[en] In three different experiments, Mn, Mo and La were used in a dual tracer system called double activation analysis (DAA), where stable elements and neutron-induced radioisotopes of that same element are combined for tracer studies in a stream mesocosm. DAA in ecological studies is successful in overcoming problems associated with the need of obtaining both immediate and long-term information in the same experiment. Analysis of the data suggests that roots growing in stream waters may serve as bioaccumulators of potentially polluting heavy metals. (author) 9 refs.; 5 figs