Macroinvertebrate drift along an elevational and stream size gradient in a southern Appalachian stream

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Virginia Tech


Drift was measured monthly at four sites on a southern Appalachian stream in order to examine spatial and temporal patterns along elevational and stream size gradients. Measurements consisted of four, one-hour samples corresponding to dawn, mid-day, dusk, and mid-night monthly for 14 months. On average, only 10% of the drifting insects were terrestrial, but terrestrial inputs became numerically important during summer and fall. There were no significant differences among sites in aquatic insect drift density, although taxa richness and total drift (#/sec) increased significantly downstream. None of the sites exhibited a consistent diel pattern. But, all of the lower three sites exhibited decreased drift density during the day. Baetis, the dominant aquatic taxon in the drift, exhibited a typical alterans patterns at UBC, LBC, and CC. Aquatic insect drift density was highest in summer when flows were lowest, while total drift (#/s) was highest in late spring and winter. FPOM and mean daily water temperature accounted for 64% of the variation in total aquatic insect drift density and 60% of Baetis drift density, suggesting both passive and active components in drift. Both variables were also highly correlated with drift density. Taxonomic composition of drift differed from that of the benthos, indicating drift was not simply random, but that taxa vary in the propensity to drift. Overall, this study found more evidence of temporal (diel and seasonal) drift patterns than spatial (along the gradient).