Terrestrial detritus subsidies are an essential source of energy and nutrients in freshwater ecosystems. In many such ecosystems, processing of detritus by macroinvertebrate shredders is key to integrating this allochthonous detritus into stream food webs. Thus, resource throughput in stream catchments depends not only on the scale and type of inputs from surrounding terrestrial ecosystems, but also on macroinvertebrate community structure in the streams themselves. We addressed how anthropogenic land use and terrestrial and aquatic biodiversity affect spatio-temporal dynamics of catchment-level leaf litter processing. We surveyed benthic leaf litter abundance and diversity at 120 sampling points distributed across four predominantly forested and six predominantly agricultural stream catchments in Central Europe. In parallel, we assessed the abundance and community structure of amphipods, a regionally-dominant macroinvertebrate driving leaf litter processing and linking to higher trophic levels. We experimentally assessed leaf consumption rates by amphipods in the lab, and then predicted leaf litter throughput for forested and agricultural catchments based on the amphipod distributions, benthic leaf litter supply, and catchment morphology.
As expected, allochthonous leaf litter was more abundant in forested catchments than in agricultural catchments. However, forested catchments received significantly less diverse inputs with the majority of benthic leaf litter consisting of relatively resource-poor beech leaves. In all catchments, benthic leaf litter diversity peaked during fall leaf drop, where litter biomass increased by an order of magnitude or more. There was a significant interaction between temporal dynamics of the input-diversity effect and land-use type, with diversity of input persisting longer in agricultural catchments. Estimates from feeding trials also suggested a selection effect of leaf litter consumption at sites and times with high litter diversity. In the experiments, increasing litter richness raised the expected consumption rate by amphipods compared to monocultures of the dominant tree, due to the inclusion of more preferred food resources. However, ~40% of sites in agricultural catchments were unoccupied by amphipods compared to 100% occupancy in the forested catchments. Thus, the expected increase in leaf processing in agricultural catchments was dampened by microsite-level extirpations or changes in the abundance of key macroinvertebrates. We conclude that different aspects of human impact interact to create generalizable patterns of subsidy processing in streams.