Sublethal effects of silver in zooplankton: importance of exposure pathways and implications for toxicity testing

In aquatic environments, organisms are exposed to contaminants via direct uptake from water and by trophic transfer. However, most toxicity tests only examine uptake via the dissolved phase. We compared the response of marine and freshwater crustacean zooplankton to silver following dissolved and food exposure. Silver, like other metals, concentrates in aquatic food chains and may exert toxicity. In standard solute exposure toxicity tests, Ag is toxic to zooplankton at concentrations of 400 nM for marine copepods and 100 nM for freshwater cladocerans, concentrations far greater than those in most waters. However, if Ag is accumulated from algal food, reproductive success decreases by >50% when algae are exposed to only 1 nM Ag in copepods and 0.5 nM Ag in cladocerans. These concentrations are within an order of magnitude of those found in contaminated estuaries. Following dietary exposure, decreased egg production and viability occur when tissue Ag concentrations increase three- to fourfold to 0.3 ppm in cladocerans and 0.5 ppm in copepods. Assimilated Ag depresses egg production by reducing yolk protein deposition and ovarian development. Our results indicate that ecologically relevant toxicity tests should consider sublethal effects of contaminants obtained from food since these effects cannot be predicted from exposures to only dissolved contaminants.