Dynamic model for the accumulation of cadmium and zinc from water and sediment by the aquatic oligochaete, Tubifex tubifex

In aquatic environments, organisms are exposed to and accumulate metals via waterborne and dietary routes including ingested sediment. A key element in understanding metal uptake and accumulation is information concerning the relative importance of the routes of uptake and the kinetics of the processes. In this work the bioaccumulation of the essential element zinc and the nonessential element cadmium were studied from the aqueous and sediment phase, in the cosmopolitan oligochaete Tubifex tubifex, using the radiotracers Cd-109 and Zn-65. A compartmental kinetic model was constructed and parametrized by fitting the model to metal body concentrations. Using the pharmacokinetic modeling approach and taking into account the distribution of the metal between water and sediment, the different routes were quantitatively separated. Under the experimental conditions, the sediment phase accounted for 9.8% of the cadmium and 52% of the zinc uptake. These values are based on the uptake of the radiotracers spiked sediments and therefore likely represent maximal values since it was shown that under the specific conditions this was the most mobile metal fraction. This difference was largely explained by the large difference in assimilation efficiency between cadmium and zinc. Simulations of different conditions showed that both dissolved and sediment-associated metal can be important sources of metal exposure for the worms and that the relative importance strongly depends on the metal and exposure conditions including the lability of the metals in the sediment phase.