Abstract

 Colloids can provide a substantial surface area for binding and transportation of hydrophobic and charged contaminants in aquatic systems. Most models vary in their consideration of colloids where free ions are calculated as the most bioavailable. However, colloids may act as an important source of contaminants, thus influencing bioavailability to freshwater invertebrates via uptake from sorption and feeding, which increases during resuspension events. The influence of colloid particulate matter on the toxicity of two model contaminants, cadmium109 and 3Hfluoranthene, to two freshwater invertebrates, Daphnia magna and Hyalella azteca, was evaluated in 48 hour laboratory bioaccumulation and 24 hour feeding inhibition tests. Test organisms were exposed to surface water collected from an Ohio stream treated by cross-flow ultrafiltration to separate 'colloidal' material (<0.45 µm - 10000 Daltons) and 'dissolved' material (<10000 Daltons) subsequently spiked with the model contaminants. Apparent bioconcentration factors correlated with particle abundance (r2=0.68) but not with organic carbon (r2=0.18). Significantly greater contaminant uptake (p<0.05) and lower feeding IC50 ([Cd]=0.005 ppm or [Flu]=0.16 ppm), measured by ß-galactosidase activity, were observed in colloidal fractions compared to dissolved fractions of test waters. These results were consistent despite changes in colloid concentrations between seasons and varied discharge events sampled. Further, uptake was greater in high flows when colloid particles were increased. Results indicate that colloid-bound contaminants can be bioavailable to aquatic invertebrates where feeding may be a method of contaminant uptake and that colloid bound contaminants play a potentially important role in toxicity that may increase invertebrate exposure from predictive models.