MEASURING ASSIMILATION EFFICIENCIES FOR SEDIMENT-BOUND PAH AND PCB CONGENERS BY BENTHIC ORGANISMS

Assimilation efficiencies (AEs) of sediment-sorbed H-3-benzo(a)pyrene (BaP) and C-14-2,2',4,4',5,5'-hexachlorobiphenyl (HCBP) were measured in Diporeia spp. (Amphipoda) and the AE for BaP was determined in Lumbriculus variegatus (Oligochaeta). For Diporeia, three methods of determining AEs were compared and for L. variegatus AEs were measured by two methods. The first method used organic carbon as a tracer based on a feeding selectivity index (SI) and the relative concentrations in the sediment and fecal material. After 10-day exposure, the AEs in Diporeia for BaP and HCBP were 45% to 57% and 46% to 58%, respectively. The AEs in L. variegatus for BaP were 0% to 26% throughout the 5-day exposure. The second method estimated assimilation from ingestion based on the feeding rate and the SI for organic carbon. The AEs in Diporeia for BaP and HCBP were 11 to 15% and 36% to 52%, respectively. For the third approach, a dual-label technique was used with C-14-polydimethylsiloxane as a non-assimilated tracer for estimating AE based on the relative ratio of the assimilated and non-assimilated tracers in the food and fecal material. This technique gave a BaP-AE of 56% for Diporeia after 10 days. The BaP-AE in L. variegatus ranged from 23% to 26% for the first day of exposure, then decreased to 10% by the end of the 5-day exposure period. Differences in the AEs for Diporeia exposed to BaP determined from these techniques result, in part, from the differential distribution of the xenobiotics among the sediment particles and the selective feeding by Diporeia.