KINETIC UPTAKE OF BIOAVAILABLE CADMIUM, SELENIUM, AND ZINC BY DAPHNIA MAGNA

Kinetic uptake of Cd, Se(IV), and Zn by Daphnia magna from the dissolved phase was determined using radiotracer techniques in moderately hard water. The metal influx rate and distribution in the soft tissue and the exoskeleton of the daphnids as influenced by metal concentration, inorganic ligands including pH, Ca²⁺ and SO₄^2-, and body size were quantified. When the metal concentrations were <180 nM for Cd and <769 nM for Zn, the concentration factor in daphnids increased linearly within the 12 h of exposure. At a higher concentration, apparent steady state was reached after 3 h of exposure. Cadmium and Zn distribution in the soft tissues was not affected by the total ambient concentrations, whereas Se distribution in the soft tissue decreased by 7 to 10% with increasing Se concentration from 16 to 643 nM. A linear positive power relationship was found between the influx rates of the metals and the ambient concentrations. The concentration factor for Se, however, decreased significantly with increasing Se concentration in water. The influx rate of metals was inversely related to the body size in a power function. When the pH in ambient water increased from 5.0 to 7.0, the influx rate of Cd, Se, and Zn increased by 2.9, 16.6, and 4.1 times, respectively. The influx rates of Cd, Se, and Zn decreased by 6.9, 8.7, and 4.4 times, respectively, with an increase in Ca²⁺ concentration from 0.6 to 5.1 mM. In contrast, the uptake rates of all three metals were not significantly affected by the SO₄^2- concentration. The majority of accumulated Se was distributed in the soft tissues after 12 h of exposure, whereas Cd and Zn were about evenly distributed in the soft tissue and exoskeleton. Any changes in pH, Ca²⁺, and SO₄^2- concentrations did not apparently affect their distributions in the daphnids. Our study provides important kinetic data necessary for delineating the exposure routes and for further development of the biotic ligand model in Daphnia. Using a bioenergetic-based kinetic model, we showed that the dissolved uptake is dominant for Zn accumulation (>50%). For Cd and Se, dietary exposure is dominant when the bioconcentration factors of these metals in phytoplankton are at the high end.