We investigated the roles of copper accumulated on leaves and in-stream copper concentrations in the inhibition of leaf decomposition in a copper-contaminated stream before and after treatment with a wetland passive treatment system. While post-treatment copper accumulations on leaves (CuL) were significantly related to in-stream copper concentrations (r2=0.42, p=0.0026), CuLwere not important determinants of the decomposition rate (r2=0.00053, p=0.92) or shredder abundance in leaf bags (r2= 0.003, p = 0.82), indicating that the inhibition of the decomposition rate in our study was not a response to CuLHowever, in a step-wise regression model that included leaf decomposition, CuL, in-stream copper concentrations, and shredders, in-stream copper concentration was the only significant effect and explained the most variation in the decomposition rate (r2=0.23, p = 0.02). The inhibition of leaf decomposition was a response to high (>10 μg/L) in-stream copper concentrations rather than CuLSince in-stream copper concentrations during most of the study were well above those known to affect microbial activity (≥ 22.5 μg/L), inhibition of leaf decomposition was probably due to effects of in-stream copper concentrations on shredding macroinvertebrates.