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.