Fungal biomass associated with decaying leaf litter in a stream.

Fungal biomass, measured as ergosterol content, was determined on alder leaf litter incubated during autumn in a softwater Pyrenean stream. The ergosterol content of the leaf litter increased rapidly to a maximum of 462 μg/g detrital dry mass. Ergosterol contents of aquatic Hyphomycetes grown in shake culture were typically ≤5 mg/g mycelial dry mass. Using the corresponding ergosterol-to-biomass conversion factor of 200, peak fungal mass accounted for 9.2% of total system mass, or 10.2% of leaf dry mass. For the period of highest activity (incubation days 7-28), net fungal production on leaf litter was estimated as 2.3 mg d g leaf mass. A conservative estimate of the growth efficiency for the same period was 105 mg mycelial mass per gram leaf mass degraded, assuming that non-leaf organic matter did not constitute an important carbon source supporting fungal production.