To facilitate comparisons among sites, body burdens were calculated for standard length (2.0 cm) mussels. Highest body burdens occurred at the outflow of the lake and at one relatively uncontaminated site downstream. Lowest body burdens occurred at sites in the industrial area, although concentrations in the tissues were high. Expected total cadmium concentrations in tissues were estimated using a bio-energetic based kinetic model. Observed cadmium concentrations were lower than predicted, suggesting that near-shore areas may comprise significant depositional areas subject to enrichment with contaminants carried in the outflow from Lake Ontario. Long-term studies of inter-annual variability in metal concentrations, metal burdens, and growth rates should enhance the usefulness of D. polymorpha as a biomonitor and help in understanding the fate of trace metals in the upper St. Lawrence River.