Breakdown, fungal colonization, and nutrient dynamics of green alder (Alnus
viridis) leaves were examined in 4 stream types of a glacial alpine floodp
lain in southeast Switzerland. Standardized leaf packs were enclosed in bot
h fine- and coarse-mesh bags to differentiate between microbial and other p
rocesses 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 coefficie
nts (k) ranged from -0.0029 to -0.0064 d(-1), indicating that leaf breakdow
n proceeded at a relatively slow but significant rate. Fungal biomass in de
composing leaves attained levels (>500 mu g ergosterol/g litter dry mass) a
s high as those found in temperate woodland streams. Sporulation of aquatic
hyphomycetes on leaves was detectable in all stream types (up to 700 conid
ia 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 composit
ion and successional trajectories of aquatic hyphomycete assemblages during
leaf breakdown showed broadly similar patterns as those reported for strea
ms 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 strea
ms in other biomes. Glacial alpine streams also provide suitable habitat fo
r aquatic hyphomycetes, the abundance of these fungi probably being control
led primarily by the limited access to suitable resources rather than the p
hysically harsh setting of the alpine environment.