Manipulating the intensity of near-bed turbulence in rivers: effects on benthic invertebrates

1. Flow conditions were modified over patches of river bed in three rivers in southwestern Australia to determine the effects of turbulence on benthic invertebrate communities. 2. Artificial structures to increase downstream turbulence were developed in a laboratory flume. In the field, these increased turbulence intensity by 35% for a 20% reduction in velocity. 3. Patches of gravel were placed in each river and turbulence-generating structures allocated randomly to half of these, creating treatment patches. An acoustic Doppler velocimeter was used to measure flow conditions over both treatment and control patches at several heights above the bed. After 6 weeks, the invertebrate fauna of the gravel patches were sampled to examine the response to modified flow conditions. 4. The treatments increased relative turbulence intensity twofold for a reduction in velocity of between 3 and 5 cm s-1, but turbulence intensity was significantly higher in only one of the three rivers. 5. There were no significant effects of increased relative turbulence intensity on any aspect of the invertebrate assemblage. This may be a result of the fairly small increase in relative turbulence intensity created during the experiment, the spatial scale of the manipulation or the types of stream community studied.