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Riparian harvesting and resource limitation in small B.C. interior streams Melody, Katherine Jill

Abstract

Leaf litter inputs and shading of small streams are altered by forest harvesting. Many of the effects of riparian modification are known for streams in coastal British Columbia, but little is known of how streams in the drier, continental areas of BC respond to logging. The main objectives of this study were to determine how the harvest of the riparian forest affects stream food resources (leaf litter and algae) and macroinvertebrate composition in small, continental streams of BC. I conducted a paired stream-reach study of one harvested and two upstream reaches in each of five streams and measured periphyton, detritus, macroinvertebrate abundance and biomass and many physical measures in both July and October of 1997. In general, harvested stream sections tended to be wider and contained more riffle areas than the upstream forested sections. Leaf litter and algae were higher in some streams and lower in other streams in the harvested stream sections but significantly greater quantities of the food resources were found in the fall season in all streams. Greater macroinvertebrate density and biomass were found in the fall and the shredder and gatherer functional feeding groups were largely responsible for this season increase. The magnitude and direction of differences seen between sites, streams or seasons were stream dependent. To test the hypothesis that changes in light and litter inputs would affect the benthic community in these small streams, as occurred following riparian forest harvesting, I studied the impacts on periphyton and macroinvertebrate abundance and biomass in artificial stream channels. In this experiment I used a 2x2 factorial design with light (shaded or full light) and leaf litter inputs (forested input rate or one quarter that rate to represent the harvested input rate) as factors. A qualitative difference was found in the periphyton, as longer algal filaments occurred in the high light treatments, but there were no quantitative algal responses in chlorophyll a or ash-free dry mass, suggesting that light is not limiting in this system. The leaf litter treatments resulted in higher densities of two macroinvertebrate shredders, Limnephilus sp. and Podmosta sp., in the high leaf litter treatments. This suggests that these shredders may be food limited. This experiment provides evidence that changes to shading and leaf inputs to small streams affect the benthos and may limit secondary production.

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