Influence of water level fluctuation below the dam on the structure of the macroinvertebrate community

The study was aimed at investigating the influence of a reservoir on lotic macroinvertebrates by comparing the upstream conditions with two sites at different distances downstream of the dam. Hydropeaking flow regime causes diurnal water level fluctuations resulting in dewatering of part of the stream bed and long-term variability of river flow below the Bmo Reservoir. Compared with the stretch above the reservoir, the number of taxa in the downstream stretch, and especially close to the dam release was significantly reduced. A decrease of density in many samples is caused by the extinction of most insect taxa except chironomids and ceratopogonids. Less significant changes of the natural temperature regime downstream from the Bmo Reservoir are connected with maximum depth of the reservoir reaching only 19 m. This fact, as well as the amelioration of peak-discharge waves by their passing through slackwater pools, is reflected in the structure of the zoobenthos community 8 km downstream from the dam. With drought exposure impacts limited to narrow marginal zones, the higher yearly average temperature and rich food base allow the existence of a zoobenthos community rich in species, and with densities very often higher than at the reference site. Regarding the species composition, there is a decrease of rhithral species and especially those (e.g. stoneflies and heptageniid mayflies) demanding high oxygen saturation, temperature not above 20 degrees C and high streamflow.