Fungal alteration of the elemental composition of leaf litter affects shredder feeding activity

Leaf litter from riparian vegetation provides the main source of matter and energy for food webs of small forest streams. Shredding macroinvertebrates mostly feed on this litter when it hasbeen colonised and conditioned by microorganisms, especially by aquatic hyphomycetes. Since shredders feed selectively, they must make foraging decisions based on the physical and chemicalcharacteristics of the food resource, which can change depending on the identity of fungal species. Here, we addressed the effect of changes in fungal assemblage structure on the elemental composition of oak (Quercus robur) leaf litter and how variation in litter quality affects the feeding of a stream shredder. Leaf discs were incubated in microcosms for 2weeks, inoculated with various fungal assemblages comprised of three species each, and offered to a shredder (Schizopelex festiva, Trichoptera: Sericostomatidae) as food. This shredder ate more leaves with a high mycelial biomass, which depended on fungal assemblage composition. Leaf litter conditioned by different fungal assemblages resulted in different litter N and P concentrations. Mycelial biomass was positively related to litter P concentration, with the lowest and highest P concentrations differing by 40% at most, but not to litter N concentration, even though the lowest and highest N concentrations differed by as much as 35%. The caddisfly larvae ate more leaves with a low C/P ratio. These findings suggest a key role of litter P concentration in eliciting fungal conditioning effects on shredder-mediated litter decomposition.