Bacteria, Fungi and the Breakdown of Leaf Litter in a Large River

We examined the dynamics of leaf mass loss and microbial biomass associated with decomposing leaf litter in a seventh order river. This was in an attempt to test the hypothesis that fungal contribution to the breakdown process is less important in major rivers than was previously found for headwater streams. Bacterial biomass was estimated from direct cell counts coupled with determinations of bacterial biovolumes. Fungal biomass was estimated on the basis of ergosterol measurements, species-specific conversion factors, and the relative abundance of the dominant fungal species. Sporulation rates of aquatic hyphomycetes were determined by counting conidia released from leaf litter during brief laboratory incubations. Compared to low-order streams, the breakdown of willow, poplar and plane leaves was slow with exponential decay coefficients k ranging from 0.0045 to 0.0091 d-1. Numbers of bacteria first increased exponentially on all leaf species but reached a plateau of almost 108 cells per mg AFDM after 4-8 weeks of leaf submergence. This corresponds to a peak bacterial biomass of 0.3-0.5% of detrital carbon. Fungal biomass attained peaks of 5-10% of detrital carbon after 4-8 weeks and greatly exceeded bacterial biomass at any instance. On average, fungi accounted for 96% of the total microbial (fungal plus bacterial) biomass in leaf litter. Dynamics of sporulation rates of aquatic hyphomycetes were characterized by early peaks of 1.2-1.4 conidia μ g-1 AFDM d-1, followed by sharp declines to about 0.2 μ g-1 d-1. Peaks occurred before the corresponding peaks in fungal biomass. Rough organic matter budgets suggest that fungi assimilated a minimum of 16-23% of the initial leaf carbon, and accounted for 42-65% of the overall carbon loss from leaves during periods of highest fungal activity. Taken together, these findings indicate that fungi play an eminently important role in the biological transformation of leaf litter even in major rivers. Bacterial contribution is likely to be small in spite of increases in biomass at advanced stages of breakdown. With regard to leaf decomposition, large fluvial systems would thus appear to behave like their headwater counterparts, suggesting that the present results can be generalized for lotic ecosystems.