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Description: The paper assesses the impact of the Jeziorsko dam reservoir on chironomid assemblages and selected environmental factors in the Warta River, Poland, by means of patterns recognized with the self-organizing map (SOM, Kohonen unsupervised artificial neural network). Over 1988-1996, in four annual cycles, a total of 233 monthly samples were collected in a seven order section of the river at two sites: WAA (backwater) located about 2 km upstream from the Jeziorsko Reservoir, and WAB (tailwater) located about 1.5 km downstream from the reservoir's dam. At each site three habitats were selected: H"1, H"2 and H"3 at WAA, and H"1"1, H"1"2 and H"1"3 at WAB. H"1 and H"1"1 were located in the depositional area close to the banks, H"2 and H"1"2 about 6-7 m towards the mid-river and H"3 and H"1"3 in the mid-river. SOM effectively vertically separated H"1 and H"1"1 (bank habitats) from H"3 and H"1"3 (the mid-river zone of both sites) and H"2 (the transition zone of the upstream site). The H"1"2 samples were scattered all over SOM but still exhibited a slight temporal gradient. At the end of the study the water discharge, especially in summers, stabilized at WAB at a level lower than natural and as a result submerged macrophytes appeared at H"1"2 making the abundance of macroinvertebrates increase very quickly. Moreover, a weaker horizontal grouping of samples by season and by site of collection (upstream or downstream from the reservoir) was observed over SOM: 1) bank upstream habitat H"1, with hydrological regime resembling natural, was separated from the downstream H"1"1, which enlarged and contracted in response to dam operation, 2) deeper habitats were less dependent on water level and this is why they underwent seasonal fluctuations. To sum up, the deepest habitats were most resistant to water level fluctuations, while the formerly most productive habitat at the tailwater WAB site, H"1"1, became the most negatively impacted. Nevertheless, the reservoir has not negatively influenced chironomid density, because the latter increased closer the mid-river, at H"1"2, where large patches of macrophytes developed. .
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