Bivalve Trends Plots of Bivalve Trends

Transplanted bivalves are valuable in assessment of long-term trends because they provide an integrated measure of contamination over a three month period. This interval is more appropriate for assessment of interannual trends than the one-hour interval represented by RMP water samples or the approximate 20 year interval represented by RMP sediment samples.

This section presents plots of RMP bivalve bioaccumulation data for trace elements and trace organics from 1993 to 1997 (Figures 5.19a, b, c, d, & e, and 5.20a, & b ). Concentrations in these plots are expressed as net bioaccumulation or depuration during the deployment period (initial concentrations prior to deployment have been subtracted from final concentrations measured after deployment). Presented in this manner, the plots are capable of showing the presence or absence of both trends and accumulation during deployment. In many cases (e.g., arsenic) there was either little accumulation or even net depuration during deployment. Mercury in clams has exhibited a consistent seasonal pattern, with higher concentrations in summer samples in all five years. The trace metals database accumulated so far is fairly noisy, and clear trends are not expected to be discernible for the near future.

Plots of Bivalve Trends