The aim of this study was to examine the differences between natural and morphologically modified stream bed sections in Hungarian midland streams by means of macroinvertebrate shredder abundance and leaf litter decomposition, and identify the key drivers of poor shredder fauna in modified water bodies. Eight sampling sites on three low order Hungarian watercourses were selected and studied between the springs of 2008 and 2009. Three types of leaf litter were collected and placed into leaf litter bags that were fixed to a metal wire net on the bottom of the river bed. Samples were taken in every four weeks. Leaf litter content and decomposition rate was determined and macroinvertebrates identified. Gammaridae dominated among shredders and their proportion did not differ significantly in bags. The shredders’ counts and the leaf litter decay rates were different in the experimental streams. The sampling sites located in modified stream sections, with limited availability of food, exhibited lower shredder densities but higher decomposition rates (k>0.01 d⁻¹) than those observed in sites of undisturbed bed morphology with rich allochtonous organic matter (k<0.0077 d⁻¹). However, no significant differences were obtained in decay rates among the leaf species. Our results suggest that shoreline vegetation and bed morphology are two equally important factors in determining functional properties of streams; and leaf litter decomposition rate is not only a function of shredder density. Thus decomposition rate can be an important functional variable for ecological status assessment of the water.
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