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Abstract
The benthic invertebrate fauna in River Aurland was investigated during 1966/67, before regulation, and during 1988/89, after regulation. The regulation strongly reduced the water flow in the inlet river to Lake Vassbygdvatn, whereas the reduction in the outlet river was lower, minimum flows of 30 m3 s-1 in summer and 3 m3 s-1 in winter. After regulation the temperature increased in the inlet river, while a small decrease was measured in the outlet river. A high increase in density of mayflies, stoneflies and chironomids was recorded, whereas the number of blackfly larvae was strongly reduced after regulation. Measurements of biomass showed small changes compared with the high increase in density. The reduction of large stonefly species and the almost extinction of blackfly larvae were the main reasons for the low response in biomass. Alternations in species dominance and changes in the life cycle of some mayflies and stoneflies were found.