| Summary: | Understanding the direction and impact of nutrient fluxes across ecosystem boundaries is fundamental to ecology. Nitrogen (N)-fixation by alder (Alnus spp.) and Pacific salmon (Oncorhynchus spp.) provide key nutrient subsidies to freshwater systems. Southwestern (SW) Alaska supports some of the greatest salmon runs in the world. Alder is also a prevalent constituent of the regional vegetation. The importance of alder-derived nutrients (ADN) in the tundra is expected to increase as alder cover expands under climate warming, and as salmon harvesting reduces marine-derived nutrients (MDN) in salmon-spawning habitats. I evaluated the drivers and impacts of alder- and salmon-derived nutrients on aquatic systems by analyzing stream and lake water chemistry from a suite of 26 streams and 13 lakes, over a four-year period in SW Alaska. Additional water samples from all the study lakes were collected for analyses of microbial community structure and function. I statistically modeled relationships among aquatic nutrient concentrations, alder and salmon abundance, physiographic features within each watershed, and meteorological conditions to determine the ultimate controls on aquatic nutrient dynamics in this region. To assess the relative impacts of alder and salmon on aquatic microbial communities, I compared shifts in bacterial community composition and microbial function with catchment physical features and lake-water chemistry. I also measured microbial metabolic responses to N, P, and N+P enrichment to assess variation in nutrient limitation. My results reveal that watershed elevation is a key factor controlling the quantities of alder-derived N in streams. Elevation was inversely related to alder cover and N yield (ρ = -0.8 and -0.73 respectively, P < 0.05 for both). Alder cover had the largest influence on stream N (β estimate = 0.56, P < 0.05). In streams, higher P was associated with lower temperatures, possibly reflecting reduced P demand under low rates of metabolic activity. Stream N:P declined with ...
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