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-mu-g/g detrital dry mass. Ergosterol contents of aquatic Hyphomycetes grown in shake culture were typically less-than-or-equal-to 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-1 g-1 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.