Evaluating alder and salmon as alternate drivers of lake nutrient availability in southwestern Alaska
Nitrogen (N)-fixing alder (Alnus spp.) and Pacific salmon (Oncorhynchus spp.) provide key nutrient subsidies to ecosystems in southwestern (SW) Alaska. This region supports the greatest salmon runs in the world. Alder is a prevalent constituent of the regional vegetation, and may contribute dissolved nutrients to adjacent aquatic systems. This importance is expected to increase as alder cover expands under climate warming, and as salmon harvesting reduces marine-derived nutrients (MDN) in salmon-spawning habitats. Such nutrient reduction threatens the resiliency of ecosystems dependent on these annual pulses of MDN. This project aims to determine the relative roles of salmon and alder in controlling lake nutrient dynamics in this region. To do this, we identified 11 lakes that are accessible by salmon, and two control lakes that are inaccessible by salmon in the Togiak National Wildlife Refuge (TNWR) in SW Alaska. Salmon-spawner density in each lake was estimated via aerial salmon counts. Measurements of alder cover and watershed features were extracted from satellite images of the TNWR in ArcGIS. Water samples from lakes were collected in the summer and fall from 2011 to 2013 for the analysis of nutrients, including dissolved inorganic nitrogen (DIN: NH4+ and NOx) and total phosphorus (TP).
Alder cover explained 30% more variation in summer lake N concentrations, as compared to the fall (average summer r2=0.62, average fall r2=0.32, p<0.05, n=52) from 2011 to 2013, likely due to greater leaching of alder-derived N from the higher precipitation received during this season. Salmon density explained successively less variation in lake P content across the three years (r2=0.731, 0.462, and 0.446, p=0.01, n=39), likely due to the declining total abundance of salmon the lakes received over that period. Lakes with larger catchment areas also had higher P content in the fall, even after the effect of salmon abundance was removed (r partial=0.699, p<0.01, n=45). Lakes with greater alder cover had higher N:P (average r=0.62, p<0.01, n=65), and mean N:P in lakes with no salmon were not significantly higher than that in lakes with salmon (9.76 vs. 10.8, F=0.07, p=0.79). These results demonstrate that P inputs from watersheds to lakes may be as important as those from salmon, and that alder is increasing N availability and P limitation in these lakes, thereby providing incomplete buffering to declining MDN. Temporal variation in alder-derived N fluxes and salmon abundance also play deterministic roles in nutrient availability in this system.