Larch needle breakdown in contrasting streams of an alpine glacial floodplain

Larch (Larex decidua Mill.) is an early successional tree species of glacia l sediments and valley side-slopes at treeline in the Swiss Alps. In such a reas, the needles from this deciduous conifer are a dominant source of part iculate organic matter to springs and streams of glacial floodplains. We ex amined the breakdown of larch needles in 5 stream types of a glacial floodp lain in the Swiss Alps in relation to macroinvertebrates, aquatic fungi, an d litter nutrient concentrations. The 5 streams ranged from a physically ha rsh pro-glacial (kryal) stream that lacked significant input of terrestrial ly derived particulate organic matter to a stable side-slope springbrook th at flowed through a mixed larch/pine forest with alder also present in the riparian zone. We hypothesized that needle breakdown las a surrogate of eco system function) would be significantly faster in the springbrook than in t he other streams, reflecting differences in macroinvertebrate and fungal as semblages present among streams. Although needle breakdown was generally sl ow, especially relative to alder leaves, parameters of litter breakdown wer e indeed higher in the springbrook than in the other streams. Concentration s of nutrients (N and P) found in decomposing needles were 2 to 4x higher i n the springbrook than in the other streams. Ergosterol levels (a measure o f fungal biomass) in needles were 4x greater in the springbrook than for ne edles in the kryal and main channel sites. Last, abundances of mac roinvert ebrates colonizing needle packs, shredders in particular, were higher in th e springbrook and a side-channel site than in the other stream types. The r esults emphasize the importance of trophic linkages in the successional dev elopment and function of lotic ecosystems.