ASSIMILATION EFFICIENCY FOR SEDIMENT-SORBED BENZO(A)PYRENE BY DIPOREIA SPP

Two methods are currently available for determining contaminant assimi lation efficiencies (AE) from ingested material in benthic invertebrat es. These methods were compared using the Great Lakes amphipod Diporei a spp. and [C-14]benzo(a)pyrene (BaP) sorbed to Florissant sediment (< 63 mum). The first approach, the direct measurement method, uses tota l organic carbon as a tracer and yielded AE values ranging from 45.9-6 0.4%. The second approach, the dual-labeled method, uses Cr-51 as a no n-assimilated tracer and did not yield AE values for our data. The ina bility of the dual-labeled approach to estimate AEs was due, in part, to the selective feeding by Diporeia resulting in a failure of the non -assimilated tracer (Cr-51) to track with the assimilated tracer ([C-1 4]BaP). The failure of the dual-labeled approach was not a result of a n uneven distribution of the labels among particle size classes, but m ore likely resulted from differential sorption of the two isotopically labeled materials to particles of differing composition. The [C-14]Ba P apparently sorbs to organic particles that are selectively ingested, while the Cr-51 apparently sorbs to particles which are selectively e xcluded by Diporeia. The dual-labeled approach would be a viable and e asier experimental approach for determining AE values if the character istics that govern selective feeding can be determined.