Multiphase biokinetic modeling of cadmium accumulation in Daphnia magna from dietary and aqueous sources.

We investigated the Cd accumulation dynamics in a freshwater cladoceran, Daphnia magna, and validated the biokinetic accumulation model under nonsteady-state conditions. All biokinetic parameters were monitored for the animals from birth to the adult stage. During the whole exposure period, two factors were observed to play critical roles in affecting Cd bioaccumulation. One was the Cd partitioning between the dissolved and dietary phases. Although the total Cd loads in the exposure media were kept constant, the increasing Cd partitioning from the water to the food promoted Cd bioaccumulation by approximately twofold. The other factor was the growth rate constant, which was comparable with the Cd efflux rate constant in daphnids and significantly influenced Cd accumulation because of its variation in different developmental stages. A multiphase modeling approach was used to simulate such a nonsteady-state process. This approach was viewed as a success, because the model simulations clearly aided in the interpretation of the experimentally observed temporal Cd accumulation in daphnids. Proper application of the kinetic bioaccumulation model undoubtedly will help us to understand time-dependent responses under the nonsteady state, which may be caused by episodic exposure or seasonal fluctuations in food/particle availability.