The impact of urbanization on the transient storage characteristics, phosphorus uptake dynamics and community metabolism of Valley Creek

This work was conducted as part of a comprehensive investigation of the effects of urbanization on the Valley Creek watershed, located in Chester County approximately 30km west of Philadelphia, Pennsylvania. The goal of this research was to monitor the impacts of perturbations caused by urbanization as those impacts were carried through the ecosystem. Results from field experiments are presented which clearly demonstrate that the fine sediment fraction of the stream bed controls hydraulic conductivity and hyporheic exchange in this gravel- and cobble-bed, piedmont stream. The laboratory results of Packman and MacKay (2003) were confirmed with results from field experiments at a site in which the fraction of the fine (d < 2 mm) bed sediment with d < 50 mum increased from 6% to 25% while the hydraulic conductivity decreased by a factor of two, the hyporheic exchange rate decreased by an order of magnitude and the hyporheic area decreased by a factor of four. These results were obtained by estimating hydraulic conductivity of the stream bed from the d10 of the fine sediment fraction and using a method presented by Worman et al. (2002) in which hyporheic residence time scaled by hydraulic conductivity and stream depth is a function of stream velocity and channel characteristics. The phosphorus uptake rate, the gross primary productivity rate, and the community respiration rate were each shown to increase linearly with the hyporheic exchange rate at this site. At a less severely impacted site, the phosphorus uptake rate was controlled by a combination of the hyporheic exchange rate, the fraction of the fine bed sediments with d < 50 mum), and the aluminum content of the fine bed sediments. These results extend and expand on the results of Mulholland et al. (1997), who reported that streams with larger hyporheic storage zones have higher phosphorus uptake rates (s-1); Fellows et al. (2001) who reported higher community and hyporheic respiration rates (mg O2 m-2 d-1 ) in streams with larger hyporheic storage zones; and Hall et al. (2002) who found no relationship between phosphorus uptake and hyporheic storage area in a stream with little biological phosphorus uptake.