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 NO x -N concentration (nitrate ? nitrite) and daily NO x -N yield using Generalized Additive Models (GAM) and compared using Akaike's Information Criterion (AIC c ). Alder cover was the only probable model and explained 75-96% of the variation in NO x -N concentration and 83-89% of the variation in daily NO x -N yield. The relationship between alder and both NO x -N concentration and daily NO x -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 Nsaturated. 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.

Pacific salmon (Oncorhynchus spp.) runs and consumer fitness: growth and energy storage in stream-dwelling salmonids increase with salmon spawner density

“…Salmon-free scrapers were the most enriched in the Cook Inlet Basin ecoregion, where finer soils associated with the meandering stream channels may foster denitrification and, in turn, an isotopically enriched nitrogen pool (Pinay et al 2003). Salmon-free scrapers were the most depleted in the Gulf of Alaska Coast ecoregion, where depleted inputs from extensive stands of nitrogen-fixing alder may dominate the nitrogen pool (Compton et al 2003;Shaftel et al 2010). Additional post hoc support for the magnitude of these ecoregional baselines came from our study streams, where our model predicted that in a stream with zero spawners, coho salmon and Dolly Varden would be enriched by about 5.4‰ and 4.7‰, respectively, over grazer macroinvertebrates.…”

Salmon nutrients are associated with the phylogenetic dispersion of riparian flowering‐plant assemblages

“…Riparian areas in larger watersheds are wider, more productive and often support early seral species such as the nitrogen fixing red alder ( A. rubra ; Shaftel et al. , Harding and Reynolds ).…”

Catchment topography and wetland geomorphology drive macroinvertebrate community structure and juvenile salmonid distributions in south-central Alaska headwater streams

“…We estimated catchment slope (WSlope) by computing the slope of each 60-m pixel in the DEM and averaging by catchment. We computed mean topographic wetness (Wetx) using both upstream contributing area (A) and local (pixel-by-pixel) slope (b) with the equation: ln(A/tanb) (Sorenson et al 2006, Shaftel et al 2012. We computed a similar metric, flow-weighted slope (FWSlope), as the sum of the product of the slope and flow accumulation value for each pixel in the catchment divided by the sum of the flow accumulation of all pixels in the catchment.…”