© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1434-2944/04/5–612– 0508 TADEUSZ FLEITUCH 1 * and MARIA LEICHTFRIED 2 1 Institute of Nature Conservation, Polish Academy of Sciences, Pl 31–016 Cracow, Al. Mickiewicza 33, Poland; e-mail: fleituch@iop.krakow.pl 2 Biological Station Lunz, Institute of Limnology, Austrian Academy of Sciences, A-3293 Lunz/See, Seehof 4, Austria; e-mail: maria.leichtfried@oeaw.ac.at Ash Tree Leaf Litter (Fraxinus excelsior L.) Breakdown in Two Different Biotopes and Streams key words: stream, leaf breakdown, biotopes, ash tree, chemical composition Abstract Coarse (0.5 mm) and fine (0.1 mm) mesh size bag methodology was used for comparing the breakdown of ash tree leaves (Fraxinus excelsior L.) in two biotopes (dry – terrestrial and wet – overflown stream zones) in two low order streams (the Oberer Seebach (OSB), Lower Austria and the Brzezowka stream (BRZ), Beskidy Mountains, southern Poland). In total, 96 bags were exposed in autumn 2000. Ash-free dry mass (AFDM) ranged in dry zones of both streams from 94–62% (OSB) and 85–53% (BRZ) respectively. The decomposition process was faster in wet zones: 96–33% (OSB) and 56–11% (B) during the study period. Significant differences in ash breakdown and its chemical content between stud- ied streams were found. Total organic carbon (TOC) and total nitrogen content (TN) of AFDM of litter showed increased differences with experiment duration between zones and between two bag types for both streams. The strongest increase of TOC and TN content (100% on average vs. initial content) for bag types, zones, and streams was observed in the first two weeks of the experiment. These results confirm the im- portance of chemical compounds for microbiological processes (biofilms) in different ecosystem biotopes. 1. Introduction Organic matter is an essential resource in most aquatic and terrestrial ecosystems (ODUM and DE LA CRUZ, 1963). Low order streams are typically heterotrophic; i.e. more energy comes from allochthonous sources than from autochthonous production (FISHER and LIKENS, 1973; MINSHALL et al., 1983). Leaf breakdown, an important component of organic matter dynamics in lotic ecosystems (CUMMINS, 1974; GESSNER et al., 1999) has been widely exam- ined in streams (see reviews by ANDERSON et al., 1979 and BOULTON and BOON, 1991). Riparian zones provide an ideal setting for the identification of factors that have a sig- nificant effect on organic matter breakdown because they contain a series of relatively dis- tinct zones that differ in abiotic and biotic factors across a relatively short distance (HUTCHENS and WALLACE, 2002). Most studies of leaf breakdown in riparian zones that have incorporated biotopes (i.e. stream and terrestrial habitats) have been done in parts of stream drainages that have relatively large, active floodplains (MERRITT and LAWSON, 1979, 1992). However, it is unclear whether riparian zones of mountain streams with little or no flood- plain development have such a wide range in leaf breakdown dynamics. Differences in leaf processing rates among ecosystems and their biotopes have been attrib- uted mainly to physical factors such as temperature (see references in IRONS et al. 1994). Other important factors to consider are tree species and their nutrition quality (CUMMINS, Internat. Rev. Hydrobiol. 89 2004 5–6 508–518 DOI: 10.1002/iroh.200410770 * Corresponding author