TEMPORAL AND SPATIAL VARIATIONS OF TRACE-METAL CONCENTRATIONS IN OYSTERS FROM THE PATUXENT RIVER, MARYLAND

Concentrations of copper (Cu), silver (Ag), and cadmium (Cd)in eastern oysters (Crassostrea virginica) from the upper Patuxent River estuary analyzed since 1986 (Cd since 1991) were high relative to concentrati ons in other sites in the United States analyzed by the National Ocean ic and Atmospheric Administration National Status and Trends program. Patuxent River oysters had above average concentrations of Ag and Cu, and unusually high concentrations of Cd. Metal concentrations were hig hest in summer, a period in which oyster meat condition index was rela tively low. Copper values were highest in 1986-1987, likely elevated b y erosion from Cu-Ni alloy condensers at a local power plant. Silver a nd Cd values exhibited more year-to-year variation. A number of factor s were examined as candidates to explain the interannual differences, including river flow, salinity, and oyster condition, but none was abl e to explain the high and low years. Samples collected in spring and l ate summer of 1996 at a number of oyster bars located along the length of the Patuxent River showed that concentrations of all three metals increased with distance up-river. For Cd, the upstream increase was li near with distance, while for Ag and Cu, there was a secondary maximum near river km 16. A close correlation was observed between Ag and Cu for individuals at each site, with a poorer correlation between Cd and either Ag or Cu.