Comparison of Fungal Activities on Wood and Leaf Litter in Unaltered and Nutrient-Enriched Headwater Streams

Abstract

Fungi are the dominant organisms decomposing leaf litter in streams and mediating energy transfer to other trophic levels. However, less is known about their role in decomposing submerged wood. This study provides the first estimates of fungal production on wood and compares the importance of fungi in the decomposition of submerged wood versus that of leaves at the ecosystem scale. We determined fungal biomass (ergosterol) and activity associated with randomly collected small wood (<40 mm diameter) and leaves in two southern Appalachian streams (reference and nutrient enriched) over an annual cycle. Fungal production (from rates of radiolabeled acetate incorporation into ergosterol) and microbial respiration on wood (per gram of detrital C) were about an order of magnitude lower than those on leaves. Microbial activity (per gram of C) was significantly higher in the nutrient-enriched stream. Despite a standing crop of wood two to three times higher than that of leaves in both streams, fungal production on an areal basis was lower on wood than on leaves (4.3 and 15.8 g C m ؊2 year ؊1 in the reference stream; 5.5 and 33.1 g C m ؊2 year ؊1 in the enriched stream). However, since the annual input of wood was five times lower than that of leaves, the proportion of organic matter input directly assimilated by fungi was comparable for these substrates ( Terrestrial detritus in the form of leaves and wood is an important carbon and energy source for microorganisms and food webs in freshwaters. Detritus-associated microorganisms also control important processes in aquatic ecosystems, including the uptake and sequestration of dissolved inorganic nutrients (33, 37). Fungi, rather than bacteria, dominate the microbial biomass on coarse particulate organic matter (CPOM), such as leaves and wood in streams, and fungal production on submerged leaves is also greater than that of bacteria (10,14,23,28,56). A recent study suggests that fungal ribotype richness levels (as determined by denaturing gradient gel electrophoresis) may be higher than those of bacteria and actinomycetes on submerged leaves (10). Thus, fungi are likely the most important biological driver of processes such as CPOM carbon dynamics and detritus-associated nutrient uptake in headwater streams.Both allochthonous leaf litter and submerged wood in streams harbor diverse communities of fungi, mainly ascomycetes and their anamorphs (hyphomycetes) (see, e.g., references 4, 39, and 51) though chytrids, zygomycetes, and basidiomycetes have also been detected by using molecular techniques (35). Communities can be species rich. For example, more than 200 species from wood in a single river have been reported (51), whereas the diversity of aquatic hyphomycetes that colonize mostly leaves is considerably lower (e.g., see reference 5). Aquatic fungi also show some substrate preferences (13, 22), so leaves and wood harbor different communities that may result in different biomass and activity. While several studies have quantified biomass and production of fungi associate...

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