Barriers to dispersal: The effect of a weir on stream insect drift

Abstract

Dams and weirs degrade river ecosystems, reducing diversity and altering the assemblage composition of aquatic biota. These structures may damage rivers by disrupting longitudinal connectivity, fragmenting rivers, and isolating populations. We tested whether a weir could impede the downstream dispersal of stream insects by comparing drift rates through natural pools and through a weir pool. For three of four of our study taxa (Offadens spp., Austrosimulium spp., and Simsonia spp.), we found the numbers of drifters were consistently reduced by the weir across multiple occasions (reduction ranging from 68 to 98%) to a higher degree than natural pools (reduction ranging from 24 to 41%). Drift o..