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, <5% of the maximum fungal biomass (77 and 70 mg dry mass/g, respectively). Aquatic hyphomycetes released up to 2.7 and 1.4 × 106 conidia·g−1·d−1, equivalent to a daily conidial production of 9.4 and 2.9 mg dry mass/g on alder and willow, respectively. Calculations based on the measured decomposer biomass and conidial production, published microbial growth efficiencies, turnover times, and shredder feeding rates indicate that shredders accounted for the largest portion of overall leaf mass loss (64% and 51% on alder and willow leaves, respectively), fungi contributed at least 15% and 18%, and bacterial contribution also was estimated to be substantial (7% and 9%). Analyses of elemental flows from allochthonous leaf litter thus clearly require a quantitative consideration of the complex decomposer consortium responsible for leaf decomposition.