Acute toxicity of five sediment-associated metals, individually and in a mixture, to the estuarine meiobenthic harpacticoid copepod Amphiascus tenuiremis

The acute effects of many individual, seawater-solubilized metals on meiobenthic copepods and nematodes are well known. In sediments, however, metals most often occur as mixtures, and it is not known whether such mixtures exhibit simple additive toxicity to meiobenthos. The estuarine meiobenthic copepod Amphiascus tenuiremis was tested in four acute (96-h) sediment bioassays to determine sediment and pore-water LC 50s for single-metal exposures to copper (Cu), lead (Pb), nickel (Ni) and zinc (Zn). Laboratory-cultured copepods were exposed to clean 98% silt:clay sediments spiked with metal chloride solutions to yield five exposure concentrations plus a control. Trimmed Spearman-Karber analysis gave sediment 96-h LC 50 values of 4.4 μmole Cu/g, 5.7 μmole Ni/g, 11.9 μmole Pb/g, 10.3 μmole Zn/g, and pore-water 96-h LC 50 values of 2 μmole/l, 11.7 μmole/l, and 5.7 μmole/l for Cu, Ni, and Zn, respectively. Male survival after exposure to Cu, Pb, and Ni was significantly less than female survival ( α=0.05). Toxicity of a combined USEPA priority metal mixture to A. tenuiremis was assessed using sediment spiked equitoxically with Cd, Cu, Ni, Pb, and Zn. The sum toxic unit that produced a median lethal dose was 0.72. The mixture had a significantly greater than additive effect on A. tenuiremis survival, with the mixture being 1.4× more toxic than that expected by simple additivity.