Alder cover drives nitrogen availability in Kenai lowland headwater streams, Alaska

Abstract

Terrestrial sources of nitrogen (N), particularly N-fixing alder, may be important for sustaining production in headwater streams that typically lack substantial subsidies of marine-derived nutrients from spawning salmon yet support upstream-dispersing juvenile salmonids. However, other physiographic characteristics, such as watershed slope and topographic wetness, also control transport of nutrients to streams and may confound apparent linkages between alder and stream N. Seasonal patterns in precipitation and temperature may interact with watershed characteristics to modulate stream N availability. We empirically modeled the effect of alder cover and other watershed physiographic variables on stream N and contrasted these relationships over the growing season among 25 first-order streams from the lower Kenai Peninsula, Alaska. For each date, percent alder cover, mean topographic wetness, and mean slope were used as watershed predictors of NOx–N concentration (nitrate + nitrite) and daily NOx–N yield using Generalized Additive Models (GAM) and compared using Akaike’s Information Criterion (AICc). Alder cover was the only probable model and explained 75–96% of the variation in NOx–N concentration and 83–89% of the variation in daily NOx–N yield. The relationship between alder and both NOx–N concentration and daily NOx–N yield changed from constant inputs in May across the range of alder cover (linear fit) to increasing inputs in July and September (non-linear fits) implying that high-alder watersheds were N-saturated. The strong linkage between alder and stream N coupled with the concurrent timing of maximum stream N from alder in the spring to salmon fry emergence indicates the potential importance of this subsidy to headwater stream ecosystems.