Macroinvertebrate Colonization of Multiplate Samplers in the Ohio River: the Effect of Dams

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Abstract
We investigated the importance of contrasting current velocities on macroinvertebrate colonization in a large river, manipulating multiplate samplers placed upstream (slow current) and downstream (fast current) from a surface-release dam. Some samplers were colonized in solely fast-water or slow-water conditions while other colonized samplers were "switched" (with the original communities intact) to the opposite condition and subsequently collected either 1 d or 1 wk later. Experimental results confirmed observations from an earlier length-of-the-river study and showed that widely different communities become established on the samplers in the two contrasting flow conditions. An amphipod, Gammarus sp., colonized slow-water samplers in relatively large numbers while hydropsychid caddisflies dominated fast-water samplers. Gammarus sp. was found in only very small numbers in the fast-water samplers while the hydropsychids were almost completely absent from the slow-water samplers. Similar striking differences in current velocity preferences were exhibited by the congeneric chironomid species Polypedilum convictum (fast-water) and P. illinoense (slow-water). The transfer portion of the experiment demonstrated that a sudden reduction in current will cause large increases in invertebrate drift, regardless of whether the amount of living space changes.Key words: freshwater macroinvertebrates, colonization, current, invertebrate drift, impoundment effects, Ohio River