Contribution of Stream Detrivores, Fungi, and Bacteria to Leaf Breakdown Based on Biomass Estimates

Linking species and ecosystems is currently one of the great challenges in ecology. To this end, we assess here the contributions of bacteria, fungi, and detritivorous invertebrates (shredders) to leaf litter breakdown, a key ecosystem-level process. We enclosed alder (Alnus glutinosa) and willow (Salix fragilis) leaves in coarse-mesh bags (5 g dry mass), placed them in a stream during peak leaf fall, and retrieved them periodically to determine leaf mass remaining and the biomass of leaf-associated organisms. Shredder biomass was derived from numbers and length-mass relationships, bacterial numbers and biomass were determined by epifluorescence microscopy, and fungal biomass was measured as ergosterol. In addition, conidial production of aquatic hyphomycetes was determined. Leaves decomposed rapidly with exponential breakdown coefficients k of 0.035 d-1 (alder) and 0.027 d-1 (willow). Leaves were also quickly colonized within the first 4 wk of decomposition, when shredder biomass reached 263 and 141 mg dry mass/litter bag, respectively. Maximum bacterial numbers (5.6 and 4.8 × 1010 g-1 detrital dry mass) were attained after 8 wk and corresponded to a biomass of 3.6 (alder) and 3.1 (willow) mg dry mass/g,