Growth and production of aquatic hyphomycetes in decomposing leaf litter

The acetate-to-ergosterol technique was used to estimate fungal productivity of three species of aquatic hyphomycetes growing in decomposing ash leaves in stream microcosms. Following a lag of 20-88 min, incorporation of acetate into ergosterol was linear for at least 10 h. Substrate saturation was reached in the mM range, and there was no indication of isotope dilution. For one species, Articulospora tetracladia, a conversion factor of 5.5 mg mycelial dry mass produced per pmol acetate incorporated was deter-mined. This was similar to the theoretical conversion factor (6.6 mg p,mol-I) deduced from pathways OF ergosterol synthesis in fungi. Thus, the acetate-toergosterol assay appears to be suitable for estimating the productivity of aquatic hyphomycetes growing in leaf litter in streams. Estimated growth rates of A. tetracladia in microcosms changed markedly over time, with the maximum being as high as 0.72 d-l at an early growth stage. After 23 d when 58% of the initial leaf mass was degraded, the fungus had produced 89 mg biomass per g of initial leaf mass. Almost half of this production was allocated to conidia. Assuming an average growth efficiency of 0.35, this would be equivalent to a fungal assimilation of 25% of initial leaf mass and account for 44% of the observed leaf mass loss. In an experiment with leaf litter colonized by fungi in a stream, acetate incorporation was linear for 6 h, but the estimated growth rate was only 0.017 d-l. A critical step in ecosystem analysis is the assessment of biomass production of the major species or functional groups. In addition, as Benke (1993, p. 15) stated in his review on animal secondary productivity in running waters: “Production is the most comprehensive representation of ‘success’ for a population.” Measuring productivity of single species and species assemblages is hence critically important in ecology, regardless of whether questions are addressed from an organismic perspective or at the ecosystem level. Not surprisingly therefore, the development and use of methods to measure in situ phytoplankton and bacterial productivity (Hurst et al. 1996) have significantly influenced current views of the structure and functioning of both pelagic and benthic aquatic systems (Hobbie 1994; Schwoerbel 1994). In streams, one of the major energy sources for overall metabolism is derived from riparian trees and shrubs, with leaf litter being a particularly important fraction (Cummins I Present address: Swiss Federal Institute for Environmental Science and Technology (EAWAG), Limnological Research Center, 6047 Kastanienbaum, Switzerland. Acknowledgments We thank S. Y. Newell and K. Suberkropp for discussion, and M. Escautier and A.-M. Jean-Louis for technical assistance during portions of this study. Financial support for this research was provided by the French Ministry of the Environment (EGPN; grant 92221), the Regional Council of Midi-Pyrences (CCRDT; grants RECH9300082 and RECH9307903), and the German Federal Ministry of Research and Technology (BMFT; grant 0339077E). Travel grants were made available through the French-German cooperation program PROCOPE (references 3 12/pro-bmbw-gg and 94074). 1988; Schwoerbel 1994). This allochthonous resource is rapidly colonzed and exploited by a consortium of fungi, bacteria, and detritivorous animals known as shredders (Webster and Benfield 1986; Boulton and Boon 1991). Although shredders may be important in the breakdown of leaf litter in streams (Maltby 1992), recent evidence suggests that a major fraction of this material is diverted to secondary production of saprotrophic fungi (Baldy et al. 1995; Suberkropp 1995) and especially of a group commonly referred to as aquatic hy phomycetes (Barlocher 1992; Suberkropp 1992a). Fungal biomass associated with decomposing leaf litter can in fact excleed 15% of total detrital mass (Gessner and Chauvet 1994; Suberkropp 1995) and accounts for more than 90% of the total microbial (bacterial plus fungal) biomass (Findlay and Arsuffi 1989; Baldy et al. 1995). In addition, aquatic hyphomycctes allocate a substantial portion of assimilated organic matter to nonsexual propagules (up to seven conidia per p,g detrital dry mass per day: Suberkropp 1991; Gessner and Chauvret 1994; Barlocher et al. 1995). Because of this allocation to, and subsequent release of, conidia, losses of mycelial mass due to fragmentation (Suberkropp and Klug 1980) and selective feeding of detritivores on fungal hyphae (Suberkropp 1992b), the overall productivity of aquatic hyphomycetes cannot be adequately assessed by simply measuring differences in fungal biomass. Until recently, a satisfactory evaluation of fungal productivity was hampered by a lack of appropriate methodology. Newell and Fallon (199 1) therefore devised a technique to measure virtually instantaneous growth rates and production of fungi colonizing standing-dead emergent macrophytes in marine and freshwater wetlands. The rationale underlying their method is similar to that adopted for estimating bac-