『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.
Keywords: Alnus;; Topographic wetness index; Nitrogen fixation;
Watershed physiography』
Introduction
Methods
Study area
Sampling and analysis
Data analysis
Results
Discussion
Acknowledgments
References