Leaf decomposition, an important component of the organic matter dynamics in streams, has been widely examined in temperate regions but much less documented in tropical regions. We report here the first study of leaf decomposition in a Sri Lankan stream. The litterbag technique was used. Coarse (8 mm) and fine (100 μm) litterbags, that included or excluded macroinvertebrates respectively, were used to enclose leaves of three dominant riparian tree species: native Ochlandra stridula (bamboo), and the introduced Alstonia macrophylla and Hevea brasiliensis (rubber). A fourth set of litterbags contained a mixture of these three species. Leaf colonization by macroinvertebrates was highest in the early stages of the decomposition process on Hevea leaves but invertebrate densities declined later. The opposite colonization effect was observed on the native Ochlandra leaves: slow colonization with continuing low densities from the beginning to the end of the process. Decomposition of all three species was significantly faster in the coarse than in the fine mesh bags. Alstonia, which has the softest leaf tissue, was most rapidly decomposed while Ochlandra, with its tough structure, was the slowest. Among the invertebrates, insects were the most important leaf colonizing animals, with Diptera, Coleoptera and Trichoptera the most dominant. The invertebrate variety in the mixed bags was higher than in the singlespecies leaf bags, where Chironomidae dominated the colonizing assemblages. This study has shown that toughness, indicated by the 'specific weight of leaf tissue', and the quality of the leaves was more important in determining breakdown rates than their origin.

Naturally occurring leaf packs in five sites along the elevation gradient in Eswathu Oya stream in Sri Lanka were sampled and analysed for species composition of leaves and benthic macroinvertebrates. Eswathu Oya is a perennial, low order, mountain stream in the wet climatic zone of Sri Lanka. Leaves of only five species of plants were found in all sites-Hevea brasiliensis, Ochlandra stridula, Alstonia macrophylla, Terminalia arjuna and Melia azedarach. In each sampling site Hevea brasiliensis and Ochlandra stridula leaves were found in higher abundance. The contribution of other leaves varied with the site. Diptera larvae of the family Chironomidae, were the dominant group of leaf colonizing animals. The relative abundance of Chironomidae decreased along the elevation gradient in the downstream. Gathering Collectors (GC) were the dominant functional feeding group (FFG) found in all leaf packs, but their abundance decreased along the downstream elevation gradient. Shredders were not present, as reported from some other tropical streams. Predators and Scrapers were increased along the downstream gradient. Filtering Collectors and Pierce-Herbivores did not show a clear pattern. The study documents that macro invertebrate communities responded to change in water quality, availability of leaves and their nutritional quality along the elevation gradient in Eswathu Oya, Sri Lanka.