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Toxicity of silver in water and sediment to the freshwater amphipod Hyalella azteca

Hyalella azteca was exposed to Ag as AgNO3 over a 10‐d period in water and two lake sediments that were selected on the basis of their differences in metal‐binding properties. The median lethal concentrations (LC50s) for waterborne exposures were 5.4 and 4.9 μg/L for total and dissolved Ag, respectively. In the sediment containing a lesser quantity of total Ag‐binding ligands (i.e., Bond Lake, Douglas County, WI, USA, sediment), an Ag‐amended sediment toxicity test resulted in a 10‐d LC50 of 0.084 g (i.e., 84,000 μg) Ag/kg dry sediment or 8.6 μg Ag/L of pore water (PW). The no‐observed‐effect concentration (NOEC) to lowest‐observed‐effect concentration (LOEC) range was 0.012 to 0.031 g Ag/kg dry sediment, or less than 5.0 to 6.0 μg Ag/L of PW. In the sediment with a greater quantity of total Ag‐binding ligands (i.e., West Bearskin Lake, Cook County, MN, USA, sediment), the 10‐d LC50 was 2.98 g Ag/kg dry sediment, and the NOEC to LOEC range was 2.15 to 4.31 g Ag/kg dry sediment. Because “dissolved” concentrations of Ag in PW were less than 5.0 μg/L at the critical exposures in the latter test, the bioavailable and toxic form of Ag may have been a weakly associated coprecipitate or colloidal complex with hydrous iron oxides that competitively partitioned to the surface of the gills.