Water residence time in the Wloclawek dam reservoir (the Vistula river, Poland) affects its macrozoobenthos structure

The aim of our study was to compare the macrozoobenthos structure as well as water and organic matter content of the bottom sediments of two hydrologically different zones of a strongly fluvial the Wloclawek Dam Reservoir. Samples were collected from the Wloclawek Dam Resevoir at six sites. Three of them were located in the upper, rheolimnic part (URP) of the reservoir and three others in its lower, limnetic part (LLP). Water transparency, sediment water content and organic matter content in the sediments were higher in the LLP. The higher number of taxa and diversity of macrozoobenthos were found in the URP of the reservoir. The density of bottom fauna was slightly higher in the LLP (90,990 ind. m(-2)) than in the UPR (73,486 ind. m(-2)), while the significantly higher biomass of macrozoobenthos was found in the URP of the reservoir (2314 g . m(-2)) than in the LLP (336 g . m(-2)). The dominant taxa of Oligochaeta and Chironomidae larvae did not show significant differences in the density between both zones. Some taxa were found only in the URP. In this zone, significantly higher densities of other taxa were observed. The only species with greater abundance in the LLP was Potamothrix hammoniensis (Oligochaeta). Due to the very short water residence time, the differences in the parameters under study between the URP and the LLP of the reservoir were smaller as well as the abundance of the macrozoobenthos was distinctly higher than in other reservoirs.