Alteration of leaf decomposition in copper-contaminated freshwater mesocosms

The influence of copper on leaf litter decomposition was examined in experimental streams. Controls and three levels of contamination (5, 25, and 75 microg/L) were tested in triplicate in 20-m-long mesocosms. Equal quantities of alder, maple, and oak leaves were enclosed in mesh bags and placed in the upper and lower mesocosm sections that exhibited different habitat characteristics (shallow with pebble substrate or deep with fine substrate and macrophytes, respectively). Decomposition rate in the upper section of the 75 micro.g/L mesocosms was significantly reduced to 28% of control values. No significant reductions in the decomposition rate were seen in the lower section. No differences in the leaf-associated mycelial biomass and sporulation rates were observed between treatments, but they were consistently higher in the upper sections. The aquatic hyphomycete community was not affected by the contamination. The abundances of total leaf-associated macroinvertebrates and the dominant shredder (Gammarus pulex) were considerably reduced in the upper sections at 75 microg/L. Therefore, the reduced leaf decomposition probably resulted from a reduction in the abundance of macroinvertebrate detritivores. In addition, local variations in physical and biological characteristics of the habitat along the mesocosms significantly influenced the effects of copper on leaf-associated decomposers.