Submersed aquatic macrophytes and associated fauna as an effect of dam operation on a large lowland river

Abstract

An atypical development of submersed aquatic macrophytes (SAM, mainly Stuckenia and Potamogeton) in the large lowland alluvial Warta River (Poland) occurred annually as an effect of discharge changes caused by dam operation. The ecological responses of SAM with their associated flora and fauna to a deep water release through the Jeziorsko dam were investigated over a 20-year period in terms of their effects on the flora and fauna: firstly those associated with SAM and secondly organisms constituting mass downstream flush out from the reservoir, whose survival is enabled by the aquatic macrophytes’ habitat.In the tailwater of the Warta River, SAM and their associated organisms disappeared at the end of each summer as an effect of water level management in the reservoir. In addition, reconstruction of this diverse and rich biota began every year at the end of May, resulting in a similar abundance of the main biological groups, such as benthos, epiphytes and zooplankton. This phenomenon applies to all of the above-mentioned ecological groups, except for fish assemblages with domination of roach, perch and ruffe, which change in terms of abundance from season to season. Thus, SAM caused an increase in the structural complexity of the alluvial tailwater, and were not only the substrate for attachment for epiphytic chironomids (mainly Orthocladiinae, Cricotopus) but also a refuge for young fish and habitat for large sized pelophilous forms of Chironomini (Chironomus and Glyptotendipes). This last ecological group developed due to trapping organic matter by water plants.