INTERACTION OF SUBSTRATE AND NUTRIENT AVAILABILITY ON WOOD BIOFILM PROCESSES IN STREAMS

We examined the effect of decomposing leaf litter and dissolved inorga nic nutrients on the heterotrophic biofilm of submerged wood in stream s with and without leaves. Leaf litter was excluded from one headwater stream in August 1993 at Coweeta Hydrologic Laboratory in the souther n Appalachian Mountains. We compared microbial processes on wood in th e litter-excluded stream to a reference stream using microbial respira tion, fungal biomass, and extracellular enzyme activity. Exclusion of leaf litter enhanced microbial respiration and extracellular enzyme ac tivity, and fungal biomass was seven times higher than in the referenc e stream. Nutrient-releasing substrates placed beneath wood veneers in dicated colimitation by nitrogen and phosphorus on biofilms in the ref erence stream. Our conclusion is that, in the absence of nutrient immo bilization by leaves, nutrients are more available for other heterotro phic processes. Nutrient limitation may have been responsible for low microbial respiration, fungal biomass, and extracellular enzyme activi ty on wood in the reference stream containing leaves. Our results sugg est that competition for nutrients may regulate heterotrophic microbia l processes in these streams.