Ecological Responses of Hydropsychid Caddisflies to Stream Regulation

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Abstract
Based on the river continuum theory and preliminary investigation in the upper Flathead River (British Columbia and Montana), we expected a sequential change in the hydropsychid fauna from large Arctopsychinae species in low-order streams to smaller Hydropsychinae in the higher-order rivers. Parapsyche elsis was found in small 1st- and 2nd-order streams and was replaced by Arctopsyche grandis as the dominant net-spinning caddisfly in 3rd- and 4th-order creeks. Three Hydropsychinae species first appeared in 4th-order creeks and became dominant in the 5th-order river, although A. grandis was still present. Site-specific degree-days (annual) increased progressively down the longitudinal gradient from 1200 degree-days in low-order, high-mountain streams to 2400 degree-days in the 5th-order river. However, hypolimnion tail waters from the regulated South Fork altered the thermal regime in the 6th-order mainstream from an expected 2600 annual degree-days to 2050 degree-days. The diurnally fluctuating discharge pattern from the hydroelectric dam also resulted in sloughing of attached algae into the river seston. The decrease in annual degree-days and increase in sestonic particulate organic matter produced conditions similar to those in 4th-order creeks. Hydropsychids responded to the effects of stream regulation by reflecting population structure found in lower-order creek reaches. Arctopsyche grandis was the dominant collector-filter feeder in the regulated mainstream river segment and Hydropsychinae abundance was significantly lower than in unregulated, 5th-order river segments. We interpret the alteration of environmental parameters and corresponding faunal response as a reset of the river continuum caused by stream regulation.Key words: Hydropsychidae, Trichoptera, river, ecosystem, stream regulation, Flathead River, stream continuum, zoobenthos, temperature, particulate organic matter

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