IMPORTANCE OF STREAM MICROFUNGI IN CONTROLLING BREAKDOWN RATES OF LEAF-LITTER

Breakdown of seven leaf species covering a broad range of litter quali ties (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 rep roductive 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) lash). Fungal colonization of litter w as generally rapid, with the fungus-specific indicator molecule ergost erol increasing from initially negligible concentrations to 375-859 mu g/g of detrital mass. Using species-specific factors relating ergoste rol concentrations to mycelial dry mass, maximum fungal biomass associ ated with litter was estimated as 61-155 mg/g of total system mass. Mi nimum 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). Ini tially, reproductive activity was largely synchronized with increases in ergosterol concentrations, but it declined dramatically after peak sporulation rates were reached, whereas ergosterol concentrations leve lled off or decreased at considerably slower rates. Periods of highest fungal productivity were thus limited to an initial breakdown stage o f approximate to 2-8 wk. Strong correlations were found between the ex ponential breakdown coefficient and each of three parameters reflectin g fungal activity in leaf litter, that is, maximum ergosterol concentr ation (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 co ntent of leaves was also significantly correlated with the rate consta nt k (P = 0.02, r = -0.83), suggesting that lignin was the primary fac tor determining litter quality and thus breakdown rate. The correlatio n 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 nit rogen (P = 0.82, r = 0.06) were poor predictors of litter decomposabil ity. These results suggest that the initial lignin content of leaves c ontrolled 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 doin g this, fungi produced substantial amounts of both mycelial and conidi al biomass that was potentially available to higher trophic levels of the food web.