The natural hydrology of many streams is being severely altered through dam construction and the diversion of water for consumptive use.  Hydrological disturbance is a dominant process structuring streams and extreme events (droughts and floods) have been shown to strongly influence macroinvertebrate and fish communities.   Escalating human water demand and climate change will likely lead to an increased frequency of low flow disturbance and the consequences of this for stream communities remains highly uncertain.  We explored the ecological impacts of low flow disturbance on aquatic insect and fish communities by experimentally manipulating stream flow over a 100m reach in three streams for three summers.

Results/Conclusions

Our water diversion reduced August mean flow by 40-80%.  The impact of lowered flow on aquatic insect communities was habitat dependent.  In riffle habitats, there was no effect on aquatic insect family richness, but family diversity increased due to the decreased density of dominant families.  In pool habitats, there was no effect of low flow on insect family richness, diversity, or dominance.  The susceptibility of aquatic insects varied among families and trait groups; some showed large shifts in density and size structure, but others showed no response.  Highly susceptible organisms included those with limited mobility, a strong preference for riffle habitats, and a collector-filterer feeding mode.  Fish communities showed increased densities and decreased size with reduced flow.