Impact of the dam reservoir on river macrobenthic community : long-term study of Jeziorsko Reservoir and the Warta River in central Poland

The riverine faunal responses was shown in relation to environmental changes arising from flow regulations in the Warta River downstream of the man-made Jeziorsko Reservoir. Two study sites were chosen for studies: one above the backwater (WAA) and the second in the tailwater (WAB) of the reservoir. The investigations were begun in 1988/1989 and repeated in the next three annual cycles: 1991/1992, 1992/1993 and 1995/1996. Chironomidae and Oligochaeta dominated in terms of density and biomass at both sites of the Warta River. At the beginning of the dam's functioning the pattern of macrobenthic spatial distribution was similar at both sites. But latter, in two last study summers water management resulted in a reduction of stream flows below their natural levels in the tailwater. These reductions in seasonal flow variability caused the greatest changes in biotic communities. Firstly the shoreline region in the tailwater enlarged and contracted in response to dam operations; the bottom was sometimes stranded (exposure to air) thus many macrobenthic groups were eliminated. Secondly Cladophora glomerata and macrophytes that developed in habitat several meters from the banks appeared to be responsible for substantial increase in abundance, especially of orthoclad midges. Additionally the retention of FPOM due to filtration by a dense bed of macrophytes created favourable conditions for pelophilus taxa, large sized taxa of Chironomini. Finally these mechanisms in opposite tendencies equalled - chironomids reached similar density in the backwater and tailwater.