Leaf-litter breakdown in urban streams of Central Amazonia: direct and indirect effects of physical, chemical, and biological factors

• Published in: Freshwater science 2015-06, Vol.34 (2), p.716-726
• Main Authors: Martins, Renato T., Melo, Adriano S., Gonçalves, José F., Hamada, Neusa
• Format: Article
• Language: English
• Subjects: Fungal biomass; Fungi; Invertebrates; Metropolitan areas; Microbial biomass; Microorganisms; Plants; Streams; Tropical Streams; Urbanization; Water temperature
• ISSN: 2161-9549; 2161-9565
• DOI: 10.1086/681086

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 leaf-litter 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.