BACTERIA, FUNGI AND THE BREAKDOWN OF LEAF-LITTER IN A LARGE RIVER

We examined the dynamics of leaf mass loss and microbial biomass assoc iated with decomposing leaf litter in a seventh order river. This was in an attempt to test the hypothesis that fungal contribution to the b reakdown process is less important in major rivers than was previously found for headwater streams. Bacterial biomass was estimated from dir ect cell counts coupled with determinations of bacterial biovolumes. F ungal biomass was estimated on the basis of ergosterol measurements, s pecies-specific conversion factors, and the relative abundance of the dominant fungal species. Sporulation rates of aquatic hyphomycetes wer e determined by counting conidia released from leaf litter during brie f laboratory incubations. Compared to low-order streams, the breakdown of willow, poplar and plane leaves was slow with exponential decay co efficients k ranging from 0.0045 to 0.0091 d(-1). Numbers of bacteria first increased exponentially on all leaf species but reached a platea u of almost 10(8) cells per mg AFDM after 4-8 weeks of leaf submergenc e. This corresponds to a peak bacterial biomass of 0.3-0.5% of detrita l carbon. Fungal biomass attained peaks of 5-10% of detrital carbon af ter 4-8 weeks and greatly exceeded bacterial biomass al any instance. On average, fungi accounted for 96% of the total microbial (fungal plu s bacterial) biomass in leaf litter. Dynamics of sporulation rates of aquatic hyphomycetes were characterized by early peaks of 1.2-1.4 coni dia mu g(-1) AFDM d(-1), followed by sharp declines to about 0.2 mu g( -1) d(-1). Peaks occurred before the corresponding peaks in fungal bio mass. Rough organic matter budgets suggest that fungi assimilated a mi nimum of 16-23% of the initial leaf carbon, and accounted for 42-65% o f 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 litt er 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 pres ent results can be generalized for lotic ecosystems.