Thursday, June 7, 2007 - 10:15 AM
374

Ammonium uptake and transformation in two arctic streams with contrasting disturbance regimes using simultaneous N tracer additions

Thomas Heatherly II1, Diane Sanzone2, Heidi Wilcox1, Jon Benstead1, Alex Huryn1, Stephanie Parker1, and Bruce J. Peterson3. (1) Biological Sciences, University of Alabama, 2620 University Blvd, Tuscaloosa, AL 35401, (2) National Park Service, 4175 Geist Rd., Fairbanks, AK 99709, (3) Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543

15N isotope addition experiments are effective for tracing the fate and turnover of nitrogen in streams without confounding enrichment effects.  Over 28 days in summer 2002, we added 15N simultaneously to contrasting, adjacent tributaries of the Ivishak River, Arctic National Wildlife Refuge, Alaska, U.S.A.  One stream (Ivishak Spring) was fed primarily by groundwater and thus had relatively stable discharge and temperature regimes.  The other stream (Ivishak Mountain) was fed mainly by snowmelt and thus was characterized by more frequent and intense disturbances.  Ivishak Spring had ammonium uptake lengths that were slightly longer than most previously studied streams (~70 m), but had the highest uptake velocities yet recorded (0.796 and 0.697 mm s-1on days 1 and 28). Ammonium uptake lengths also were relatively long in Ivishak Mountain (84 and 250 m), which also had the third highest uptake velocities yet recorded (0.346 and 0.116 mm s-1 on days 1 and 28).  These high rates of transformation of inorganic nitrogen to organic suggest that these streams have a high biological demand for ammonium, likely because of low natural concentrations and relatively high primary producer biomass.

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