The role of colloids in regulating element transport,behavior, and bioavailability in aquatic systems is now wellestablished. It appears that further progress in this research field is being slowed by (i) a limited integration between the geochemical and the biological aspects of the research on colloids and (ii) a persistent gap between wellcontrolled laboratory studies and real field situations. This paper presents a simultaneous evaluation of the role of colloids in controlling element environmental fate and bioavailability at the confluence between a major river and a polluted tributary. Fractionation of trace elements among suspended particulate matter, colloids, and true solution suggests that colloids may play a role in the removal of trace elements from the water column to bed sediments during the mixing of the two rivers. Toxicity testing of water samples indicates that, in this specific system, contaminants associated with colloids can contribute to water toxicity for the rotifer Brachionus calyciflorus but not for the green alga Pseudokirchneriella subcapitata. To the best of our knowledge, the results for B. calyciflorus are the first ones pointing to the possible contribution of colloid-bound contaminants to water toxicity in environmental samples. Despite the uncertainties associated with field variability, the results of chemical analysis and toxicity testing show several points of convergence. Following these observations, a few innovative research approaches are suggested to improve the understanding of trace element biogeochemistry in real field situations.