Response of benthic macroinvertebrate communities to El Ni(n)over-tildeo related drought events in six upland streams in south-central Ontario

We examined the effects of extreme drought events on benthic macroinvertebrate ( BMI) community structure in six forested upland streams in south- central Ontario, Canada, during a 9- year period. Variation in the mean winter El Ni (n) over tildeo - Southern Oscillation Index was strongly correlated with drought conditions ( zero flow days) in the study streams. Drought onset and duration varied among study streams and among years. Below- average precipitation coincided with the occurrence of drought, although it remains unclear if snowfall and rainfall contributed equally to the impact of decreased precipitation. Increased relative abundance of Ephemeroptera, Plecoptera, and Trichoptera ( EPT) one year following drought and decreased relative abundance two years after drought indicated high resistance but poor resilience. In contrast, chironomids showed poor resistance and high resilience. Although these patterns were not consistent across all streams, temporal coherence among streams was found in percent EPT, percent chironomids, and percent dipterans, suggesting that drought acts as a disturbance mechanism that simplifies benthos community assemblages. Biocriteria developed from 22 nearby reference streams indicated that abnormal BMI communities occurred only after recurring episodes of drought, indicating that the effects of drought are cumulative. Headwater streams may prove to be sentinel ecosystems for monitoring the impacts of climate change.