The effects of nutrient enrichment on the responses of alpine stream macroinvertebrates to disturbance were studied using 2 field experiments. In experiment 1, individual stones were physically disturbed in 3 different streams at intervals of 0 (i.e., no disturbance), 4, 8, and 16 d over a 32-d period. In experiment 2, nutrients were added to 1 of 2 channels in the same stream, and the same disturbance regime as in experiment 1 was applied in both channels. The stones were collected at the end of each experiment. Periphyton chlorophyll a and ash-free dry mass (experiment 2 only), macroinvertebrate diversity and density, and densities and relative abundances of baetid mayflies, nemourid stoneflies, and chironomid and simuliid dipterans were determined for each stone. In addition, C, N, and P contents of periphyton, benthic organic matter, grazing mayflies, and detritivorous stoneflies and dipterans were determined for each channel in experiment 2. In experiment 1, disturbance frequency affected the relative abundances of baetids, nemourids, chironomids, and simuliids, but no other variable was significantly affected. In experiment 2, disturbance frequency effects in the unfertilized channel were similar to the effects observed in experiment 1, whereas no significant disturbance effects were observed in the fertilized channel. Nutrient enrichment caused a shift in community structure in the fertilized channel, and chironomids constituted >85% of the assemblage. Periphyton, grazing mayflies, and detritivorous chironomids had significantly higher P concentrations and lower molar N:P ratios in the fertilized than in the unfertilized channel. Periphyton N content also was higher in the fertilized channel than in the unfertilized channel. Nutrient enrichment appeared to change the disturbance responses of stream macroinvertebrates by changing the community structure of primary consumers.
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