EFFECTS OF HYDROPOWER-INDUCED FLOW PERTURBATIONS ON MAYFLY (EPHEMEROPTERA) RICHNESS AND ABUNDANCE IN NORTH SWEDISH RIVER RAPIDS

Mayfly species richness and abundance were investigated at 52 Swedish river rapids. These were either unregulated, or regulated with or with out reduced discharge. Sites impacted by regulation had lower mayfly r ichness and abundances than unregulated reference sites. The relative importance of reduced discharge, daily fluctuations in flow, flow cons tancy, and the distance to nearest rapid (as a measure of isolation) w as evaluated in multiple regression analyses. These indicated negative effects of diel fluctuations on both abundance and richness, whereas flow constancy was favourable for richness. Neither the distance to ne arest rapid nor reduced flow were significantly related to mayfly rich ness and abundance. In total, 26 mayfly species were recorded. Only Ba etis rhodani was found at all sites, but another five species were pre sent at more than 40 sites. The strongest effects were found for speci es within the Heptageniidae. Nineteen of 20 mayfly species present in both the regulated (with unreduced flow) and unregulated reference rap ids were on average more common in the unregulated ones. Mayfly assemb lage structure was primarily influenced by regional factors and nutrie nt status, although daily fluctuations in flow together with rapids di mensions also had a significant influence. No rare species appears to be threatened by hydropower regulation though it is conceivable that d epressed abundances in regulated rivers indirectly influence predators and periphyton.