Microbial activity on wood in streams: exploring abiotic and biotic factors affecting the structure and function of wood biofilms

In this examination of microbial colonization of wood and factors affecting the structure and function of wood biofilms in streams, the first two chapters summarize research conducted in New Zealand and compare the composition of biofilm colonizing wood to that colonizing rocks and leaves. Similar biofilms developed on wood veneer, natural twigs, and beech leaves, but fungi did not colonize stones where diatoms were the predominant colonizer. Comparing wood incubated on the streambed surface to wood buried beneath the streambed, fungal hyphae dominated the biofilm but actinomycetes and bacteria were also present. An assay of microbial activity (¹⁴C glucose uptake) indicated that surface biofilms were more active than biofilms on buried wood. There was no relationship between ¹⁴C glucose uptake and stream pH indicating that acidity did not affect wood biofilm activity in these streams. Biofilm activity on wood buried at 3-9 cm in the streambed was not significantly different than that buried at 19-25 cm.

Chapters 3, 4, and 5 examine the processes governing wood biofilms in the presence and absence of leaf litter in 2 small mountain streams at Coweeta Hydrologic Laboratory in the southern Appalachians. Microbial respiration, fungal biomass, extracellular enzyme activity, and the effect of nutrient addition were used as descriptors of wood biofilms. Exclusion of leaf litter from a headwater stream enhanced extracellular enzyme activity, and fungal biomass was 7 times higher than that in the reference stream. Relative activities of selected extracellular enzyme activities suggested that the biofilm in the reference stream was nutrient limited. Also, nutrient releasing substrates placed beneath wood veneers indicated co-limitation of nitrogen and phosphorus on biofilms in the reference stream. Competition for nutrients by microbial biofilms may play a regulatory role in detrital processing in these streams.

Laboratory feeding studies using Tallaperla sp. were conducted to explore the suitability of wood biofilms as a food resource for shredders. There were no differences in Jallaperla growth rates on wood and leaves, and Tallaperla grew equally well on wood incubated for 1 or 2 months. At the end of each study, fungal biomass on wood in the feeding chambers was not different from fungal biomass at the beginning indicating that Tallaperla nymphs were not food limited and fungal production was able to compensate for invertebrate grazing. In the absence of leaf litter, stream shredders such as Tallaperla can survive and grow on the microbial biofilm on wood. The carbon stored in wood in streams is transferred to higher trophic levels via microbial biofilms.