Effects of nutrient enrichment and leaf quality on the breakdown of leavesin a hardwater stream

1. The breakdown of leaf litter in streams is influenced strongly by leaf q uality and the concentration of dissolved nutrients, primarily inorganic ni trogen (N) and phosphorus (P) in the water. We examined the effect of nutri ent enrichment on the breakdown of three species of leaves in a hardwater, nutrient-rich stream. The rate of microbial respiration was also measured o n the decomposing leaves. 2. The breakdown rates of dogwood (Cornus stolonifera), aspen (Populus trem uloides) and birch (Betula occidentalis), k-values of 0.0461, 0.0307 and 0. 0186 day(-1), respectively, were unaffected by nutrient enrichment and gene rally faster than reported previously. Microbial respiration on the leaves was greater than reported previously for leaves of congeneric species. It a ppears that leaf breakdown in the study stream was not nutrient limited. 3. Nitrogen-based measures of leaf quality, such as percentage N and carbon (C)/nitrogen ratio, did not correspond to measured breakdown rates among t he three leaf types. The best predictors of relative breakdown rates were p ercentage lignin and the percentage of the total carbon that occurred as li gnin. We suggest that, when leaf breakdown is not nutrient limited, measure s of carbon quality (i.e. lignin-based measures) are a better assessment of overall leaf quality than are N-based measures. 4. Previous studies have indicated that the enzymes produced by aquatic hyp homycetes (microfungi) operate most efficiently at a basic pH and in the pr esence of calcium ions. The hardwater conditions (pH = 8.6, total hardness > 300 mg CaCO3 L-1) and abundance of dissolved NO3 and soluble reactive pho sphorous (SRP) (approximately 50 mug L-1, each) in the study stream appear to have provided conditions that resulted in a high respiration rate and ra pid breakdown of leaf litter.