Limnol. Oceanogr., 44(7), 1999, 1763–1771

Although freshwater insects are known to accumulate trace metals in the laboratory from both water and food, the relative importance of metal sources for these animals, as well as the rate at which they take up and eliminate their metal, has not been measured in nature. We describe a novel in situ approach that allowed us to determine that trophic transfer is the main source of cadmium for larvae of a common lake-dwelling animal, the phantom midge Chaoborus punctipennis. We transferred C. punctipennis larvae from a low-cadmium to a high-cadmium lake, where they were exposed in 64-mm-mesh mesocosms to the prevailing high-Cd concentrations in water and to various quantities of prey collected from the Cd-rich lake. Our experimental design ensured exposure of C. punctipennis larvae to realistic Cd concentrations in water and in a natural mixture of prey types. Our results indicate that larvae take up their Cd mainly from prey. Thus models of metal dynamics and effects on these invertebrates are likely to be more realistic if they include food as a metal source. Using the same mesh mesocosm design, we also determined that C. punctipennis larvae transferred from a high-Cd to a low-Cd lake lost their Cd slowly. Combining our information on Cd uptake and loss from C. punctipennis allowed us to model Cd exchange between this insect and its surroundings. Aquatic systems directly influenced by mining, metal smelting, and other industrial activities have been contaminated by the thousands with potentially toxic trace metals such as cadmium, copper, and lead (Pacyna et al. 1995). Trace metals are also transported long distances in the atmosphere (Nriagu and Pacyna 1988), thereby reaching aquatic ecosystems distant from local sources (Franzin et al. 1979; Verta et al. 1986). The wide dispersion and potential toxicity of metals require that their impact on aquatic organisms be evaluated. The biological effects of metals on aquatic animals are most often assessed by exposing individuals in the laboratory to aqueous metal in the absence of metal-contaminated food. Exposures of this type assume by design that metal uptake from food is negligible (Luoma 1995). Rigorous demonstrations to support this assumption are few, owing in large part to the technical difficulties involved in unambiguously separating food and water as metal sources to animals (Kay 1985; Fisher and Reinfelder 1995). One exception is a recent laboratory experiment in which water and food (a planktonic crustacean) were successfully separated as cadmium (Cd) sources for the predatory insect Chaoborus punctipennis (Munger and Hare 1997). In this experiment, the predator was shown to accumulate Cd solely from its prey, which brings into question the realism of experiments in 1 Corresponding author.