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 <1 to >5 h.

Benthic macroinvertebrate distribution, abundance, and composition were observed in a fluctuating (7.7 m) central Wisconsin, U.S.A., reservoir during 1973-74. Chironomidae and Oligochaeta represented 98% of the total fauna by number. The chironomid, Chironomus plumosus, and an oligochaete, Limnodrilus, each averaged 36% of the total benthic biomass. Annual mean numbers and biomass in areas exposed to air, exposed to ice cover, or remaining inundated were 3025/m^ (1.8g/m^), 4311/m^ (4.5g/m^), and 8558/m( 16.0 g/m^), respectively. A substantial portion ofthe benthic fauna vi'as stranded and subsequently decreased rapidly in drying and frozen substrates exposed to air. Total benthic numbers and biomass were greatest immediately below the drawdown limit.

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The defecation rate of the tubificid oligochaete, Limnodrilus hoffmeisteri Claparede was measured by using inverted and upright defecation chambers. Worms cultured using the upright method consistently produced larger amounts of feces (45 to 110%) than those in the inverted method (P < 0.01). The average defecation rate for the upright method was 0.69 ± 0.058 (95% CL) mg feces mg' dry weight h-l compared with 0.41 + 0.033 (95% CL) mg feces mgl dry weight h' for worms using the inverted method.Oligochaetes can have significant effects on the physico-chemical status of the uppermost sediments and presumably on the overlying water. The feeding activity of tubificids is well known as an important process in vertically mixing sediments in the Great Lakes (e.g. Davis, 1974;Robbins et al., 1979). These oligochaete worms feed anterior end down with,t.he posterior end extending a few millimeters to several centimeters above the sediment water interface where the defecated material is deposited.During an on-going research program to determine contaminant mobility mediated by biota, a comparison of methods for estimating the defecation rate of Limnodrilus hoffmeisteri Claparede was made to determine the best technique to be used in studying the transfer rates for particlebound organic pollutants. The comparison was made between the method originally described by Alsterberg (1922) and later used by Appleby & Brinkhurst (1970) which relies on inverting vials containing worms for the collection of feces and a method we developed in which the worms remain in their normal orientation. This paper reports the defecation rates of L. hoffmeisteri using both methods. Materials and methodsThe worms were collected from the Milwaukee Harbor, in 6-7 m of water with an Ekman grab. The worms and sediment were placed in buckets with lake water, returned to the laboratory and stored at room temperature (23 C) for about eight months.Worms were hand-picked from 250 m screened samples of Milwaukee Harbor sediment. Approximately 350 L. hoffmeisteri were placed into a cylinder for 12 h containing 250 Mm screened Lake Michigan sediment. This sediment, used in the experiments, was collected by Ponar grab from a depth of 100 m in Lake Michigan at a site located approximately 11 km northeast of Milwaukee Harbor. Mud samples were washed through a 250 Mm screen into buckets to eliminate large organisms. The screened residues were allowed to settle overnight, after which the supernatant was removed and the mud thoroughly homogenized.Following the 12 h adjustment period, the Lake Michigan sediment samples were screened a second time. Immediately thereafter, L. hoffmeisteri were randomly sorted into each of 20 petri dishes containing lake water. An attempt was made to select healthy specimens of approximately the same size Hydrobiologia 111, 181 184 (1984).

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