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The effects of in-stream structure placements on a macroinvertebrate community : testing of reach-scale responses and mechanisms with regard to flow refugia Negishi, Junjiro N.

Abstract

The effects of placements of in-stream structures on a macroinvertebrate community were examined in Spring Creek, a second-order stream of British Columbia, Canada. I tested the responses of organic matter and macroinvertebrates to structural changes at two spatial scales: reach-scale responses to the placements of boulder clusters; and the mechanisms through which in-stream structure affects macroinvertebrates as flow refugia at a microhabitat scale. The reach-scale study was conducted in three sections (length 40-56 m) within a 300-m reach with at least 20 m buffers between them. Two upstream reference sections were in relatively natural condition with channel meanders and abundant woody debris whereas the downstream treatment section had a relatively straight stream channel with less woody debris. In the treatment section, 6 boulder clusters were installed to manipulate habitat heterogeneity. These three sections were studied for three months prior to and 1.2 year following the placements of boulder clusters. Mean (140 %) and coefficient of variation (115 %) of velocity increased in the treatment section whereas the reference section remained relatively unchanged after the treatment. Through the increased organic matter storage (550 %), total macroinvertebrate abundance, which was dominated by detritus feeders, increased (280 %) in the treatment section to become similar in level to that in the reference sections almost one year after that treatment. However, the effect of placements of boulder clusters on taxonomic richness was negligible. Effects of food resource values on macroinvertebrate colonization of flow refugia during floods were examined in the downstream 180-m reach during three floods in 1999 and 2000. Substrate cages were used as an experimental unit, and were assigned to combinations of three types of treatments: food (natural or artificial leaves); flow refugia (reduced velocity or exposed); and flood (retrieval before or after a flood). Low and stable antecedent flow conditions resulted in significant responses of animals to the flood whereas high and variable antecedent flow resulted in little response. During the flood with the highest peak discharge observed among the three trials, two detritivorous taxa Paraleptophlebia and Despaxia accumulated in refugia and their colonization was disproportionally higher in the cage provided with high food resource value (natural leaves). Nevertheless, the accumulation of macroinvertebrates in flow refugia was species-specific, and was not consistent across the community. It was concluded that instream structures (boulder clusters) could be used as a remedy to restore macroinvertebrate community productivity.

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