Canyon-shaped reservoirs are often characterised by longitudinal gradients of environmental factors (including trophic level, phytoplankton and zooplankton biomass and abundance of planktivorous fish) affecting the taxonomic composition of the pelagic community. We tested the hypothesis that the spatial distribution of different species and interspecific hybrids of the Daphnia longispina species complex is non-random under such conditions. During the summer stratification, we sampled crustacean

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... zooplankton from 11 reservoirs, covering both longitudinal (upstream, middle, dam) and vertical (epi-, meta-and hypolimnion) environmental gradients. Allozyme electrophoresis was used to discriminate among different Daphnia taxa. All three frequently hybridizing European species of the complex (galeata, cucullata, longispina ¼ hyalina) and hybrids with Daphnia galeata were commonly recorded. Smaller-bodied Daphnia cucullata and its hybrids, when present, preferred mostly the nutrient-and food-rich upstream regions; D. longispina and its hybrids were more commonly found in the downstream part, and often dominated in the meta-or hypolimnion. Redundancy analyses confirmed significant differences in the Daphnia taxon composition, as well as in spatial distribution of other crustacean species, along both gradients. For the first time, we demonstrate regular patterns in the horizontal distribution of Daphnia species and hybrids within a water body, thus accepting our hypothesis. Such spatial distributional patterns may strongly impact local hybridization processes.