Infl uence of shredder feeding and nutrients on fungal activity and community structure in headwater streams

In stream detrital food webs, interactions occur between aquatic hyphomycetes associated with decomposing leaves and shredders consuming those leaves. However, few studies have examined how the feeding activity of shredders affects aquatic hyphomycetes. We examined the effect of shredder feeding on aquatic hyphomycete communities associated with submerged leaves in two southern Appalachian headwater streams in Coweeta Hydrologic Laboratory, North Carolina, USA. Coarse (allowing shredder access) and fi ne (preventing shredder access) mesh litter bags containing red maple (Acer rubrum) leaves were placed in the treatment stream (C54) which was enriched with nitrogen (N) and phosphorus (P), and in the reference stream (C53) and were retrieved monthly. Both shredder feeding and nutrient enrichment enhanced breakdown rates. The breakdown rates of leaves in coarse mesh bags in the reference stream (k = 0.0275) and fi ne mesh bags in the nutrient enriched stream (k = 0.0272) were not signifi cantly different, suggesting that the higher fungal activity stimulated by nutrient enrichment could increase the relative contribution of fungi to leaf breakdown to the level similar to that of shredders in the reference stream. Macroinvertebrate abundance and biomass were higher in the litter bags submerged in the treatment stream. Fungal sporulation rates and biomass were higher in the treatment stream than in the reference stream, but neither fungal biomass nor sporulation rate was affected by shredder feeding in either stream. The enrichment with N and P altered fungal community composition more than shredder feeding. Species richness was higher in the nutrient enriched stream than in the reference stream, and fungal assemblages from fi ne and coarse mesh bag treatments within a stream were more similar to each other than the fungal assemblages from the same mesh bag treatments but from different streams.