Geomorphic control and temporal variability of Alnus-derived nutrients: Implications for aquatic ecosystem resilience

Background/Question/Methods

Nitrogen-fixing alder (Alnus spp.) and Pacific salmon (Oncorhynchus spp.) provide key nutrient subsidies to ecosystems in southwestern (SW) Alaska. This region supports the greatest wild 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. We investigate how geomorphic and temporal factors control the extent to which alder-derived nutrients can buffer aquatic systems from declining MDN in 26 streams in the Togiak National Wildlife Refuge (TNWR) in SW Alaska. Alder cover and watershed features were measured using satellite images and topographic maps in ArcGIS. Water samples from streams were collected in each summer and fall from 2010 through 2013 for nutrient analyses, including NH₄⁺, NO_x total dissolved N (TDN), and total dissolved phosphorus (TDP). Lysimeters were installed in soils under alder-dominated and non-alder communities to assess N and P leaching from alder. 

Results/Conclusions

The strength of the relationship between alder cover and stream N increases from 2010 to 2012 (r=0.63 to 0.92 for NO_x, p<0.01, n=78) likely due to the successively later timing of snowmelt over this period. This relationship weakens in 2013 (NO_x; r=0.69, p<0.01, n=26), probably resulting from widespread insect defoliation, reducing alder-N fixation rates. Removing watershed elevation most significantly weakened the relationship of alder cover and stream N (r_partial reduction = 0.27). Variable importance ranking identified alder cover, year and watershed elevation as major drivers in the model derived for stream N availability (R²=36%). Stream N:P is consistently related to alder cover (average r=0.71, p<0.01, n=26). In 2011 and 2012, alder patches leached 25-35kg NO_x ha^-1 yr^-1 more than non-alder patches (F=29.2, p<0.01), while DRP flux from beneath alder patches was not significantly larger than that from non-alder patches (2.66kg PO₄ ha^-1 yr^-1, F=0.61, p=0.45). These results demonstrate strong regulation of alder-derived nutrient contributions to aquatic systems by inter-annual variations in weather and watershed elevation. Alder increases stream N availability and P limitation, thereby providing incomplete buffering to declining MDN. Geomorphic factors, in particular watershed elevation, play a deterministic role in nutrient availability and thereby ecological resilience in these systems.