Urbanization alters water physical and chemical variables and may affect leaf-litter breakdown in streams. Higher temperature and nutrient inputs in urban streams can stimulate microbial biomass, which can increase leaflitter breakdown rates over rates in nonurban streams. On the other hand, urbanization can reduce leaf-litter breakdown rates by eliminating shredders. We evaluated physical, chemical, and biological factors that may directly and indirectly affect leaf-litter breakdown of Coussapoa trinervia and Mabea speciosa in 42 urban streams in Central Amazonia. We used structural equation modeling to assess whether: 1) shredder activity is more important than microbes for leaf-litter breakdown of plant species with softer tissues, 2) microbes (as adenosine triphosphate [ATP] concentration) and fungi (as ergosterol concentration) positively influence leaf-litter breakdown rate, 3) water velocity positively affects leaf-litter breakdown rate, and 4) effects of shredders and microbes, including fungi, on leaf-litter breakdown are mediated by the effects of urbanization. Leaf-litter breakdown of M. speciosa and C. trinervia was fastest in the least urbanized streams. Fungi had a direct positive effect on leaf-litter breakdown of both species, but shredders were the most important factor for leaf-litter breakdown in M. speciosa (softer leaf tissues). Water velocity had a slight indirect effect on leaf-litter breakdown of C. trinervia through its effect on fungi. Microbes were not important for leaf-litter breakdown rates of either species. Urbanization indirectly affected leaf-litter breakdown via negative effects on shredder and fungal biomass. Our study provides evidence for multiple direct and indirect pathways by which urbanization can decrease leaf-litter breakdown rates in tropical streams, mainly through negative effects on the fungal and shredder biomass. © 2015 by The Society for Freshwater Science.
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