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 u ptake 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 fo od 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 10 0 nM for freshwater cladocerans, concentrations far greater than those in m ost waters. However, if Ag is accumulated from algal food. reproductive suc cess decreases by >50% when algae are exposed to only 1 nM Ag in copepods a nd 0.5 nM Ag in cladocerans. These concentrations are within an order of ma gnitude of those found in contaminated estuaries. Following dietary exposur e, decreased egg production and viability occur when tissue Ag concentratio ns increase three- to fourfold to 0.3 ppm in cladocerans and 0.5 ppm in cop epods. Assimilated Ag depresses egg production by reducing yolk protein dep osition and ovarian development. Our results indicate that ecologically rel evant toxicity tests should consider sublethal effects of contaminants obta ined from food since these effects cannot be predicted from exposures to on ly dissolved contaminants.