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Abstract

Larch (Larex decidua Mill.) is an early successional tree species of glacial sediments and valley side-slopes at treeline in the Swiss Alps. In such areas, the needles from this deciduous conifer are a dominant source of particulate organic matter to springs and streams of glacial floodplains. We examined the breakdown of larch needles in 5 stream types of a glacial floodplain in the Swiss Alps in relation to macroinvertebrates, aquatic fungi, and litter nutrient concentrations. The 5 streams ranged from a physically harsh pro-glacial (kryal) stream that lacked significant input of terrestrially derived particulate organic matter to a stable side-slope springbrook that flowed through a mixed larch/pine forest with alder also present in the riparian zone. We hypothesized that needle breakdown (as a surrogate of ecosystem function) would be significantly faster in the springbrook than in the other streams, reflecting differences in macroinvertebrate and fungal assemblages present among streams. Although needle breakdown was generally slow, especially relative to alder leaves, parameters of litter breakdown were indeed higher in the springbrook than in the other streams. Concentrations of nutrients (N and P) found in decomposing needles were 2 to 4 x higher in the springbrook than in the other streams. Ergosterol levels (a measure of fungal biomass) in needles were 4 x greater in the springbrook than for needles in the kryal and main channel sites. Last, abundances of macroinvertebrates colonizing needle packs, shredders in particular, were higher in the springbrook and a side-channel site than in the other stream types. The results emphasize the importance of trophic linkages in the successional development and function of lotic ecosystems.