The role of copper accumulations on leaves in the inhibition of leaf decomposition in a mountain stream

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 (Cu-L) were significantly related to in-stream copper concentrations (r(2) = 0.42, p = 0.0026), Cu-L were not important determinants of the decomposition rate (r(2) = 0.00053, p = 0.92) or shredder abundance in leaf bags (r(2) = 0.003, p = 0.82), indicating that the inhibition of the decomposition rate in our study was not a response to Cu-L However, in a step-wise regression model that included leaf decomposition, Cu-L, in-stream copper concentrations, and shredders, in-stream copper concentration was the only significant effect and explained the most variation in the decomposition rate (r(2) = 0.23, p = 0.02). The inhibition of leaf decomposition was a response to high (>10 mu g/L) in-stream copper concentrations rather than Cu-L Since in-stream copper concentrations during most of the study were well above those known to affect microbial activity (greater than or equal to 2.5 mu g/L), inhibition of leaf decomposition was probably due to effects of in-stream copper concentrations on shredding macroinvertebrates.