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This study evaluated for one year the effects of three small dams located in low order sections of the Bzura, Mrożyca and Mroga Streams (central Poland) on their benthic communities (Chironomidae). Six sampling sites were established in these streams (one above and one below each dam reservoir) at a distance of several dozen km from one another, all these lotic ecosystems being a part of the Vistula drainage basin. The composition of benthic fauna at the upstream and downstream sites of each river was compared using multivariate techniques (CLUSTER and NDS). The downstream sites of these streams display different seasonal hydrological regimes due to management of the reservoirs and the amount of forested versus agricultural land use in their catchments. The highest macrobenthic density was observed at the upstream site in the Bzura Stream (over 25 000 ind.m-2) while at the other sites macrobenthos was less numerous. At each stream benthic macroinvertebrates were dominated by Oligochaeta and Chironomidae (midges constituted from over 30% to over 90% of the total macrobenthic density). In the upstream reaches due to small fluctuations of abiotic parameters (discharge) large populations of a small number of chironomid species (mainly Chironomini) dominated in the benthos, while in the downstream reaches a moderate disturbance enabled a much higher number of species to develop and coexist, but at a lower level of density than at the upstream sites (the Bzura and Mrożyca Streams). In turn when the size of inorganic substrate particles was larger (gravel) at the downstream site not only a higher number of species but also their higher density than in upstream site (the Mroga Stream) might be observed.
Published online: 17 October 2012.
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