Changes in mechanisms of summer detritus processing between wooded and agricultural sites in a Michigan headwater stream

Abstract

The mechanisms of leaf decay, leaf-associated macroinvertebrate community structure, leaf-associated microbial activity and physicochemical stream characteristics were investigated on a mid-Michigan headwater stream in summer. An undisturbed wooded site was compared with two agriculturally perturbed sites. Discharge, total suspended particulates, and nutrients were all higher and more variable throughout the season within the agricultural reaches. Leaf decay rates were higher at the agricultural sites and results suggest discharge abrasion was the major leaf processing mechanism at these sites while microbial decay and macroinvertebrate shredding appear to be the primary mediators of leaf weight loss at the wooded site. Total macroinvertebrate densities on leaf packs at the agricultural sites were 1.9 times the densities at the wooded site. It is suggested that experimentally introduced leaf packs acted as a lure for net-spinning invertebrates limited by stable substratum at the agricultural sites. Species shifts were observed from wooded reaches where Pychnopsyche spp., Gammarus, Ephemeroptera, Bezzia, and Nigronia serricornis were important, to downstream agricultural reaches which were dominated by Cheumatopsyche, Chironomidae, Elmidae, Hydracarina, Hemerodromia, and Caecidotea.