Leaf Breakdown in Streams of an Alpine Glacial Floodplain: Dynamics of Fungi and Nutrients

Breakdown, fungal colonization, and nutrient dynamics of green alder (Alnus viridis) leaves were examined in 4 stream types of a glacial alpine floodplain in southeast Switzerland. Standardized leaf packs were enclosed in both fine- and coarse-mesh bags to differentiate between microbial and other processes contributing to leaf mass loss. Fungal biomass was quantified from ergosterol concentrations. Apart from 1 site where shredder consumption of leaves greatly accelerated leaf mass loss, exponential breakdown coefficients (k) ranged from -0.0029 to -0.0064 d-1, indicating that leaf breakdown proceeded at a relatively slow but significant rate. Fungal biomass in decomposing leaves attained levels (>500 μg ergosterol/g litter dry mass) as high as those found in temperate woodland streams. Sporulation of aquatic hyphomycetes on leaves was detectable in all stream types (up to 700 conidia mg-1 litter dry mass d-1), although it was depressed at the 2 sites nearest the glacier (maximum of 200 conidia mg-1 d-1). Species composition and successional trajectories of aquatic hyphomycete assemblages during leaf breakdown showed broadly similar patterns as those reported for streams at lower elevations. Thus, where apparent, the divergence in patterns of leaf breakdown between glacial and temperate woodland streams was gradual rather than fundamental. This result suggests that streams above tree line are capable of processing leaf litter much like their forested counterparts at lower altitudes, and hence share basic functional attributes with streams in other biomes. Glacial alpine streams also provide suitable habitat for aquatic hyphomycetes, the abundance of these fungi probably being controlled primarily by the limited access to suitable resources rather than the physically harsh setting of the alpine environment.