Macrozoobenthos in some rapids in a lowland river in Finland before and after the construction of a hydroelectric power plant

Abstract

AbstractIn the beginning of the 1980s a hydroelectric power plant was constructed in the middle course of the lowland river Perhonjoki in western Finland. Some shallow lakes upstream of the power plant were enlarged for use as a reservoir and some rapids downstream were dredged and remodelled. As a consequence of the construction, the flow was almost completely diverted from a 2.5 km stretch of the river and the former rapids in this area were remodelled with weirs. The discharge from the power plant was regulated in the short term during low flow seasons in winter and summer. The effects of this project on the stream bottom fauna has been studied. The sampling started in 1980 before any major impact took place and has continued until 1992.In the low flow stretch of the river the lotic fauna of the former rapids has withdrawn to small rivulets and several species have disappeared, including the large caddisfly Arctopsyche ladogensis. In the remodelled rapids downstream from the power plant large numbers of black fly larvae have appeared every spring since the power plant was completed. The reason for this was probably dredging of the rapids, which wiped out the natural moss vegetation on the stones in the rapids and generated large areas of bare stone surfaces in places with suitable current velocities for the larvae. A lake outlet effect might also be part of the reason, even if samples of the phytoplankton could not demonstrate higher amounts of plankton downstream of the power plant reservoir than upstream. In late summer the number of filter‐feeding trichopterans has increased and the species composition has changed. Neurclipsis bimaculata and Ceratopsyche nevae have become more abundant and Arctopsyche ladogensis as well as several species commonly found among water moss have disappeared.