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 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. We investigate broad-scale spatial and temporal drivers of ADN, MDN, and freshwater stoichiometry in SW Alaska. To do this, we identified 26 streams and 13 lakes in the Togiak National Wildlife Refuge (TNWR) in this region. Salmon numbers in each lake were estimated via aerial salmon counts. Measurements of alder cover and watershed features were extracted from satellite images of the TNWR in ArcGIS. Stream and lake water samples were collected in the spring and summer from 2010 to 2013 for nutrient analyses, including total N (TN), dissolved inorganic N (DIN: NH4+ and NOx), and total phosphorus (TP). We also calculated annual growing season length (AGSL) from weather records.
Results/Conclusions
Elevation was inversely related to alder cover and N content (alder; ρ = -0.8 (lakes and streams); N content ρ = -0.66 (lakes) and -0.73 (streams), P < 0.05). Salmon abundance declined with elevation (ρ = -0.55, P = 0.06). Alder cover had the largest influence on lake and stream N (β estimates = 0.41 and 0.56 resp., 95% CIs). In the spring, there were significant interactions among alder cover, lake and stream N, and GSL from the previous year (effect sizes = 0.2 and 0.18 resp., 95% CIs). This implies seasonal legacy effects on ADN content. In streams, higher P was associated with lower temperatures, possibly reflecting reduced P demand under low rates of metabolic activity. In lakes, catchment area and salmon abundance drove lake P concentrations (β estimates = 0.48 and 0.39 resp., 95% CIs). N:P declined with elevation in lakes and streams (ρ = -0.62 and -0.71, P = 0.06 and 0.01 resp.). These results demonstrate that spatial variation in alder cover associated with elevation is a stronger regulator of nutrient limitation than temporal variation in growing season conditions. Therefore, the nutrient regimes of freshwater systems at low elevations are more resilient than those at high elevations to climate change.