J. R. WEBSTER 1 , E. F. BENFIELD 1 , J. J. HUTCHENS 2 , J. L. TANK 3 , S. W. GOLLADAY 4 , and J. C. ADAMS 5 1 Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, U.S.A. 2 U.S. Environmental Agency, 26W Martin Luther King Dr., Cincinnati, OH 45268, U.S.A. 3 Department of Biological Sciences, University of Notre Dame, Notre Dame, IN 46556, U.S.A. 4 J. W. Jones Ecological Research Center, Route 2, Box 2324, Newton, Georgia 31770, U.S.A. 5 PO Box 2, DAW Park, SA 5041, Australia Do Leaf Breakdown Rates Actually Measure Leaf Disappearance from Streams? key words: stream, leaves, breakdown, leaffall, decomposition Abstract We measured leaf input, leaf breakdown, and benthic leaf standing stock in Hugh White Creek, a second-order, Appalachian Mountain stream in North Carolina, U. S. A. Leaf input and leaf breakdown data were used in a computer model to predict standing stocks. Predicted standing stocks were then compared with measured values. Once the model was modified to include leaves in four breakdown rate categories, leaf blow-in, and temperature effects on leaf breakdown, agreement between model predic- tion and measurement was quite good. 1. Introduction The earliest measurements of leaf breakdown rates in streams (e.g., MATHEWS and KOWALCZEWSKI, 1969; WITKAMP and FRANK, 1969; THOMAS, 1970; KAUSHIK and HYNES, 1971) followed techniques developed in terrestrial studies (e.g., FALCONER et al., 1933; LUNT, 1935; JENNY et al., 1949; SHANKS and OLSON, 1961). With various modifications such as using leaf packs rather than bags (PETERSON and CUMMINS, 1974), measurement of leaf breakdown rates has become a valuable and extensively used tool of stream ecologists. Breakdown rates can be used to provide three types of information. First, they can be used to study mechanisms of leaf breakdown. Second, breakdown rates provide a metric to com- pare streams or to evaluate a stream response to some modified conditions (e.g., BENFIELD et al. this issue). Third, breakdown rates can be used to estimate a component of the organic matter dynamics of streams, i.e., how fast natural leaves in the stream break down. This rate cannot be directly measured by quantifying the change in standing stock of leaves in the stream through time because leaf inputs are continuous. So mass loss from known quanti- ties of leaves that are somehow confined, tied together, or otherwise identifiable is used as a surrogate for natural leaves in the stream. Our objective in this paper is to evaluate the adequacy of leaf breakdown rate as a measure of what is occurring naturally in the stream. We have done this by measuring inputs and breakdown of leaves and using them in a com- puter simulation model. Results of this simulation are then compared with measurements of leaf standing stock in the stream. Internat. Rev. Hydrobiol. 86 2001 4–5 417– 427 III. Methods in Litter Breakdown © WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 1434-2944/01/4-507-0417 $ 17.50+.50/0