We sampled periphyton communities in a highly productive stream to characterize how longitudinal changes in watershed geology and land use affect periphyton nutrient status and elemental composition. Nutrient status was evaluated from measures of periphyton nutrient composition (carbon, nitrogen, and phosphorus), stable isotope signatures (δ15N and δ13C), and the response of periphyton to experimental enrichment with nitrogen. Biomass and nutrient content increased dramatically from the headwaters to downstream, while tissue nutrient ratios (C:P and C:N) were more consistent and did not indicate strong N- or P-limitation. Nitrogen enrichment experiments did not exhibit a consistent response upstream or downstream, and periphyton C:N:P stoichiometry showed no significant response to N-enrichment. Absolute densities of periphyton N were 5- to 90-fold greater than the overlying N concentrations in stream water (159- to 353-fold greater for P), and the δ15N signal indicates downstream enrichment from likely watershed sources (urban and agriculture land-use). These results suggest that periphyton in Spring Creek are not N-limited and store large quantities of both N and P, which in turn can be transported downstream during high flow events.
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Acknowledgments We thank M. Johnston-Greenwald and C. Rilk for their technical assistance to the project. CMG was supported in part by funding from the Intercollege Graduate Program in Ecology at Penn State. This research was supported by grants from the United States Geological Survey (Grant # 01HQGR0099) and the Pennsylvania Department of Environmental Protection (Growing Greener grant # 4100034590). Two anonymous reviewers provided beneficial comments on this manuscript.