Effects of consumers and nitrogen availability on heterotrophic microbial activity during leaf decomposition in headwater streams

1. Microbial processes in streams may be influenced by bottom-up factors, such as nutrient availability, and by top-down factors, such as the activity of upper trophic levels. The latter pathway has been well described between autotrophs and grazers, but the effects of consumers on heterotrophic basal trophic levels may also be important. Consumers may influence microbial activity through consumption of microbial biomass and by altering nutrient availability via consumer nutrient recycling (CNR). 2. We investigated the effects of two shredder taxa, Gammarus spp. and Tipula spp., on nitrogen (N) uptake during organic matter decomposition by leaf-associated heterotrophic microbial assemblages under conditions of ambient and enriched N availability in stream mesocosms. Colonised red maple leaves were placed in mesocosms with low (<20 mu g L-1) or high (>800 mu g L-1) nitrate concentration in the presence of amphipods, tipulids or no shredders. To test the importance of consumption and CNR, leaf packs were constructed from both coarse and fine mesh, which respectively allowed or prevented shredders from consuming detritus. 3. Nitrogen enrichment generally increased red maple breakdown rates, while shredder influence on leaf breakdown was taxon specific. Feeding by tipulids increased red maple breakdown rate compared with amphipods, although not significantly. While feeding by amphipods did not directly influence breakdown rates, amphipod nutrient recycling stimulated leaf breakdown rate by the same magnitude as the N-enrichment treatment. 4. Fungal biomass increased with N enrichment. However, this did not lead to changes in N uptake, which was extremely variable and did not respond significantly to either treatment. In general, N uptake (per leaf mass) was greatest under enriched N conditions when tipulids were present. 5. Our results indicate that both top-down and bottom-up factors influence microbial activity in heterotrophic systems and suggest that consumers may play important roles in detrital processing, not only through direct consumption but also through nutrient recycling.