Relative importance of water and food as cadmium sources to an aquatic insect (Chaoborus punctipennis): Implications for predicting Cd bioaccumulation in nature

The relative importance of water and food as Cd sources to the predatory insect Chaoborus punctipennis was established using a three-link planktonic food chain composed of larvae of the insect, its crustacean prey (Ceriodaphnia dubia), and the prey's algal food (Selenastrum capricornutum). To ensure that our laboratory results were reliable and could be extrapolated to field conditions, we introduced Cd at the bottom of the food chain; we used Cd concentrations to which animals might be exposed in nature; we measured Cd loss from the food to the water compartment; and we controlled the availability and concentrations of Cd in water and food. There was no significant difference between the Cd concentrations in larvae exposed to the metal in their food alone versus those exposed to Cd in both food and water, indicating that Cd bioaccumulation from water was negligible. These results suggest that, although Cd concentrations in the predator can be correlated with those in lake water, direct uptake from water is probably unimportant; instead, the correlation is likely a consequence of Cd uptake from water by organisms at a lower level in the food chain. The importance of food as a Cd source suggests that models designed to predict Cd concentrations in animals will be more complete and probably more reliable if they consider metal uptake through the food chain.