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Assimilation and retention of a PCB congener, 2,4,5,2′,4′,5′-hexachlorobiphenyl (HCBP), by the oppossum shrimp, Mysis relicta, was followed during sediment ingestion and defecation using a 14C label. Uptake was rapid and essentially linear, with mysids reaching a [14C]HCBP specific activity equivalent to that in the labeled sediment within 2 wk. Assimilation efficiencies calculated from the relative depletion of the label in the fecal material averaged 53% (±7%). Sediment ingestion rates calculated from the activity required to supply the label retained were approximately 1 mg∙mysid−1∙d−1, in agreement with published estimates. The distribution of HCBP as a function of particle size indicates that particle-size selective feeding by detritivores can have a significant effect on calculated assimilation efficiencies. Given the vertical migration behavior of M. relicta, sediment ingestion is a potentially important pathway for the reintroduction of sediment-associated contaminants into the pelagic environment of deep lakes.

The assimilation of [14C]2,4,5,2′,4′,5′-hexachlorobiphenyl (HCBP) from Lake Michigan sediments by oligochaete worms was determined in laboratory microcosms using dual tracer techniques. Particle size selective feeding by oligochaetes makes single tracer calculations of assimilation from bulk feces subject to errors resulting from the changing distribution coefficients of adsorbed constituents as a function of particle size. 51Cr3+ adsorbed to sediments passes through the guts of worms without being assimilated and serves as a conservative tracer of ingestion. Assimilation efficiencies for HCBP decreased from 36 to 15% over the initial 10 d of active feeding and was inversely related to average defecation rate which increased from 0.05 to 0.25 mg sediment∙mg worm−1∙h−1 over the same period. In combination with measured defecation rates, assimilation efficiencies were used to estimate HCBP uptake rates of 3.9–8.1 pmol∙mg worm−1∙h−1. Assimilation efficiencies appear to be dependent upon gut clearing times which are a function of both gut volume and feeding rate and which are estimated to vary from 5 h.