Nutrients stimulate leaf breakdown rates and detritivore biomass: bottom-up eVects via heterotrophic pathways

Most nutrient enrichment studies in aquatic systems have focused on autotrophic food webs in systems where primary producers dominate the resource base. We tested the heterotrophic response to longterm nutrient enrichment in a forested, headwater stream. Our study design consisted of 2 years of pretreatment data in a reference and treatment stream and 2 years of continuous nitrogen (N) + phosphorus addition to the treatment stream. Studies were conducted with two leaf species that diVered in initial C:N, Rhododendron maximum (rhododendron) and Acer rubrum (red maple). We determined the eVects of nutrient addition on detrital resources (leaf breakdown rates, litter C:N and microbial activity) and tested whether nutrient enrichment aVected macroinvertebrate consumers via increased biomass. Leaf breakdown rates were ca. 1.5 and 3£ faster during the Wrst and second years of enrichment, respectively, in the treatment stream for both leaf types. Microbial respiration rates of both leaf types were 3£ higher with enrichment, and macroinvertebrate biomass associated with leaves increased ca. 2–3£ with enrichment. The mass of N in macroinvertebrate biomass relative to leaves tended to increase with enrichment up to 6£ for red maple and up to 44£ for rhododendron leaves. Lower quality (higher C:N) rhododendron leaves exhibited greater changes in leaf nutrient content and macroinvertebrate response to nutrient enrichment than red maple leaves, suggesting a unique response by diVerent leaf species to nutrient enrichment. Nutrient concentrations used in this study were moderate and equivalent to those in streams draining watersheds with altered land use. Thus, our results suggest that similarly moderate levels of enrichment may aVect detrital resource quality and subsequently lead to altered energy and nutrient Xow in detrital food webs.