Importance of Stream Microfungi in Controlling Breakdown Rates of Leaf Litter

Breakdown of seven leaf species covering a broad range of litter qualities (lignin: 7—31% of leaf dry mass; tannin: 0.0—6.7%; nitrogen: 0.5—2.6%; phosphorus: 0.017—0.094%) and dynamics of fungal biomass and reproductive activity were studied in a softwater mountain stream. Litter breakdown proceeded at exponential rates k ranging from 0.0042 d — 1 (evergreen oak) to 0.0515 d — 1 (ash). Fungal colonization of litter was generally rapid, with the fungus—specific indicator molecule ergosterol increasing from initially negligible concentrations to 375—859 mg/g of detrital mass. Using species—specific factors relating ergosterol concentrations to mycelial dry mass, maximum fungal biomass associated with litter was estimated as 61—155 mg/g of total system mass. Minimum estimates of net mycelial production during active growth varied between 0.3 and 3.8 mg°g — 1 °d — 1 , and maximum sporulation rates of aquatic hyphomycetes ranged from 760 to 7500 conidia°mg — 1 °d — 1 . Initially, reproductive activity was largely synchronized with increases in ergosterol concentrations, but it declined dramatically after peak sporulation rates were reached, whereas ergosterol concentrations levelled off or decreased at considerably slower rates. Periods of highest fungal productivity were thus limited to an initial breakdown stage of °2—8 wk. Strong correlations were found between the exponential breakdown coefficient and each of three parameters reflecting fungal activity in leaf litter, that is, maximum ergosterol concentration (P = 0.002, r = 0.96), net mycelial production (P = 0.02, r = 0.92), and sporulation rate (P < 0.001, r = 0.99). The initial lignin content of leaves was also significantly correlated with the rate constant k (P = 0.02, r = —0.83), suggesting that lignin was the primary factor determining the litter quality and thus breakdown rate. The correlation was even stronger when data were logarithmically transformed (P < 0.01, r = 0.95). Tannin concentration was significantly correlated with k only when two high—lignin species were excluded from the analysis (P = 0.19, r = 0.56 compared with P = 0.05, r = —0.88), while initial concentrations of phosphorus (P = 0.17, r = 0.58) and particularly nitrogen (P = 0.82, r = 0.06) were poor predictors of litter decomposability. These results suggest that the initial lignin content of leaves controlled litter breakdown rate through a kinetic limitation of carbon sources for saprotrophic microfungi. The decomposer activity of these organisms, in turn, would then have governed breakdown rates. In doing this, fungi produced substantial amounts of both mycelial and conidial biomass that was potentially available to higher trophic levels of the food web.