Interaction of substrate and nutrient availability on wood biofilm processes in streams

We examined the effect of decomposing leaf litter and dissolved inorganic nutrients on the heterotrophic biofilm of submerged wood in streams with and without leaves. Leaf litter was excluded from one headwater stream in August 1993 at Coweeta Hydrologic Laboratory in the southern Appalachian Mountains. We compared microbial processes on wood in the litter-excluded stream to a reference stream using microbial respiration, fungal biomass, and extracellular enzyme activity. Exclusion of leaf litter enhanced microbial respiration and extracellular enzyme activity, and fungal biomass was seven times higher than in the reference stream. Nutrient-releasing substrates placed beneath wood veneers indicated colimitation by nitrogen and phosphorus on biofilms in the reference stream. Our conclusion is that, in the absence of nutrient immobilization by leaves, nutrients are more available for other heterotrophic processes. Nutrient limitation may have been responsible for low microbial respiration, fungal biomass, and extracellular enzyme activity on wood in the reference stream containing leaves. Our results suggest that competition for nutrients may regulate heterotrophic microbial processes in these streams.