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Using a forested headwater stream system as a model, the effects of inter-annual variation in summer discharge regimes on aquatic insect communities were investigated. More specifically, the benthic invertebrate community response to the intensity, minimum discharges, frequency, duration and abruptness of summer low-flow events were examined. We hypothesized that intensification of summer low-flow events, both in duration and magnitude, have some negative impacts on benthic macroinvertebrate communities in riffles. Examples of negative impacts include reduction in their abundance and/or biodiversity. First, the abundance and functional trait data of the benthic macroinvertebrates in the three streams in the Malcolm Knapp Research Forest, British Columbia, Canada, were analyzed with respect to the low-flow events. Second, population models were built to simulate the potential responses of lotic aquatic insect communities to future climate change scenarios that differ in the rate of intensifications in extreme flow events: a low-flow event scenario within the current range versus 10% increase in intensity. The summer low-flow events were found to have a significant relationship with benthic macroinvertebrate communities through three-table ordinations of the empirical data. The community structure was correlated with a major ocean-atmosphere regime shift (Pacific Decadal Oscillation). The intensity and duration of low-flow events explained the observed shift in community structure favouring r-selected traits (e.g. short life cycle, high reproduction rate). The two low-flow severity scenarios showed the significant differential impacts on the aquatic insect community structures when individual populations were modeled according to their traits. Aquatic insects could be separated into three groups according to their sensitivities, measured by extinction rates, toward the two scenarios.
Headwater streams have some of the best quality water in the country (Dissmeyer 2000). Because headwaters eventually flow into other downstream water bodies like rivers and lakes, they are important places to focus research and conservation efforts. All streams in this study had good water quality based on several assessments of macroinvertebrate communities. 1. Judging from metrics of abundance, richness, EPT, and HBI, all study streams had very good water quality. Thirty-nine total families were observed among the three streams. Whittier Stream was the least healthy of the three, most likely as a result of human disturbances such as a road crossing, fishpond, and mill remnants. It is probable these habitat alterations are having an effect on downstream benthic communities. 2. The riparian zone of each stream was forested and stable, but there were some eroding banks at some points along the study reaches. The abundance of sensitive macroinvertebrates (measured using representative Orders Ephemeroptera, Plecoptera, and Trichoptera) increased with increasing percent canopy cover and decreased with high percentages of sand characterizing the streambed. 3. Macroinvertebrates can be categorized into functional feeding groups based on feeding behavior. Shredders should hypothetically be in high abundance in headwater streams due to high riparian organic matter inputs. However, observed shredders were limited. This may be due to seasonal changes in functional feeding group ratios. In addition, all study streams had a predator/prey ratio that was higher than the ratio expected for a typical stream, which suggests that there might be an abundance of prey biomass in the study stream headwaters. 4. Based on results from stable isotope analysis of carbon and nitrogen, most macroinvertebrates within the study streams seem to be omnivorous. The most complex food web was observed at Stony Brook. Food webs of Beaver Brook and Stony Brook showed similar trophic clustering, a pattern that might be indicative of streams with excellent water quality.