Leaf breakdown and the ecosystem functioning of alpine streams

Leaf breakdown, a key functional process, was examined in 3 glacial streams, 3 alpine springbrooks, and a rock-glacier stream in the Swiss Alps. Environmental conditions within each stream type were expected to significantly affect leaf breakdown rates among streams. Individual leaf packs (7 g fresh mass) of alder (Alnus viridis) were immersed in each stream and replicate samples were collected at periodic intervals over 49 d. Collected leaves were assessed for N and P, ergosterol (a measure of fungal biomass), % lignin (an indicator of recalcitrance), and aquatic macroinvertebrates. Leaf breakdown was faster in the springbrooks (k = -0.0085) and rock-glacier stream (k = -0.0073) than in the glacial streams (k = -0.0027) when expressed in terms of time. However, leaf breakdown rates based on degree days were similar among streams, ranging from k -0.0011 to -0.0020. Leaf P levels increased over time in all sites, and reflected respective increases in ergosterol. Leaf ergosterol levels increased from 62 mu g/g to similar to 300-500 mu g/g leaf dry mass during the study, and were highest in the springbrooks and rock-glacier stream. The % lignin of leaves displayed a similar pattern as breakdown rates, reaching 20 (glacial streams) to 30% (springbrooks) leaf dry mass by day 49. Chironomids dominated leaf packs in glacial streams, whereas leaf packs in springbrooks were additionally colonized by Ephemeroptera, Plecoptera, and Trichoptera. We conclude that differences in environmental conditions (interaction between abiotic and biotic properties) strongly influence ecosystem functioning, as inferred from leaf breakdown characteristics, among alpine streams.