RESEARCH PAPER A conceptual model of litter breakdown in low order streams Manuel A. S. Graça 1,2 , Verónica Ferreira 2 , Cristina Canhoto 1,2 , Andrea C. Encalada 2,3 , Francisco Guerrero-Bolaño 4 , Karl M. Wantzen 5 and Luz Boyero 6,7,8 1 Department of Life Sciences, University of Coimbra, Portugal 2 IMAR-Marine Research Centre, MARE-Marine and Environmental Sciences centre, University of Coimbra, Coimbra, Portugal 3 Laboratorio de Ecología Acuática, Colegio de Ciencias Biológicas y, Ambientales, Universidad San Francisco de Quito, Quito, Ecuador 4 Instituto Geofísico, Pontificia Universidad Javeriana, Bogotá, Colombia 5 Interdisciplinary Research Center for Cities and Territories, Université François Rabelais, Parc Grandmont, Tours, France 6 Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Leioa, Spain 7 IKERBASQUE, Basque Foundation for Science, Bilbao, Spain 8 School of Marine and Tropical Biology, James Cook University, Australia The detrital-based food web of many streams and rivers plays a fundamental role in the cycling and retention of carbon and nutrients. However, we still need to understand which global mechanisms underlie the biogeochemical pathways that control energy transfer from the detrital pool through local food webs into nutrient and energy cycles and storage. Previous attempts to understand the variability in litter breakdown rates have included the search for latitudinal variation patterns and analysis of the influence of different factors. Here we complement those studies by developing a conceptual model to predict litter breakdown dynamics in low order streams. According to the model, litter breakdown rates and the relative role of microbial decomposers and shredder detritivores on this process are hierarchically governed by interactions between climate/ hydrology and geology acting upon plant traits, nutrient and leaf availability to decomposers, and metabolism of microbial decomposers and shredders. The model explains variations in leaf litter breakdown rates and shredder abundance across large geographic areas, allowing the formulation of predictions of how anthropogenic pressures may affect litter breakdown rates. Received: June 2, 2014 Revised: October 27, 2014 Accepted: October 30, 2014 Keywords: Decomposition / Organic matter / Processing / Streams / Shredders / Aquatic hyphomycetes 1 Introduction 1.1 Litter breakdown is an important ecosystem process Typically 90% of the energy fixed by primary producers in terrestrial systems and many fresh waters falls into the detrital pool (Cebrian, 1999; Abelho, 2001). Food webs relying strongly on detrital pathways are known as “brown food webs” (Kaspari, 2004). Brown food webs are directly involved in the mineralization of organic matter, a key ecosystem process. The detrital-based food web of many streams and rivers plays a fundamental role in the cycling of C. Battin et al. (2009) estimated that organic carbon inputs from land to freshwater ecosystems is 2.7 Pg C year 1 ; freshwaters in turn would be responsible for carbon dioxide (CO 2 ) emissions of about 1.2 Pg C year 1 . A critical role for forested low-order streams in those land–water–atmosphere C-fluxes should be expected for two reasons. First, due to their size, low order streams Handling Editor: Björn Gücker Correspondence: Dr. Manuel A. S. Graça, Dep of Life Scienes, University of Coimbra, Apartado 3046, Coimbra 3001-401, Portugal. E-mail: mgraca@ci.uc.pt Fax: þ 351 239855789 International Review of Hydrobiology 2015, 100,1–12 DOI 10.1002/iroh.201401757 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1