Comparative in situ and laboratory sediment bioassays with juvenile Mercenaria mercenaria

Sediment toxicity assays were conducted with juvenile Mercenaria mercenaria to compare the results of laboratory assays and in situ deployments. Juvenile clams were deployed for one week at a variety of degraded and undegraded sites in Charleston Harbor, South Carolina. USA, during the summers of 1998, 1999, and 2000. Parallel laboratory assays were conducted with sediments collected from the deployment sites. Mortality and a sublethal endpoint, seed clam growth rate, were used to compare toxicity between reference and degraded sites. Growth rates of field-deployed clams tended to be higher than growth rates for laboratory assays, especially at the reference sites. Field studies indicated a higher potential for toxicity than did the laboratory studies at degraded sites. These studies suggest that laboratory assays may underestimate potential sediment toxicity at degraded sites. However, field growth rates may be affected by natural environmental factors (e.g., pH, dissolved oxygen, and salinity), so regression normalization techniques were used to distinguish the effects of these variables from those of contaminants.