Dynamic multipathway modeling of Cd bioaccumulation in Daphnia magna using waterborne and dietborne exposures

We tested the predictive ability of the dynamic multipathway bioaccumulation model (DYMBAM) to characterize Cd accumulation in Daphnia magna, a species commonly used in toxicity tests and because of its sensitivity, particularly to metals, a species that is relied upon in ecological risk assessments. We conducted chronic exposure experiments in which D. magna were exposed to either dietborne Cd alone or to both dietborne and waterborne Cd. In the food-only treatments, the algae Chlamydomonas reinhardtii or Pseudokirchneriella subcapitata were pre-exposed to free Cd ion concentrations, [Cd(2+)], from 0.001 to 100nM (0.001-11microgL(-1)) then, on a daily feeding renewal basis, fed to D. magna over 21 days. In the water plus food treatment, D. magna were exposed for 21 days to the same range of [Cd(2+)] and fed with the same algal species that had been exposed to Cd at various concentrations. In the algal exposure media, Cd concentrations in algae were directly related to those in water and were characterized by a linear regression model using the log transformed concentration of the WHAM predicted Cd(2+) concentration. The DYMBAM was used with estimated values of the model constants for ingestion rate (0.08-0.34gg(-1)day(-1)) and growth rate (0.085-0.131day(-1)) based on our experimental data and with literature values for rate constants of Cd influx and efflux as well as Cd assimilation efficiency. Measured Cd concentrations in D. magna agreed with model predictions within a factor of 3. Using the model, we predict that food is an important contributor of Cd burden to D. magna, particularly at lower Cd exposure concentrations over an environmentally realistic gradient of free Cd in water. However, this cladoceran also takes up Cd from water and this exposure route becomes increasingly important at very high concentrations of free Cd (>10nM or 1.1microgL(-1)). Nevertheless, Cd produced lethal effects in D. magna that were exposed to this metal in water and diet, but exposure to Cd in food only did not result in toxic effects (as measured by survival and reproduction).