LEAF BREAKDOWN AND ASSOCIATED MACROINVERTEBRATES IN ALPINE GLACIAL STREAMS

1. The relationship between macroinvertebrate assemblages and the brea kdown of alder [Alnus viridis (Chaix), Dc.] leaves was examined by exp osing leaf packs in four streams in an alpine glacial floodplain over 8 months. Although glacially fed, the four sites (pro-glacial, glacial lake outlet, main channel, and a side-channel with a mix of water sou rces) differed physically and contained different benthic communities. 2. Leaf breakdown and associated fungal properties differed widely am ong sites. Leaf decay rate varied by an order of magnitude (k ranged f rom 0.0029 to 0.0305 day(-1)), and was fastest at the lake outlet (< 2 0% leaf mass remaining by day 45) and slowest at the pro-glacial site (> 75% remaining on day 45). Rapid processing at the lake outlet was b ecause of the presence of Acrophylax zerberus Brauer, a shredding cadd isfly. 3. There were few macroinvertebrate taxa at the pro-glacial sit e (two to four taxa present in packs) and leaf breakdown was attribute d primarily to micro-organisms. Leuctra abundance in leaf packs was st rongly correlated with fungal biomass but not with the sporulation act ivity of any specific aquatic hyphomycete. Other taxa, such as Baetis and chironomids, showed no relationship with any leaf characteristic, suggesting that leaf packs were used mainly as a habitat and not as a food resource. 4. The predatory stonefly Isoperla was significantly as sociated with the abundance of macroinvertebrate prey (Baetis, Chirono midae and Leuctra) in leaf packs at the main and side-channel sites. T he results indicate that leaf breakdown can vary widely in alpine loti c environments, reflecting site-specific differences in habitat charac teristics, and in macroinvertebrate and fungal composition.