The objective of this study was to assess the effects of industrial discharges, contaminated primarily with trace metals, on populations of Lampsilis radiata (Bivalvia: Unionidae) in the upper St. Lawrence River. Effects on shell growth were examined by (a) comparing growth curves derived from analyses of external annual rings of clams sampled from sites located upstream and downstream of four localized sources of discharges, and (b) measuring shell growth for one year from clams that were transplanted into contaminated and uncontaminated sites. Allozyme frequencies in clams from above and below discharges were determined for glucose-phosphate isomerase (GPI) and phosphoglucomutase (PGM), two enzyme systems reported to be sensitive to selection by polluted conditions. Field collected clams and sediment were analysed for trace metal content to correlate soft tissue burdens with degree of environmental contamination. Toxicity tests with glochidia larvae were performed with zinc to test for differential resistance based on the source (upstream versus downstream of a zinc-enriched discharge), and the GPI and PGM genotypes of the mother clams.;Minor depressions were detected in shell growth patterns of clams from downstream of discharges relative to clams from upstream in three out of four discharge areas. However, no shell growth depression was observed in clams transplanted into contaminated sites. Shell growth of transplanted clams was strongly affected by their collection source, suggesting that genetic factors or irreversible physiological compensation is more important in controlling shell growth than environmental contaminants. There was no evidence for differential selection of allozymes in clams from upstream and downstream of discharges. Metal burdens were elevated in clam tissue only in those from the most polluted site. Although glochidia of clams from polluted and unpolluted sites exhibited significant among-clam variability in resistance to zinc toxicity, differences were not related to source (site) or genotype of mother clams.;These results suggest that populations of L. radiata in the upper St. Lawrence River are either resilient to effects of industrial discharges, or that gene flow between sites is sufficiently high to offset selective pressure by contaminants
