A Coupled Field and Modeling Approach for the Analysis of Nitrogen Cycling in Streams


The stable isotope stream tracer model (SISTM) calculates the expected 15N content in various stream ecosystem N compartments over distance and time during and after 15N additions to streams. SISTM is a steady state compartment model that predicts δ 15N values based on N stocks and fluxes and the experimental rate of 15N addition. Predicted δ 15N values are compared with observed δ 15N values from a field tracer addition to evaluate our understanding of the N cycle. We demonstrated the use of this tool with information collected from field measurements and a 6-wk 15N-NH4+ addition to the Kuparuk River, Alaska, during the summer of 1991. SISTM was used to run a series of model calibrations that reflected increased information as the experiment progressed. Results of an a priori calibration (using pre-1991 data) yielded a predicted NH4+ uptake length (Sw) of 5.2 km compared with the observed Sw of 0.84 km, and underestimated the δ 15N values of biota in all cases. When discharge and NH4+ concentrations measured during the 1991 experiment were added (model calibration Update 1), the predicted Sw dropped to 0.44 km, indicating that the modeled fluxes overestimated the rate of NH4+ removal by the stream bottom. Adding N stocks and fluxes measured during the tracer addition (Update 2) did not improve predicted Sw, indicating faulty assumptions in our a priori calibration. The observed isotope data were used to estimate the form (NH4+ vs NO3-) of N taken up by primary producers and to improve our representation of the epilithon compartment (Update 3). Including this information brought the predicted Sw to 0.71 km compared with the observed 0.84 km, and resulted in a reasonable correspondence between predicted and observed δ 15N values over the 6-wk addition. SISTM can be used as a framework to 1) summarize N-cycle information prior to a tracer addition, 2) generate testable predictions for field isotope studies, 3) improve our understanding of the N cycle using the field isotope data as constraints on flux estimates, and 4) explore hypothetical N-cycle characteristics. The combined modeling and field tracer experiment approach efficiently provided a synoptic view of the N cycle in streams and rivers.

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Journal of the North American Benthological Society


University of Chicago Press

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