In ecosystems in which the food web is primarily based on deciduous leaf litter, the rate of breakdown of this litter is a critical process and alterations of this rate by disturbance can lead to dramatic changes in ecosystem functioning. Several previous studies of urban stream organic matter have found lower benthic organic matter standing stocks and higher rates of decomposition than comparable forested streams, and have attributed these differences to a combination of reduced inputs, higher temperatures, and higher hydrologic losses in hydrologically altered urban streams.  We quantified benthic organic matter standing stocks and distribution in 9 streams across an urbanization gradient in the Piedmont of North Carolina.  Despite similar standing stocks, we observed that buried organic matter in the urban streams consisted primarily of intact, blackened leaves, while the organic matter in forested streams was dominated by partially decomposed organic muck.  This led us to wonder if decomposition rates of organic matter are different in these urban and forested streams compared to those previously studied.  We deployed surface and buried (approximately 20 cm) leafpacks of Acer rubrum leaves in a subset of two urban and two forested streams. Based on the timing of previous studies, we collected the surface leafpacks at intervals to 80 days and the buried leafpacks at intervals to 245 days. 

Results/Conclusions

Contrary to previous studies, we found similar ranges of benthic organic matter across both urban (3-10 mg AFDM cm^-3) and forested (2-11 mg AFDM cm^-3) streams.  Both temperatures and nitrogen concentrations were consistently higher in the urban streams.  In contrast to previous studies, we measured slightly reduced rates of breakdown of surface leafpacks in the urban streams (k = 0.0052 and 0.0062) versus forested streams (k = 0.0071 and 0.0101) and greatly reduced rates of breakdown for leafpacks buried in sediments in the urban streams (k = 0.0012 and 0.0019) when compared to the forested streams (k = 0.0038 and 0.0068).  The reduced decomposition of buried organic matter could be due to low dissolved oxygen or heightened exposure to urban contaminants in the urban stream sediments.  To explore this hypothesis, we measured short-term decomposition rates for substrates placed in 80 streams across a gradient of watershed imperviousness in the NC Piedmont and in which we simultaneously measured trace metal content and redox potential within the sediments.