Microbial activity during leaf decomposition in an Alaskan subarctic stream

Fungal biomass and growth and microbial respiration were studied for two field seasons in a second‐order subarctic stream where water temperature is 0°C for approximately 6 months. Leaf packs (5‐g) of alder Alnus tenuifolia , birch Betula papyrifera and willows Salix alaxensis and Salix arbusculoides immersed in autumn of 1979 and 1980 were sampled until June 1980 and January 1981, respectively. Fungal growth and microbial respiration occurred in submerged detritus at 0°C. Total and FDA‐active hyphal lengths were measured, the active proportion averaging 25% of the total (all leaf species, both years). Generally, microbial respiration peaked in all leaf species after two weeks in the stream. As water approached 0°C, respiration declined by 20–50% depending on leaf species, but often increased later in decomposition (at 0°C). Seasonal trends in microbial respiration and FDA‐active hyphal lengths were not similar although maximal respiration usually occurred as FDA‐active hyphae were growing most rapidly. The calculated leaf weight loss due to microbial respiration was small (7–10%) in all leaf species, compared with total weight loss over 98 d. Scanning electron microscopy provided a visual record of leaf surface microorganisms and apparent leaf cuticle dissolution by fungi and bacteria.