Here, we present the results of a LINX 1-style 15N tracer addition conducted in the summer of 2002 in two adjacent headwater tributaries of the Ivishak River in the Arctic National Wildlife Refuge, Alaska. Our objective was to assess how contrasting disturbance regimes drive differences in stream N-uptake dynamics. The Ivishak spring-stream is groundwater-fed and maintained relatively constant discharge during the 15N addition (110-180 L/s). The Ivishak mountain stream is fed by precipitation and experienced more variable discharge through the addition period (60-2200 L/s). The isotope tracer (15NH4Cl) was added simultaneously to both reaches for 4 weeks to quantify N-uptake dynamics and food-web interactions. Preliminary results show that 15N uptake was rapid in both streams. Uptake lengths of 15N-NH4 (Sw) in the mountain stream were high at the beginning of the tracer release (7 km) due to elevated discharge but decreased significantly thereafter (200-400 m). Uptake lengths and their temporal variability in the spring stream were consistently lower (80-150 m). Most of the added 15N-NH4 in the spring-stream was taken up by bryophytes, whereas uptake of N in the mountain stream was dominated by filamentous algae. Chironomids appeared to be feeding primarily on epilithon and filamentous algae, and simuliids fed on seston in both streams. Additionally, the spring-stream contained a predatory stonefly (Isoperla petersoni), which became enriched midway through the addition. We are currently analyzing this unpublished dataset with the Bayesian hidden Markov model in the R package ‘IsoTracer’ to quantitatively compare N-dynamics in streams with differing disturbance regimes.