Pathways of trace metal uptake in the lugworm Arenicola marina

Radiotracer techniques were used to determine the rates of trace metal (Ag, Cd and Zn) uptake and elimination (33 psu, 10 degrees C) from water and sediment by the deposit-feeding polychaete Arenicola marina, proposed as a test species for estuarine-marine sediments in whole-sediment toxicity tests. Metal uptake rates from solution increase with increasing dissolved metal concentrations, with uptake rate constants (+/-SE) (lg(-1) d(-1)) of 1.21+/-0.11 (Ag), 0.026+/-0.002 (Zn) and 0.012+/-0.001 (Cd). Assimilation efficiencies from ingested sediments were measured using a pulse-chase radiotracer feeding technique in two different lugworm populations, one from a commercial supplier (Blyth, Northumberland, UK) and the other a field-collected population from the outer Thames estuary (UK). Assimilation efficiencies ranged from 2 to 20% for Zn, 1 to 6% for Cd and 1 to 9% for Ag for the Northumberland worms, and from 3 to 22% for Zn, 6 to 70% for Cd and 2 to 15% for Ag in the case of the Thames population. Elimination of accumulated metals followed a two-compartment model, with similar efflux rate constants for Zn and Ag and lower rates of elimination of Cd from the slow pool. Efflux rate constants (+/-SE) of Zn and Ag accumulated from the dissolved phase were 0.037+/-0.002 and 0.033+/-0.006 d(-1) whereas Cd was eliminated with an efflux rate constant one order of magnitude lower (0.003+/-0.002 d(-1)). When metals were accumulated from ingested sediments, the efflux rate constants for the slow-exchanging compartment were of the same order of magnitude for the three metals, and of the same order of magnitude as those derived after the dissolved exposure for Zn and Ag (0.042+/-0.004 and 0.056+/-0.012 d(-1) for Zn and 0.044+/-0.012 and 0.069+/-0.016 d(-1) for Ag for the Northumberland and Thames populations, respectively). Cd accumulated from ingested sediments was eliminated with a rate constant not different from the fast-exchanging compartment after the water-only exposure (0.025+/-0.012 and 0.020+/-0.004 d(-1) for the Northumberland and Thames populations, respectively). A biodynamic model was used to estimate the relative importance of the dissolved phase versus ingested sediment as source of metal for the worms, showing that more than 90% of the Zn and Cd and more than 70% of Ag in lugworms is accumulated from sediment ingestion at realistic environmental concentrations. The model also shows that metal accumulation is highly dependent on the ingestion rate and assimilation efficiency.