The vertical distribution of macro‐invertebrates in the substratum of the upper reaches of the River Wye, Wales

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Abstract
SUMMARY. A study of the vertical distribution of benthic macro‐invertebrates in the upper Wye, Wales, using basket samplers, indicated no difference between the number of organisms collected, at one sampling time, after 28, 61 and 93 days. However, densities in samples collected after periods greater than 370 days were generally lower than might have been expected. Overall, 59.4%, 22.6% and 18.0% of the benthic invertebrates were recorded in the top (0–11 cm), middle (12–22 cm) and bottom (23–33 cm) levels of the basket samplers. Some organisms were generally confined to the top level (e.g. Ephemeroptera, Simuliidae, Mollusca), others were more evenly distributed (e.g. Oligochaeta, Chironomidae) and some increased with depth at certain times (Sericostoma personatum). Except for organisms confined to the upper layer it was not possible to predict with any certainty the vertical distribution of most organisms within the substratum. The community structure and abundance of invertebrates in the basket samplers differed from collections obtained with a conventional surface sampler.