Effects of beaver dams on the zooplankton assemblages in four temperate lowland streams (NW Poland)

Beaver ponds favour lentic species over the original lotic animals. The typical lentic animals are zooplankton, which can be good predictors of hydrological changes in streams caused by beaver activity. We analysed the effects of beaver dams on the zooplankton communities in small lowland streams in stream-beaver pond-stream systems. All spatial changes in the composition of zooplankton reflected the effects of physical changes introduced by the beaver dams. The rapid increase in the density of the aforementioned taxa was possible because the features typical of stagnant water reservoirs are present in beaver ponds. The number of taxa and the abundance and biodiversity of zooplankton, especially planktonic rotifers, were higher in the dam and downstream sites than in the upstream sites. Therefore, the impact of beaver ponds on the composition of zooplankton in streams below the dams is considerable, and there is an increase in the organic matter downstream, which leads to trophic changes in the stream.