Effects of litter diversity on decomposition and biological colonization of submerged litter in temperate and tropical streams

Detrital food webs of woodland streams depend on terrestrial litter input and, thus, are susceptible to changes in riparian cover. We assessed effects of litter species richness and quality on decomposition and associated biological communities in temperate deciduous forest and tropical rainforest streams. Three native litter species were incubated in each stream in all combinations (7 litter treatments, 3 richness levels) in coarse- (invertebrate access) and fine-mesh bags (no invertebrate access) and were sampled 5 times over 74 (temperate stream) or 94 d (tropical stream). Decomposition, and fungal biomass, sporulation, and species richness were measured for each treatment. Alnus glutinosa litter was incubated in both streams to assess effects of environmental and biological differences between streams on litter decomposition. Biological colonization (number of fungal species, fungal biomass) and activity (conidial production) were lower in the tropical than the temperate stream, despite its higher water temperature (24 vs 8°C). Mass loss for individual species reached 95% in the temperate and 60% in the rainforest stream. Decomposition rates in mixtures were unaffected by litter richness but could be predicted from their initial N, phenol, and lignin concentrations (leaf quality). In the temperate stream, Alnus decomposition in coarse-mesh bags was positively related to litter richness, and Alnus stimulated decomposition of mixtures. Microbial O2 consumption, fungal biomass accrual, aquatic hyphomycete sporulation rate and richness, and shredder abundance and richness were insensitive to litter richness. In the temperate stream, presence of tough litter inhibited invertebrate colonization of mixtures, whereas in the tropical stream, presence of soft litter stimulated invertebrate colonization of mixtures. Litter quality (species identity), not richness, was the main controller of decomposition of litter mixtures, and decomposition of litter in mixtures may differ from decomposition of individual species. Thus, disappearance or introduction of key species might affect organic matter processing in streams.