Cadmium sources and exchange rates for Chaoborus larvae in nature

Although freshwater insects are known to accumulate trace metals in the lab oratory 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 a pproach that allowed us to determine that trophic transfer is the main sour ce of cadmium for larvae of a common lake-dwelling animal, the phantom midg e Chaoborus punctipennis. We transferred C. punctipennis larvae from a low- cadmium to a high-cadmium lake, where they were exposed in 64-mu m-mesh mes ocosms to the prevailing high-Cd concentrations in water and to various qua ntities of prey collected from the Cd-rich lake. Our experimental design en sured exposure of C. punctipennis larvae to realistic Cd concentrations in water and in a natural mixture of prey types. Our results indicate that lar vae take up their Cd mainly from prey. Thus models of metal dynamics and ef fects on these invertebrates are likely to be more realistic if they includ e food as a metal source. Using the same mesh mesocosm design, we also dete rmined that C. punctipennis larvae transferred from a high-Cd to a low-Cd l ake lost their Cd slowly. Combining our information on Cd uptake and loss f rom C. punctipennis allowed us to model Cd exchange between this insect and its surroundings.