Dual Tracer Studies of the Assimilation of an Organic Contaminant from Sediments by Deposit Feeding Oligochaetes

Abstract

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.