Calibrating the data logger…
During summers 1984, 1985, and 1988 we tested the effects of stainless steel mesh enclosures on the microhabitat and composition of the invertebrate community (community structure) at a third-order site on the East River, Gunnison County, Colorado. Comparisons of substrate composition, interstitial flow rates, periphyton on stones, coarse detritus accumulation, and benthic invertebrate community structure were made between enclosed and unenclosed habitats, and among enclosures remaining in the stream for different lengths of time. Surface flow rates were not a good predictor of interstitial flow through substrates inside or outside enclosures, and interstitial flow rates inside enclosures were significantly lower than those of the adjacent unenclosed habitat. Due to reduced flow rates, enclosures were detritus sinks, accumulating significantly more coarse detritus than in surrounding substrates. This difference became more pronounced as enclosures were exposed for longer periods. Similarly, although there were no significant differences in particle size distributions of coarse substrates inside versus outside enclosures, enclosures accumulated fine sediments, especially following storms. Densities of diatoms, blue-green, golden-brown, and green algae were similar on glass slides after exposure inside and outside enclosures for six days. Further, chlorophyll a values were the same on stones exposed inside or outside enclosures over 3, 6, and 9 days. Despite apparently unchanged chlorophyll a levels, macroinvertebrate community structure in enclosures was markedly changed from that of the surrounding benthos after seven days. Communities initially dominated by grazers became dominated by shredders after two weeks, probably due to accumulation of detritus in enclosures. A concurrent decline in grazers suggests that the periphyton resource in enclosures became either less suitable or less accessible for grazing mayflies; the mechanism of this effect remains to be tested. These data support the recommendation that experiments testing for invertebrate community-level effects using these or similar enclosures in open-canopied mountain streams be implemented over the short term. Longer-term experiments may be possible in streams where changes in benthic communities occur more slowly, and time before enclosure artifacts occur is longer.
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