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The benthic organisms Melita plumulosa (amphipod) and Tellina deltoidalis (bivalve) are currently being used as test species for assessing contamination of estuarine-marine sediments in whole-sediment toxicity tests in Australia. Radiotracer techniques were used to determine the rates of accumulation (at 21 ± 3°C) of cadmium and copper by these organisms from water, algae and sediment sources. A biokinetic model was then developed to separately quantify influx and efflux rates from the metal sources. For exposures to seawater, metal uptake (1 g -1 d -1 ) and efflux rates (% d -1 ) were 0.028 and 6.0 (Cd) and 0.12 and 16 (Cu) for M. plumulosa and 0.012 and 8.5 (Cd) and 0.19 and 11 (Cu) for T. deltoidalis. Net accumulation of metals from the dissolved phase was greater for Cu than Cd for both organisms. For sediment exposures, metal assimilation efficiencies (AEs, %) and efflux rates (% d -1 ) were 22 ± 10 and 19 ± 3 (Cd) and 7.8 ± 3 and 31 ± 4 (Cu) for M. plumulosa, and 25 ± 3 and 17 ± 3 (Cd) and 30 ± 2 and 20 ± 4 (Cu) for T. deltoidalis. AEs and efflux rates for algae were 56 ± 4 and 9.8 ± 1 (Cd) and 33 ± 5 and 16 ± 4 (Cu) for M. plumulosa, and 73 ± 3 and 5.0 ± 2 (Cd) and 49 ± 4 and 4.3 ± 1 (Cu) for T. deltoidalis. For M. plumulosa, the presence of algae increased the rate of accumulation of both Cu and Cd from sediments, and indicated that M. plumulosa fed more rapidly, but not necessarily selectively, in the presence of algae. Ingestion rates of sediments were estimated as 0.20 and 0.08 g g -1 d -1 for M. plumulosa and T. deltoidalis, respectively. The biokinetic models indicated that metal accumulation by the organisms would increase greatly as partitioning to the pore waters increased or if organisms increased their ingestion of metal-contaminated algae in preference to sediments. If it is assumed that the toxic effect of accumulated metals is independent of accumulation pathway, then changes in sediment properties or in an organism's feeding behaviour would be expected to greatly affect the toxicity of metals to the organism. The study highlights the need for careful scrutiny of metal partitioning in sediments when data are to be used to estimate toxicity thresholds or effects concentrations, before such values are adopted in sediment quality guidelines.

Short-term accumulation of Cd and Cu from water, sediment and algae by the amphipod Melita plumulosa and the bivalve Tellina deltoidalis