Nitrogen uptake and food web nitrogen transfer in a restored urban stream compared to an unrestored urban stream and a pristine stream

摘要

Urbanization of low-order streams can cause an increase in nitrogen (N) concentration and a decrease in biotic diversity (Lenat 1993). These impacted streams are less efficient at N cycling, and have slower N uptake rates (Meyer et al. 2005). Because over $ 1 billion is spent in the United States annually to restore urbanized streams, the hydrological and biological results of stream restoration are often monitored and studied (Muotka et al. 2002). In contrast, only recently have there been studies examining changes in N retention and transport in restored streams (Bukaveckas 2007). The short-term addition of N heavy stable isotope (~(15)N) into streams can be used to measure N uptake rates (Mul-holland et al. 2002); long-term additions can trace N storage in biotic compartments and trophic interactions (Tank et al. 2000). The ~(15)N tracer additions have been shown to label stream epilithon more rapidly and at a higher level compared to detrital components and can therefore be used to estimate N uptake parameters at whole-reach scales (Mulholland et al. 2000, Wollheim et al. 2001). Based on the distribution of ~(15)N stored in the benthic epilithon, estimated N travel length and uptake rate can be compared between streams. Our aim was to determine if restoration of an urban stream increases the uptake of N into the biotic components when compared to an unrestored urban stream and a pristine Piedmont stream by conducting tracer additions of NH_4~+-~(15)N.